Fuel storage rack system for underwater storage of spent nuclear fuel

Abstract

A fuel rack for wet storage of nuclear fuel in a fuel pool in one embodiment comprises a baseplate and a cellular body formed of interlocked stacked slotted plates coupled to the baseplate. The body comprises tightly-packed upwardly open cells which each hold a nuclear fuel assembly. Open flux traps are formed between at least some cells which fill with water when the rack is submerged to act as a neutron moderator to control reactivity in the rack. In one embodiment, a flux trap is interspersed between adjacent cells running in a first direction along a first horizontal axis of the rack. However, a flux trap is not interspersed between adjacent cells running in a perpendicular second direction along a second horizontal axis. Other flux trap and cell arrangements are provided. Tension elements extend through some flux traps which compress the stack of plates together for stability.

Description

FUEL STORAGE RACK SYSTEM FOR UNDERWATER STORAGE OF SPENT NUCLEAR FUELCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims the benefit of priority to U.S. Provisional Application No. 63 / 558,778 filed February 28, 2024, and U.S. Provisional Application No. 63 / 492,586 filed March 28, 2023; the entireties of which are incorporated herein by reference.BACKGROUND

[0002] The present invention generally relates to systems for wet storage of spent nuclear fuel (SNF) underwater, and more particularly to an improved nuclear fuel storage rack system for use in a fuel pool in a nuclear’ generation plant.

[0003] Storing spent nuclear fuel (SNF) discharged from the nuclear reactor in a deep pool of water referred to as the “fuel pool” is a standard practice for light water reactors. The physical embodiment of the nuclear fuel storage device is simply called the “fuel rack” in the art. Fuels racks are designed to sit on the floor of the nuclear fuel pool. The fuel rack comprises a means to hold the fuel assemblies in a specified horizontal / lateral pitch guided by the reactivity limit permitted by the nuclear regulatory guidelines.

[0004] A conventional free-standing, high density nuclear fuel storage rack (“fuel rack” for brevity) is a multi-cellular structure typically supported on a set of pedestals from the floor or bottom slab of the water-filled spent fuel pool. The bottom extremity of the structure which defines the array of fuel storage cells is welded to a common baseplate which is seated on the floor of the fuel pool. The baseplate serves to provide the support structure which carries the weight of cell structural members and fuel assemblies stored in the cells, and in turn transmits the weight load to the reinforced concrete floor of the fuel pool. The cells each comprise vertically elongated prismatic cavities which are each configured in cross-sectional area and height to accept only a single nuclear fuel assembly therein comprising a plurality of new or spent nuclear fuel rods.

[0005] The term “active fuel region" denotes the vertical space above the baseplate in the intermediate portion of the fuel rack between the top and bottom extremities where the enriched uranium is located due to the design of the fuel assemblies. This is the most radiation active (reactive) region of the fuel rack. The top and bottom extremities of the rack are less reactive regions.

[0006] Fuel racks used to store spent nuclear fuel assemblies hold them upright in the pool of water which serves to remove the generated heat, protect them against damage under seismic conditions, and controls reactivity. There are two styles of fuel rack and corresponding fuel assemblies in use. The nuclear fuel assemblies used in most Russian-origin reactors are elongated structures with a hexagonal cross sectional shape. To store the hexagonal cross section fuel assemblies, it is desirable to have a rack module that has hexagonal cells in cross- sectional shape such that the quantity of water in the storage cavity and around it is precisely controlled, which is necessary to achieve the desired sub-criticality of the stored fuel array. The fuel must also be elevated above the pool liner (floor or bottom slab) such that there is a water plenum underneath the rack which the rack’ s design configuration must facilitate to deliver cold water to the space around the fuel and through its inter-rod spaces by natural convective thermosiphon action flow.

[0007] Fuel assemblies used in Western-origin reactors are similarly elongates structures, but have a rectilinear (e.g., square) cross-sectional shape. Storage of this type of fuel assembly requires cells with a corresponding rectilinear cross-sectional shape.

[0008] Since the available floor area in the fuel pool is limited, the guiding principle in the design of fuel storage devices such as fuel racks is to maximize the density of nuclear fuel storage. Thus the fuel assemblies, which are vertically elongated structures that hold a plurality of uranium fuel rods in tightly packed relationship, are oriented and arrayed vertically in the fuel storage cells of the fuel racks with as little lateral space between the cells as possible to pack as many fuel assemblies into each rack as possible, and concomitantly pack as many fuel racks in the fuel pool as possible for high density SNF storage.

[0009] The fuel rack designs used in the past have relied on a stainless-steel cell structure to provide the structural strength for fuel storage. Reactivity between cells and fuel assemblies in the fuel rack however is controlled with non- structural boron-containing or bearing absorber materials which control neutron radiation transmission and concomitantly criticality control of the nuclear fuel stored in the rack. These materials have low strength in comparison to the steel cell structure and are relatively brittle from an impact standpoint.

[0010] The minimum permissible spacing between storage cells in each fuel rack is controlled by the reactivity of the fuel in the fuel assemblies and the availability of the B-10 (Boron- 10 isotope) content in the neutron absorber which is typically made in the form of plates of analuminum alloy impregnated with boron carbide powder referred to as borated aluminum. A widely used type of borated aluminum is sold under the trade name Mctamic™ available from Holtec International of Camden, New Jersey, which is an aluminum boron carbide metal matrix composite material. In fuel racks formed entirely of stainless steel to form the fuel storage cells, neutron absorber panels are inserted in various spaces or pockets formed between each metallic cell. The fuel storage device must also fulfill other requirements such as keeping the fuel cool (spent fuel rods emit heat energy as its nuclides undergo radioactive decay) and possessing sufficient structural strength to withstand natural environmental phenomena such as earthquakes or impacts due to an accidental drop of an object carried over the pool by cranes.

[0011] The conventional all metal cell fuel rack design suffers from several drawbacks, as follows. While stainless steel is highly amenable to welding, stainless weldments suffer from considerable weld-induced distortion which forces the nominal cell openings to be made larger than necessary to accommodate the loss of the prismatic fuel assembly storage openings (i.e. cells) in the fuel rack due to weld distortion. This directly and adversely impacts the density of storage. As a fully welded structure, the prior fuel racks have a low structural damping which is a critical factor in dampening the structural response of the rack (a high structural damping being a desirable attribute).

[0012] An alternative fuel rack design minimizes the amount of welding in the fuel rack (and hence weld-induced distortion in the storage cells) and creates a true “egg-crate” construction is disclosed in commonly-owned U.S. Patent No. 10,650,933 assigned to Holtec International, which is incorporated herein by reference. This fuel rack is formed from a plurality of slotted panels or plates which are mechanically interlocked to construct the fuel storage cells from vertically stacked rows of the plates rising upwards from the baseplate. Vertically elongated spaces created internally within the fuel rack by the intersecting slotted plates form the array of fuel storage cells.

[0013] Additional design requirements for the fuel rack are: (a) the hexagonal cavities must provide a smooth interface for the fuel to enable unobstructed insertion and removal operations; (b) the side walls of the storage cavities must be capable of withstanding the lateral loading from the rattling of the fuel assemblies during a seismic event; and (c) in the case of free-standing modules (i.e. not fastened to spent fuel pool floor slab), the racks must have sufficient flexural rigidity to withstand the nuclear plant’s Design Basis Earthquake without excessive movement.

[0014] Improvements in fuel racks formed of interlocked slotted plates to create fuel storage cells for wet storage of SNF in the fuel pool is desired.SUMMARY

[0015] The present application discloses several design variations of fuel racks suitable for wet storage of spent nuclear fuel (SNF) in a spent fuel pool of a nuclear generation facility. Improvements over past designs are provided which increases both the structural strength of the fuel racks to withstand impacts and seismic loads, and fuel storage density. In one embodiment, each rack comprises a baseplate configured for placement on the floor of fuel pool and a cellular body supported and rising upwards from a top surface of the baseplate. The body comprises a plurality of interlocked slotted panels or plates which when assembled defines an array of fuel storage cells each configured in cross-sectional dimension and area to only hold a single fuel assembly. Each fuel rack includes a combination of stainless steel slotted plates for structural strength and boron-containing neutron absorber slotted plates for reactivity control in the active fuel storage region of the rack.

[0016] The fuel racks disclosed herein include hexagonal and rectilinear (e.g., square) cells for storage of Russian and western type fuel assemblies, respectively. Fuel racks of the flux trap and non-flux trap designs are provided and further described herein.

[0017] Depending on the reactivity of the fuel being stored, it is possible to array the fuel storage cells in the middle “active fuel region” of the fuel rack with a single wall of neutron absorber material (e.g., Metamic™ or other) between them when the fuel assembly energy is low. These are called “non-flux trap” racks in the art. Flux is the amount of radiation (e.g., neutron, alpha particles, etc. or energy emitted from the nuclear fuel contained in the fuel assemblies. In the case of fuel assemblies of a large cross section such as those found in PWRs (pressurized water reactors) which contain a large amount of nuclear fuel and concomitantly higher flux, it may be necessary to locate the storage cells with a vertically extending small space or gap (called “flux trap”) provided between them. These are called “flux trap” racks in the art. The gaps or flux traps are filled with water when the fuel racks are submerged in the fuel pool to ameliorate radiation flux between the fuel assemblies in adjacent cells of the racks. Neutron particles emitted from the fuel assemblies are moderated by the water-filled traps. The design anatomy of both flux trap and non-flux trap racks is similar but not identical. The commonalities and differences between these two type racks will be described further herein.

[0018] In order to increase the number of fuel assemblies which can be stored in the fuel rack, and concomitantly the entire fuel pool, a rectilinear cell fuel rack is disclosed having a unidirectional flux trap configuration in which: (1) a flux trap is present between adjacent ones of the fuel storage cells in a first direction along a X-axis of a Cartesian coordinate system; and (2) a flux trap is not present between adjacent ones of the fuel storage cells in a second direction along a Z-axis of the Cartesian coordinate system. The flux traps are dimensioned so that a Ineffective factor for the fuel rack is less than 1, and preferably 0.95 or less as further described herein to maintain the subcriticality state of the fuel rack when the fuel rack is fully loaded with a stored array of nuclear fuel assemblies. The phrase “fuel rack is fully loaded with a stored array of nuclear fuel assemblies” means that a spent nuclear fuel assembly is positioned within each and every fuel storage cell of the rack.BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The features of the exemplary embodiments will be described with reference to the following drawings where like elements are labeled similarly, and in which:

[0020] FIG. 1 is a side view of a spent nuclear fuel pool showing a fuel rack representative of several embodiments of fuel racks disclosed herein for wet storage of nuclear fuel assemblies;

[0021] FIG. 2 is a top perspective view of a first embodiment of a fuel rack according to the present disclosure having a unidirectional flux trap arrangement and rectilinear cells;

[0022] FIG. 3 is a bottom perspective view thereof;

[0023] FIG. 4 is a top exploded perspective view thereof showing the three sections of slotted plates of the rack;

[0024] FIG. 5 is a bottom exploded perspective view thereof;

[0025] FIG. 6 is a first side view thereof;

[0026] FIG. 7 is a second side view thereof

[0027] FIG. 8 is a top view thereof;

[0028] FIG. 9 is a bottom view thereof showing the baseplate;

[0029] FIG. 10 is a perspective view of a first slotted plate of the fuel rack;

[0030] FIG. 11 is a perspective view of a second slotted plate of the fuel rack;

[0031] FIG. 12 is a perspective view of a third slotted plate of the fuel rack;

[0032] FIG. 13 is a perspective view of a fourth slotted plate of the fuel rack;

[0033] FIG. 14 is a first perspective view in a sequence of assembling the fuel rack of FIG. 1 showing the lowermost bottom section of the fuel rack comprising the baseplate and first tier of structural slotted plates;

[0034] FIG. 15 is a second perspective view in a sequence of assembling the fuel rack of FIG. 1 showing the second tier of slotted plates comprising neutron absorber slotted plates stacked on and interlocked with the first tier of structural slotted plates;

[0035] FIG. 16 is a third perspective view in a sequence of assembling the fuel rack of FIG. 1 showing the third tier comprising neutron absorber slotted plates stacked on and interlocked with the neutron absorber slotted plates in the second tier;

[0036] FIG. 17 is a fourth perspective view in a sequence of assembling the fuel rack of FIG. 1 showing completion of all of the tiers of neutron absorber slotted plates which forms the middle section of the fuel rack;

[0037] FIG. 18 is a fifth perspective view in a sequence of assembling the fuel rack of FIG. 1 showing the addition of the structural slotted plates which forms the top section of the fuel rack on top of the stack of neutron absorber slotted plates in FIG. 17;

