Long silicon carbide fuel elements

Abstract

The present invention describes an elongated fuel element having a silicon carbide cladding enclosing a fuel, such as UO2, wherein the fuel is sized relative to the cladding to define a sufficiently large gap at each lateral end of the shell so that, when swollen in use, the fuel does not increase the strain on the cladding beyond the cladding strain tolerance limit. The lateral gaps at the ends of the fuel allow for lateral expansion during swelling, which reduces the strain on the cladding.

Description

[0001] Cross-references to related art

[0002] This application claims the benefit of 16 / 459,764, filed July 2, 2019, entitled “Elongate SIC Fuel Elements,” the contents of which are incorporated herein by reference.

[0003] Government Rights Statement

[0004] This invention was made with Government support under Contract No. DE-NE0008824 awarded by the Department of Energy. The U.S. Government has certain rights in this invention. Technical Field

[0005] The present invention relates to fuel elements for nuclear power plants, and more particularly to elongated fuel segments contained within elongated SiC cladding. Background Art

[0006] Silicon carbide (SiC) has been shown to be a very promising cladding for accident-tolerant fuel (ATF), but one of the main challenges of SiC is that the fuel pellets contained in the SiC cladding tube cannot be in close contact with the cladding, otherwise the SiC cladding will lose its airtightness.

[0007] UO2 fuel has a long and successful history of use in the nuclear industry. Although UO2 has low reactivity with light water coolant, moderate swelling, and a stable structure during irradiation, it suffers from low thermal conductivity, which limits its use to small-diameter fuel rods. During use, fission products will cause the UO2 to swell, and unless there is a sufficiently large initial gap between the fuel pellets and non-ductile cladding (such as SiC), the inevitable swelling can lead to cladding failure due to the strong mechanical interaction (PCMI) between the pellets and cladding. Due to the low thermal conductivity of helium in the gap and the reduced uranium loading, increasing the initial pellet-cladding gap to address the PCMI problem will lead to even higher fuel centerline temperatures. This disadvantage becomes apparent when UO2 is used in fuel rods with advanced accident-tolerant cladding, such as SiC, which requires minimal mechanical interaction between the fuel and cladding to avoid cracking and loss of hermeticity. Therefore, using UO2 in SiC cladding is not possible for cylindrical fuel rod geometries without severe economic losses.

[0008] On the other hand, fuels such as U3Si2, UN, and uranium alloys can mitigate the PCMI problem by leaving a larger pellet-cladding gap due to their high fuel thermal conductivity and reduced fuel centerline temperature, but at the expense of reduced uranium loading. Furthermore, these fuels have poor resistance to aqueous corrosion, which is detrimental during a leak event in a light water reactor because it compromises the fuel's accident tolerance.

[0009] For metal cladding such as Zr, which has a strain capacity of 2% to 4%, the gap is not a PCMI issue and only needs to be large enough to hold the fuel pellets. However, if a leak occurs, for example, U3Si2 will oxidize, increasing the internal volume of the Zr cladding. This increased volume causes the cladding to expand, which can block the coolant path.

[0010] Compared to conventional Zr-based materials, SiC cladding offers improved high-temperature performance during accident conditions. However, rod-type fuel designs have limited the potential for SiC materials because SiC has limited strain capacity (~0.04 to 0.1%) to mitigate local stress buildup and minimize mechanical interaction between the fuel and cladding. A solution to this problem is to leave a large fuel-cladding gap. However, a large gap significantly increases the fuel centerline temperature, especially in low-thermal-conductivity fuels such as UO2, leading to large fission gas releases and / or swelling, and also reduces the uranium charge, resulting in severe economic losses.

[0011] Plate fuels, such as those disclosed in U.S. Patents Nos. 3,173,843, 3,297,543, 3,586,745, 3,855,061, 4,038,135, and 4,224,106, were initially developed in the 1960s and 1970s and use metal cladding (e.g., aluminum or zirconium alloys) and metal fuel or fuel dispersed in a metal matrix (e.g., UZr or UMo). Such plate fuels were found to be unsuitable for commercial reactors and were abandoned, but are currently used in test reactors and have been evaluated for use in small modular reactors. See Mark D. DeHart, “Fuel Element Design and Analysis for Potential LEU Conversion of the Advanced Test Reactor,” INL / JOU-17-41538 Revision 0, April 2018; Samuel J. Miller et al., “Evaluation of U 10 Mo Fuel Plate Irradiation BehaviorVia Numerical and Experimental Benchmarking 10Irradiation Behavior of Mo Fuel Plates,” INL / CON-12-25074, November 2012; and Min-Gil Kim et al., “Comparison of Thermal Hydraulic Performances of Rod Type Fuel to Plate-Type Fuel for Small Modular Reactor Application,” Proceedings of ICAPP, April 2014. Summary of the Invention

[0012] The following summary of the invention is provided herein to facilitate understanding of some innovative features unique to the disclosed embodiments and is not intended to be a complete description. A full understanding of the various aspects of the embodiments can be obtained by taking the entire specification, claims, and abstract as a whole.

