Method and apparatus for improving the performance of RCCA and CEA for mitigating cladding strain in the high-fluence region.

Abstract

The present disclosure generally relates to methods, devices, and systems for improving the performance of a control rod cluster assembly (RCCA) and / or a control element assembly (CEA) for mitigating cladding distortion during normal operating conditions and accident conditions, particularly in high fluence regions. In various aspects, a powder collection and interruption device is disclosed. The device can be positioned between the upper and lower absorbers of the control rods of the RCCA and / or CEA. The device can be configured to prevent powder generated by the upper absorber from passing through the lower section of the control rod. In another aspect, the plenum volume of the control rod can be increased by incorporating an axial hole in the top plug extension and / or the bottom plug.

Description

Technical Field

[0001] Cross - reference to related applications This application claims the benefit and priority of 35 U.S.C. § 120. U.S. Patent Application No. 17 / 353,016, filed on June 21, 2021, "Methods and Apparatus for Improving the Performance of RCCA and CEA to Reduce Cladding Strain in High - Fluence Regions". The entire content thereof is incorporated herein by reference.

[0002] The present disclosure generally relates to methods, apparatus, and systems for improving the performance of rod cluster control assemblies (RCCA) and control element assemblies (CEA) to mitigate cladding strain in high - fluence regions between normal operating conditions and accident conditions.

Summary of the Invention

[0003] The following summary is provided to facilitate an understanding of some of the innovative features specific to the aspects disclosed herein and is not intended as a complete description. The various aspects can be fully understood by taking the entire specification, claims, and abstract as a whole.

[0004] In various aspects, methods, apparatus, and systems for improving the performance of rod cluster control assemblies (RCCA) and / or control element assemblies (CEA) to mitigate cladding strain in high - fluence regions between normal operating conditions and / or accident conditions are disclosed herein. One method can include incorporating a device such as a powder collection barrier between the upper absorber and the lower absorber of the RCCA and / or CEA. Another method can include increasing the plenum volume by incorporating an axial hole in the upper end plug. Another method can include increasing the plenum volume by incorporating an axial hole in the lower end plug and optionally incorporating a radial groove at the bottom of the lower absorber to ensure that the opening of the lower end plug is not blocked by the lower absorber, providing a flow path for gas expansion or generation.

[0005] In various embodiments, reactor control rod cluster assemblies (RCCAs) or control element assemblies (CEAs) for mitigating cladding strain are disclosed herein.

[0006] In various embodiments, the reactor control rod cluster (RCCA) or control element assembly (CEA) may be equipped with devices to improve the performance of the RCCA in order to mitigate cladding strain, for example, cladding strain in the high-fluence region of the RCCA or CEA.

[0007] In various embodiments, the RCCA or CEA may comprise at least one control rod comprising a cladding tube having an upper section and a lower section. In various embodiments, at least one control rod may comprise an absorbent material housed within the cladding tube, which may consist of an upper absorbent material housed within the upper section of the cladding tube and a lower absorbent material housed within the lower section of the cladding tube. The cladding tube may further comprise a device positioned between the upper and lower absorbent materials within the cladding tube.

[0008] In various embodiments, the control rod may be configured to have a first annular gap between the upper absorbent and the cladding tube; and a second annular gap between the lower absorbent and the cladding tube. In one embodiment, the first annular gap has the same width as the second annular gap.

[0009] In various embodiments, the device may be a powder collection and containment device. In one embodiment, the powder collection and containment device may be located inside a cladding tube and / or incorporated into the upper or lower absorbent material within the cladding of RCCA or CEA. In one embodiment, the powder collection and containment device may be located between the upper and lower absorbent materials within the cladding.

[0010] In various embodiments, the RCCA or CEA may further comprise one or more powder collection and shutoff devices incorporated into the upper absorbent. In one embodiment, one or more powder collection and shutoff devices may be evenly distributed in the upper absorbent along the axial length of the cladding tube or at least one control rod. Each of the powder collection and shutoff devices may be configured to collect powder from the absorbent generated above each device. Powder may be generated by the absorbent during normal operating hours and / or during failure times exceeding the design standard (BDB). The device may be configured to reduce powder accumulation in the absorbent at the bottom of the RCCA or CEA and / or at the lower end of the cladding tube. In one embodiment, one or more powder collection and shutoff devices may be evenly distributed or at different intervals in the upper absorbent along the axial length of the cladding tube or at least one control rod.

[0011] In various embodiments, the RCCA or CEA may further comprise one or more powder collection and blocking devices incorporated into the lower absorbent along the axial length of the cladding tube or at least one control rod, and distributed evenly or at different intervals.

[0012] In various embodiments, the RCCA or CEA may further comprise at least one powder collection and blocking device incorporated in the upper absorbent material within the cladding tube; and at least one powder collection and blocking device incorporated in the lower absorbent material within the cladding tube.

[0013] In various embodiments, the RCCA or CEA comprises a plurality of powder collection and blocking devices incorporated into the absorbent material within the cladding tube, which may be distributed evenly or at different intervals along the axial length of the cladding tube or at least one control rod.

[0014] In various embodiments, the powder collection and blocking device may comprise a spacer and a girder spring. In one embodiment, the spacer is configured to hold the girder spring, provide a support surface between the upper and lower absorbent materials, and provide an axially open space for collecting powder generated by the upper absorbent material. In one embodiment, to compensate for thermal and irradiation expansion of the cladding tube, as well as creep-down of the cladding due to external pressure, the outward force of the girder spring keeps the girder spring firmly held against the inner wall of the cladding, preventing powder from passing outside the girder spring within the cladding.

[0015] In various embodiments, the garter spring may have a dense coil configured to prevent powder from passing through the cladding tube, for example, from the upper section to the lower section of the cladding, but not to prevent the refilling gas and gases generated during irradiation from moving through the control rod.

[0016] In various embodiments, the garter spring may have a spring coil pitch that is too coarse to block the powder. In one embodiment, the powder collection blocker may further include a fine screen mesh. The garter spring is configured to hold the screen mesh firmly against the inner wall of the cladding tube to prevent a gap from forming between the screen mesh and the inner wall of the cladding tube, and thus to prevent the powder from passing through the cladding tube, for example, from the upper section to the lower section of the cladding tube outside the garter spring or screen mesh.

[0017] In various embodiments, the powder collection and blocking device further comprises a screen mesh configured to be inserted on a garter spring, the garter spring being configured to firmly hold the screen mesh against the inner wall of the cladding tube to prevent a gap from forming between the screen mesh and the inner wall of the cladding tube.

[0018] In various embodiments, the screen mesh has a diameter that precisely matches the inner diameter of the cladding tube and a mesh size that is fine enough to prevent powder from passing through.

[0019] In various embodiments, the upper absorbent is a ceramic absorbent.

[0020] In various embodiments, the upper absorbent is boron carbide (B4C).

[0021] In various embodiments, the lower absorbent is a ceramic absorbent.

[0022] In various embodiments, the lower absorbent is boron carbide (B4C).