[0038] FIG. 19 is a sixth perspective view in a sequence of assembling the fuel rack of FIG. 1 showing the additional of a perimetrically extending open girdle frame to the slotted plates in the top section of the rack in FIG. 18;

[0039] FIGS. 20 and 21 are top and bottom perspective views respectively in a sequence of assembling the fuel rack of FIG. 1 showing the addition of a vertically-extending corner tension member to the fuel rack;

[0040] FIG. 22 is a perspective view in a sequence of assembling the fuel rack of FIG. 1 showing the additional of a horizontally-extending strap member to the fuel rack;

[0041] FIG. 23 is an enlarged detail of a portion of the top section of the fuel rack of FIG. 1 showing an array of unidirectional flux traps further described herein;

[0042] FIG. 24 is a perspective view of a portion of the top section of the fuel rack of FIG. 1 showing an internal tension member and associated lateral restraint element coupled to one of the flux traps;

[0043] FIG. 25 is a perspective view of the bottom portion of the fuel rack of FIG. 1 showing the baseplate with under-girder beams;

[0044] FIG. 26 is a perspective view of a western style nuclear fuel assembly with square cross- section;

[0045] FIG. 27 is a first partial vertical cross sectional view of the fuel rack of FIG. 1 showing the internal tension member coupled to the top section and baseplate of rack;

[0046] FIG. 28 is a second partial vertical cross sectional view of the fuel rack of FIG. 1 showing the internal tension member coupled to the top section and baseplate of rack;

[0047] FIG. 29 is a top perspective view of a portion of the top section of fuel rack of FIG. 1 showing the top end of the internal tension member, lateral restraint element, and locking nut in exploded view;

[0048] FIG. 30 is a top perspective view of the lateral restraint element;

[0049] FIG. 31 is a bottom perspective view of the lateral restraint element;

[0050] FIG. 32 is a first transverse cross-sectional view taken along the length of the lateral restraint element;

[0051] FIG. 33 is a second transverse cross-sectional perspective view taken across the width of the lateral restraint element;

[0052] FIG. 34 is a top perspective view of a second embodiment of a fuel rack according to the present disclosure with rectilineal- cells and no flux traps;

[0053] FIG. 35 is a bottom perspective view thereof;

[0054] FIG. 36 is a top view thereof;

[0055] FIG. 37 is a bottom view thereof;

[0056] FIG. 38 is a top perspective view of a third embodiment of a fuel rack according to the present disclosure with rectilineal- cells and a bidirectional arrangement of flux traps;

[0057] FIG. 39 is a bottom perspective view thereof;

[0058] FIG. 40 is a top view thereof;

[0059] FIG. 41 is a bottom view thereof;

[0060] FIG. 42 is a top perspective view of a fourth embodiment of a fuel rack according to the present disclosure with hexagonal cells and triangular flux traps;

[0061] FIG. 43 is a bottom perspective view thereof;

[0062] FIG. 44 is a first side view thereof;

[0063] FIG. 45 is a second side view thereof;

[0064] FIG. 46 is a top view thereof;

[0065] FIG. 47 is a bottom view thereof;

[0066] FIG. 48 is a partial top perspective view of the fuel rack of FIG. 42 showing a Russian style hexagonal fuel assembly disposed in one of the hexagonal cells of the rack;

[0067] FIG. 49 is a partial perspective view of the fuel rack of FIG. 42 showing the first tier of structural slotted plates attached to the baseplate;

[0068] FIG. 50 is a partial top perspective view of the fuel rack of FIG. 42 showing an internal tension member and associated triangular-shaped lateral restraint element disposed in one of the triangular flux traps;

[0069] FIG. 51 is a top perspective view of the triangular- lateral restraint elements;

[0070] FIG. 52 is a bottom perspective view thereof;

[0071] FIG. 53 is a perspective view of a slotted plate used to construct the fuel rack of FIG. 42;

[0072] FIG. 54 is a top perspective view of a Russian style hexagonal fuel assembly; and

[0073] FIG. 55 is a bottom perspective view thereof.

[0074] All drawings are schematic and not necessarily to scale. Parts shown and / or given a reference numerical designation in one figure may be considered to be the same parts where they appear in other figures without a numerical designation for brevity unless specifically labeled with a different part number and described herein.DETAILED DESCRIPTION

[0075] The features and benefits of the invention are illustrated and described herein by reference to exemplary embodiments. This description of exemplary embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description. Accordingly, the disclosure expressly should not be limited to such exemplary embodiments illustrating some possible non-limiting combination of features that may exist alone or in other combinations of features. Furthermore, all features and designs disclosed herein may be used in combination even if not explicitly described as such.

[0076] In the description of embodiments disclosed herein, any reference to direction or orientation is merely intended for convenience of description and is not intended in any way to limit the scope of the present invention. Relative terms such as "lower," "upper," “horizontal,” “vertical,”, “above,” “below,” “up,” “down,” “top” and “bottom” as well as derivative thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative termsare for convenience of description only and do not require that the apparatus be constructed or operated in a particular orientation. Terms such as “attached,” “affixed,” “connected,” “coupled,” “interconnected,” and similar refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise. It will be appreciated that any numerical ranges that may be described herein shall be understood to include the lower and upper numerical terminus values or limits of the cited range, and any numerical values included in the cited range may serve as the terminus values.

[0077] Referring initially to FIG. 1, a nuclear facility which may be a nuclear generating plant includes a water-impounded spent fuel pool 40 according to the present disclosure configured for wet storage of nuclear fuel such as in individual nuclear fuel racks. The fuel rack shown represents any of the fuel racks 100, 200, 300, and 400 disclosed herein. The fuel pool 40 comprise a plurality of vertical sidewalls 41 rising upwards from an adjoining substantially horizontal bottom floor wall or slab 42 (recognizing that some slope may intentionally be provided in the upper surface of the floor slab for drainage toward a low point if the pool is to be emptied and rinsed / decontaminated at some time and due to installation tolerances). The floor slab 42 and sidewalls 41 may be formed of steel reinforced concrete in one non-limiting embodiment. The fuel pool floor slab 42 may be formed in and rest on soil or engineered fill. The floor slab 42 may be located at grade, below grade, or elevated above grade. In some embodiments contemplated, the floor slab 42 and sidewalls 41 may be at least partially embedded in soil and / or engineered fill surrounds the outer surfaces of the side walls. Any of the foregoing arrangements or others may be used depending on the layout of the nuclear facility and does not limit of the invention.

[0078] In one embodiment, the fuel pool 40 may have a rectilinear shape in top plan view. Four sidewalls 41 may be provided in which the pool has an elongated rectangular shape (in top plan view) with two longer opposing sidewalls and two shorter opposing sidewalls (e.g. end walls). Other configurations of the fuel pool 40 are possible such as square shapes, other polygonal shapes, non-polygonal shapes, and combinations thereof which may vary from nuclear facility to nuclear facility.

[0079] The sidewalls 41 and floor slab 42 of the fuel pool 40 define an upwardly open well or cavity 43 configured to hold pool water W and the plurality of submerged nuclear fuel rackswhich may be fuel racks 100, 200, 300, or 400 each holding multiple nuclear fuel assemblies such as fuel assemblies 30A (western style square cross-sectional shape) shown in FIG. 26 or fuel assemblies 30B (Russian style hexagonal cross-sectional shape) shown in FIGS. 54-55, depending on the particular rack selected. Each fuel assembly in turn contains multiple individual spent uranium fuel rods. The fuel racks storing the fuel assemblies are emplaced on the floor slab 42 in a high-density arrangement in horizontally-abutting manner or with minimal space between racks in various embodiments.

[0080] Referring to FIG. 26, the conventional western style fuel assembly 30A includes an elongated body 34A with square cross-sectional shape terminated with a top flow nozzle 32A and bottom flow nozzle 33A. The assembly contains a plurality of fuel rods 31A which contain the uranium.

[0081] Referring to FIGS. 54-55, the conventional Russian style fuel assembly 30B includes an elongated body 34B but with hexagonal cross-sectional shape terminated with a top flow nozzle 32B and bottom flow nozzle 33B. The assembly contains a plurality of fuel rods 3 IB which contain the uranium.

[0082] FIGS. 2-25 show a first embodiment of a fuel rack 100 according to the present disclosure and various features / details thereof for use with western style fuel assemblies 30A described above. Fuel rack 100 is a cellular upright module or unit comprising a vertically- extending cellular- body 101 including an array of vertically-extending fuel storage cells 110. The cellular body has a rectilinear cross-sectional shape in this embodiment. The cells 110 of fuel rack 100 define openings which extend for the full height of the cellular body rack from top102 of the body to bottom 103 of the body at the top surface of the baseplate 140. This is referred to as the cellular region of the fuel rack.

[0083] Fuel rack 100 includes four lateral sides 104 extending vertically from top 102 to bottom103 of the cellular body 101. The fuel rack defines a vertical centerline VC passing through the geometric center of the rack from side to side. Cellular body 101 rises upwards from and is supported by baseplate 140. Baseplate 140 is configured for placement on the floor slab 42 of fuel pool 40, as further described herein. The weight of the fuel rack body 101 and fuel assemblies stored in cells 110 is carried by baseplate 140 and transferred to the floor of the fuel pool. Accordingly, the baseplate is preferably formed of a ductile and corrosion resistant metal such as stainless steel.

[0084] The structure of the cellular body 101 of fuel rack 100 and array of fuel storage cells 110 is formed by a plurality of the interlocked and orthogonally intersecting slotted plates 120 in the present embodiment. The present rack is comprised of four different plate shown in FIGS. 10-13 explained further below. The elongated slotted plates 120 each define a centerline longitudinal axis LA extending along the length LI of the plate. Slotted plates 120 are horizontally elongated in length each having a length LI substantially greater than its height Hl as shown (e.g. at least 4 times the height or more). The plates 120 are oriented horizontally and transversely to vertical centerline VC of fuel rack 100 when installed. The lateral sides 104 of the fuel rack are collectively defined by the perimetrically extending and outermost slotted plates 120 which define outward facing exterior peripheral side surfaces 105 of the rack. Plates 120 may each be continuous monolithic structures which extend from one corner 106 of the fuel rack 200 to the opposite comer. The corners of the rack are defined by the perpendicular intersection of the plates.

[0085] Slotted plates 120 each include flat and parallel opposing vertical major sides surfaces 124, a top longitudinal edge 122, bottom longitudinal edge 123, and opposing ends 125 defining peripheral side edges of the plates. To interlock the plates, a plurality of longitudinally spaced apart vertical slots 121 are formed perpendicularly to longitudinal axis LA in the top longitudinal edge, bottom longitudinal edge, or both depending on the location of the plates in and configuration of the rack. The slots have a height which is less than the total height Hl of the slotted plates. Any suitable number, height (i.e. vertical depth), and arrangement of slots 121 may be provided to form the fuel rack structure shown and concomitantly the intended size of the cells 110 of the fuel rack 100 which store the fuel assemblies which contain the spent nuclear fuel (SNF).

[0086] Slotted plates 120 plates are oriented horizontally / laterally and interlocked with each other via the slots 121 to form a stacked structure comprised of multiple horizontal levels or tiers of plates rising from baseplate 140 and stacked to the desired height of the fuel rack. The lowermost plates 120 in the fuel rack only require slots formed in their top edges 122 to interlock with next level / tier of plates above since the bottom edges 123 of the lowermost plates are coupled to the baseplate. The uppermost slotted plates 120 in the top tier of the fuel rack may comprise slotted plates 120 having only downwardly open slots 216, both downwardly and upwardly open slots 121, or a combination thereof as needed to finish the top of the rack. Thoseslotted plates in the intermediate or middle tiers of the fuel rack between the top and bottom tiers of plates comprise both top and bottom slots 121 to form interlocked relationship with plates both above and below. As shown, some of the slots 120 extend only partially through the entire height Hl of the slotted plates, approximately 50% or less of height Hl in some embodiments. Not all slots may have the same height. The plates 120 may have the same or different heights and lengths depending on their location with the plate stack of the fuel rack structure.

[0087] The fuel storage cells 110 of fuel rack 100 includes a grid array of tightly packed and vertically-extending cells in its interior. Each cell is configured and dimensioned in cross- sectional area, height, and shape to hold only a single western style fuel assembly 28 (which contains a multitude of spent nuclear fuel rods 28a). An exemplary fuel assembly of this type having a conventional rectilinear cross-sectional configuration is shown in FIG. 26. Such fuel assemblies are well known in the industry. The cells 120 of the fuel rack defined by the orthogonally intersecting slotted plates 120 therefore have a concomitantly rectilinear cross- sectional shape (e.g. square).