[0013] To address the problems caused by pellet-cladding mechanical interaction between SiC cladding and nuclear fuel, while at the same time providing the benefits of increased uranium loading and better safety margins, particularly in light water reactors, a new fuel element is described herein that includes an elongated fuel with an elongated SiC cladding.

[0014] In various aspects, a fuel element for a nuclear reactor includes a silicon carbide cladding and nuclear fuel contained in the cladding. The cladding has two opposing elongated walls and two opposing end walls that are shorter than the elongated walls. The elongated walls and the end walls together define a shell. The shell has a length from end wall to end wall and a width from elongated wall to elongated wall, and the cladding has a longitudinal axis and a transverse axis perpendicular to the longitudinal axis. The nuclear fuel has two elongated surfaces and two end surfaces that are shorter than the elongated surfaces. Before swelling during use, each of the two elongated surfaces is minimally spaced apart from the other of the elongated walls of the cladding by a first distance (e.g., between about 0.05 and 2.0 mm) and is in a facing relationship therewith, and each of the end surfaces is spaced apart from the other of the end walls of the cladding by a second distance and is in a facing relationship therewith. The second distance is greater than the first distance. The first distance and the second distance are sized so that, in use, when the fuel swells, the swollen fuel swells beyond the first distance and to, but not beyond, the second distance to deform the elongate wall of the cladding but not the end wall of the cladding and without straining the cladding by more than 0.1%.

[0015] The strain on the cladding caused by the swelling of the fuel is no greater than 0.05% in any area.

[0016] The second distance defines a gap between the fuel end surface and the cladding end wall, sized to accommodate the maximum swelling of the selected fuel plus space for fission gases produced during use. A person skilled in the art can determine the second distance by calculating the maximum swelling that the selected fuel will experience during use and the amount of fission gases that will be produced during use, either by known methods or through historical usage. The first distance defines a gap between each of the elongated walls of the cladding and the facing elongated surface of the fuel. The first distance gap is sufficient to allow the fuel to be inserted into the cladding prior to use, typically about 0.07 mm.

[0017] In certain aspects, the fuel can be configured as an ellipse in cross-section, defining an apex at each of the shorter end surfaces, wherein the two elongated surfaces are convexly curved. The ellipse has a major axis extending from apex to apex and a minor axis perpendicular to the major axis. Each convexly curved surface has a common apex at the minor axis and curves from the common apex to each of the apex points, intersecting each of the apex points. In this embodiment, the first distance is minimum between the common apex and the elongated wall of the cladding.

[0018] In certain aspects of the fuel element, the elongated walls of the cladding can be curved inwardly toward the fuel. Prior to deformation during use, the elongated walls of the cladding can be curved concavely toward the fuel and deform outwardly during use. In various aspects, the end walls of the cladding can be flat prior to use and remain flat during use.

[0019] Prior to swelling during use, the elongated surfaces of the fuel may be concavely curved inwardly toward one another and swell outwardly during use to form convexly curved elongated surfaces that extend beyond a first distance to deform the elongated walls of the cladding. Each concavely curved elongated surface defines a common vertex at the minor axis and curves from the common vertex to intersect each end surface. Prior to swelling during use, the end surfaces of the fuel may be flat and swell outwardly during use to form the convexly curved end surfaces. Alternatively, prior to swelling during use, the end surfaces of the fuel may be flat and remain flat during use.

[0020] In various aspects, before swelling during use, the fuel can be configured in a rectilinear shape in cross section, defining a flat end surface and a flat elongated surface. The rectilinear shape defines a major axis extending from end surface to end surface and a minor axis perpendicular to the major axis. During use, the fuel swells outward along both the major and minor axes to form an elliptical configuration in cross section. Before swelling during use, the cladding can be configured in a rectilinear shape in cross section, defining a flat end wall and a flat elongated wall, which deform outward during use as the fuel swells.

[0021] In various aspects, the end walls of the cladding are thicker in cross-section than the elongate walls of the cladding.

