Self-healing liquid pellet-coated gap heat transfer filler

Abstract

To provide design of an improved nuclear fuel rod.SOLUTION: The improved fuel rod includes a cladding tube, a plurality of fuel pellets stacked within the cladding tube, and a liquid material filling the gap between the fuel pellets and the cladding tube. The liquid material is selected from those having a thermal conductivity higher than the thermal conductivity of helium, a melting point lower than about 400°C, a boiling point higher than 1600°C, and which are capable of wetting both the fuel pellets and the cladding sufficient to form a protective layer over the pellets and to infiltrate into openings that may form in the cladding.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] (Government Rights Statement) This invention was made with government support under Contract No. DE-NE0008824 awarded by the Department of Energy. The United States Government has certain rights in this invention.

[0002] (cross reference) This application claims priority to U.S. Provisional Patent Application No. 62 / 814,943, filed March 7, 2019, the contents of which are incorporated herein by reference.

[0003] The present invention relates to nuclear fuel rods, and more particularly to fuel rods with a liquid material between a solid fuel and a solid cladding surrounding the fuel. [Background technology]

[0004] In a typical nuclear reactor, the reactor core contains numerous fuel assemblies, each consisting of multiple elongated fuel rods. Each fuel rod contains fissile nuclear fuel material, such as at least one of uranium dioxide (UO2), plutonium dioxide (PuO2), uranium nitride (UN), and / or uranium silicide (U3Si2), and is typically in the form of a stack of nuclear fuel pellets surrounded by a gas, such as helium (He) or hydrogen (H2). The fuel rods are encased in cladding, which acts as a containment for the fissile material. In a nuclear reactor, the fuel rods are organized into arrays configured to provide a neutron flux within the core sufficient to support high efficiency of nuclear fission and the release of a large amount of energy in the form of heat. A coolant, such as water, is pumped through the core to extract the heat for useful work. Nuclear fuel and cladding present challenges. Summary of the Invention [Problem to be solved by the invention]

[0005] A solution to the challenges presented by the particular combination of nuclear fuel and cladding is provided by the improved fuel rod design described herein, in particular the use of a particular liquid material to fill the gaps between the fuel pellets and the cladding. [Means for solving the problem]

[0006] In a nuclear fuel rod comprising a cladding tube, at least one fuel pellet, and in various embodiments a plurality of fuel pellets, stacked within the cladding tube, and a radial gap defined between the fuel pellet and the cladding tube, the improvement includes a liquid material filling the gap between the fuel pellet and the cladding tube, the liquid material having a thermal conductivity greater than that of helium, a melting point less than about 400°C, and a boiling point greater than 1600°C, and capable of wetting both the cladding and the fuel pellets sufficiently to penetrate cracks, if any, in the cladding and to form a protective layer over the pellets.

[0007] In various embodiments, the liquid material may be selected from the group consisting of lead-based alloys and tin-based alloys, and the metals tin, zinc, lead, and bismuth. Exemplary alloys include Pb-Sn-Bi and Pb-Bi systems, Li-Be fluoride, and eutectic alloys containing 10-18 atomic % Zn with residual Sn. Exemplary lead-based alloys may be selected from the group consisting of Pb-Sn-Bi and Pb-Bi systems. Exemplary tin-based alloys may be selected from the group consisting of Sn-Zn or Sn.

[0008] The cladding may be made of silicon carbide, such as a silicon carbide composite. In various embodiments, the fuel may be selected from the group consisting of UO2, UN, U3Si2, and combinations thereof.

[0009] The nuclear fuel rods can be used in any type of nuclear reactor having fuel rods. In various embodiments, the nuclear fuel rods can be used in a light water reactor, such as a boiling water reactor or a pressurized water reactor, a CANDU reactor, a molten salt reactor, or a fast reactor. If the reactor is a boiling water reactor, the liquid material can be a lead-based alloy, such as a Pb-Sn-Bi or Pb-Bi alloy. [Brief explanation of the drawings]

[0010] The features and advantages of the present disclosure may be better understood with reference to the accompanying drawings.

[0011] The accompanying drawing is a schematic diagram of a cross section of a SiC fuel rod showing microcracks in the cladding originating from the inside and the liquid material filling the cracks to prevent leakage through the cladding to the coolant surrounding the fuel rod. DETAILED DESCRIPTION OF THE INVENTION

[0012] 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 to more than one (i.e., to at least one) of the grammatical object of the article. By way of example, "an element" means one element or more than one element.