[0088] Baseplate 140 comprises a horizontally broad and flat planar body which may be rectilinear (i.e. square or rectangular) in configuration in the present illustrated embodiment. The baseplate defines four peripheral lateral sides 140a which collectively define the perimeter of the baseplate. The peripheral sides 140a may be linear and straight. Baseplate 140 is preferably made of a corrosion resistant metal plate such as stainless steel to withstand corrosion when submerged in the fuel pool. The baseplate has suitable thickness to support the entire weight of the fuel rack 100 (i.e. slotted plates 120) and fuel assemblies when stored therein. A typical thickness is about + / - 4 inches.

[0089] In some embodiments, the baseplate 140 of fuel rack 100 may include a plurality of legs or pedestals 141 which support the rack from the floor slab 42 of the fuel pool 40 (see, e.g., FIGS. 1 and 35 ). Pedestals 141 may each have a flat bottom end to engage the pool floor slab 42 and a top end fixedly attached to the bottom of baseplate 140 such as via welding or bolting. The pedestals 141 protrude downwards from the flat body of baseplate 140 and a e laterally spaced apart from each other and located at appropriate points on the baseplate to properly support fuel rack 100. Pedestals 141 are provided at least at all four comers on the underside of the baseplate, and for larger racks may be provided between the corner pedestals.

[0090] Pedestals 141 elevate and vertically space the baseplate 140 of the rack off the floor slab 42 of fuel pool 40, thereby forming a gap therebetween which defines a bottom flow plenum P beneath rack 100 (see, e.g., FIG. 1). More specifically, when heat-emitting fuel assemblies 30A are positioned in the cells 110 in the submerged environment within fuel pool 40, the water within the cells surrounding the fuel assemblies becomes heated, thereby rising due to decrease in density and increased buoyancy creating a natural upflow pattern. As this heated water rises and exits the cells 110 via their open top ends, cooler water W in the fuel pool 40 is drawn into the bottom of the cells through flow holes 142 in baseplate 140 at the bottom of the cells and flows upward through the cells and fuel assemblies to cool the nuclear fuel. This heat induced water flow and circulation pattern along the fuel assemblies then continues naturally to dissipate heat generated by the fuel assemblies. Pedestals 141 may therefore have a height selected to form a bottom flow plenum P of generally commensurate height to ensure that sufficient thermally-induced natural circulation is created to adequately cool the fuel assemblies thereby protecting the SNF contained therein. In one non-limiting example, the height of the plenum P may be about 2 to 2.5 inches (including the listed values and those therebetween of this range). Other suitable heights may be used however depending on the requirement of the fuel rack installation and reactivity of the fuel assemblies.

[0091] Flow holes 142 in baseplate 140 therefore create inlet passageways from plenum P below the baseplate 140 into the cells 110 formed by the stacked slotted plates 120 of the rack 100. For fuel racks such as fuel rack 100 which includes flux traps, a flow hole may also provided in baseplate 140 for each flux trap to introduce cooling pool water through traps. Each cell 110 and trap has an associated flow hole 142 in baseplate 140. Flow holes 142 may be circular in some embodiments; however, other shapes of holes may be used. For example, some flow holes 143 associated with a cell 110 may be configured differently than the normal majority of flow holes in baseplate 140 to also provide rigging attachment points for raising and lowering empty fuel racks out from and into fuel pool 40. These rigging-associated flow holes however function in the same manner to introduce pool water W into the bottom of the cells for cooling the fuel assemblies 30 which are stored therein.

[0092] Preferably, at least a single flow hole 142 is provided for each cell 110; however, additional holes may be used as needed to create sufficient flow through the tubes to cool the fuel assemblies 30A therein.

[0093] With specific reference to FIGS. 4-5, fuel rack 100 includes a top section 100A, bottom section 100B, and intermediate or middle section 100C of slotted plates 120; the middle section being disposed between the top and bottom sections of plates. All slotted plates from each section collectively define the fuel storage cells 110 from top to bottom when slideably interlocked and assembled to build the cellular body 101 of fuel rack 100. However, the plurality of slotted plates 120 are formed of two different materials for different purposes and have several configurations shown in FIGS. 10-13 which occupy different positions in the fuel rack 100. The plates when assembled as shown in FIGS. 2-3 form the cellular body 101 of the rack.

[0094] For a first type of plate, fuel rack 100 includes corrosion resistant structural metallic slotted plates 120A preferably formed of a strong and corrosion resistant weldable metal such as stainless steel for structural support and rigidity (these may alternatively be referred to as simply “structural plates” herein for brevity). Plates 120A are used at the top and bottom sections 100A, 100B of the rack’s cellular body 101, as further described herein. For example, FIG. 14 shows the half-height starter plates 120A welded directly to the baseplate 140 which form the starter tier of slotted plates. These lowermost structural slotted plates along with the baseplate 140 collectively form a stable lower weldment 100B-1 at the bottom section 100B of slotted plates of the rack. At the top of the rack, FIGS. 10-11 show the half-height and full height slotted plates 120A that are assembled to form the top section 100A of the rack. After being mechanically and slideably interlocked, these uppermost structural slotted plates are preferably welded together to form a stable upper weldment 100A-1 for the top section 100A. Slotted plates 120A are therefore considered structural slotted plates.

[0095] A second type of slotted plate is necessary for reactivity control in the fuel rack 100. Accordingly, the fuel rack also includes non- structural boron-containing metallic neutron absorber slotted plates 120B in the intermediate or middle section 100C of the rack, which is the “active fuel region” from a radiation emission standpoint (these may alternatively be referred to as simply “neutron absorber plates” herein for brevity). For clarity, neutron absorber plates 120B are represented with a dot pattern in some of the figures which show a completed stack of these plates (or portions thereof) to visually distinguish them from the structural plates 120A which are not dotted for clarity. In other views the neutron absorber plates might not be dotted for clarity of depiction. The neutron absorber plates 120B reduce inter-cell neutron radiationtransmission from the fuel assemblies 30A in cells 110 therein to ensure the fuel rack remains in negative reactivity indicating a subcritical state (criticality being the condition where a sufficient number of neutrons are released by fission of the nuclear fuel to create a self-sustaining chain reaction). FIGS. 11 and 12 show two different configured of neutron absorber plates 120B used to form the middle section 100C of the fuel rack cellular body 101. In one embodiment, the neutron absorber plates 120B may be without limitation borated aluminum plates sold under the trade name Metamic™ available from Holtec International of Camden, New Jersey. Other metallic neutron absorber plate materials however may be used in other embodiments.

[0096] Reactivity between cells 110 of the rack and fuel assemblies 30A therein is controlled with the non-structural boron-containing absorber material which controls neutron radiation and provides criticality control of the nuclear fuel stored in the rack. However, these boron- containing metal plates 120B have lower strength than the stainless steel structural slotted plates 120A and are also relatively brittle by comparison to the steel slotted plates from an impact standpoint as previously described herein due to their Boron- 10 (B- 10) content. The boron- containing neutron absorber slotted plates 120B are further not particularly amenable to welding to each other. Accordingly, these slotted plates are held together by only the slideable mechanical interlock formed between the plates via slots 121. Notably, the neutron absorber slotted plates 120B are metallurgically incompatible for welding to the stainless steel slotted plates 120A which do not contain boron.

[0097] It bears noting that the closely spaced pairs of slots in FIGS. 11-13 for both slotted plates 120A and 120b are associated with the formation of flux traps 150, further described herein.

[0098] The structural slotted plates 120A at top and bottom sections 100A, 100B formed of stainless steel along with the stainless steel baseplate 140 and other structural support members described herein are rigidly coupled together to build the structural framework of fuel rack 100 which supports, retains, and protects the intermediate neutron absorber slotted plates 120B in the active fuel region of the fuel rack 100. In addition to structural slotted plates 120A and baseplate 140, the other structural members of the fuel rack structural framework includes a plurality of vertically-extending tensions members which function to couple and draw the upper and lower weldments together in a manner which compresses the neutron absorber plates 120B in the middle section 120 together. In some embodiments, the tension members include internal tension members 130 and / or vertically-extending external comer tension members 131. Otherstructural members provided to strengthen and structurally reinforce the fuel rack include latcrally / horizontally-cxtcnding strap members 132 and top girdle frame 133. These structural members may be formed of stainless steel like the structural slotted plates 120A and baseplate 140, and are further described below.

[0099] Internal tension members 130 may be used for fuel racks which include flux traps. Each internal tension member is coupled to and extends vertically between the top section 100A of slotted plates 120A (upper weldment 100A-1) and the baseplate 140 in the interior cellular region of fuel rack 100 at plural locations between fuel storage cells 110 within the flux traps. The tension members may be of any suitable type, shape, and construction so long as the members can be secured to the rack by mechanical or welding processes. Examples of suitable tension members 130 include tension rods 130-1 as shown, hollow tubes, steel cables, or others. In one preferred but non-limiting embodiment, the internal tension members 130 may threaded at the top and bottom ends for detachable coupling to slotted plates 120A in top section 100A of the rack (upper weldment 100A-1) via lateral restraint elements 130A and threaded nuts 134, and to the baseplate 140 at bottom via threaded locking nuts 134. The locking nuts thus threadably couple each tension member to the lateral restraint elements and baseplates. The tension members 130 are located within the interior cellular region of fuel rack 100 and extend vertically through the open gaps between certain selected cells 110 in fuel rack 100 which are referred to in the art as flux traps 150, further described herein. Placement within the flux traps avoids interference with the space within the cells dedicated to inserting and removing fuel assemblies. The tension members 130 provide a tensioning function to draw slotted plates 120A in top section 100A downwards towards the baseplate 140, thereby compressing the stack of non- structural neutron absorber slotted plates 120B in middle section 100C together to stabilize engagement between these plates which are not welded to each other, but only mechanically interlocked. In essence, the tension members therefore function to prevent the upper and lower weldments 100A-1, 100B-1 from separating to lock the stack of intermediate slotted neutron absorber plates 120B in position in the rack. Tension members 130 further advantageously enhance the axial load bearing capacity of the rack structure. Any suitable number of tension members may be provided and arranged to achieve that function. The number selected will in part be based on the horizontal dimensions / size of the rack. Tension rods 130-1 when used for tension members 130 may have a cylindrical shape with circular cross-section in oneembodiment; however, other shaped tension rods including those with rectilinear, hexagonal, or other cross-sections may be used.

[0100] The vertically-extending external comer tension members 131 are located at the comers 106 of the fuel rack; one comer tension member at each corner (four in total). Tension members 131 may be elongated straight bar- shaped structural members in one embodiment with rectilineal- cross-sectional shape. The top ends of the corner tension members 131 may be welded to the structural slotted plates 120A in top section 100A of fuel rack 100 (i.e. upper weldment 100A-1), or the top girdle frame 133, or both. When welded to the girdle frame 133, the corner tension members 131 may be welded to the underside thereof in some embodiments as the girdle frame projects laterally beyond the exterior peripheral side surfaces of the rack (see, e.g., FIG. 20). The bottom ends of the external comer tension members 131 at the comers are preferably welded to the thick baseplate 140, such as to the sides thereof in one embodiment. These top and bottom fixation locations are the only points of securement of the comer tension members to the fuel rack. No coupling is made to the middle section 100C of the neutron absorber slotted plates 120B.

[0101] It bears noting that the corner tension members 131 are not merely attached to the comers of the rack, but are tensile elements installed to place tension on the slotted structural plates 120A in the rack, which in turn applies a concomitant tensile force on the stack of slotted neutron absorber plates 120B in the middle section 100C of the rack 100 to draw and hold them together since the neutron absorber plates are not welded together. Accordingly, the comer tension members 131 are configured and operable to compress the stack of neutron absorber plates 120B together.

[0102] The plurality of the laterally / horizontally-extending strap members 132 in one non-limiting embodiment may be formed by straight bar- shaped structural members with rectilineal’ cross-sections which are fixedly attached at their ends to the external corner tension members 131 as shown such as via welding. Mechanical couplings other than welding (e.g., threaded fasteners such as bolting or others) may also be used in other embodiments to secure the strap members to the corner tension members. Each strap member spans horizontally and perpendicularly from one corner tension member to another comer tension member transversely to the vertical centerline VC of the rack 100. The strap members are vertically spaced apart as shown. Any suitable number of strap members 132 may be provided as needed to laterally 1stabilize and secure the middle neutron absorber slotted plates 120B together. Because the fuel rack 100 after assembly is shipped to the nuclear generation plant installation site on its side via flatbed truck or rail car, the strap members 132 advantageously also help to keep the un-welded middle section 100B of neutron absorber plates 120B together in mechanically interlocked relationship until the rack is up-righted for maneuvering to and lowering into the fuel pool. The strap members also help keep the neutron absorber plates 120B from shifting laterally out of place during a seismic event when in operation submerged in the fuel pool.