[0023] In various embodiments, the spacer is made of a material comprising at least one of 304 stainless steel, nickel-based alloy, Inconel 625, Inconel 718, ceramic absorbent, and B4C absorbent.

[0024] In various embodiments, the garter spring is made of a material comprising at least one of 304 stainless steel, nickel-based alloy, Inconel 625, and Inconel 718.

[0025] In various embodiments, the screen mesh is made of a material comprising at least one of 304 stainless steel, nickel-based alloy, Inconel 625, and Inconel 718.

[0026] In various aspects, a reactor control rod cluster assembly (RCCA) or CEA for mitigating clad strain is disclosed herein. In various aspects, the RCCA or CEA may comprise: at least one control rod. The at least one control rod comprises: a cladding tube having an upper end and a lower end; an absorber accommodated within the cladding tube; and a spring disposed within the cladding tube above the absorber. In some aspects, the spring forms a plenum above the absorber, and the plenum is composed of a plenum volume. In some aspects, the plenum region is formed around the absorber inside the cladding tube, and the plenum region has a plenum volume. The RCCA and / or CEA may further comprise a lower end plug configured to be attached to the lower end of the cladding tube, an upper end plug configured to be attached to the upper end of the cladding tube, and an axial hole in the upper end plug, the axial hole in the upper end plug increasing the plenum volume.

[0027] In various aspects, a reactor control rod cluster assembly (RCCA) or CEA for mitigating clad strain is disclosed herein. In various aspects, the RCCA may comprise: at least one control rod. In some aspects, the control rod may comprise: a cladding tube including an upper end and a lower end; an absorber accommodated within the cladding tube; and a plenum region formed around the absorber inside the cladding tube and having a plenum volume. In various aspects, the RCCA and / or CEA may further comprise a lower end plug configured to be attached to the lower end of the cladding tube; the lower end plug comprising an axial hole configured to increase the plenum volume.

[0028] In various aspects, a portion of the absorber proximate to the lower end of the cladding tube may comprise a radial groove formed therein, the radial groove being configured to allow gas to flow into the axial hole. In some aspects, the radial groove at the bottom of the lower absorber can provide a flow path for gas expansion or generation to ensure that the lower absorber does not block the opening of the lower end plug.

[0029] In various aspects, a nuclear reactor control element assembly (CEA) for mitigating clad strain is disclosed herein. In various aspects, the control element assembly (CEA) may comprise one or more, or all, of the features of the rod cluster control assemblies (RCCA) disclosed above herein.

[0030] In various aspects, a method for mitigating clad strain in a nuclear reactor RCCA or CEA is disclosed herein. In various aspects, the method may include: providing an RCCA or CEA comprising one or more, or all, of the features of the RCCA or CEA disclosed above herein.

[0031] In various aspects, the RCCA or CEA may comprise at least one control rod comprising a clad having an upper section and a lower section, an upper absorber housed within the upper section of the clad, and a lower absorber housed within the lower section of the clad. The method may further include incorporating a device between the upper absorber and the lower absorber within the clad. In various aspects, the device may be a powder collection barrier device. In various aspects, the powder collection barrier device may comprise: a spacer; and a garter spring. In various aspects, the spacer holds the garter spring, provides a bearing surface between the upper absorber and the lower absorber, and provides an axially open space for collecting powder from the upper absorber generated between normal operating conditions and accident conditions. In various aspects, due to the outward force of the garter spring, the spacer and the garter spring remain firmly held against the inner wall of the clad, preventing powder from passing from the upper section to the lower section of the clad outside the garter spring. In various aspects, the garter spring has closely spaced coils that prevent powder from passing from the upper section to the lower section of the clad, but do not prevent refill gas and gas generated during irradiation from moving through the control rod.

[0032] In various embodiments, at least one control rod is configured to have a first annular gap between the upper absorber and the inner wall of the cladding; and the same annular gap between the lower absorber and the inner wall of the cladding. In various embodiments, these gaps can provide sufficient space for thermal and irradiation expansion of the absorbers, including the upper and lower absorbers.

[0033] In various embodiments, the method may further include incorporating one or more powder collection and blocking devices into the upper absorbent material within the upper section of the cladding.

[0034] In various embodiments, the method may further include incorporating one or more powder collection and blocking devices into the lower absorbent material within the lower section of the cladding.

[0035] In various embodiments, the method may further include distributing powder collection and blocking devices evenly across the absorbent material along the axial length of the cladding.

[0036] In various embodiments, methods for relieving cladding strain in control rod cluster assemblies (RCCAs) or CEAs are disclosed herein. In various embodiments, the method may include providing a control rod cluster assemblies (RCCAs) or CEAs comprising: a cladding having upper and lower ends; an absorbent material housed within the cladding; a spring housed within the cladding; and a plenum housed within the cladding and further within the spring, comprising at least one control rod. In various embodiments, the RCCA or CEA may further include a lower end plug configured to be attached to the lower end of the cladding; and an upper end plug configured to be attached to the upper end of the cladding. In various embodiments, the method may include incorporating an axial hole in the upper end plug, the axial hole in the upper end plug increasing the plenum volume of the RCCA or CEA.

[0037] In various embodiments, methods for relieving cladding strain in a control rod cluster assembly (RCCA) or CEA are disclosed herein. In various embodiments, the method may include providing a control rod cluster assembly (RCCA) or CEA comprising: a cladding having an upper and lower end; an absorbent material housed within the cladding; a spring housed within the cladding; and a plenum housed within the cladding and further within the spring, for at least one control rod. In various embodiments, the RCCA or CEA may further include a lower end plug configured to be attached to the lower end of the cladding; and an upper end plug configured to be attached to the upper end of the cladding. In various embodiments, the method may include incorporating an axial hole in the lower end plug, the axial hole in the lower end plug increasing the plenum volume of the RCCA or CEA.

[0038] In various embodiments, the method may include incorporating a radial groove at the bottom of the absorbent material and providing a passage for gas expansion or generation to ensure that the lower absorbent does not block the opening of the lower end plug.

[0039] In various embodiments, methods for relieving strain in the cladding of a reactor control element assembly (CEA) are disclosed herein. In various embodiments, methods for relieving strain in the cladding of a control element assembly (CEA) may have features of one, more, or all of the methods for relieving strain in the cladding of a control rod cluster assembly (RCCA) disclosed herein above.

[0040] The methods, apparatus, and systems disclosed herein may be used in reactors such as water-cooled reactors, light water reactors (LWRs), heavy water reactors (HWRs), pressurized water reactors (PWRs), boiling water reactors (BWRs), and supercritical water reactors (SCWRs).

[0041] These and other objects, features and characteristics of the present invention, as well as the methods and functions of the relevant structural elements, and the economics of the assembly and manufacture of the parts, will become more apparent by considering the following description and the appended claims with reference to the appended drawings, all of which form part of this specification, and similar reference numerals indicate corresponding parts in the various drawings. However, it should be explicitly understood that the drawings are for illustrative and illustrative purposes only and are not intended to define the limitations of the present invention. [Brief explanation of the drawing]

[0042] Various features of the embodiments described herein are specifically described in the appended claims. However, various embodiments relating to both the organization and the operating methods, along with their advantages, can be understood in conjunction with the following appended drawings and in accordance with the following description.