[0013] In this application, including the claims, unless otherwise indicated, all numbers expressing quantities, values, or properties are understood to be modified in all instances by the term "about." Thus, even if the term "about" is not explicitly stated with that number, the number can be read as if preceded by the term "about." Accordingly, unless otherwise indicated, any numerical parameter set forth in the following description may vary depending on the desired properties sought to be obtained in the compositions and methods according to the present disclosure. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter set forth herein should, at the very least, be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

[0014] Furthermore, any numerical range recited herein is intended to include all subranges subsumed therein. For example, a range of "1 to 10" is intended to include any and all subranges between (and including) the recited minimum value of 1 and the recited maximum value of 10, i.e., having a minimum value of 1 or greater and a maximum value of 10 or less.

[0015] As used herein, "wet" means to lower the surface tension so that a liquid can spread across or penetrate more easily the surface of a solid.

[0016] As used herein, "eutectic alloy" means a homogeneous mixture of at least one metal and at least one other substance, which may be a second metal, where the metal and other substance melt or solidify at a single temperature below the melting point of either of the components. The eutectic temperature is the lowest possible melting temperature across all mixture ratios for the component species involved. A eutectic alloy has two or more materials and has a eutectic composition. When a eutectic alloy melts, it does so at a single, distinct temperature, as opposed to a non-eutectic alloy, where the components solidify at different temperatures and exhibit a plastic melting range.

[0017] Efforts to improve the safety and performance of nuclear reactors are driving research into improved cladding materials and fuels. SiC and SiC composites are very promising accident-resistant fuel cladding materials. Experimental ceramic-type materials such as SiC monoliths, fibers, and combinations thereof are taught in U.S. Patent Nos. 6,246,740, 5,391,428, 5,338,576, 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.

[0018] One of the main challenges in using SiC as cladding for nuclear fuel rods is that the fuel pellets cannot tolerate mechanical strain on the SiC cladding. Because SiC cladding is not as flexible as metallic cladding, it loses its hermetic seal when subjected to strains of more than about 0.05%. In ceramic cladding, thermal and mechanical stresses can cause microcracks to form, while in metallic cladding, debris or wear due to abrasion can cause pinholes.

[0019] Although UO2 is an excellent fuel, its low thermal conductivity leads to high pellet temperatures, which causes excessive thermal expansion of the pellet and ultimately leads to central melting of the pellet during power transients. Fission products, such as cesium and strontium isotopes, are less dense than the original uranium fuel and cause the fuel pellet to expand, which can crack the cladding unless the pellet-cladding gap is sufficiently wide. An increase in the pellet-cladding gap can result in increased fuel temperatures and fission gas release due to the low thermal conductivity of the helium gas gap, significantly reducing fuel melting and rod internal pressure margins. For this reason, employing UO2 with SiC cladding and using helium to fill the gap would be impossible.

[0020] On the other hand, U3Si2 and UN have very high thermal conductivity, so they do not expand much, avoiding contact between the pellet and the cladding. The gap between the pellet and the cladding can be relatively wide while still avoiding center melting of the pellet. However, one drawback to the use of U3Si2 and UN fuel is the low water corrosion resistance of these two materials, which can make their use in light water reactors problematic. In the event of a leak, U3Si2 and UN pellets react with water to produce UO2 and U3O8, which causes the pellets to expand and damage the cladding.

[0021] In the 1980s, sodium metal liquid filling gaps was reported in fast reactors (FBRs) with sodium-based coolant, stainless steel cladding, uranium fuel, and large gaps between the pellets and cladding to accommodate excessive expansion of the fuel pellets. However, because sodium often reacts violently with water, the approach attempted in fast reactors is not possible in light-water reactors (LWRs), where even a small leak event could cause contact between water from the coolant and the sodium-based liquid metal in the gaps to generate excessive heat, potentially leading to central fuel melt. Additionally, increasing the radial gap size reduces thermal conductivity within the gaps, resulting in increased central heat within the fuel pellets. Metal fillers such as lead are prohibited due to corrosion problems with high-nickel alloys in fast reactors.

[0022] In current fuel rods in water reactors, helium fills the gaps between the UO2 pellet stack and the zirconium-based cladding. Early in the reactor fuel's life, approximately 35% of the thermal resistance is through the helium-filled gaps. The low thermal conductivity of UO2, combined with the low thermal conductivity of helium in the gaps, causes thermal expansion of the pellets. Additionally, fission products produced from the fuel cause the UO2 pellets to expand, blocking the size or depth of the gaps as the fuel is used in the reactor.

[0023] Alternative fuels such as U3Si2 and UN have been proposed for SiC cladding. However, both fuels have poor resistance to aqueous corrosion. In the event of a leak, the volume increase caused by the oxidation of U3Si2 to UO2 / U3O8 can cause the fuel rod to bulge in the case of metal cladding or fracture in the case of ceramic cladding (e.g., SiC), blocking the coolant path.