[0103] Top girdle frame 133 is an open frame (i.e. open center, perimetrically continuous body) which extends perimetrically around the top of the fuel rack 100. The frame is welded to the side faces of the uppermost slotted structural plates 120A in the top section 100A of the rack (upper weldment 100A-1) to form a fixed and rigid coupling thereto (see, e.g., FIG. 20). Frame 133 is spaced slightly below the top edges of the perimetrically extending uppermost slotted plates 120A which define the four exterior sides of the fuel rack 100. In one embodiment, the frame may be formed of four steel structural bars arranged orthogonally and welded together at their ends to create a rectilinear shaped frame which conforms to the shape and dimensions of the top of the fuel rack. The structural bars may be preassembled and welded together before being welded to the rack, or the structural bars may be welded one-by-one directly to the fuel rack for form the completed girdle frame.

[0104] The foregoing steel structural framework including the upper / lower weldments 100A-1, 1 — B-l, internal tension members 130, comer tension members 131, strap members 132, and girdle frame 133 collectively forms a cage or exoskeleton enclosing and protecting the more brittle neutron absorber slotted plates 120B in the middle section 100C of fuel rack 100 which are only mechanically interlocked. In the case of a seismic event, the slotted plates 120B are prevented from laterally shifting and becoming displaced by the structural members which can adversely affect the structural integrity of the fuel rack and containment of the spent nuclear fuel (SNF) stored in the cells of the rack. From a general standpoint, the steel structural framework provides strength to and protection of the rack in general during, transport, lifting, handling, installation in the fuel pool, and during a seismic event. When the rack is lifted and handled by an overhead crane, the strong structural framework bears the weight load which is not transferred to the more brittle stack of neutron absorber slotted plates 120B in the middle section 100C of the rack.

[0105] The general process or method for assembling fuel rack 100 will now be briefly described.

[0106] First, lower weldment is formed by welding the structural slotted plates 120A in bottom section 100B of fuel rack 100 onto the top major surface I40B of the baseplate 140 (see, e.g., FIG. 14). A plurality of slotted plates are arranged in parallel to each other and spaced horizontal apart as shown. The slotted plates extend in a first direction perpendicular to a pair of opposing peripheral sides 140a of the baseplate 140. As seen, it is not necessary to provide slots in the bottom edges of these slotted plates 120 A as the bottom edges abuttingly engage the baseplate. An orthogonal array of partial-depth grooves 136 may be machined into the top surface of the baseplate to precisely control the spacing between the steel structural plates during installation of this lowermost level or tier of slotted plates which forms the foundation for receiving the first tier or level of the neutron absorber slotted plates 120B.

[0107] In one embodiment, baseplate 140 may be additionally structurally reinforced and stiffened by a plurality of under-girder beams 144 fixedly attached via welding or bolting to the bottom major surface 140C of baseplate (see, e.g., FIGS. 3, 9, and 14). The beams form an integral structural part of the lower weldment 100B-1 along with the slotted plates 120A welded to the opposite top surface of the baseplate. Because, the under-girder beams structurally reinforce and support the baseplate from the fuel pool floor, the thickness of the baseplate itself (measured from top to bottom surface thereof) can be decreased and still support the weight load of the entire fuel rack loaded with fuel assemblies. Typical un-reinforced baseplates may typically have a thickness of about 4 inches due to the fuel rack weight load. However, a baseplate reinforced with under-girder beams can have a thickness which is less.

[0108] Beams 144 are horizontally spaced apart and parallel to each other. In one embodiment, the under-girder beams extend from continuously from one side 140A of the baseplate to an opposite side of the baseplate as shown. In other embodiments, the under-girder beams may have a length which does not continuously extend from one side of the baseplate to the opposite side (i.e. less than the width of the baseplate measured from side to side). The under-girder beams are preferably arranged and oriented perpendicularly to the starter tier of structural slotted plates 120A welded to the top surface of the baseplate described above. Improved baseplate stiffness is advantageously achieved by the starter slotted plates 120A running in one direction and the array of under-girder beams 144 running in a second orthogonaldirection so that each starter plate is supported by multiple beams. The under-girder beams are configured and have a height to engage the floor 42 of the fuel pool 40 so as to elevate the baseplate to form flow plenum P alone without the use of pedestals 141. For this reason, the beams 144 may include flow holes in some embodiments to enhance circulation of pool water W beneath the baseplate (see, e.g., FIG. 1).

[0109] In various embodiments and arrangements, the baseplate 140 may include pedestals alone, beams alone, or a combination of both (see, e.g., FIG. 35) depending on the configuration of the fuel storage pool. When both are provided, the beams 144 preferably have the same height and project downwards from the bottom surface of baseplate 140 by the same distance as the pedestals 141 so that the beams and pedestals engage the floor of the fuel pool and distribute the weight of the fuel rack and fuel assemblies stored therein across the floor. Because the beams have a greater contact length and surface area with the underside of the baseplate than the generally cylindrical pedestals, the baseplate is less likely to bow in certain regions under the weight of the rack and fuel assemblies due to the added stiffness. The undergirder beams 144 also help to prevent bowing of the baseplate 140 when fuel rack 100 is being raised from or lowered into the fuel pool suspended from the fuel pool crane.

[0110] Referring to FIG. 15 now, the lowermost tier of neutron absorber slotted plates 120B are then stacked on and mechanically interlocked with the starter tier of structural slotted plates 120A welded to the baseplate 140 as described above. Neutron absorber slotted plates 120B are arranged orthogonally / perpendicularly to the structural slotted plates 102A as shown. The neutron absorber plates are held in place on the structural plates only via the interlocking slots. The boron-containing neutron absorber plates 120B are metallurgically incompatible for welding to the stainless steel structural slotted plates 120A. FIG. 16 shows the second tier of neutron absorber slotted plates 120B stacked on and mechanically interlocked with the first tier of neutron absorber plates. Plates 120B in the second tier of neutron absorber plates are arranged orthogonally to the plates 120B in the first tier of neutron absorber plates. Additional levels or tiers of neutron absorber slotted plates 120B are then added to the stack of plates in the same manner to gradually build the neutron absorber middle section 100C of fuel rack 100 up to the height necessary to complete coverage of the active fuel region of the rack, thereby controlling reactivity between the cells and fuel assemblies 30A therein. This process gradually increasesthe height of fuel storage cells 110 tier by tier. The middle section of the rack is then completed once all neutron absorber plates 120B arc in place (sec, c.g., FIG. 17).

[0111] Once all the neutron absorber slotted plates 120B which are needed are added to the rack, the structural slotted plates 120A in the top section 100A of the fuel rack (e.g., upper weldment 100A-1) are stacked onto and mechanically interlocked with the uppermost tier of neutron absorber slotted plates (see, e.g., FIG. 18). Unlike the single starter tier of structural slotted plates 120A welded to baseplate 140 as described above which extend in only a single direction, the final upper weldment comprises two tiers of orthogonally intersecting structural slotted plates 120 A which extend in two different orthogonal directions. Plates 120A shown in FIGS. 10 and 11 are used for forming the upper weldment. The half-height plates 120A in FIG. 10 with downwardly open slots creates a level top edge of the fuel rack’s cellular body 100. These stainless steel slotted plates 120A are welded to each other after being mechanically interlocked via slots 121 to complete the upper weldment structure. The upper weldment 100A- 1 may be formed in place by installing the two tiers of slotted plates 120A on top of the uppermost tier of neutron absorber plates 120B followed by welding, or alternatively the slotted plates 120A may be welded together first to complete the weldment, which is then mounted on top of the neutron absorber plate stack. Either approach is possible and none of these structural slotted plates 120A are welded to the top tier of neutron absorber plates 120B.

[0112] As shown, it bears noting that each level or tier of slotted plates 120A and 120B in fuel rack 100 are intentionally vertically offset in elevation from those above and below and interlock to eliminate continuous horizontal shear' paths through the slotted plates. This prevents a horizontal shift or displacement of one or more tiers of slotted plates out of plane with others during a seismic event which could disrupt the straightness of the storage cells 110 in the rack and adversely affect the structural integrity of the fuel rack to protect the fuel assemblies. In other words, one set of slotted plates in one tier is vertically offset from the adjoining interlocked set of slotted plates in adjacent tiers above and / or below as applicable by about half a plate height.

[0113] Referring to FIG. 19, the top girdle frame 133 may next be welded to the outward facing side surfaces of the structural slotted plates 120A in the top section 100A of fuel rack 100 around the perimeter. The girdle frame acts as a bumper to protect the slotted plates at the top end of the fuel rack 100 from damage when either in use in the fuel pool or when laid on its sideduring shipping and handling. Girdle frame 133 may be prefabricated and then installed on the stack of slotted plates, or alternatively may be provided in four separate linear pieces which arc each in turn welded to the fuel rack. Either installation approach may be used.

[0114] Next, the vertically-extending comer tension members 131 can be positioned on the corners 106 of the rack and welded at each end to top girdle frame 133 at top and welded to baseplate 140 at bottom (see, e.g., FIGS. 20 and 21). The top ends of the comer tension members 131 may be welded to the underside of the girdle frame in one embodiment. The bottom ends of members 131 be welded to the sides of the baseplate. Alternatively, the corner tension members 131 may be bolted to the girdle frame and baseplate in other embodiments.

[0115] Next, the laterally / horizontally-extending strap members 132 are added and welded at each end to one of the comer tension members 131 (see, e.g., FIG. 22). Strap members 132 are oriented perpendicularly to the vertically-extending corner tension members 131. It bears noting that the order of installation for the strap members and corner tension members may be reversed and is at the assembler’s discretion. Fillet welds and groove welds may be used in some embodiments for coupling the strap members to the comer tension members.

[0116] As shown in FIGS. 2, 8, 23, and 24, fuel rack 100 comprises a plurality of flux traps 150. Flux traps are open spaces or gaps intentionally formed between fuel storage cells 110 of the rack to control reactivity between cells and fuel assemblies stored therein. Flux traps 150 are filled with water when the rack is submerged in the fuel pool 40; the water acting as a radiation (e.g., neutron) moderator to reduce reactivity between the cells. The flux traps define open areas which extend for the full height of the cellular body 101 of the fuel rack from top 102 to bottom 103 terminating at the top surface of baseplate 140. The cross-sectional area and volume of the flux traps are predetermined to provide the necessary degree of reactivity control between cells 110 required (when filled with water) depending on the energy level of the fuel in the fuel assemblies to be stored in the rack. The cross-sectional area of the flux traps 150 is too small to hold a fuel assembly.

[0117] Referring to FIG. 19, each flux trap 150 has a horizontally-elongated configuration defined by a length L2 and a width W2; each dimension being measured in a horizontal direction and plane. Length L2 is greater than width W2. Width W2 defines a water- filled gap between adjacent cells separated by a flux trap (e.g., between cells 110 in each rowextending along the horizontal X axis shown in FIG. 8). Length L2 is coextensive with the dimension of cell 110 measured along the length of the flux trap. Accordingly, the width W2 determines the horizontal spacing and gap between cells separated by the flux trap.

[0118] Referring to FIGS. 24-25 and 27-29, assembly of fuel rack 100 is completed by next installing the vertically elongated internal tension members 130 such as tension rods 130-1 in the illustrated but non-limiting embodiment. Tension rods 130-1 are installed and inserted through at least some of the flux traps 150 from top to bottom thereof and preferably spaced apart horizontally around the interior of the cellular body or region of the rack. Each tension rod is inserted through a respective one of the flux traps 150 at select locations and extend from the top of the rack at upper weldment 100A-1 down through the open trap to the baseplate 140.

[0119] A lateral restraint element 130A provided for the top of each tension rod 130-1 is configured to fit over the selected flux trap and engages the top edges 120A-1 of the uppermost slotted structural plates 120A that circumscribe and define the top of each rectilineal’ flux trap at the top of the rack, as shown in FIG. 24. The threaded top end of tension rod 130-1 passes through a mounting hole 135F in the restraint element and is secured thereto with a threaded locking nut 134. The threaded bottom end of the tension rod 130-1 passes through a respective through hole in baseplate 140 positioned within the flux trap 150 and is secured thereto with another threaded locking nut 134 on the underside of the baseplate (see, e.g., FIG. 25).

[0120] When at least one of the opposing nuts 134 on tension rod 130-1 are tightened, tension is created in the tension rod which draws the vertical stack of slotted plates 120A and 120B together in compression. This ensures that the plates are snuggly fit together and locked in place via the interlocking slots 121 in the plates. Importantly, the compressive force applied to the middle section 100C of the neutron absorber plates 120B sandwiched between the upper and lower weldments 100A-1, 100B-1 helps ensure that these un- welded intermediate plates do become laterally displaced with respect to each other either during transport or via vibrations imparted to the fuel rack when submerged in the fuel pool and resting on the floor thereof during a seismic event.