[0022] The fuel may be a fissile material selected from the group consisting of UO2, U3Si2, UN and uranium alloys.

[0023] Elongated fuels with elongated SiC claddings enable the use of the proven benefits of fuels such as UO2, as well as the accident tolerance characteristics of SiC cladding materials, without compromising fuel performance both during normal operation and under transient conditions. The proposed elongated SiC claddings with elongated fuels and end gaps that allow for fuel expansion provide benefits such as improved heat transfer with small gaps or direct contact between the fuel and cladding. The elongated SiC claddings with elongated fuels will also result in lower fuel temperatures due to the minimal gaps throughout the fuel's service life, which will lead to reduced fission gas release and reduced swelling, reduced deformation on the cladding during transient events due to low temperature rise, and reduced tensile stresses during hard contact between the fuel and cladding due to the increased span of the allowed cladding protrusions. The elongated cladding surface can be selectively pre-curved to further mitigate local stress buildup. The SiC-clad elongated fuels described herein allow the use of UO2 fuel without compromising the economics of UO2 or the accident tolerance characteristics of SiC. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The nature and advantages of the present disclosure may be better understood by referring to the accompanying drawings.

[0025] Figure 1 is a schematic diagram of a top plan cross-sectional view of an embodiment of an elongated fuel element in its initial configuration.

[0026] Figure 2 is a schematic diagram of a top plan cross-sectional view of an alternative embodiment of an elongated fuel element showing a convex fuel segment and elongated cladding (solid line) in their initial configuration compared to a configuration in which the swollen fuel segment (dashed line) and cladding are deformed (dashed line) during use.

[0027] Figure 3 is a schematic diagram of a top plan cross-sectional view of another embodiment of a fuel element showing the pre-bent cladding wall and pre-bent fuel segments in their initial configuration (dashed lines) and after the fuel has swollen and the cladding has deformed (solid lines).

[0028] Figures 4A to 4D Schematic diagrams of a front cross-sectional view (A), a top cross-sectional view (B), a side cross-sectional view (C), and an alternative front cross-sectional view (D) of an embodiment of a fuel segment in an assembly where the fuel can be manufactured as a single piece (D) or in stacked segments (A) and inserted into a cladding.

[0029] Figures 5A to 5C Schematic diagrams of front (A), top (B), and side (C) cross-sectional views of the fuel element assembly of FIG. 4 inserted into a reactor core are shown.

[0030] Preferred embodiments

[0031] As used herein, the singular forms "a," "an," and "the" include plural references unless the context clearly dictates otherwise. Thus, the articles "a" and "an" are used herein to refer to one or more than one (i.e., to at least one) of the grammatical objects of the article. By way of example, "an element" means one element or more than one element.

[0032] Directional phrases used herein, such as, but not limited to, top, bottom, left, right, lower, up, down, downward, front, back, and variations thereof, shall relate to the orientation of elements as shown in the drawings and not to the claims unless expressly stated otherwise.

[0033] In this application (including claims), unless otherwise indicated, all numbers representing quantities, values ​​or characteristics are understood to be modified by the term "about" in all cases. Therefore, even if the term "about" may not appear explicitly with a number, the number can be interpreted as if it were preceded by "about". Therefore, unless otherwise indicated, any numerical parameters set forth in the following description can vary according to people's attempts to obtain desired properties in the compositions and methods according to the present disclosure. At least, and without attempting to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter described in this specification should be interpreted at least according to the number of significant figures reported and using ordinary rounding techniques.

[0034] In addition, any numerical range recited herein is intended to include all subranges contained therein. For example, a range of "1 to 10" is intended to include any and all subranges between the recited minimum value of 1 (inclusive) and the recited maximum value of 10 (inclusive), i.e., having a minimum value equal to or greater than 1 and a maximum value equal to or less than 10.

[0035] refer to Figure 1, shows a fuel element 10 in its initial configuration, as before use. The fuel element 10 comprises an elongated SiC cladding 12 enclosing a nuclear fuel segment 14. The cladding 12 is shown in cross-section as an elongated rectangular structure for accommodating the elongated fuel segment 14. In its initial configuration, the cladding 12 has two opposing elongated walls 18 and two opposing shorter end walls 16. The fuel segment 14 also has two initially parallel or substantially parallel elongated side surfaces 20 and two shorter end surfaces 22 relative to the segment's elongated side surfaces. The fuel segment 14 is sized relative to the cladding 12 such that a small gap 32 exists at a first distance between the cladding's elongated walls 18 and the elongated side surfaces 20 of the fuel segment 14, allowing just enough space to load and stack the segment within the cladding 12. At each end of the cladding 12, a larger gap 34 exists at a second distance between the cladding's end walls 16 and the segment 14's end surfaces 22. The larger gap 34 is larger than the small gap 32. The gap 34 is sized to allow the fission gases to accumulate at a pressure below the reactor design pressure (eg, 2200 psia at 320° C. for a conventional pressurized water reactor (PWR)).