[0024] To make SiC cladding a very promising accident-resistant cladding in cooperation with UO2 as reactor fuel, high thermal conductivity and reduced stored energy in the fuel are required. To make SiC cladding work with U3Si2 or UN as fuel in light water reactors, a protective film on the fuel pellets is required to prevent or at least delay contact of the U3Si2 or UN fuel with steam or coolant during a leakage event.

[0025] A solution to the aforementioned problem can be provided by allowing a self-repair mechanism to occur within the cladding during operation to prevent coolant and / or steam, which may be introduced into the cladding by microcracks that form in the cladding, from reacting with the pellets. By replacing the helium gas or other gas currently used to surround the fuel pellets within the cladding with a liquid material to fill the gap between the fuel pellets and the cladding, the liquid material will flow into any cracks, microcracks, fissures, deformations, voids, pinholes, or other openings that form in the cladding that have the potential to form a path through the cladding to the coolant (collectively or individually referred to as a "crack").

[0026] Referring to the drawings, a cross-sectional schematic of a fuel rod 10 shows fuel pellets 12 centrally disposed within a cladding tube 16. A radial gap G separates an inner surface 20 of the cladding tube 16 from an outer surface 22 of the fuel pellets 12. An exemplary crack 18 is shown extending from an opening in the inner surface 20 into the body of the cladding tube 16. Liquid material 14 fills the gap G, enters the opening in the inner cladding 20, and fills the crack 18. A protective layer 24 may be formed covering the outside of the fuel pellets 12. While not shown for simplicity, those skilled in the art will understand that there may be multiple cracks 18 that form in the cladding 16 during use. Liquid material 14 will flow into and fill each such crack that forms in the inner cladding surface 20. As cracks form from the outside of the cladding 16 and eventually reach the inside of the cladding, the liquid material reacts with water vapor to form a solid protective oxide layer 24 covering the outer surface of the pellets 12. Layer 24 is shown in the figures for illustrative purposes, but one skilled in the art will understand that in various embodiments, layer 24 will not form unless a reaction with the coolant occurs.

[0027] Various embodiments of the cladding tube 16 may be made of a SiC composite material. SiC composites often take the form of a wrapping or braiding of SiC fibers around a core tube or SiC tube to form a fuel rod cladding. Continuous SiC fiber-reinforced SiC matrix composites are disclosed in U.S. Patent Application Publication No. 2015 / 0078505 or Y. Katoh et al., "Continuous SiC fiber, CVI SiC matrix composites for nuclear applications: Properties and irradiation effects," Journal of Nuclear Materials, vol. 448, pp. 448-476 (2014). Commercial sources of SiC fibers are Hi-Nicalon® Type S fiber (manufactured by Nippon Carbon, Tokyo, Japan) and Tyranno® SA3 fiber (manufactured by Ube Industries, Ube, Japan), both of which are listed in Table 1 of Y. Katoh et al., Journal of Nuclear Materials, vol. 448, p. 450.

[0028] In various embodiments, the pellets 12 may be uranium pellets, such as UO2. Alternatively, the fuel pellets 12 may be: UN Or it may be U3Si2.

[0029] In some embodiments, the liquid material is liquid within a temperature range of 315° C. to 1600° C., and in some embodiments, within a temperature range of 400° C. to 1600° C. The liquid material 14 can have a melting point below about 400° C. and a boiling point above 1600° C. The liquid material 14 can sufficiently wet both the fuel pellets 12 and the cladding 16, form a protective layer 24 over the pellets 12, and penetrate into cracks 18 in the cladding 16, if any.

[0030] In various embodiments described herein, a liquid material 14 having high thermal conductivity is used to fill the gap G between the exterior 22 of the pellet 12 and the interior surface 20 of the cladding 16, allowing compatibility of different uranium-based fuels with the SiC-based cladding, and is particularly useful in light water reactors, but may be used in any nuclear reactor with fuel rods. When the cladding 16 is made of SiC or a SiC composite, the gap-filling liquid material 14 makes the SiC compatible with the UO2 by reducing the thermal resistance within the gap, which in turn reduces the thermal expansion of the fuel and reduces the chance of center melting of the UO2 fuel during thermal cycling.

[0031] In various embodiments, the liquid material 14 acts as a protective layer such that in the event of a leak, the liquid 14 reacts with steam to form a solid protective oxide layer 24 that covers the exterior 22 of the pellets 12, preventing the steam from contacting the pellets 12. The liquid material also fills cracks 18 in the cladding 16, and again, in the event of a leak, reacts with water passing through the crack to form a plug for the crack or to form a protective layer that covers one or both of the fuel pellets 12 and the interior 20 of the cladding when in contact with the coolant.