[0121] FIGS. 30-33 show one non-limiting embodiment of a lateral restraint element 130A in isolation and detail. Each restraint element is configured to securely engage a respective flux trap 150 in a manner which prevents relative lateral / horizontal movement between the restraint element, tension rod 130-1, and flux trap. In one embodiment, restraint elements 130Amay be a generally elongated flat bar with a rectangular shape to match the shape of the flux traps 150. Element 130 includes a top surface 135 A, opposing bottom surface 135B, opposing ends 135C, and opposing sides 135D. Bottom surface 135B has a stepped configuration in one embodiment defining a perimetrically extending downward facing lip 135E and downwardly extending anti-rotation protrusion 135G. Lip 135E defines a downward bearing surface 135H which abuttingly engages the top edges 120A-1 of the four orthogonally arranged structural slotted plates 120A that define the flux trap 150 to which the restraint element is mounted. When tension rod 130-1 is tightened via nuts 134, the rod draws lateral restraint element 130A downwards against the top edges 120A-1 to apply a compressive force against the stack of slotted plates 120A, 120B. Accordingly, the lateral restraint elements 130A also act as compression plates in addition to preventing lateral movement of the restraint elements.

[0122] Anti-rotation protrusionl35G is complementary configured to the flux trap opening such that the protrusion extends downwards below top edges 120A-1 around the flux trap 150 (see, e.g., FIGS. 27-28). Protrusion 135G has horizontal dimensions (e.g., width W3 and length L3) sized to fit closely to but inside the flux trap 150. The locking protrusion prevents twisting of the lateral restraint element 130A relative to the flux trap when the locking nuts are rotatably tightened. The lateral restraint element further prevent horizontal / lateral movement between both the restraint element and tension rod 130-1 relative to the flux trap 150 and slotted plates 120A in upper section 100A of upper weldment 100A-1. This keeps the tension rod centered in the flux trap.

[0123] It bears noting that other configurations and types of lateral restraint elements may be provided so long as horizontal / lateral movement between lateral restraint element 130A and tension rod 130-1 relative to the flux trap 150 and slotted plates 120A in upper section 100A of upper weldment 100A-1 is prevented.

[0124] Unidirectional Flux Trap Arrangement

[0125] In order to increase the number of fuel assemblies which can be stored in each fuel rack 100, the inventors have discovered that flux traps 150 can be provided in a unidirectional (i.e. single direction) arrangement while advantageously still maintaining the subcriticality state of the rack with respect to meeting the required K-effective factor prescribed by the Nuclear Regulatory Commission (NRC), explained further below. This means that flux traps can be provided between adjacent fuel storage cells 110 of the rack in only a singleorthogonal direction (see, e.g., FIGS. 2, 8, 23, and 24). An X-Y-Z Cartesian coordinate system provided in FIGS. 2 and 8 facilitate description (with Y axis being vertical, and X and Z axes being horizontal). In the unidirectional flux trap arrangement, flux traps 150 are only located and interspersed between each pair of adjacent cells in rows of cells in the rack arranged along the horizontal X axis in the first orthogonal direction (X direction). However, there are no flux traps provided between each pair of adjacent cells in rows arranged along the orthogonal horizontal Z axis in a second orthogonal direction (Z direction) perpendicular to the first direction. This means that a greater number of cells may be packed in rows of cells along the Z axis which increases the total number of cells and fuel assemblies stored in the fuel rack in contrast to providing flux traps in rows along both the X and Z axis as is conventional practice when flux traps must be provided due to nuclear fuel reactivity. The axial pitch between the fuel storage cells 110 in the two orthogonal directions is variable such that, in the limit, the flux traps in one orthogonal direction (e.g., Z direction along the Z axis) is zero (i.e. no flux traps) leading to a unidirectional flux trap configuration with flux traps provided only between cells along the X axis.

[0126] As background, spent nuclear fuel (SNF) discharged from the reactor and stored in the fuel pool has varying quantities of U-235 “burned” and “integral burnable poison” consumed depending on how the reactor has been operated. The reactivity of each discharged fuel is a complex function of its burn- up and decay time after cessation of fission in the core. The main function of the fuel racks in the pool is to ensure that the neutrons generated due to ongoing fuel decay in the pool is less than the number being absorbed by the neutron absorber plates. The neutron absorber plates can capture neutrons if they have been slowed down by collisions with hydrogen atoms in the water. The slow moving neutrons are also called thermal neutrons. By adjusting the amount of water around the fuel (which is determined by the cross sectional opening of the storage cells), the extent of thermal neutrons as a portion of the total population of neutrons is controlled. By setting a “flux trap” around the cell, the first absorber plate captures the thermalized neutrons produced by the water in the cell and then the rest get thermalized in the water- filled flux trap followed by their capture by the second plate of boron- containing neutron absorber plate material (e.g., Metamic™ or other). Thus, a flux trap design is needed if the population of fast neutrons is too high to be thermalized and captured by only one wall of neutron absorber material plates.

[0127] Because the objective in the rack design is to kill large enough numbers of neutrons so that their population docs not rise with time (making the stored array critical), the designer chooses the right amount of flux trap in both X and Y-directions to meet the subcriticality target. Because neutron capture is a volumetric effect as opposed to directional, one can make the storage cells rectangular thus providing more water gap in orthogonal direction compared to the other. Likewise, the flux trap may or may not be needed and if needed, can be unequal in the two orthogonal directions. In one extreme case, the flux trap can be absent entirely in one direction but not the other. For some fuel that generates a small quantity of fast (energetic neutrons), flux traps may not be needed at all (e.g., a non-flux trap fuel rack). Optimization is needed to maximize the number of storage cells in each rack and collectively the fuel pool with a fixed floor slab area.

[0128] According to NRC regulations, the K-effective of the spent fuel storage racks must be less than or equal to 0.95. K-effective of less than 1.0 represents that the ratio of the number of neutrons released by the decaying spent nuclear fuel in the fuel rack is less than the number which are being absorbed (killed) by the neutron absorber plates. This ensures that the nuclear’ fuel in the fuel rack remains in a subcritical state incapable of sustaining a fission chain reaction. Accordingly, fuel rack 100 preferably has a K-effective of less than 1.0, and more preferably 0.95 or less to meet the NRC requirement.

[0129] A unidirectional flux trap array as described above can therefore be used to increase the number of fuel assemblies which can be stored in the fuel rack with sufficiently sized flux traps to yield a K-effective of less than 1.0 due to the volumetric effect of the neutron capture for each cell via the neutron absorber plates and water in the flux traps. The ability of the unidirectional flux trap arrangement to meet the NRC K-effective standard has been verified by the inventors using K-effective computer code (algorithm) MCNP - A General Monte Carlo N-Particle Transport Code, Version 5, LA-UR-03-1987, by Los Alamos National Laboratory. Other commercially-available codes for modeling K-effective are available.

[0130] As an illustrative but non-limiting example shown in FIG. 8, eight (8) fuel assemblies can be stored in cells 110 in each row of cells of fuel rack 100 along the X axis and direction (the flux trap direction), whereas eleven (11) fuel assemblies are be stored in each row along the Z axis and direction (the non-flux trap direction). Accordingly, 24 additional fuel assemblies can be stored in rack 100 compared to a conventional flux trap type rack with fluxtraps provided in both orthogonal directions. Other numbers of fuel assemblies may be packed into the fuel rack in other cases depending on the total horizontal dimensions of the rack in both the X and Y directions and does not limit the invention.

[0131] It bears noting that depending on the reactivity of the fuel assemblies to be stored in the rack, flux traps 150 for the unidirectional flux trap arrangement might not be needed between every pair of cells 110 in the rows extending along the X axis in order to achieve a Ineffective factor of less than 1.0. Accordingly, some pairs of cells in one or more rows along the X axis might not require a flux trap between them in some embodiments. In addition, in other embodiments, at least two of the flux traps in one or more rows of cells extending along the X axis may have a different width W2 than each other. Accordingly, numerous variations of the arrangement and size of the flux traps in a unidirectional flux trap arrangement are possible taking into account the reactivity of the specific fuel assemblies to be stores in fuel rack 100.

[0132] FIGS. 34-37 show a second embodiment of a rectilinear cell fuel rack 200 which is ostensibly similar to fuel rack 100 and includes many of the same features; however, rack 200 does not include any flux traps by comparison. The non-flux trap rack design can be used when for wet storage of low energy fuel assemblies as previously described herein. The rectilinear cells 110 are all separated by only a single neutron absorber plate as opposed to two neutron absorber plates and a water-filled flux trap therebetween. Fuel rack 200 therefore does not include internal tension members 130 as there are no flux traps for inserting these members within the cellular region of the rack. The fuel rack 200 includes structural members of the fuel rack structural framework including vertically-extending external corner tension members 131, laterally / horizontally-extending strap members 132, and top girdle frame 133 similarly to rack 100 as previously described herein. Fuel rack relies on the corner tension members 131 to provide the tension function and apply a compressive force to the stack of plates. During assembly of the rack to tension the corner tension members, the stack of structural slotted plates 120A and neutron absorber plates 120B may be temporarily compressed together via mechanical clamping means such as cables, hydraulic jacks, a large vice mechanism, or other. The corner tension members 131 may then be welded to the corners of the rack in the same way as previously described herein. When clamping means are then released after welding, which puts the corner tension members in tension as the stack of slotted plates relaxes attempts to stretchT1back to its original height. The comer tension members however maintain compression of the middle section of neutron absorber plates 120B between the upper and lower weldments.

[0133] Fuel rack 200 also includes a combination of both pedestals 141 and under-girder beams 144. In other embodiments, only one type of these fuel rack support elements may be provided (e.g., pedestals or beams) similar' to that shown for fuel rack 100. Beams 144 in this embodiment omit the flow holes 144a.

[0134] FIGS. 38-41 show a third embodiment of a rectilinear cell fuel rack 300 which is ostensibly similar to fuel rack 100 and includes many of the same features. By contrast, fuel rack 300 includes bidirectional flux traps 150 such that each rectilinear cell 110 is separated from adjacent cells by a flux trap. The flux traps run in two orthogonal directions along two horizontal axes (X axis and Z axis). The cells are separated by two neutron absorber plates and a water- filled flux trap therebetween.

[0135] Fuel rack 300 includes internal tension members 130 as there flux traps 150 available for routing the tension members therethrough within the cellular region of the rack.The fuel rack 300 also includes structural members of the fuel rack structural framework including vertically-extending external comer tension members 131, laterally / horizontallyextending strap members 132, and top girdle frame 133 similarly to rack 100 as previously described herein. Assembly of fuel rack 300 may follow the same sequence as previously described herein for rack 100.

[0136] Fuel rack 300 uses only under-girder beams 144 to elevate the baseplate 140 of the rack above the floor of the fuel pool. Pedestals however may be used in addition to or instead of the beams.

[0137] FIGS. 42-53 show a fourth embodiment of a fuel rack 400 with hexagonal cells 410 in cross-section configured to received the Russian style hexagonal fuel assemblies 30B shown in FIGS. 54-55. Rack 400 is formed by a plurality of obliquely arranged, interlocked and slotted flat plates comprising structural slotted plates 420A and neutron absorber slotted plates 420B seen in FIG. 53. This single slotted plate configuration may be used for both types of these plates 420A, 420B. Plates 420A are similar to slotted plates 120A previously described herein and may be made of stainless steel. Plates 420B are similar to slotted plates 120B and may be made of boron-containing aluminum such a Metamic™ or another boron-containing metal plate. Accordingly, slotted plates 420A, 420B will not be further described for sake of brevity.

[0138] The structural and neutron absorber slotted plates 420A, 420B are interlocked via slots 121 (sec, e.g., FIG. 53) and vertically stacked in interlocked arrangement and tiers similar to plates 120A, 120B of fuel rack 100 previously described herein. This similarly forms upper section 100A, lower section 100B, and middle section 100C of slotted plates of the hexagonal fuel rack 400. The assembly process is the same; however, three rows of differently oriented plates are needed to form cells 410 with a hexagonal cross-section in lieu of the rectilineal- cells like in fuel racks 100, 200, and 300.

[0139] Accordingly, two sets 400A and 400B of horizontally elongated slotted plates 400A and 400B would be arranged which are interlocked in a classic harlequin (rhombus) gridded pattern in which the parallel plates of the first set obliquely intersect the parallel plates of the second set to first form elongated diamond- shaped cells (see, e.g., FIG. 46). To produce the final hexagonal- shaped cells 410, a third set 400C of horizontally elongated parallel plates would be arranged to obliquely interlock and intersect the first and second sets 400A, 400B of plates in which the third set of parallel plates extend through the previously formed diamond-shaped cell openings at two places. The end result is an array of hexagonal cells 410 seen in FIG. 46. Each slotted plate may include an appropriate number of slots formed in the top and bottom sides of the plates to form the interlocked plate structure. The sets of plates may be either structural slotted plates 420A or neutron absorber slotted plates 420B depending on which section 100A, 100B, or 100C of the fuel rack the plates are located in a similar vane to fuel rack 100 previously described herein.