[0043] [Figure 1] This is a side elevation view of a prior art fuel assembly, with a prior art control rod spider assembly positioned above the fuel assembly, with the components partially divided and shown in cross-section for clarity.

[0044] [Figure 2] Figure 1 is a plan view of a prior art control rod spider assembly of the type shown.

[0045] [Figure 3] This is an elevation view of the control assembly taken along line 3-3 in Figure 2, with the components partially divided.

[0046] [Figure 4A] A schematic diagram of a powder collection and blocking device within a cladding, according to at least one non-limiting aspect of the present disclosure, is shown. [Figure 4B] A schematic diagram of a powder collection and blocking device within a cladding, according to at least one non-limiting aspect of the present disclosure, is shown.

[0047] [Figure 5A]A cross-section of a powder collection and blocking device within a cladding, according to at least one non-limiting aspect of the present disclosure, is shown. [Figure 5B] A cross-section of a powder collection and blocking device within a cladding, according to at least one non-limiting aspect of the present disclosure, is shown.

[0048] [Figure 6] An example of an RCCA with increased control rod cluster (RCCA) plenum volume is shown according to at least one non-limiting aspect of this disclosure.

[0049] [Figure 7] Another example of an RCCA with increased control rod cluster (RCCA) plenum volume is shown according to at least one non-limiting aspect of this disclosure.

[0050] [Figure 8A] An example of an axial hole and radial groove incorporated into a lower end plug, included in the bottom of the lower absorber of an RCCA, is shown in at least one non-limiting aspect of the present disclosure. [Figure 8B] An example of an axial hole and radial groove incorporated into a lower end plug, included in the bottom of the lower absorber of an RCCA, is shown in at least one non-limiting aspect of the present disclosure.

[0051] Corresponding reference letters indicate the corresponding parts through multiple figures. The examples described herein illustrate various aspects of the invention in one form and are not intended to limit the scope of the invention. [Modes for carrying out the invention]

[0052] Numerous specific details are provided to allow for a full understanding of the overall structure, function, manufacture, and use of the embodiments described in this disclosure and shown in the accompanying drawings. Well-known operations, components, and elements are not described in detail so as not to obscure the embodiments described in the specification. The reader will understand that the embodiments described and illustrated herein are non-limiting examples, and therefore certain structural and functional details disclosed herein may be representative and illustrative. Various modifications and changes thereto may be made without departing from the claims. Furthermore, it should be understood that terms such as “front,” “rear,” “left,” “right,” “up,” “down,” “top,” “bottom,” “upper,” and “lower” are for convenience only and should not be interpreted as limiting terms. In the following description, the same reference letters refer to the same or corresponding parts through several figures in the drawings.

[0053] Before describing in detail the various embodiments disclosed herein, it should be noted that exemplary embodiments are not limited to the details disclosed in the accompanying drawings and specification. It should be understood that exemplary embodiments may be introduced or incorporated into other embodiments, variations, and modifications, and may be practiced or implemented in various ways. Furthermore, unless otherwise indicated, the terms and expressions used herein have been selected for the convenience of the reader to illustrate exemplary embodiments and are not intended to limit them.

[0054] One common assembly that uses control rods in conjunction with a fuel assembly is found in U.S. Patent No. 8,483,346 ('346) and U.S. Patent No. 9,053,824 ('824) granted to McCarty et al. These patents describe a control rod spider assembly comprising several control rods and a spider structure supporting the control rods at their upper ends. A typical structure of a control rod cluster assembly (RCCA) is shown in Figure 1. The contents of the '346' and '824' patents are incorporated herein by reference only to the extent that the incorporated contents do not contradict existing definitions, descriptions, or other disclosed materials contained herein, for any purpose whatsoever.

[0055] Referring next to the drawings, particularly Figure 1, an elevation view of a reactor fuel assembly is shown, represented in a vertically compressed form and the whole assembly is indicated by the number 10. The fuel assembly 10, of the type used in PWRs, basically comprises a bottom end structure or bottom nozzle 12 for supporting the assembly in the lower core plate (not shown) within the core region of a reactor (not shown), and a plurality of longitudinally extending guide tubes or thimbles 14 projecting upward from the bottom nozzle 12. The assembly 10 further comprises a plurality of lateral grids 16 spaced axially along the guide thimbles 14, and an organized array of elongated fuel rods 18 that are spaced laterally and supported by the grids 16. The assembly 10 also has an instrumentation guide tube 20 located at its center and an upper end structure or upper nozzle 22 detachably attached to the upper end of the guide thimbles 14, forming a one-piece assembly that can be handled conventionally without damaging the assembly components.

[0056] As described above, the fuel rods 18 in the array within the assembly 10 are held apart from each other by grids 16 spaced along the length of the fuel assembly. Each fuel rod 18 contains a nuclear fuel pellet 24, and both ends of the rod are closed by an upper end plug 26 and a lower end plug 28 to seal the rod. Generally, a plenum spring 30 is placed between the upper end plug 26 and the pellet 24 to maintain the pellets in a tightly stacked state within the tire rod 18. The fuel pellets 24, composed of fissile material, play a role in generating the reactor's reactance. A liquid moderator / coolant, such as water or water containing boron, is sent upward through the fuel assemblies in the core to extract the heat generated therefor to produce a useful amount of work.

[0057] Figures 2 and 3 show additional diagrams of a conventional control rod spider assembly of the type shown in Figure 1, all denoted by reference letter 32. In its basic configuration, the control assembly 32 comprises a plurality of control rods 34 and a spider structure 36 that supports the control rods at their upper ends. The spider structure 36 holds the control rods 34 in a pattern that matches the pattern of the guide thimble 14, which is configured to be inserted below the guide thimble 14 through the upper nozzle 22 of the PWR fuel assembly 10. The spider structure 36 is connected to a control mechanism (not shown) that is operable in known ways to move the control rods 34 to adjust the core power.

[0058] In a typical configuration, each control rod 34 of the control assembly 32 consists of an elongated metal cladding tube 38 in which a neutron absorber is placed, and upper and lower plugs 40 and 42 attached to both ends of the cladding tube 38 to seal the absorber inside. The spider structure 36 of the control assembly 32 typically comprises a central hub 46 supported by a plurality of radially extending flukes or vanes 44 spaced circumferentially around the central hub 46. Cylindrical control rod connecting fingers 48 are attached to and supported by the vanes 44. Some vanes 44 have only a single connecting finger 48 attached, while others have a pair of spaced-apart connecting fingers 48.

[0059] In one aspect of this disclosure, RCCAs and CEAs having a powder collection blockage device between an upper absorber and a lower absorber are disclosed herein to mitigate cladding strain in the high fluence region.