[0036] In use, at the beginning of the fuel life cycle, the fuel segments 14 swell. Fuel is isotropic and therefore will expand uniformly in all directions. For the elongated cladding and fuel configurations described herein, the elongated walls 18 provide a force that prevents the fuel from expanding in the transverse direction (i.e., along the fuel line). Figure 1 and Figure 2 The fuel is free to swell along the Y-axis shown. Due to the presence of small gaps 32 along the long sides of the wall 18, the expanding fuel will initially press against the wall 18 of the cladding 12 and deform the elongated wall 18 outward. However, it is believed that the resistance of the elongated wall 18 to swelling means that the fuel will become anisotropic and will expand along the longitudinal axis of the cladding (i.e., along the longitudinal axis as shown). Figure 1 and Figure 2The swollen fuel segment 14 expands more in the direction of less resistance (as shown in the X-axis). Due to the larger end gap 34 and the length of the elongated wall 18 of the cladding 12, the swollen fuel segment 14 will expand into the gap 34, and the force exerted on the elongated wall 18 along the axis 44 by the swollen segment 14 will be spread over a longer distance (from the axis 44 to the end wall 16), resulting in a reduction in the total stress on the cladding 12 over the distance from the axis 44 to the end wall 16. The larger end gap 34 allows for longitudinal expansion of the segment 14 without applying any appreciable force or hard contact between the end surface 22 of the segment 14 and the end wall 16 of the cladding 12. As used herein, hard contact refers to contact between the segment 14 and the wall of the cladding, and particularly contact that applies any force to the cladding wall. SiC can withstand strains of about 0.04% to 0.1% and in various regions about 0.05% to 0.07% before cracking, so the strains caused by swelling of the extended fuel segment 14 over the longer length of the wall 18 limit the overall increase in strain above the applicable percentage threshold.

[0037] In various aspects, the fuel segments may have a convex shape, such as Figure 2 As shown, the reason for this is that the center of the containment wall 18 can undergo large displacements in the Y direction along the axis 44 due to the width of the fuel (in the direction from the axis 44 to the end wall 16).

[0038] refer to Figure 2 , shows an alternative embodiment of the fuel element 100. In this embodiment, the cladding 12 is Figure 1 The cladding 12 of the illustrated fuel elements 10 is identical. The cladding 12 of the fuel element 100 is an elongated rectangular structure in cross section, also formed of SiC, and in its initial configuration has two opposing elongated walls 18 and two opposing shorter end walls 16, shown in solid lines. The fuel segments 114 of the fuel element 100 in its initial configuration are oval or elliptical in cross section, with two convexly curved elongated surfaces 120 and two end curves or vertices 122. The major axis is defined as the length along the elliptical segment 114 from vertex to vertex 122, and the minor axis, shorter than and perpendicular to the major axis, is defined as the width across the ellipse.

[0039] In the preferred loading arrangement, the major axis of the segment 114 will be aligned and coaxial with the longitudinal axis 40 of the cladding 12, and the minor axis will be coaxially aligned with the lateral axis 44 of the cladding. As described above, some deviation from the preferred alignment can be tolerated as long as the end gap 34 allows the segment 114 to swell without increasing the strain on the cladding 12.

[0040] The fuel segments 114 are sized relative to the cladding 12 such that a small gap 32 exists between the elongated wall 18 of the cladding and the widest area of ​​the longer convexly curved elongated surface 120 of the fuel segments 114 (referred to as the common vertex along the minor axis of the ellipse). This allows just enough space at the widest area of ​​the pellets to load and stack the segments 114 in the cladding 12 without applying contact pressure to the inner surface 28 of the cladding 12 during loading. Due to the elliptical shape, the gap 32 widens into a gap 36 as the segments 114 bend from each common vertex toward the end or vertex 122. The fuel segments 114 are sized relative to the cladding 12 such that a larger gap 34, relative to the small gap 32, exists at each end of the cladding 12 between the end wall 16 of the cladding 12 and the end curve or vertex 122 of the segments 114. This shape promotes lateral expansion of the fuel 114 toward the end gap 34 and maximizes fuel loading. Since there is no fuel present at the end gap 34 , the SiC cladding 12 at the end wall 16 can be thickened and strengthened without affecting heat transfer and fuel temperature.