[0140] Each of the slotted plates in sets 420A, 420B may have a lateral width which extends from one peripheral side 140a of the baseplate 140 to an opposite side as shown in FIG. 46. The slotted plates are therefore horizontally elongated having a greater lateral width than height (see, e.g., FIG. 30). The full height is achieved by vertically stacking and interlocking tiers of the plates in the manner disclosed in the two above-referenced patents.

[0141] Similarly to rectilinear cell fuel rack 100, hexagonal cell fuel rack 400 defines a top 402, opposite bottom 403, and plurality of lateral sides 404 extending vertically therebetween. The fuel rack defines a vertical centerline VC passing through the geometric center of the rack from side to side. Cellular body 401 rises upwards from and is supported by baseplate 140. Baseplate 140 is configured for placement on the floor slab 42 of fuel pool 40 and may include pedestals 141 as shown and / or under-girder beams 144.

[0142] The slotted plates 420A, 420B define a plurality of triangular shaped flux traps 450 around each cell 410. The flux traps define spaces for inserting tension rods 130-1 therethrough in a similar manner to flux traps 150 in fuel rack 100 previously described herein. Triangular shaped versions of lateral restraint elements 130A previously described herein are used to couple the top ends of tension rods 130-1 to the slotted plates 420A in top section 100A of fuel rack 400. Specifically, triangular lateral restraint elements 430A are provided for inserting into the triangular flux traps 450.

[0143] Lateral restraint elements 430A provide the same function as elements 130A to securely engage a respective flux trap 150 in a manner which prevents relative lateral / horizontal movement between the restraint element, tension rod 130-1, and flux trap. However, triangular lateral restraint elements 430A are configured and mounted in a different manner than elements 130A previously described herein. The refueling machine for fuel racks holding Russian style hexagonal fuel assemblies 30B requires that there be no protrusions extending above the top face (horizontal plane) of the fuel rack to avoid interference. Accordingly, lateral restraint elements 430A are configured to fit entirely within the triangular flux trap 450 and are welded directly to the slotted plates on three sides as to not protrude above the rack face.

[0144] Lateral restraint elements 430A each include a flat top surface 435A, opposing flat bottom surface 435B, mounting hole 435F extending therebetween, and obliquely oriented peripheral sides 435D. Corners 435M formed between the sides may be chamfered as shown to improve fit within the flux traps 450. When mounted inside a triangular flux trap 450, top surface 435 A may be at most flush with the top edges 420A-1 of the three slotted plates 420A that define the trap and top face or plane of the rack, or recessed slightly therebelow (see, e.g., FIG. 50). No portion of the lateral restraint elements 430A or the top of tension rod 130-1 protrude above top edges 420A-1 of the plates. To ensure the threaded top end of the tension rod does not protrude beyond the top face of the rack, the top end of the rod and threaded nut 134 are recessed inside the upper portion of the rod mounting hole 435F as shown. Hole 435F has a stepped cross-sectional shaped defining a larger diameter upper portion than a smaller diameter lower portion; the latter of which is sized to be diametrically smaller than the nut but larger than the rod so it can pass through the lateral restraint element 430A (see also FIGS. 51 and 52). The nut 134 cannot pass through the lower diameter and bears down on an annular seating surface 435K defined within hole 435F when the tension rod 130-1 is tightened to compress the stack ofslotted plates 420A, 420B together. The upper portion of hole 435F of course has a diameter sized larger than the rod and nut so that they can be completely nested inside the hole as describe above.

[0145] Example Claims

[0146] Following are example claims for the foregoing described inventions and aspects thereof.

[0147] 1. A fuel rack for wet storage of nuclear fuel in a fuel pool, the fuel rack comprising: a baseplate; a rectilinear gridwork of plates extending vertically from the baseplate along a Y axis of a Cartesian coordinate system, the rectilinear gridwork of plates forming: a plurality of vertically-extending fuel storage cells, each of the plurality of vertically-extending fuel storage cells having a horizontal cross-sectional profile configured to receive a nuclear fuel assembly therein; and a unidirectional flux trap configuration in which: (1) a flux trap is present between adjacent ones of the fuel storage cells in a first orthogonal direction along a X axis of the Cartesian coordinate system; and (2) a flux trap is not present between adjacent ones of the fuel storage cells running in a second orthogonal direction along a Z axis of the Cartesian coordinate system.

[0148] 2. The fuel rack according to claim 1, wherein the flux traps are dimensioned so that when the fuel rack is fully loaded with a stored array of the plurality of nuclear fuel assemblies, the stored array has a K-effective factor that is less than 1.

[0149] 3. The fuel rack according to claim 2, wherein the horizontal cross-sectional profile of each of the fuel storage cells is configured to hold only a single nuclear fuel assembly.

[0150] 4. The fuel rack according to claim 3, wherein for the adjacent ones of the fuel storage cells running in the second direction along the Z axis, a single plate separates the adjacent ones of the fuel storage cells.

[0151] 5. The fuel rack according to claim 2, wherein the horizontal cross-sectional profile of each cell is rectilineal- in shape, and each flux trap has a rectilinear cross-sectional shape.

[0152] 6. The fuel rack according to claim 2, wherein the cell and flux traps are each vertically elongated and extend for a full height of the cellular body.

[0153] 7. The fuel rack according to any one of claims 2-6, wherein the gridwork of plates comprises a plurality of slideably interlocked and orthogonally intersecting slotted plates.

[0154] 8. The fuel rack according to claim 7, wherein the slotted plates comprise: a bottom section of first slotted plates fixedly attached to a top surface of the baseplate, the first slotted plates being welded to the baseplate to form a lower weldment; a middle section of second slotted plates stacked on top of the bottom section of slotted plates, the second slotted plates in the middle section formed of a neutron absorber material comprising boron; and a top section of third slotted plates stacked on top of the middle section of second slotted plates, the third slotted plates being welded together to form an upper weldment.

[0155] 9. The fuel rack according to claim 8, further comprising: a plurality of vertically-extending external corner tension members, each comer tension member being disposed at a corner of the fuel rack, the corner tension members each being fixedly coupled via welding at a bottom end to the baseplate; a plurality of horizontally-extending and vertically spaced apart strap members, the strap members being oriented perpendicularly to the comer tension members, each strap member being fixedly coupled via welding at a first end to one of the corner tension members and at a second end to another one of the corner tension members.

[0156] 10. The fuel rack according to claim 9, further comprising a top girdle frame with open center fixedly coupled to the upper weldment around a perimeter thereof, a top end of the comer tension members each being welded coupled to the top girdle frame.

[0157] 11. The fuel rack according to claim 9, further comprising a plurality of vertically-extending internal tension members having a top end coupled to the upper weldment and a bottom end coupled to the baseplate, each of the internal tension members being disposed in one of the flux traps.

[0158] 12. The fuel rack according to claim 11, wherein each internal tension member comprises a vertically elongated tension rod detachably coupled to the upper weldment via a lateral restraint element disposed engaged with a top of one of the flux traps.

[0159] 13. The fuel rack according to claim 12, wherein opposing top and bottom ends of each tension rod arc threaded, the top ends of the tension rods being secured to the lateral restraint elements via threaded nuts, and the bottom ends of the tension rods being secured to the baseplate via threaded nuts.

[0160] 14. The fuel rack according to any one of claims 8-13, wherein the second slotted plates are formed of an aluminum boron carbide metal matrix composite material which is metallurgically incompatible for welding.

[0161] 15. The fuel rack according to claim 14, wherein the second slotted plates in the middle section are comprised of borated aluminum, and the first and third slotted plates are formed of stainless steel.

[0162] 16. The fuel rack according to any one of claims 8-14, wherein the upper weldment comprises a single tier of horizontally spaced apart third slotted plates welded to the top surface of the baseplate, the slotted plates in the single tier being oriented parallel to each other.

[0163] 17. The fuel rack according to claim 16, wherein the baseplate further comprises a plurality of horizontally spaced apart under-girder beams fixedly attached to a bottom surface of the baseplate, the under-girder beams being configured to be placed on a floor of the fuel pool and elevate the baseplate to form a flow plenum between the baseplate and the floor.

[0164] 18. The fuel rack according to claim 17, wherein the under-girder beams are oriented parallel to each other and extend from one side of the baseplate to an opposite side of the baseplate.

[0165] 19. The fuel rack according to claim 18, wherein the under-girder beams are arranged orthogonally to the third slotted plates in the single tier on the top surface of the baseplate.

[0166] 20. The fuel rack according to any one of claims 17-19, wherein the under-girder beams each comprise a plurality of flow holes and a flow hole is formed in the baseplate at a bottom of each fuel storage cell and flux trap.

[0167] 21. The fuel rack according to claim 17, wherein the under-girder beams comprise stainless steel bars with a rectilinear cross-sectional shape, the under-girder beams being welded to the bottom surface of the baseplate.

[0168] 22. The fuel rack according to any one of claims 1-21 , wherein the flux traps each have a length and a width which defines a gap between adjacent cells in the first direction along the first horizontal axis, and wherein at least two of the flux traps have a different width than each other.

[0169] 23. The fuel rack according to any one of claims 17-22, further comprising a plurality of pedestals depending downward from the bottom surface of the baseplate, the pedestals each configured to engage the floor of the fuel pool and elevate the baseplate above the floor.

[0170] 24. The fuel rack according to claim 1, wherein an axial pitch between the fuel storage cells in the in the two orthogonal X and Z directions is variable such that flux traps in the second orthogonal direction is zero leading to a unidirectional flux trap configuration.

[0171] 25. A fuel rack for wet storage of nuclear fuel in a fuel pool, the fuel rack comprising: a baseplate formed of a metal; a cellular body extending from the baseplate, the cellular body comprising a plurality of fuel storage cells configured to receive a fuel assembly therein, the cellular body comprising a vertical stack comprising: a bottom section of first slotted plates welded atop the baseplate to form a lower weldment; a middle section comprising second slotted plates stacked atop the bottom section, the second slotted plates formed of a boron-bearing material acting as a neutron absorber, the second slotted plates being mechanically interlocked; a top section of third slotted plates welded together to form an upper weldment; and a plurality of flux traps formed between at least some of the fuel storage cells; a plurality of tension members, each of the plurality of tension members comprising a top end coupled to the upper weldment and a bottom end coupled to the lower weldment, the tension members each extending through one of the flux traps.

[0172] 26. The fuel rack according to claim 25, wherein each internal tension member comprises a vertically elongated tension rod detachably coupled to the upper weldment via a lateral restraint element configured to engage a top of one of the flux traps.

[0173] 27. The fuel rack according to claim 26, wherein opposing top and bottom ends of each tension rod are threaded, the top ends of each tension rod being detachably coupled to one of the lateral restraint elements via a threaded nut, and the bottom ends of each tension rod being detachably coupled to the baseplate via a threaded nut.

[0174] 28. The fuel rack according to claim 27, wherein the internal tension members are each tightenable via rotating the threaded nuts to draw the vertical stack of slotted plates together and compress the middle section of second slotted plates between the lower and upper weldments.

[0175] 29. The fuel rack according to claim 26, wherein each lateral restraint element comprises a mounting hole which receives the threaded top end of tension rod therethrough for coupling to one of the threaded nuts at a top surface of the lateral restraint element.

[0176] 30. The fuel rack according to claim 26, wherein: the first, second, and third slotted plates form an orthogonal grid which defines the fuel storage cells and the flux traps, the fuel storage cells and flux traps each having a rectilinear cross-section in shape; the lateral restraint elements each having a stepped bottom surface configured to lockingly engage one of the flux traps to prevent relative lateral movement between the lateral restraint element, tension rod, and flux trap.

[0177] 31. The fuel rack according to claim 30, wherein the bottom surface of the lateral restraint element includes a perimetrically extending downward facing lip which engages top edges of the third slotted plates of the upper weldment which circumscribe a top of each flux trap, and downwardly extending locking protrusion which is insertably received inside the flux trap.

[0178] 32. The fuel rack according to any one of claims 25-31, further comprising: a plurality of vertically-extending external corner tension members, each comer tension member being disposed at a corner of the fuel rack, the corner tension members each having a bottom end fixedly coupled to the baseplate;a plurality of horizontally-extending and vertically spaced apart strap members, the strap members being oriented perpendicularly to the comer tension members, each strap member being fixedly coupled at a first end to one of the comer tension members and at a second end to another one of the corner tension members.