[0060] Reactor-Resistant Condensed ATF (ATF) reactor fuel assemblies (RCCAs) must withstand temperatures significantly higher than those during normal operation, resulting from over-design-critical (BDB) accidents. These temperatures are too high for the commonly used absorbent, silver-indium-cadmium alloy (Ag-In-Cd), to be incorporated into RCCAs because its melting point is lower than the temperatures experienced during a BDB accident.

[0061] Ceramic materials are also used as RCCA absorbers. Hybrid RCCAs use boron carbide (B4C) material in the upper section of the RCCA (the lowest fluence region of the RCCA). However, Ag-In-Cd material is still used in the lower section (the highest fluence region of the RCCA) under the aforementioned thermal limitations, due to the problem of cracking in the cladding.

[0062] For combustion engineering plants, a control element assembly (CEA) was manufactured with boron carbide (B4C) material placed in both the upper section (lowest fluence region) and the lower section (highest fluence region) of the CEA. However, there were operational problems because powder from the B4C absorber accumulated at the bottom of the CEA rods. This powder reduced the effective gap between the cladding and the absorber, leaving insufficient space for thermal and irradiation expansion of the B4C absorber. As a result, strain occurred in the cladding, eventually leading to cracking during normal operation.

[0063] However, since the B4C absorbent meets the temperature requirements of the BDB, if there is a means to prevent powder accumulation in the high-fluence region near the bottom of the RCCA, the B4C pellets can be given sufficient space to accommodate thermal and irradiation growth without excessive cladding strain. This means that in high-temperature (HT) RCCAs for ATF problems, the Ag-In-Cd absorbent in the lower high-fluence region may be replaced with B4C.

[0064] The powder collection and blocking device (Figures 4 and 5) prevents B4C powder from accumulating in the high-fluence lower section after exiting the upper section of the control rod, thus ensuring that the annular gap necessary for the thermal and irradiation growth of B4C is not restricted.

[0065] In various embodiments, control rods for RCCA or CEA are shown in Figures 4A and 4B. Figure 4A shows a control rod 100 comprising a cladding 110, an absorbent material 120 within the cladding 110, a powder collection and containment device 150, and a lower end plug 116. The cladding 110 includes an upper section 112 and a lower section 114. The powder collection and containment device 150 is configured to be attached to the inner wall of the cladding 110. Figure 4B shows a cross-sectional view of the powder collection and containment device 150 within the cladding 110.

[0066] In various embodiments, parts of an RCCA or CEA control rod, including details of the powder collection and containment device 150, are shown in Figures 5A and 5B. Figures 5A and 5B show parts of a control rod 100 having the powder collection and containment device 150 within a cladding 110, and further show details of the powder collection and containment device 150. The control rod 100 comprises: a cladding 110 including an upper section 112 and a lower section 114; absorbent material including upper absorbent material (upper B4C pellets) 122 housed in the upper section 112 of the cladding 110 and lower absorbent material (lower B4C pellets) 124 housed in the lower section of the cladding 110; and the powder collection and containment device 150. The control rod 100 is configured to have an annular gap or annular open space (not shown) between the absorbent material (B4C pellets) 120 (122, 124) and the cladding 110 (112, 114) to accommodate strain due to thermal and irradiation growth of the absorbent material (B4C pellets) 120 and / or cladding 110.

[0067] Referring again to Figures 5A and 5B, the powder collection blockage device 150 comprises a spacer 156, a garter spring 152, and optionally a fine screen mesh 154. The spacer 156 holds the garter spring 152 and provides a support surface between the upper absorber section 122 and the lower absorber section 124, providing an axially open space for B4C powder collection. The garter spring 152, with its densely packed coils, prevents powder from passing from the upper B4C absorber section 122 to the lower B4C absorber section 124, but does not prevent the refilling gas and gases generated during irradiation from moving through the rod. If the coil pitch of the spring is too coarse to block the powder, a fine screen mesh 154 may be provided. To compensate for thermal and irradiation expansion of the cladding 110, as well as creep-down of the cladding due to external pressure, the outward force of the girder spring 152 keeps the girder spring 152 firmly in place against the inner wall of the cladding 110, preventing powder from passing outside the girder spring 152. If a screen mesh 154 is required, the girder spring 152 is configured to firmly hold the screen mesh 154 against the inner wall of the cladding 110 to prevent gap formation.

[0068] Multiple powder collection and blocking devices can be used at different axial positions on the control rod to prevent the annular open space from becoming filled with powder. However, if the annular open space becomes filled and the powder begins to close the annular gap between the cladding and the upper absorbent section, the fluence must be sufficiently low in the upper section of the rod above the powder collection and blocking devices in order for the cladding to cope with the thermal and irradiation-induced strain of the B4C.

[0069] Spacer 156 may be made of stainless steel, or B4C material if the presence of an axial gap in the absorbent stack does not meet the requirements for nuclear power.

[0070] Because RCCA or CEA for ATF must withstand temperatures significantly higher than those during normal operation due to BDB accidents, this new method is beneficial for products under development such as HTRCCA or CEA. These temperatures are too high for RCCA or CEA to incorporate commonly used Ag-In-Cd absorbers, because the melting point of Ag-In-Cd is lower than the temperature in BDBs.

[0071] Expansion in the tip areas of RCCA and CEA has been a problem in the past. Current hybrid RCCA and CEA designs using B4C absorbers incorporate Ag-In-Cd absorbers in the lower sections of the control rods to accommodate expansion in the areas with the highest fluence. However, RCCA or CEA previously incorporated designs in which the tips of the B4C pellets were wrapped in "felt metal" to prevent migration of the B4C powder and provide a pulverizable material that would absorb the expansion of the B4C without straining the cladding. While this design was an improvement over B4C pellets alone, it was not as effective as initially thought, and some plants experienced cracking in the control rods.

[0072] The advantage of the powder collection and blocking device shown in Figures 4 and 5 is that it collects and blocks powder moving to the lower section of the control rod to accommodate the expansion of the absorbent in the highest fluence region, by having a girder spring that prevents a gap from forming in the tubular section between the pellet and the cladding. If the spring is too coarse to block the powder, a finer screen can be incorporated (Figures 4 and 5).

[0073] As disclosed above, RCCA or CEA for ATF must withstand temperatures significantly higher than those during normal operation due to BDB accidents. These temperatures are too high for RCCA or CEA to incorporate commonly used Ag-In-Cd absorbents because the melting point of Ag-In-Cd is lower than the temperature in BDB.

[0074] Ceramic materials like B4C meet the temperature requirements of BDB. However, an operational problem has arisen where the cladding of RCCA or CEA made entirely of B4C material cracks. This is because B4C powder released from the upper section of the RCCA or CEA accumulates in the highest fluence region near the bottom of the RCCA or CEA, blocking the annular gap necessary for the thermal and irradiation growth of B4C.

[0075] Powder collection blockage devices are beneficial for new products under development, such as HTRCCA or HTCEA for ATF problems, because they mitigate cladding strain in the high-fluence lower region by preventing ceramic powder from filling the annular gap between the cladding and pellets.