[0041] Figure 2 Also shown in dashed lines is the configuration of the cladding 12 and fuel segment 114 after a period of use at the beginning of the fuel's useful life cycle, when the segment 114 swells to a swollen configuration 124, thereby deforming the elongated wall 18 of the cladding 12 to a deformed configuration 26. As the fuel segment 114 swells, the convexly curved elongated sides 120 expand outward, contacting the elongated sidewall 18 of the cladding 12 at contact region 30, pushing the cladding wall outward at the widest point along the width of the segment 114 (the common vertex). The size of the larger gap 34 between the segment vertex 122 and the cladding endwall 16 is such that, when the segment 114 swells to its maximum size, the vertex 122 can (but need not) merely contact the endwall 16 without applying any appreciable or effective force to the endwall 16 of the cladding 12 that would increase the strain on the cladding 12. Alternatively, when the segment 114 swells to its maximum size, the vertex 122 does not contact the endwall 16, leaving a smaller gap 34'. In either case, the end wall 16 is not deformed by the swelling of the segments 114. Even when the segments 114 swell within the rectangular cladding 12, the curved shape of the segments 114 leaves gaps 36 to accommodate fission gases released during operation of a reactor using the fuel elements 100.

[0042] Before any swelling, the second distance defining the gap 34 between the fuel end surface 22 / 122 and the cladding end wall 16 in the initial configuration is sized to allow for the maximum swelling of the selected fuel plus room for fission gases generated during use. Those skilled in the art can determine the maximum swelling that the selected fuel will experience during use and the amount of fission gas that will be generated during use to determine the second distance by calculation using known methods or through historical usage. The gap 34 is sized to allow for the accumulation of fission gases at a pressure below the reactor design pressure (e.g., 2200 psia at 320° C. for a conventional pressurized water reactor (PWR)).

[0043] Before any swelling, a gap 32 is defined between the elongate wall 18 of the cladding 12 and the elongate surface 20 / 120 of the fuel in the initial configuration. The first distance gap 32 is sufficient to allow the fuel to be inserted into the cladding prior to use, typically about 0.07 mm.

[0044] In various aspects, because the SiC cladding 12 will bulge outward after a short irradiation time, the cladding's elongated walls 18 can be pre-curved concavely inward toward the fuel segments to accommodate displacement during irradiation and mitigate localized stress buildup. This is because the initial expansion of the fuel segments exerts compressive forces on the cladding, rather than tensile forces. The concave curve as an initial configuration mitigates stress buildup caused by the SiC swelling gradient. Additionally or alternatively, the fuel can be concavely shaped to minimize stress in the direction of the minor axis 44.

[0045] refer to Figure 3 , an alternative initial configuration of the fuel element 200 is shown in dashed lines, and the configuration after swelling is shown in solid lines. In various aspects, the fuel element 200 includes a fuel segment 214 having a concave elongated surface 220 in the initial configuration that swells to a convex surface 220′. The fuel element 214 has a flat or curved end surface 222 that expands outwardly as it swells to a flat surface 222′. The cladding 212 can have straight, flat walls as in the fuel element 10, or can include an elongated wall 218 having an initial concave configuration (dashed lines) that swells to move the wall 218 to a flatter, elongated wall 218′ configuration (solid lines), thereby giving the cladding 212 an overall rectangular shape. The wall 218 can also expand outwardly to a convex shape, in which case the final shape is lens-shaped rather than straight. Figure 3 Both the cladding and the fuel elements are shown in an initial concave configuration. However, the fuel elements 214 may be inserted into the cladding 12 through the top of the fuel elements, as shown in FIG. Figure 1 and Figure 2 shown.

[0046] As with fuel elements 10 and 100, in the initial configuration, a small gap 232 is defined between the elongated wall 218 of the cladding and the elongated surface 220 of the fuel segment. During swelling, as the fuel element elongated concave surface 220 expands in the Y direction to the elongated convex surface 220', the gap 232 closes at the contact area 230 between the wall 218' and the surface 220' along the transverse axis 44. In the initial configuration, a larger gap 234 is defined between each end wall 16 of the cladding 214 and the facing end surface 222 of the fuel segment 214. During swelling, as the fuel element 214 expands in the X direction along the longitudinal axis 40, the larger end gap 234 at each end closes to a smaller, but still present, gap 234'. Due to the width of the fuel segment 214 (in the direction from the axis 44 to the end wall 16), the center of the cladding wall 218 can accommodate large displacements in the Y direction along the axis 44. This shape will promote lateral expansion of the fuel 214 toward the end gap 234 and maximize fuel loading. Since there is no fuel at the end gap 234, the SiC cladding 212 at the end wall 16 can be thickened and strengthened without affecting heat transfer and fuel temperature.