[0179] 33. The fuel rack according to claim 32, further comprising a top girdle frame with open center fixedly coupled to the upper weldment around a perimeter thereof, the top end of the corner tension members each being fixedly coupled to the top girdle frame.

[0180] 34. The fuel rack according to claim 26, wherein: the first, second, and third slotted plates form a hexagonal grid which defines the fuel storage cells and the flux traps, the fuel storage cells having a hexagonal cross-section in shape and the flux traps each having a triangular cross-section in shape; the lateral restraint elements each being triangular in shape and welded to the third slotted plates in the upper weldment inside the flux traps so that the lateral restraint elements do no project above the top plate of the upper weldment.

[0181] 35. A method for assembling a fuel rack for underwater storage of nuclear fuel in a fuel pool, the method comprising: forming a lower weldment and an upper weldment, the lower weldment comprising a baseplate and a plurality of first slotted plates welded thereto, the upper weldment comprising a plurality of third slotted plates welded together; stacking a plurality of second slotted plates atop the first slotted plates, the second slotted plates being mechanically interlocked together; stacking the upper weldment on top of the second slotted plates; the first, second, and third slotted plates collectively defining a plurality of fuel storage cells each configured to hold a nuclear fuel assembly, and a plurality of flux traps disposed between at least some of the cells; inserting an elongated tension member through at least some of the flux traps; coupling a top end of each tension member to the upper weldment; coupling a bottom end of each tension member to the lower weldment;creating tension in the tension members; drawing the upper and lower weldments together via the tension members; and compressing the second plates between the upper and lower weldments.

[0182] 36. The method according to claim 35, wherein the step of coupling the top end of each tension member to the upper weldment includes threadably coupling a threaded top end of the tension member to a lateral restraint element engaged with the upper weldment.

[0183] 37. The method according to claim 36, wherein the step of coupling the bottom end of each tension member to the lower weldment includes threadably coupling a threaded bottom end of the tension member to the lower weldment.

[0184] 38. The method according to claim 37, wherein each tension member has a lock nut threadably engaged with the top of the tension member and a lock nut engaged with the bottom ends of the tension member.

[0185] 39. The method according to claim 38, wherein the step of creating tension in the tension members comprises tightening one or both lock nuts on each tension member.

[0186] 40. The method according to any one of claims 38-39, wherein the tension members are tension rods and the lateral restraint elements are plates with the mounting hole, the threaded top end of each tension rod insertable through the hole to engage one of the lock nuts.

[0187] 41. A fuel rack for wet storage of nuclear fuel in a fuel pool, the fuel rack comprising: a baseplate; a gridwork of plates extending from the baseplate, the gridwork of plates forming: a plurality of fuel storage cells, each of the plurality of fuel storage cells having a horizontal cross-sectional profile configured to receive a single nuclear fuel assembly therein; and a flux trap configuration in which: (1) a flux trap is present between some adjacent ones of the fuel storage cells; and (2) a flux trap is not present between other adjacent ones of the fuel storage cells; and wherein the flux traps are dimensioned so that when the fuel rack is fully loaded with a stored array of nuclear fuel assemblies, the stored array has a K-effective factor that is less than 1.

[0188] 42. A method for manufacturing a fuel rack for wet storage of nuclear fuel in a fuel pool, the method comprising: providing a baseplate having a top major surface, a bottom major surface, and a side edges extending between the top and bottom major surface; welding a plurality of first slotted plates to the top surface of the baseplate in a substantially parallel arrangement to one another, each of the plurality of first slotted plates comprising a first side major surface and a second side major surface, each of the first and second side major surfaces being orthogonal to the top major surface of the baseplate; and welding a plurality of under-girder beams to the bottom surface of the baseplate in a substantially parallel arrangement to one another, thereby forming a lower weldment.

[0189] 43. The method according to claim 42, wherein each of the plurality of slotted plates extend along a plate axis and each of the plurality of under-girder beams extend along a beam axis, the beam axes being orthogonal to the plate axes.

[0190] 44. The method according to claim 43, wherein each of the under-girder beams has a continuous body which extends from one of the lateral sides of the baseplate to an opposite lateral side of the baseplate.

[0191] 45. The method according to claim 43, wherein under-girder beams project downwards from the bottom surface of the baseplate to form a plenum below the baseplate.

[0192] 46. The method according to any one of claims 42-45, wherein the under-girder beams include a plurality of flow holes.

[0193] 47. The method according to claim 43, wherein the slotted plates are welded to the top surface of the baseplate, and the under-girder beams are welded to the bottom surface of the baseplate.

[0194] 48. The method according to any one of claims 47, wherein the baseplate, slotted plates, and under-girder beams are formed of stainless steel.

[0195] 49. The method according to any one of claims 42-48, further comprising coupling a plurality of pedestals to the bottom surface of the baseplate.

[0196] 50. The method according to any one of claims 42-49, further comprising: stacking a grid array of second slotted plates formed of a formed a boron-bearing metal matrixcomposite material on top of the lower weldment, the second slotted plates being metallurgically incompatible for welding to the first slotted plates of the lower weldment; and stacking a grid array of third slotted plates on top of the second slotted plates, the third slotted plates being welded together; wherein the first, second, and third slotted plates collectively define a plurality of fuel storage cells each configured to hold a nuclear fuel assembly.

[0197] 51. The method according to claim 50, further comprising forming a plurality of flux traps between at least some of the cells in a first orthogonal direction, and not forming a plurality of flux traps between the cells in a second orthogonal direction.

[0198] 52. A method of manufacturing a fuel rack for wet storage of an array of nuclear fuel assemblies in a fuel pool, wherein each nuclear fuel assembly of the array is to be stored in a fuel cell, the method comprising: varying axial pitch between adjacent fuel cells in two orthogonal directions and dimensioning flux traps in a single one of the orthogonal directions until a K-effective factor for the array is less than 1, thereby resulting in a unidirectional flux trap configuration; and building the fuel rack with the unidirectional flux trap configuration.

[0199] 53. The method according to claim 52, wherein during step a), dimensions of flux traps in the other one of the orthogonal directions is maintained at zero.

[0200] 54. A fuel rack for wet storage of nuclear fuel in a fuel pool, the fuel rack comprising: a lower weldment comprising a baseplate, a plurality of first slotted plates welded to a top surface of the baseplate, and a plurality of under-girder beams fixedly coupled to a bottom surface of the baseplate; a middle section comprising a gridwork of second slotted plates, the second slotted plates formed a boron-bearing metal matrix composite material; an upper weldment comprising a gridwork of third slotted plates; the middle section sandwiched between the upper and lower weldments, the first slotted plated, the gridwork of second slotted plated, and the gridwork of third slotted plates collectively defining a plurality of fuel storage cells each configured to hold a nuclear fuel assembly;a plurality of vertically-elongated tension members comprising a top end coupled to the upper weldment and a bottom end coupled to the lower weldment.

[0201] 55. The fuel rack according to claim 54, wherein the tension members comprise comer tension members, each one of the comer tension members being disposed at a corner of the fuel rack.

[0202] 56. The fuel rack according to claim 55, further comprising a plurality of horizontally-extending and vertically spaced apart strap members, the strap members being oriented perpendicularly to the corner tension members, each strap member being fixedly coupled at a first end to one of the corner tension members and at a second end to another one of the corner tension members.

[0203] 57. The fuel rack according to claim 56, further comprising a top girdle frame with open center fixedly coupled to the upper weldment around a perimeter thereof, the top end of the corner tension members each being fixedly coupled to the top girdle frame.

[0204] 58. The fuel rack according to claims 55 or 56, wherein the tension members further comprise a plurality of internal tension members extending vertically between the fuel storage cells, each internal tension member being disposed in one of a plurality of vertically- extending flux traps formed between at least some of the fuel storage cells of the fuel rack.

[0205] 59. The fuel rack according to claim 58, wherein each internal tension member comprises a vertically elongated tension rod having a top end detachably coupled to the upper weldment via a lateral restraint element engaging a top of one of the flux traps, and a bottom end detachably coupled to the baseplate.

[0206] 60. The fuel rack according to any one of claims 54-59, wherein the first slotted plates and the third slotted plates are formed of stainless steel.

[0207] While the foregoing description and drawings represent exemplary embodiments of the present disclosure, it will be understood that various additions, modifications and substitutions may be made therein without departing from the spirit and scope and range of equivalents of the accompanying claims. In particular, it will be clear’ to those skilled in the art that the present invention may be embodied in other forms, structures, arrangements, proportions, sizes, and with other elements, materials, and components, without departing from the spirit or essential characteristics thereof. In addition, numerous variations in the methods / processes described herein may be made within the scope of the present disclosure.One skilled in the art will further appreciate that the embodiments may be used with many modifications of structure, arrangement, proportions, sizes, materials, and components and otherwise, used in the practice of the disclosure, which are particularly adapted to specific environments and operative requirements without departing from the principles described herein. The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive. The appended claims should be construed broadly, to include other variants and embodiments of the disclosure, which may be made by those skilled in the art without departing from the scope and range of equivalents.

Claims

CLAIMSWhat is claimed is:

1. A fuel rack for wet storage of nuclear fuel in a fuel pool, the fuel rack comprising: a baseplate; a rectilinear gridwork of plates extending vertically from the baseplate along a Y axis of a Cartesian coordinate system, the rectilinear gridwork of plates forming: a plurality of vertically-extending fuel storage cells, each of the plurality of vertically-extending fuel storage cells having a horizontal cross-sectional profile configured to receive a nuclear' fuel assembly therein; and a unidirectional flux trap configuration in which: (1) a flux trap is present between adjacent ones of the fuel storage cells in a first orthogonal direction along a X axis of the Cartesian coordinate system; and (2) a flux trap is not present between adjacent ones of the fuel storage cells running in a second orthogonal direction along a Z axis of the Cartesian coordinate system.

2. The fuel rack according to claim 1, wherein the flux traps are dimensioned so that when the fuel rack is fully loaded with a stored array of the plurality of nuclear fuel assemblies, the stored array has a K-effective factor that is less than 1.

3. The fuel rack according to claim 2, wherein the horizontal cross-sectional profile of each of the fuel storage cells is configured to hold only a single nuclear fuel assembly.

4. The fuel rack according to claim 3, wherein for the adjacent ones of the fuel storage cells miming in the second orthogonal direction along the Z axis, a single plate separates the adjacent ones of the fuel storage cells.

5. The fuel rack according to claim 2, wherein the horizontal cross-sectional profile of each cell is rectilinear in shape, and each flux trap has a rectilinear cross-sectional shape.

6. The fuel rack according to claim 2, wherein the cell and flux traps are each vertically elongated and extend for a full height of the cellular body.

7. The fuel rack according to any one of claims 2-6, wherein the gridwork of plates comprises a plurality of slidcably interlocked and orthogonally intersecting slotted plates.

8. The fuel rack according to claim 7, wherein the slotted plates comprise: a bottom section of first slotted plates fixedly attached to a top surface of the baseplate, the first slotted plates being welded to the baseplate to form a lower weldment; a middle section of second slotted plates stacked on top of the bottom section of slotted plates, the second slotted plates in the middle section formed of a neutron absorber material comprising boron; and a top section of third slotted plates stacked on top of the middle section of second slotted plates, the third slotted plates being welded together to form an upper weldment.

9. The fuel rack according to claim 8, further comprising: a plurality of vertically-extending external corner tension members, each comer tension member being disposed at a corner of the fuel rack, the corner tension members each being fixedly coupled via welding at a bottom end to the baseplate; a plurality of horizontally-extending and vertically spaced apart strap members, the strap members being oriented perpendicularly to the comer tension members, each strap member being fixedly coupled via welding at a first end to one of the corner tension members and at a second end to another one of the corner tension members.

10. The fuel rack according to claim 9, further comprising a top girdle frame with open center fixedly coupled to the upper weldment around a perimeter thereof, a top end of the corner tension members each being welded coupled to the top girdle frame.

11. The fuel rack according to claim 9, further comprising a plurality of vertically-extending internal tension members having a top end coupled to the upper weldment and a bottom end coupled to the baseplate, each of the internal tension members being disposed in one of the flux traps.

12. The fuel rack according to claim 11 , wherein each internal tension member comprises a vertically elongated tension rod detachably coupled to the upper weldment via a lateral restraint element disposed engaged with a top of one of the flux traps.

13. The fuel rack according to claim 12, wherein opposing top and bottom ends of each tension rod are threaded, the top ends of the tension rods being secured to the lateral restraint elements via threaded nuts, and the bottom ends of the tension rods being secured to the baseplate via threaded nuts.