[0076] Other materials are being considered as alternatives to Ag-In-Cd, and powder collection and containment devices are beneficial for any ceramic absorber. However, using a material like B4C, which has extensive operating experience in Westinghouse PWRs, would significantly reduce costs by avoiding the need to test and characterize new materials. Furthermore, it would also avoid the risks associated with incorporating new materials into reactor operation.

[0077] In another aspect of this disclosure, an RCCA or CEA having an end plug extension with increased plenum volume is disclosed herein.

[0078] The control rods of RCCA or CEA incorporate a plenum region to provide volume for the growth of the absorber due to irradiation and thermal expansion. The plenum also provides volume for the expansion of the gas at operating temperatures. The gas is introduced during manufacturing or generated when the absorber is irradiated. Furthermore, the plenum accommodates the expansion of the absorber so that even if the absorber melts due to high temperatures associated with an accident, cracks will not form in the cladding.

[0079] The initial plenum volume is determined by the geometry of the cladding and absorbers and decreases during operation due to irradiation and thermal expansion of the absorbers. Therefore, methods to increase the plenum volume are beneficial for the operation of RCCA or CEA.

[0080] As shown in Figure 6, the plenum volume can be increased by incorporating an axial hole 210 into the upper plug 212. This design is applicable to new RCCA or CEAs designed to provide the largest possible plenum volume, and is also applicable to existing RCCA or CEAs where additional plenum volume is required without modifying the existing proven design of the plenum spring and tube.

[0081] Referring to Figure 6, a portion of the control rod 200 of an RCCA or CEA is shown. The control rod 200 comprises a cladding 202, an absorbent material 204 within the cladding 202, an upper plug extension 212 configured to be attached to the cladding 202, an axial hole 210 incorporated in the upper plug extension 212, a spring 206 within the cladding 202, and a plenum 208 within the cladding 202 and further within the spring 206. The plenum 208 and spring 206 are configured to be above the absorbent material 204.

[0082] The initial plenum volume is determined by the geometry of the cladding and absorbers and decreases during operation due to irradiation and thermal expansion of the absorbers. Therefore, methods to increase the plenum volume are beneficial for the operation of RCCA or CEA. The RCCA or CEA with control rods shown in Figure 6 increases the plenum volume by incorporating axial holes in the upper plug and thus has advantages over conventional RCCA or CEA.

[0083] The novel method disclosed above is beneficial for such products, because new products under development, such as high-temperature RCCA or CEA, require as much plenum volume as possible, as the high temperatures during a BDB accident condition can cause the absorbent to melt.

[0084] In another aspect of this disclosure, an RCCA or CEA having a lower end plug with increased plenum volume and a lower section absorbent having radial channels is disclosed herein.

[0085] Many power companies are interested in flexible power output (FPO) or load-following operation, which requires the insertion of control rods deep into the active fuel core for extended periods, resulting in a smaller plenum than normal base-load operation due to increased irradiation growth of absorbers. Therefore, it is also beneficial for existing products as it allows for the provision of additional plenum volume in a new way without requiring changes to the proven operating designs of plenum springs and tubes.

[0086] The control rods of RCCA or CEA incorporate a plenum region to provide volume for the growth of the absorber due to irradiation and thermal expansion. The plenum also provides volume for the expansion of the gas at operating temperatures. The gas is introduced during manufacturing or generated when the absorber is irradiated. Furthermore, the plenum accommodates the expansion of the absorber so that even if the absorber melts due to the high temperatures associated with a BDB accident, cracks will not form in the cladding.

[0087] The initial plenum volume is determined by the geometry of the cladding and absorbers and decreases during operation due to irradiation and thermal expansion of the absorbers. Therefore, methods to increase the plenum volume are beneficial for the operation of RCCA or CEA.

[0088] As shown in Figures 7, 8A, and 8B, the plenum volume can be increased by incorporating an axial hole 308 into the lower end plug 310. This design is applicable to new RCCA or CEAs designed to provide the largest possible plenum volume, and is also applicable to existing RCCA or CEAs where additional plenum volume is required without modifying the proven design of the plenum spring and tube.

[0089] Referring here to Figures 7, 8A, and 8B, a portion of the control rod 300 of an RCCA or CEA is shown. The control rod 300 comprises a cladding 302, an absorbent material 304 having a radial groove 306 at the bottom of the absorbent material 304, and a lower end plug 310 configured to be attached to the cladding 302 having an axial hole 308 incorporated into the lower end plug 310.

[0090] The plenum volume is increased by incorporating an axial hole 308 into the lower end plug 310 in the manner shown in Figures 7, 8A, and 8B. Furthermore, a radial groove 306 may optionally be provided at the bottom of the lower section absorbent 304 to provide a passage for gas expansion or generation to ensure that the lower section absorbent does not block the opening of the lower end plug as shown in Figures 7, 8A, and 8B.

[0091] The initial plenum volume is determined by the geometry of the cladding and absorbers and decreases during operation due to irradiation and thermal expansion of the absorbers and / or cladding. Therefore, methods for increasing the plenum volume are beneficial for the operation of RCCA or CEA. The plenum volume is increased by incorporating an axial hole in the lower end plug, as shown in Figure 7, and in Figures 8A and 8B.

[0092] This new method is beneficial for such products, because new products under development, such as high-temperature RCCA or CEA, require as much plenum volume as possible because the high temperatures during a BDB accident can cause the absorbent to melt.

[0093] Many power companies are interested in flexible power output (FPO) or load-following operation, which requires the insertion of control rods deep into the active fuel core for extended periods, resulting in a smaller plenum than normal base-load operation due to increased irradiation growth of absorbers. Therefore, it is also beneficial for existing products as it allows for the provision of additional plenum volume in a new way without requiring changes to the proven operating designs of plenum springs and tubes.

[0094] In various embodiments, methods, apparatus, and systems for improving the performance of control rod cluster assemblies (RCCAs) and / or control element assemblies (CEAs) for mitigating cladding strain in the high-fluence region between normal operating conditions and fault conditions are disclosed herein. One method may include incorporating a device such as a powder collection blockage device between the upper and lower absorbents of the RCCA and / or CEA. Another method may include increasing the plenum volume by incorporating an axial hole in the upper plug. Another method may include increasing the plenum volume by incorporating an axial hole in the lower plug and optionally incorporating a radial groove at the bottom of the lower absorbent to provide a passage for gas expansion or generation, so as to ensure that the lower absorbent does not block the opening of the lower plug.

[0095] In various embodiments, reactor control rod cluster assemblies (RCCAs) for mitigating cladding strain are disclosed herein.

[0096] In various embodiments, the reactor control rod cluster assembly (RCCA) or CEA may be equipped with devices to improve the performance of the RCCA or CEA in order to mitigate cladding strain, for example, cladding strain in the high-fluence region of the RCCA or CEA.

[0097] In various embodiments, the RCCA or CEA may comprise at least one control rod comprising a cladding having an upper section and a lower section. In various embodiments, the at least one control rod may comprise an absorbent material housed within the cladding, comprising an upper absorbent material housed within the upper section of the cladding and a lower absorbent material housed within the lower section of the cladding; and a device. In one embodiment, the device may be housed within the cladding.