[0047] The SiC cladding may preferably be made of a material such as a SiC monolith, SiC fibers, or a combination thereof, as taught in U.S. Patent Nos. 6,246,740, 5,391,428, 5,338,576, and 5,182,077, and U.S. Patent Application Publication Nos. 2006 / 0039524, 2007 / 0189952, and 2015 / 0078505, the relevant portions of which are incorporated herein by reference. In various aspects, the ceramic yarn is formed from SiC fibers. The SiC fibers may preferably be SiC fibers containing primarily Si and C with some trace or relatively small amounts of O. Exemplary amounts may include:

[0048] Si: 50% to 70% by weight (more preferably, 68% to 70%)

[0049] C: 30% to 40% by weight (more preferably, 30% to 32%)

[0050] O: 0.01% to 14% by weight (more preferably, 0.01% to 0.5%).

[0051] The SiC fiber cladding will utilize SiC in the form of fiber tows having 500 to 5000 fibers per tow and a thickness between 100 and 600 microns.

[0052] The fuel used in the fuel element 10, 100, or 200 can be selected from known fissile materials used in commercial nuclear reactors for energy production, such as, but not limited to, UO2, U3Si2, UN, and uranium alloys and other oxide fuels. The fuel element design described herein, while capable of accommodating any suitable known fuel, is particularly useful for UO2, which has heretofore been unsuitable for use with SiC cladding.

[0053] like Figure 4D In the embodiment where the fuel segment is a single piece, the fuel segment 14 (either 114 or 214) will be inserted into the elongated cladding 12 column, as shown in FIG. Figure 4A As shown in FIG, a plurality of fuel segments 14 will be stacked one above the other in an elongated column of cladding 12. In each case, the fuel segments or segments 14, 114, or 214 will generally be centered along a vertical axis (an axis extending in the Z direction perpendicular to axes 40 and 44) ​​and a transverse axis 44 (an axis extending along the width and passing through the middle of the cladding 12 and pellets 14). A plurality of fuel elements 10, 100, or 200 are inserted into a fuel assembly 50 in a nuclear reactor core, as shown in FIG. Figures 5A to 5C shown.

[0054] As indicated above, silicon carbide is a very promising material for accident-tolerant fuel cladding, but there are challenges in applying SiC to conventional rod-type fuel. The fuel element configuration described herein overcomes these challenges. To prevent excessive strain on the cylindrical rod-type cladding, a uniform annular gap exists between the cylindrical rod-type cladding and the cylindrical fuel pellet. This uniform annular gap must be large enough to accommodate the radial swelling of the fuel after a period of time in the reactor (typically greater than six months). When the SiC cladding swells at the beginning of the fuel's service life cycle, it further expands the cladding, thereby increasing the initial annular gap. At the beginning of the service life cycle, the fuel contracts. Because the fissile content is highest at the beginning of the service life, the fuel pellets generate the most heat at this time. Combined with the initially shrinking pellets and the swelling cladding, the widened initial gap poses the risk that the fuel pellet centerline temperature will exceed its melting point early in the service life. As the fuel cycle progresses, the pellets stop shrinking and begin to expand due to the accumulation of fission products, which are less dense than the original uranium fuel. Since the cladding swelling stops early in the cycle, the swelling pellets have the potential to contact and damage the SiC cladding. Since fuels such as UO2 do not have very good thermal conductivity, a wider gap adversely affects energy production by requiring the reactor to operate at lower power.

[0055] In the embodiments described herein, the fuel cladding gap is minimized, thereby minimizing the centerline temperature rise during transients and reducing the stored energy in the fuel. Furthermore, the fuel elements 10, 100, and 200 described herein provide a mechanism for the cladding 12 to accommodate thermal expansion and swelling of the fuel without mechanical failure due to the forces and resulting high strains that would otherwise be applied to the cladding in previous rod-type configurations.