[0032] The liquid material 14 can be selected from low melting point metals or eutectic metal alloys. In various embodiments, the liquid material preferably has the following characteristics: 1. Melting point below 400℃ during operation. 2. Boiling point above 1600℃ to avoid internal pressure rise of the rod in case of transient changes or accidents exceeding the design criteria. 3. Self-repairing microcrack plugs can be formed in the SiC coating. 4. Solidification around the fuel not only protects U3Si2 and UN from the release of fission products but also from corrosion by water and steam. 5. In some embodiments, liquid metal or alloy can solidify at coolant temperatures. Because its melting temperature is slightly higher than typical coolant temperatures (~315°C), it can fill SiC cracks and solidify when it comes into contact with the coolant. 6. The liquid fill material wets both the fuel pellets and the cladding, causing it to penetrate into the cracks 18.

[0033] Exemplary liquid materials 14 include, but are not limited to, Pb-based alloys (for boiling water reactors only), tin-based alloys, zinc-based eutectic alloys, and tin, zinc, lead, and bismuth metals. Exemplary lead-based alloys include Pb-Sn-Bi and Pb-Bi. Exemplary tin-based alloys include Sn-Zn or Sn. Exemplary zinc-based eutectic alloys include 10-18 atomic % Zn with residual Sn (melting point = 200°C).

[0034] Pb-based alloys, such as Pb-Sn-Bi and Pb-Bi, are suitable for use in boiling water reactors. However, in the pressurized water reactor environment, lead-based alloys are not the best choice. The limit for lead in the coolant is 10 ppb, according to Electric Power Research Institute guidelines, due to the potential for lead to corrode steam generator tubes.

[0035] The liquid material 14 filling the pellet-cladding gap G allows for the use of a wider selection of fuels. For example, if the fuel of choice is UO2, the use of the liquid material 14 filling the gap G reduces the fuel temperature, reducing thermal expansion of the fuel and the risk of center melting. The liquid material 14 has a thermal conductivity higher than that of helium, therefore allowing for a larger gap design to avoid contact between the pellets 12 and the cladding 16, reducing fuel temperature and reducing fission gas release. The reduced fission gas release would allow for higher uranium-235 enrichment and longer fuel life, as the internal pressure of the rods is reduced and maintained below the coolant pressure for longer fuel burnup.

[0036] If the fuel of choice is U3Si2 or UN, the use of liquid material 14 to fill gap G is expected to provide a protective layer that prevents or at least delays contact with water vapor, making the fuel water reactor compatible, and filling internal cladding cracks, allowing the cladding to "self-heal." If a crack occurs on the outside of the cladding, liquid material 14 can fill crack 18 and fill the crack after reacting with water. This "self-healing" mechanism prevents further leak events.

[0037] The improved fuel rod design described herein uses SiC clad UO2, UN and U3Si2, particularly in light water reactors. The improved design can also mitigate and prevent leakage mechanisms in SiC composites and Cr-clad zirconium cladding.

[0038] The following advantages are realized by using the liquid material 14 as described herein to fill the gap G between the fuel pellets 12 and the SiC-based cladding 16. 1. Increased thermal conductivity in the gap, 2. Allowing for a wide gap (e.g., up to 0.1 mm) that can be used to ensure there is no pellet-cladding mechanical contact due to fission products accumulating within the pellet, while reducing the thermal expansion of the fuel pellet to allow the UO2 fuel to work within the SiC-based cladding; 3. Reduced fission gas release due to lower fuel temperatures; 4. Protect U3Si2 and UN fuels from water vapor and allow them to be used in light water reactors. 5. It fills microcracks in SiC-coated, Cr-coated Zr alloys, and Zr alloy coatings, and reacts with water to form insoluble plugs in the cracks, thereby reducing leakage in SiC-coated, Cr-coated Zr alloys, and Zr alloy coatings.

[0039] All patents, patent applications, publications, or other disclosure materials mentioned herein are incorporated herein by reference in their entirety, as if each individual reference were expressly incorporated by reference. All references and any material, or portions thereof, that are said to be incorporated herein by reference are incorporated herein only to the extent that the incorporated material does not contradict existing definitions, explanations, or other disclosure materials set forth in this disclosure. Therefore, to the extent necessary, the disclosure set forth herein supersedes any conflicting material incorporated herein by reference and the disclosure expressly set forth in the control of this application.