[0098] In various embodiments, the control rod may be configured to have a first gap or annular gap between the upper material and the cladding; and a second gap or annular gap between the lower absorbent and the cladding. In one embodiment, the first gap is the same as the second gap. These annular gaps can provide sufficient space for thermal and irradiation expansion of the absorbent, including the upper and lower absorbent.

[0099] In various embodiments, the device may be a powder collection and containment device. In one embodiment, the powder collection and containment device may be housed within the cladding and incorporated into the upper or lower absorbent material within the cladding of RCCA or CEA. In one embodiment, the powder collection and containment device may be housed between the upper and lower absorbent materials within the cladding.

[0100] In various embodiments, the RCCA or CEA may further comprise one or more powder collection and containment devices incorporated into the upper absorbent. In one embodiment, the one or more powder collection and containment devices may be evenly distributed in the upper absorbent along the axial length of the cladding or at least one control rod. Each powder collection and containment device collects absorbent powder generated above each device during normal operation and also during failure times exceeding the design criteria (BDB) of the RCCA or CEA, reducing the accumulation of absorbent powder at the lower end of the cladding. In one embodiment, the one or more powder collection and containment devices may be evenly distributed or at different intervals in the upper absorbent along the axial length of the cladding or at least one control rod.

[0101] In various embodiments, the RCCA or CEA may further comprise one or more powder collection and blocking devices incorporated into the lower absorbent along the axial length of the cladding or at least one control rod and distributed evenly or at different intervals.

[0102] In various embodiments, the RCCA or CEA may further comprise at least one powder collection and containment device incorporated into the upper absorbent material within the cladding; and at least one powder collection and containment device incorporated into the lower absorbent material within the cladding.

[0103] In various embodiments, the RCCA or CEA comprises a plurality of powder collection and blocking devices incorporated into the absorbent material within the cladding, which may be distributed evenly or at different intervals along the axial length of the cladding or at least one control rod.

[0104] In various embodiments, the powder collection blockage device may comprise: a spacer and a girder spring. In one embodiment, the spacer is configured to hold the girder spring, provide a support surface between the upper and lower absorbent materials, and provide an axially open space for collecting powder from the upper absorbent material that occurs during normal operating conditions and RCCA or CEA fault conditions. In one embodiment, to compensate for thermal and irradiation expansion of the cladding, as well as creep-down of the cladding due to external pressure, the outward force of the girder spring keeps the girder spring firmly held against the inner wall of the cladding, preventing powder from passing outside the girder spring within the cladding.

[0105] In various embodiments, the garter spring has a dense coil that prevents powder from passing through the cladding, for example, from the upper section to the lower section of the cladding, but does not prevent the refilling gas and gases generated during irradiation from moving through the control rods.

[0106] In various embodiments, the garter spring may have a spring coil pitch that is too coarse to block the powder. In one embodiment, the powder collection blocker may further include a fine mesh screen. The garter spring is configured to hold the screen mesh firmly against the inner wall of the cladding to prevent a gap from forming between the screen mesh and the inner wall of the cladding, and thus to prevent the powder from passing through the cladding, for example, from the upper section to the lower section of the cladding outside the garter spring or screen mesh.

[0107] In various embodiments, the powder collection blockage device further comprises a screen mesh configured to be inserted on a garter spring, the garter spring being configured to firmly hold the screen mesh against the inner wall of the cladding to prevent the formation of a gap between the screen mesh and the inner wall of the cladding.

[0108] In various embodiments, the screen mesh has a diameter that precisely matches the inner diameter of the cladding and a mesh size that is fine enough to prevent powder from passing through.

[0109] In various embodiments, the upper absorbent is a ceramic absorbent.

[0110] In various embodiments, the upper absorbent is boron carbide (B4C).

[0111] In various embodiments, the lower absorbent is a ceramic absorbent.

[0112] In various embodiments, the lower absorbent is boron carbide (B4C).

[0113] In various embodiments, the spacer is made of a material selected from the group consisting of 304 stainless steel, nickel-based alloys, Inconel 625, Inconel 718, ceramic absorbents, and B4C absorbents.

[0114] In various embodiments, the garter spring is made of a material selected from the group consisting of 304 stainless steel, nickel-based alloys, Inconel 625, and Inconel 718.

[0115] In various embodiments, the screen mesh is made of a material selected from the group consisting of 304 stainless steel, nickel-based alloys, Inconel 625, and Inconel 718.

[0116] In various embodiments, reactor control rod cluster assemblies (RCCAs) or CEAs for mitigating cladding strain are disclosed herein. In various embodiments, an RCCA or CEA may comprise: at least one control rod; the at least one control rod comprising: a cladding having an upper and lower end; an absorbent material housed within the cladding; a spring housed within the cladding; and a plenum housed within the cladding and further within the spring. The RCCA or CEA may further comprise: a lower end plug configured to be attached to the lower end of the cladding; an upper end plug configured to be attached to the upper end of the cladding; and an axial hole to the upper end plug, the axial hole to the upper end plug increasing the plenum volume of the RCCA or CEA.

[0117] In various embodiments, reactor control rod cluster assemblies (RCCAs) or CEAs for mitigating cladding strain are disclosed herein. In various embodiments, an RCCA or CEA may comprise at least control rods. In various embodiments, a control rod may comprise: a cladding including upper and lower ends; an absorbent material housed within the cladding; a spring housed within the cladding; and a plenum housed within the cladding and further within the spring. In various embodiments, an RCCA or CEA may further comprise: a lower end plug configured to be attached to the lower end of the cladding; an upper end plug configured to be attached to the upper end of the cladding; and an axial hole to the lower end plug, the axial hole to the lower end plug increasing the volume of the plenum.

[0118] In various embodiments, the control rod cluster assembly (RCCA) or CEA may include radial grooves at the bottom of the lower absorber to provide a passage for gas expansion or generation, ensuring that the lower absorber does not block the opening of the lower end plug.

[0119] In various embodiments, reactor control element assemblies (CEAs) for mitigating cladding strain are disclosed herein. In various embodiments, a control element assembly (CEA) may have features of one or more, or all, of the control rod cluster assemblies (RCCAs) disclosed herein above.

[0120] In various embodiments, methods for relieving cladding strain in a reactor control rod cluster assembly (RCCA) or controlled rod assembly (CEA) are disclosed herein. In various embodiments, the method may include providing a control rod cluster assembly (RCCA) or CEA. In various embodiments, the RCCA comprises one, more, or all of the features of the RCCA or CEA disclosed herein above.