[0056] The elongated fuels 14, 114, and 214 described herein with gaps 34 and 234 and an elongated SiC cladding 12 can accommodate contact between the swelling fuel and the elongated cladding walls 18 because the cladding 12 has the freedom to move laterally (in the direction of the axis 44 in the Y direction) and can accommodate the strain from the fuel 14 / 114 / 214 to a greater extent due to the large expansion across the width of the elongated cladding (the distance from the axis 44 to the respective end walls 16). The elongated fuel design described herein also provides for more expansion (in the direction of the axis 40 in the X direction) of the fuel segments 14 / 114 / 214 within the longer walls 18 of the cladding 12 under contact (depending on the length of the end gaps 34 on both end walls 16). Figure 1 、 Figure 2 and Figure 3 The various embodiments of the fuel elements 10, 100, and 200, respectively, have the potential to significantly reduce fuel temperatures and, therefore, reduce contact loads on the cladding during transients. Since the overall gap between the cladding and the fuel can be minimized, the elongated designs described herein have the potential to load more fuel, improving fuel cycle costs.

[0057] In summary, fuel elements 10, 100, and 200 provide the following benefits:

[0058] 1. Compared with cylindrical fuel rods, the gap or direct contact between the fuel and cladding is smaller, thereby improving heat transfer;

[0059] 2. Greater fuel volume per reactor volume because there is no excessive initial gap to accommodate future fuel swelling;

[0060] 3. The fuel temperature is lower due to less fission gas release and swelling caused by smaller gaps;

[0061] 4. Due to the large span of the cladding surface, the strain at the local deformation of the cladding during a transient period is low;

[0062] 5. Reduction of tensile stress during hard contact between fuel and cladding;

[0063] 6. The long walls of the cladding surface can be pre-curved to further alleviate local stress buildup, or the fuel can be convex;

[0064] 7. Allows the use of UO2 fuel without additives to increase thermal conductivity for accident tolerance characteristics without additional manufacturing cost;

[0065] 8. Work with other innovative fuel materials such as U3Si2, UN, and U alloys; and

[0066] 9. For concave design, the contact pressure is converted into bending stress, especially at the end, to ensure compressive stress at the outer layer to achieve air tightness.

[0067] The fuel elements 10, 100, and 200 described herein can utilize SiC cladding with UO2 fuel, with a minimal initial gap between the cladding and the fuel. The gap 34 / 234 at the end of the fuel element 10 / 100 / 200 provides volume for expansion of fission gases and fuel segments 14 / 114 / 214 along the major axis 40, which reduces stress on the cladding 12 when the fuel segments 14 / 114 / 214 come into contact with the cladding 12 (compared to a rod-type design). In certain aspects, the elongated sidewalls 18 of the SiC cladding can be pre-curved to mitigate stress accumulation caused by SiC swelling gradients and / or the fuel can be convex or concave to minimize stress in the direction of the minor axis 44.

[0068] The fuel elements 10, 100, and 200 described herein can significantly reduce stress buildup in SiC cladding and mitigate mechanical failure of SiC caused by fuel-cladding mechanical interactions. The fuel elements 10, 100, and 200 make SiC cladding compatible with UO2 in light water reactors and are also capable of operating with other high thermal conductivity fuels such as U3Si2, UN, and uranium alloys.

[0069] The present invention has been described based on several examples, which are intended to be illustrative in all respects and not restrictive. Therefore, the present invention is capable of many variations in detailed embodiments, which can be derived from the description contained herein by a person skilled in the art.

[0070] All patents, patent applications, publications, or other public materials mentioned herein are incorporated herein by reference in their entirety, just as if each individual reference were expressly incorporated by reference. All references and any materials or portions thereof stated to be incorporated herein by reference are incorporated herein only to the extent that the incorporated material does not conflict with existing definitions, statements, or other public materials set forth in this disclosure. Therefore, and to the extent necessary, the disclosure as set forth herein supersedes any conflicting material incorporated herein by reference, and the disclosure expressly set forth in this application prevails.

[0071] The present invention has been described with reference to various exemplary and illustrative embodiments. The embodiments described herein are understood to be illustrative features of the different details of the various embodiments of the disclosed invention; and therefore, it should be understood that, to the extent possible, unless otherwise stated, one or more features, elements, components, ingredients, raw materials, structures, modules and / or aspects of the disclosed embodiments can be combined, separated, interchanged and / or reconfigured with or relative to one or more other features, elements, components, ingredients, raw materials, structures, modules and / or aspects of the disclosed embodiments without departing from the scope of the disclosed invention. Therefore, it will be appreciated by those skilled in the art that various replacements, modifications or combinations may be made to any of the exemplary embodiments without departing from the scope of the present invention. In addition, those skilled in the art will recognize or be able to determine many equivalents of the various embodiments of the invention described herein using only routine experiments when reading this specification. Therefore, the present invention is not limited to the description of the various embodiments, but is limited to the claims.