[0040] The present invention has been described with reference to various exemplary and illustrative embodiments. The embodiments described herein are understood to provide illustrative features of various details of various embodiments of the disclosed invention, and therefore, unless otherwise specified, it should be understood that, to the extent possible, one or more features, elements, components, compositions, ingredients, structures, modules, and / or aspects of the disclosed embodiments can be combined, separated, interchanged, and / or rearranged with one or more other features, elements, components, compositions, ingredients, structures, modules, and / or aspects of the disclosed embodiments without departing from the scope of the disclosed invention. Accordingly, those skilled in the art will recognize that various substitutions, modifications, or combinations of any of the exemplary embodiments can be made without departing from the scope of the invention. Moreover, those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the various embodiments of the invention described herein upon reviewing this specification. Therefore, the present invention is not limited by the description of the various embodiments, but rather by the scope of the claims. The following items are elements of the claims as filed: (Item 1) a cladding tube; a plurality of fuel pellets stacked within the cladding; a liquid material filling gaps between the fuel pellets and the cladding, the liquid material having a thermal conductivity greater than that of helium, a melting point less than about 400°C, and a boiling point greater than 1600°C, the liquid material being capable of wetting both the cladding and the fuel pellets sufficiently to penetrate cracks, if any, in the cladding and to form a protective layer over the pellets. (Item 2) Item 2. The nuclear fuel rod according to item 1, wherein the liquid material is selected from the group consisting of lead-based alloys and tin-based alloys, and the metals tin, zinc, lead, and bismuth. (Item 3) 2. The nuclear fuel rod according to item 1, wherein the nuclear fuel rod is used in a light water reactor. (Item 4) Item 10. The nuclear fuel rod of item 1, wherein the cladding is a silicon carbide composite cladding and the fuel is selected from the group consisting of UO2, UF, U3Si2, and combinations thereof. (Item 5) Item 2. The nuclear fuel rod according to item 1, wherein the nuclear fuel rod is used in a boiling water reactor and the liquid material is a lead-based alloy. (Item 6) 6. The nuclear fuel rod according to item 5, wherein the lead-based alloy is selected from the group consisting of Pb—Sn—Bi and Pb—Bi alloys. (Item 7) Item 2. The nuclear fuel rod according to item 1, wherein the liquid material is a metal alloy selected from the group consisting of Pb-Sn-Bi system, Pb-Bi system, Li-Be fluoride, and eutectic alloy containing 10-18 atomic % Zn with residual Sn. (Item 8) Item 2. The nuclear fuel rod according to item 1, wherein the tin-based alloy is selected from the group consisting of Sn-Zn or Sn.

Claims

1. a silicon carbide cladding tube; UN and / or U ３ Si ２ a plurality of fuel pellets stacked within the cladding tube; a gap filler material located in gaps between the fuel pellets and the cladding; the gap-filling material has a thermal conductivity greater than that of helium, a melting point greater than 315°C and less than 400°C, and a boiling point greater than 1600°C; the gap between the fuel pellets and the cladding tube is 0.1 mm or less; nuclear fuel rods.

2. The gap filling material is heated at a temperature equal to or higher than the melting point. and / or forming a solid protective layer over at least one of the fuel pellets; The nuclear fuel rod of claim 1 configured to be wetted.

3. 3. The nuclear fuel rod of claim 2, wherein said gap filler material is a Sn-Zn alloy.

4. 10. The nuclear fuel rod of claim 1, wherein said fuel pellets are solid fuel pellets.

5. a silicon carbide cladding tube; UN and / or U ３ Si ２ fuel pellets within the cladding; a Sn-Zn alloy that melts into a liquid state to fill gaps between the fuel pellets and the cladding tube; The molten Sn—Zn alloy is wetting the cladding sufficiently to penetrate cracks in the cladding and to form plugs in the cracks by contacting water through the cracks; wetting the fuel pellets so that contact with the water forms a solid protective layer over the fuel pellets; It is configured as follows: the gap between the fuel pellets and the cladding tube is 0.1 mm or less; nuclear fuel rods.

6. 6. The nuclear fuel rod of claim 5, wherein the alloy has a thermal conductivity greater than that of helium, a melting point greater than 315°C and less than 400°C, and a boiling point greater than 1600°C.

7. a silicon carbide cladding tube; a plurality of fuel pellets stacked within the cladding; a gap filler material comprising an alloy and positioned in gaps between the plurality of fuel pellets and the cladding; It is equipped with The first fuel pellet of the plurality of fuel pellets is UN, U ３ Si ２ or a combination thereof, The alloy is Sn—Zn; A nuclear fuel rod, wherein the gap between the plurality of fuel pellets and the cladding tube is 0.1 mm or less.

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