[0121] In various embodiments, the RCCA or CEA may comprise at least one control rod comprising a cladding having an upper section and a lower section, an upper absorbent housed within the upper section of the cladding, and a lower absorbent housed within the lower section of the cladding. The method may further include incorporating a device between the upper and lower absorbents within the cladding. In various embodiments, the device may be a powder collection blockage device. In various embodiments, the powder collection blockage device may comprise: a spacer and; a girder spring. In various embodiments, the spacer holds the girder spring, providing a support surface between the upper and lower absorbents and an axially open space for collecting powder from the upper absorbent that occurs during normal operation and fault conditions. In various embodiments, the outward force of the girder spring keeps the spacer and girder spring firmly held against the inner wall of the cladding, preventing powder from passing from the upper section to the lower section of the cladding outside the girder spring. In various embodiments, the garter spring has a dense coil that prevents powder from passing from the upper section to the lower section of the cladding, but does not prevent the refilling gas and gases generated during irradiation from moving through the control rod.

[0122] In various embodiments, at least one control rod is configured to have a first annular gap between the upper absorbent and the inner wall of the cladding; and the same annular gap between the lower absorbent and the inner wall of the cladding.

[0123] In various embodiments, the method may further include incorporating one or more powder collection and blocking devices into the upper absorbent material within the upper section of the cladding.

[0124] In various embodiments, the method may further include incorporating one or more powder collection and blocking devices into the lower absorbent material within the lower section of the cladding.

[0125] In various embodiments, the method may further include distributing powder collection and blocking devices evenly across the absorbent material along the length of the cladding.

[0126] In various embodiments, methods for relieving cladding strain in control rod cluster assemblies (RCCAs) or CEAs are disclosed herein. In various embodiments, the method may include providing a control rod cluster assemblies (RCCAs) or CEAs comprising: a cladding having upper and lower ends; an absorbent material housed within the cladding; a spring housed within the cladding; and a plenum housed within the cladding and further within the spring, comprising at least one control rod. In various embodiments, the RCCA or CEA may further include a lower end plug configured to be attached to the lower end of the cladding; and an upper end plug configured to be attached to the upper end of the cladding. In various embodiments, the method may include incorporating an axial hole in the upper end plug, the axial hole in the upper end plug increasing the plenum volume of the RCCA or CEA.

[0127] In various embodiments, methods for relieving cladding strain in a control rod cluster assembly (RCCA) or CEA are disclosed herein. In various embodiments, the method may include providing a control rod cluster assembly (RCCA) or CEA comprising: a cladding having an upper and lower end; an absorbent material housed within the cladding; a spring housed within the cladding; and a plenum housed within the cladding and further within the spring, for at least one control rod. In various embodiments, the RCCA or CEA may further include a lower end plug configured to be attached to the lower end of the cladding; and an upper end plug configured to be attached to the upper end of the cladding. In various embodiments, the method may include incorporating an axial hole in the lower end plug, the axial hole in the lower end plug increasing the plenum volume of the RCCA or CEA.

[0128] In various embodiments, the method may include incorporating a radial groove at the bottom of the absorbent material and providing a passage for gas expansion or generation to ensure that the lower absorbent does not block the opening of the lower end plug.

[0129] In various embodiments, methods for relieving strain in the cladding of a reactor control element assembly (CEA) are disclosed herein. In various embodiments, methods for relieving strain in the cladding of a control element assembly (CEA) may have features of one, more, or all of the methods for relieving strain in the cladding of a control rod cluster assembly (RCCA) disclosed herein above.

[0130] The use of the apparatus and methods for mitigating cladding strain in reactor RCCA and CEA disclosed herein may result in improved safety and reduced risk, enhanced production continuity, and annual manufacturing cost savings of millions of dollars compared to current RCCA and CEA apparatus and methods.

[0131] All patents, patent applications, publications, or other disclosure materials described herein and / or listed in the application data sheet are incorporated herein by reference in whole, as if each individual reference were expressly incorporated by reference. All references and any material or any part thereof that are to be incorporated herein by reference are incorporated herein only to the extent that the incorporated material does not conflict with existing definitions, descriptions, or other disclosure materials described herein. Accordingly, to the extent necessary, disclosures described herein take precedence over any conflicting content incorporated herein by reference, and disclosures expressly described in the comparison of this application take precedence.

[0132] The present invention has been described with reference to various exemplary and illustrative embodiments. The embodiments described herein are understood to provide exemplary features of various details of various embodiments of the disclosed invention. Therefore, unless otherwise specified, it should be understood that, to the extent possible, one or more features, elements, components, constituents, components, structures, modules, and / or embodiments of the disclosed embodiments can be combined, separated, and combined. Without departing from the scope of the disclosed invention, one or more other features, elements, components, constituents, components, structures, modules, and / or embodiments of the disclosed embodiments can be replaced and / or rearranged. Therefore, it will be understood by those skilled in the art that various substitutions, modifications, or combinations of any of the exemplary embodiments are possible without departing from the scope of the invention. Furthermore, by examining this specification, those skilled in the art will recognize many equivalents to the various embodiments of the invention described herein, or can confirm them through routine experimentation alone. Therefore, the present invention is limited by the claims, not by the descriptions of the various embodiments.

[0133] A person skilled in the art will generally recognize that the terms used herein, and in particular in the appended claims (e.g., the body of the appended claims), are generally intended as “public” terms (e.g., the term “includes” should be interpreted as such; the term “includes but not limited to” should be interpreted as such; the term “have” should be interpreted as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such as such that the term “includes but not limited to” should be interpreted as such that the term “includes” should be interpreted as such that the term “includes” should be interpreted as such that the term “includes” should be interpreted as such that the term “includes” should be interpreted as such that the particular claim containing such an introduced claim contains only one such claim. The same claim may include introductory phrases such as "one or more" or "at least one" and indefinite articles such as "a" or "an" (for example, "a" and / or "an" should generally be interpreted as meaning "by"); the same applies to the use of definite articles used to introduce the quotation of a claim.

[0134] Furthermore, even if a specific number is explicitly stated in a statement introduced in a claim, a person skilled in the art will recognize that such a statement should generally be interpreted as meaning at least the stated number (for example, the statement “the statement that is 2” without other modifiers usually means at least two statements, or two or more statements). Furthermore, where a convention similar to “at least one of A, B, C, etc.” applies, such a construction is generally intended to mean that a person skilled in the art will understand the convention (for example, “a system having at least one of A, B, and C” includes, but is not limited to, systems having only A, only B, only C, A and B together, A and C together, B and C together, and / or A, B, and C together). Where a convention similar to "at least one of A, B, or C" applies, such a construction is generally intended to mean that a person skilled in the art would understand the convention (for example, "a system having at least one of A, B, or C" includes, but is not limited to, systems with only A, only B, only C, A and B together, A and C together, B and C together, and / or A, B, and C together). Furthermore, in either the specification or the claims, typically disjunctive words and / or phrases that present two or more alternative terms would be understood by a person skilled in the art.

[0135] With respect to the attached claims, those skilled in the art will understand that the operations described in those claims can generally be performed in any order. Furthermore, while the claims are presented in a set order, it should be understood that various operations may be performed in an order other than that described, or simultaneously. Examples of such alternative orders may include, unless otherwise indicated in the context, repetition, interleaving, interruption, reordering, incrementing, preparing, supplementing, simultaneous, reverse, or other variations in order. Moreover, terms such as "corresponding to," "related to," or other past tense adjectives are not usually intended to exclude such variations unless otherwise indicated in the context.