Claims

1. An elongated fuel element for use in a nuclear reactor, comprising: a silicon carbide cladding having two opposing elongated walls and two opposing end walls shorter than the elongated walls, the elongated walls and the end walls defining a shell, the shell having a length from end wall to end wall and a width from elongated wall to elongated wall, the cladding having a longitudinal axis and a transverse axis perpendicular to the longitudinal axis; as well as nuclear fuel located in the shell, the fuel having two elongate surfaces and two end surfaces shorter than the elongate surfaces, each of the two elongate surfaces being minimally spaced a first distance from and in facing relationship with the other of the elongate walls of the cladding prior to swelling in use, and each of the end surfaces being spaced a second distance from and in facing relationship with the other of the end walls of the cladding prior to swelling in use; wherein the second distance is greater than the first distance, and the first distance and the second distance are sized so that, in use, when the fuel swells, the swollen fuel swells beyond the first distance and reaches but does not exceed the second distance to deform the elongated wall of the cladding but does not deform the end wall of the cladding and does not strain the cladding by more than 0.1%.

2. The fuel element according to claim 1, wherein The fuel is selected from the group consisting of UO2, U3Si2, UN and uranium alloys.

3. The fuel element according to claim 1, wherein The fuel is UO2.

4. The fuel element according to claim 1, wherein The fuel is configured in cross-section as an ellipse defining an apex at each of the two end surfaces, wherein each of the two elongated surfaces is convexly curved, the ellipse having a major axis extending from apex to apex and a minor axis perpendicular to the major axis.

5. The fuel element according to claim 4, wherein Each convexly curved elongated surface defines a common vertex at the minor axis and curves from the common vertex to and intersects each vertex, and the first distance is a minimum between the common vertex and the elongated wall of the enclosure.

6. The fuel element according to claim 1, wherein Prior to deformation in use, the elongate wall of the cladding is concavely curved towards the fuel and deforms outwardly in use.

7. The fuel element according to claim 6, wherein: The elongate surfaces of the fuel curve concavely towards each other prior to swelling in use and swell outwardly in use to form convexly curved elongate surfaces extending beyond the first distance to deform the elongate wall of the cladding.

8. The fuel element according to claim 7, wherein Prior to swelling in use, the end surface of the fuel is flat and swells outwardly in use to form a convexly curved end surface.

9. The fuel element according to claim 7, wherein: Prior to swelling in use, the end surface of the fuel is flat, and in use, the end surface remains flat.

10. The fuel element according to claim 6, wherein Prior to swelling in use, the end wall of the enclosure is flat.

11. The fuel element according to claim 10, wherein In use, the end wall remains flat.

12. The fuel element according to claim 1, wherein The end wall of the cladding is thicker in cross section than the elongate wall of the cladding.

13. The fuel element according to claim 1, wherein The strain on the cladding caused by swelling of the fuel is no greater than 0.05%.

14. The fuel element according to claim 1, wherein The elongate surfaces of the fuel are concavely curved inwardly towards each other prior to swelling in use and swell outwardly in use to form convexly curved elongate surfaces extending beyond the first distance to deform the elongate wall of the cladding, the fuel having a major axis extending from end surface to end surface and a minor axis perpendicular to the major axis.

15. The fuel element according to claim 14, wherein Each concave curved surface defines a common vertex at the minor axis and curves from the common vertex to and intersects each end surface.

16. The fuel element according to claim 15, wherein Prior to swelling in use, the end surface of the fuel defines a flat surface and swells outwardly in use to form a convexly curved end surface.

17. The fuel element according to claim 15, wherein Prior to swelling in use, the end surface of the fuel defines a flat surface, and in use, the end surface remains flat.

18. The fuel element of claim 1, wherein: Prior to swelling in use, the fuel is configured in cross-section into a rectilinear shape to define a flat end surface and a flat elongate surface, the rectilinear shape having a major axis extending from end surface to end surface and a minor axis perpendicular to the major axis.

19. The fuel element according to claim 18, wherein In use, the fuel swells outwardly along both the major axis and the minor axis to form an elliptical configuration in cross-section.

20. The fuel element of claim 18, wherein Prior to swelling in use, the cladding is configured in a rectilinear shape in cross-section to define a flat end wall and a flat elongate wall which deforms outwardly in use as the fuel swells.

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