14. The fuel rack according to any one of claims 8-13, wherein the second slotted plates are formed of an aluminum boron carbide metal matrix composite material which is metallurgically incompatible for welding.

15. The fuel rack according to claim 14, wherein the second slotted plates in the middle section are comprised of borated aluminum, and the first and third slotted plates are formed of stainless steel.

16. The fuel rack according to any one of claims 8-14, wherein the upper weldment comprises a single tier of horizontally spaced apart third slotted plates welded to the top surface of the baseplate, the slotted plates in the single tier being oriented parallel to each other.

17. The fuel rack according to claim 16, wherein the baseplate further comprises a plurality of horizontally spaced apart under-girder beams fixedly attached to a bottom surface of the baseplate, the under-girder beams being configured to be placed on a floor of the fuel pool and elevate the baseplate to form a flow plenum between the baseplate and the floor.

18. The fuel rack according to claim 17, wherein the under-girder beams are oriented parallel to each other and extend from one side of the baseplate to an opposite side of the baseplate19. The fuel rack according to claim 18, wherein the under-girder beams are arranged orthogonally to the third slotted plates in the single tier on the top surface of the baseplate.

20. The fuel rack according to any one of claims 17-19, wherein the under-girder beams each comprise a plurality of flow holes and a flow hole is formed in the baseplate at a bottom of each fuel storage cell and flux trap.

21. The fuel rack according to claim 17, wherein the under-girder beams comprise stainless steel bars with a rectilineal’ cross-sectional shape, the under-girder beams being welded to the bottom surface of the baseplate.

22. The fuel rack according to any one of claims 1-21, wherein the flux traps each have a length and a width which defines a gap between adjacent cells in the first orthogonal direction along the first horizontal axis, and wherein at least two of the flux traps have a different width than each other.

23. The fuel rack according to any one of claims 17-22, further comprising a plurality of pedestals depending downward from the bottom surface of the baseplate, the pedestals each configured to engage the floor of the fuel pool and elevate the baseplate above the floor.

24. The fuel rack according to claim 1, wherein an axial pitch between the fuel storage cells in the two orthogonal X and Z directions is variable such that flux traps in the second orthogonal direction is zero leading to a unidirectional flux trap configuration.

25. A fuel rack for wet storage of nuclear fuel in a fuel pool, the fuel rack comprising: a baseplate formed of a metal; a cellular body extending from the baseplate, the cellular body comprising a plurality of fuel storage cells configured to receive a fuel assembly therein, the cellular body comprising a vertical stack comprising: a bottom section of first slotted plates welded top the baseplate to form a lower weldment; a middle section comprising second slotted plates stacked atop the bottom section, the second slotted plates formed of a boron-bearing material acting as a neutron absorber, the second slotted plates being mechanically interlocked; a top section of third slotted plates welded together to form an upper weldment; and a plurality of flux traps formed between at least some of the fuel storage cells;a plurality of tension members, each of the plurality of tension members comprising a top end coupled to the upper weldment and a bottom end coupled to the lower weldment, the tension members each extending through one of the flux traps.

26. The fuel rack according to claim 25, wherein each tension member comprises a vertically elongated tension rod detachably coupled to the upper weldment via a lateral restraint element configured to engage a top of one of the flux traps.

27. The fuel rack according to claim 26, wherein opposing top and bottom ends of each tension rod are threaded, the top ends of each tension rod being detachably coupled to one of the lateral restraint elements via a threaded nut, and the bottom ends of each tension rod being detachably coupled to the baseplate via a threaded nut.

28. The fuel rack according to claim 27, wherein the tension members are each tightenable via rotating the threaded nuts to draw the vertical stack of slotted plates together and compress the middle section of second slotted plates between the lower and upper weldments.

29. The fuel rack according to claim 26, wherein each lateral restraint element comprises a mounting hole which receives the threaded top end of tension rod therethrough for coupling to one of the threaded nuts at a top surface of the lateral restraint element.

30. The fuel rack according to claim 26, wherein: the first, second, and third slotted plates form an orthogonal grid which defines the fuel storage cells and the flux traps, the fuel storage cells and flux traps each having a rectilineal’ cross-section in shape; the lateral restraint elements each having a stepped bottom surface configured to lockingly engage one of the flux traps to prevent relative lateral movement between the lateral restraint element, tie rod, and flux trap.

31. The fuel rack according to claim 30, wherein the bottom surface of the lateral restraint element includes a perimetrically extending downward facing lip which engages top edges of the third slotted plates of the upper weldment which circumscribe a top of each flux trap, and downwardly extending locking protrusion which is insertably received inside the flux trap.

32. The fuel rack according to any one of claims 25-31, further comprising:a plurality of vertically-extending external corner tension members, each comer tension member being disposed at a corner of the fuel rack, the corner tension members each having a bottom end fixedly coupled to the baseplate; a plurality of horizontally-extending and vertically spaced apart strap members, the strap members being oriented perpendicularly to the comer tension members, each strap member being fixedly coupled at a first end to one of the comer tension members and at a second end to another one of the corner tension members.

33. The fuel rack according to claim 32, further comprising a top girdle frame with open center fixedly coupled to the upper weldment around a perimeter thereof, the top end of the comer tension members each being fixedly coupled to the top girdle frame.

34. The fuel rack according to claim 26, wherein: the first, second, and third slotted plates form a hexagonal grid which defines the fuel storage cells and the flux traps, the fuel storage cells having a hexagonal cross-section in shape and the flux traps each having a triangular cross-section in shape; the lateral restraint elements each being triangular in shape and welded to the third slotted plates in the upper weldment inside the flux traps so that the lateral restraint elements do no project above the top plate of the upper weldment.

35. A method for assembling a fuel rack for underwater storage of nuclear fuel in a fuel pool, the method comprising: forming a lower weldment and an upper weldment, the lower weldment comprising a baseplate and a plurality of first slotted plates welded thereto, the upper weldment comprising a plurality of third slotted plates welded together; stacking a plurality of second slotted plates atop the first slotted plates, the second slotted plates being mechanically interlocked together; stacking the upper weldment on top of the second slotted plates; the first, second, and third slotted plates collectively defining a plurality of fuel storage cells each configured to hold a nuclear fuel assembly, and a plurality of flux traps disposed between at least some of the cells;inserting an elongated tension member through at least some of the flux traps; coupling a top end of each tension member to the upper weldment; coupling a bottom end of each tension member to the lower weldment; creating tension in the tension members; drawing the upper and lower weldments together via the tension members; and compressing the second plates between the upper and lower weldments.

36. The method according to claim 35, wherein the step of coupling the top end of each tension member to the upper weldment includes threadably coupling a threaded top end of the tension member to a lateral restraint element engaged with the upper weldment.

37. The method according to claim 36, wherein the step of coupling the bottom end of each tension member to the lower weldment includes threadably coupling a threaded bottom end of the tension member to the lower weldment.

38. The method according to claim 37, wherein each tension member has a lock nut threadably engaged with the top of the tension member and a lock nut engaged with the bottom ends of the tension member.

39. The method according to claim 38, wherein the step of creating tension in the tension members comprises tightening one or both lock nuts on each tension member.

40. The method according to any one of claims 38-39, wherein the tension members are tension rods and the lateral restraint elements are plates with the mounting hole, the threaded top end of each tension rod insertable through the hole to engage one of the lock nuts.

41. A fuel rack for wet storage of nuclear fuel in a fuel pool, the fuel rack comprising: a baseplate; a gridwork of plates extending from the baseplate, the gridwork of plates forming: a plurality of fuel storage cells, each of the plurality of fuel storage cells having a horizontal cross-sectional profile configured to receive a single nuclear fuel assembly therein; anda flux trap configuration in which: (1) a flux trap is present between some adjacent ones of the fuel storage cells; and (2) a flux trap is not present between other adjacent ones of the fuel storage cells; and wherein the flux traps are dimensioned so that when the fuel rack is fully loaded with a stored array of nuclear fuel assemblies, the stored array has a K-effective factor that is less than 1.

42. A method for manufacturing a fuel rack for wet storage of nuclear fuel in a fuel pool, the method comprising: providing a baseplate having a top major surface, a bottom major surface, and a side edges extending between the top and bottom major surface; welding a plurality of first slotted plates to the top surface of the baseplate in a substantially parallel arrangement to one another, each of the plurality of first slotted plates comprising a first side major surface and a second side major surface, each of the first and second side major surfaces being orthogonal to the top major surface of the baseplate; and welding a plurality of under-girder beams to the bottom surface of the baseplate in a substantially parallel arrangement to one another, thereby forming a lower weldment.

43. The method according to claim 42, wherein each of the plurality of slotted plates extend along a plate axis and each of the plurality of under-girder beams extend along a beam axis, the beam axes being orthogonal to the plate axes.

44. The method according to claim 43, wherein each of the under-girder beams has a continuous body which extends from one of the lateral sides of the baseplate to an opposite lateral side of the baseplate.

45. The method according to claim 43, wherein under-girder beams project downwards from the bottom surface of the baseplate to form a plenum below the baseplate.

46. The method according to any one of claims 42-45, wherein the under-girder beams include a plurality of flow holes.

47. The method according to claim 43, wherein the slotted plates are welded to the top surface of the baseplate, and the under-girder beams are welded to the bottom surface of the baseplate.

48. The method according to any one of claims 47, wherein the baseplate, slotted plates, and undcr-girdcr beams arc formed of stainless steel.

49. The method according to any one of claims 42-48, further comprising coupling a plurality of pedestals to the bottom surface of the baseplate.

50. The method according to any one of claims 42-49, further comprising: stacking a grid array of second slotted plates formed of a formed a boron-bearing metal matrix composite material on top of the lower weldment, the second slotted plates being metallurgically incompatible for welding to the first slotted plates of the lower weldment; and stacking a grid array of third slotted plates on top of the second slotted plates, the third slotted plates being welded together; wherein the first, second, and third slotted plates collectively define a plurality of fuel storage cells each configured to hold a nuclear fuel assembly.

51. The method according to claim 50, further comprising forming a plurality of flux traps between at least some of the cells in a first orthogonal direction, and not forming a plurality of flux traps between the cells in a second orthogonal direction.

52. A method of manufacturing a fuel rack for wet storage of an array of nuclear fuel assemblies in a fuel pool, wherein each nuclear fuel assembly of the array is to be stored in a fuel cell, the method comprising: a) varying axial pitch between adjacent fuel cells in two orthogonal directions and dimensioning flux traps in a single one of the orthogonal directions until a K-effective factor for the array is less than 1, thereby resulting in a unidirectional flux trap configuration; and b) building the fuel rack with the unidirectional flux trap configuration.

53. The method according to claim 52, wherein during step a), dimensions of flux traps in the other one of the orthogonal directions is maintained at zero.

54. A fuel rack for wet storage of nuclear fuel in a fuel pool, the fuel rack comprising:a lower weldment comprising a baseplate, a plurality of first slotted plates welded to a top surface of the baseplate, and a plurality of under-girder beams fixedly coupled to a bottom surface of the baseplate; a middle section comprising a gridwork of second slotted plates, the second slotted plates formed a boron-bearing metal matrix composite material; an upper weldment comprising a gridwork of third slotted plates; the middle section sandwiched between the upper and lower weldments, the first slotted plated, the gridwork of second slotted plated, and the gridwork of third slotted plates collectively defining a plurality of fuel storage cells each configured to hold a nuclear fuel assembly; a plurality of vertically-elongated tension members comprising a top end coupled to the upper weldment and a bottom end coupled to the lower weldment.

55. The fuel rack according to claim 54, wherein the tension members comprise comer tension members, each one of the corner tension members being disposed at a corner of the fuel rack.

56. The fuel rack according to claim 55, further comprising a plurality of horizontallyextending and vertically spaced apart strap members, the strap members being oriented perpendicularly to the corner tension members, each strap member being fixedly coupled at a first end to one of the corner tension members and at a second end to another one of the corner tension members.

57. The fuel rack according to claim 56, further comprising a top girdle frame with open center fixedly coupled to the upper weldment around a perimeter thereof, the top end of the comer tension members each being fixedly coupled to the top girdle frame.

58. The fuel rack according to claims 55 or 56, wherein the tension members further comprise a plurality of internal tension members extending vertically between the fuel storage cells, each internal tension member being disposed in one of a plurality of vertically-extending flux traps formed between at least some of the fuel storage cells of the fuel rack.

59. The fuel rack according to claim 58, wherein each internal tension member comprises a vertically elongated tension rod having a top end detachably coupled to the upper weldment via alateral restraint element engaging a top of one of the flux traps, and a bottom end detachably coupled to the baseplate.

60. The fuel rack according to any one of claims 54-59, wherein the first slotted plates and the third slotted plates are formed of stainless steel.

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