[0136] It is worth noting that references to “one aspect,” “one example,” and so on, mean a specific feature, structure, or characteristic described in relation to “one aspect.” An aspect is included in at least one aspect. Therefore, the phrases “in one aspect,” “in one aspect,” “in one example,” and “in one example,” appearing in various places throughout the specification, do not necessarily all refer to the same aspect. Furthermore, a particular feature, structure, or characteristic can be combined in any suitable way in one or more aspects.

[0137] As used herein, the singular forms of "a," "an," and "the" include plural references unless the context clearly indicates otherwise.

[0138] The directional terms used herein, such as up, down, left, right, front, back, and variations thereof, relate to the orientation of the elements shown in the accompanying drawings. Furthermore, unless otherwise specified, they do not limit the scope of the claims.

[0139] As used in this disclosure, the terms “about” or “approximately” mean an acceptable error of a particular value determined by a person skilled in the art, unless otherwise specified, and this depends in part on how the value is calculated, measured, or determined. In certain embodiments, the terms “about” or “approximately” mean within one, two, three, or four standard deviations. In certain embodiments, the terms “about” or “approximately” mean 50%, 200%, 105%, 100%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, or 0.05% of a given value or range.

[0140] In this specification, unless otherwise indicated, all numerical parameters should be understood to be preceded and modified in all cases by the term “approximately,” and numerical parameters have inherent variability characteristics of the underlying measurement technique used to determine the numerical value of the parameter. At the very least, without limiting the application of the doctrine of equivalents to the claims, each numerical parameter described herein should be interpreted by applying ordinary rounding, taking into account at least the reported number of significant figures.

[0141] The numerical ranges described herein include all subranges contained within the described range. For example, the range "1 to 100" includes all subranges between (and including) the stated minimum value of 1 and the stated maximum value of 100. That is, the minimum value greater than or equal to 1, and all ranges described herein include the endpoints of the described range. For example, the range "1 to 100" includes the endpoints 1 and 100. The maximum numerical limit described herein is intended to include all lower numerical limits contained within it, and the minimum numerical limit described herein shall include the following numerical limits: all higher numerical limits contained within it. Accordingly, the applicant reserves the right to modify this specification, including the claims, to explicitly enumerate any subranges contained within the explicitly enumerated ranges. All such ranges are essentially described herein.

[0142] The terms “includes” (and any form of including such as “includes” and “contains”), “have” (and any form of having such as “have” and “contains”), “includes” (and any form of include such as “includes” and “contains”), and “contain” (and any form of contain such as “contains” and “contains”) are non-restrictive conjunctions. As a result, a system that “includes,” “haves,” “contains,” or “contains” one or more elements possesses, but is not limited to possessing only, those one or more elements. Similarly, an element of a system, device, or apparatus that “includes,” “haves,” “contains,” or “contains” one or more features possesses, but is not limited to possessing, those one or more features.

Claims

1. A control rod cluster (RCCA) for mitigating cladding strain: A cladding tube comprising an upper section and a lower section, defining the inner wall; The upper absorbent material housed within the upper section of the cladding tube; and Lower absorbent material housed within the lower section of the covering pipe A control rod equipped with, The control rod is configured to have a first annular gap between the upper absorbent and the inner wall of the cladding tube, and a second annular gap between the lower absorbent and the inner wall of the cladding tube; A device disposed within the covering tube between the upper absorbent and the lower absorbent: A garter spring configured to exert an outward force on the inner wall of the cladding tube, wherein the garter spring comprises a coil configured to prevent powder from the upper absorbent from passing through the lower section of the cladding tube, and the coil is further configured to allow gas to pass between the upper section and the lower section of the cladding tube; and A spacer configured to hold the garter spring and provide a support surface to the upper absorbent, wherein the spacer defines a plurality of axial openings for collecting the powder generated by the upper absorbent. The apparatus comprising and RCCA is equipped with this.

2. The RCCA according to claim 1, wherein the upper absorbent is a ceramic absorbent.

3. The RCCA according to claim 1, wherein the upper absorbent is boron carbide (B4C).

4. The RCCA according to claim 1, wherein the lower absorbent is a ceramic absorbent.

5. The RCCA according to claim 1, wherein the lower absorbent is boron carbide (B4C).

6. The RCCA according to claim 1, wherein the spacer comprises at least one of 304 stainless steel, nickel-based alloy, Inconel 625, Inconel 718, ceramic absorbent, and boron carbide (B4C).

7. The RCCA according to claim 1, wherein the garter spring comprises at least one of 304 stainless steel, nickel-based alloy, Inconel 625, and Inconel 718.

8. The RCCA according to claim 1, further comprising a plurality of the devices, each disposed inside the cladding tube.

9. The RCCA according to claim 8, wherein the device is distributed along the axial length of the cladding tube.

10. The RCCA according to claim 1, wherein the first annular gap has a first gap width, the second annular gap has a second gap width, and the first gap width and the second gap width are the same.

11. The control rod further comprises a spring disposed within the upper section of the cladding tube above the upper absorbent, the spring forming a plenum with a plenum volume above the upper absorbent, The RCCA further comprises an upper end plug extension configured to be attached to the upper end of the upper section of the cladding tube, and having an axial hole configured to increase the plenum volume. The RCCA according to claim 1.

12. The control rod further comprises a plenum region having a plenum volume, formed inside the lower section of the cladding tube and surrounding the lower absorbent, The RCCA is configured to be attached to the lower end of the lower section of the cladding tube and further comprises a lower end plug having an axial hole configured to increase the plenum volume. The RCCA according to claim 1.

13. The RCCA according to claim 12, wherein the portion of the lower absorbent material adjacent to the lower end of the lower section of the cladding tube is provided with a radial groove formed therein, and the radial groove is configured to allow gas to flow into the axial hole.

14. The RCCA according to claim 12, further comprising an upper end plug extension configured to be attached to the upper end of the upper section of the cladding tube, wherein the axial hole is a first axial hole, and the upper end plug extension comprises a second axial hole configured to increase the plenum volume.

15. The plurality of axial openings comprises a plurality of openings spaced apart in the circumferential direction, The spacer comprises radial projections defining a plurality of openings spaced apart in the circumferential direction. The RCCA according to claim 1.

16. The spacer is The upper disk and The lower disk and A main body extending between the upper disk and the lower disk, Equipped with, The spacer is held in place by the garter spring between the upper disc and the lower disc. The upper disk has a plurality of openings spaced apart in the circumferential direction, The RCCA according to claim 15.

17. The RCCA according to claim 16, further comprising a screen mesh positioned on the garter spring, wherein the screen mesh is configured to prevent the powder generated from the upper absorbent from passing through the lower section of the cladding tube.

18. The RCCA according to claim 17, wherein the screen mesh comprises at least one of 304 stainless steel, nickel-based alloy, Inconel 625, and Inconel 718.

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