Self-healing liquid pellet-cladding 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 many fuel assemblies, each of which is made up of multiple elongated fuel rods. 2 ), plutonium dioxide (PuO 2 ), uranium nitride (UN), and / or uranium silicide (U 3 S 2 ), typically helium (He) or hydrogen (H 2 Nuclear fuel is in the form of a stack of nuclear fuel pellets surrounded by a gas such as uranium. The fuel rods are encased in cladding, which acts as containment for the fissile material. In a nuclear reactor, the fuel rods are organized into arrays configured to provide a sufficient neutron flux into the core 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 designs 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, 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] The liquid material may, in various embodiments, be selected from the group consisting of lead-based 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. 2 , U.N., U. 3 S 2 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 light water reactors, such as boiling water reactors or pressurized water reactors, CANDU reactors, molten salt reactors, or fast reactors. 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 description 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 liquid material filling the cracks to prevent leakage through the cladding to the coolant surrounding the fuel rod. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[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 the number, the number can be read as if preceded by the term "about". Thus, 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 be construed at least in light of the number of reported significant digits and by applying ordinary rounding techniques.

[0014] Moreover, 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 more 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 points of any of the components. The eutectic temperature is the lowest possible melting temperature over all of the mixture ratios for the component species involved. Eutectic alloys have two or more materials and have a eutectic composition. When a eutectic alloy melts, it does so at a single well-defined temperature, in contrast to non-eutectic alloys, whose components solidify at different temperatures and exhibit a plastic melting range.

[0017] Efforts to improve reactor safety and performance are behind research into improved cladding materials and fuels. SiC and SiC composites are very promising accident tolerant fuel claddings. 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 a cladding for nuclear fuel rods is that the fuel pellets cannot tolerate mechanical strain on the SiC cladding. SiC cladding is not as flexible as metallic cladding and therefore loses its hermeticity if it is 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 from abrasion can cause pinholes.

[0019] UO 2 UO is an excellent fuel, but its low thermal conductivity causes high pellet temperatures, which causes excessive thermal expansion of the pellets and central melting of the pellets during power transients. Fission products such as cesium and strontium isotopes are less dense than the original uranium fuel and cause the fuel pellets to expand, which can cause the cladding to crack unless the pellet-cladding gap is wide enough. An increase in the pellet-cladding gap can result in increased fuel temperatures and release of fission gases due to the low thermal conductivity of the helium gas gap, significantly reducing fuel melting and the internal pressure margin of the rod. For this reason, helium is used to fill the gaps and UO is used in conjunction with SiC cladding. 2 It would be impossible to adopt.

[0020] On the other hand, U 3 S 2U and UN have very high thermal conductivity, so they do not expand much and contact between the pellet and the cladding can be avoided. The gap between the pellet and the cladding can be made relatively wide while still avoiding center melting of the pellet. However, U 3 S 2 One drawback to the use of UN and UN fuel is the poor water corrosion resistance of these two materials, which can make their use in light water reactors problematic. 3 S 2 and UN pellets react with water to form UO 2 and U 3 O 8 This causes the pellet to expand and damage the coating.

[0021] In the 1980s, sodium metal liquid filling the gaps was reported in fast reactors 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, where even a small leakage event could cause contact between water from the coolant and sodium-based liquid metal in the gaps to generate excessive heat, leading to central fuel melting. In addition, increasing the radial gap size reduces the thermal conductivity in the gaps, resulting in increased central heat in the fuel pellets. Metal fillers such as lead are prohibited due to corrosion problems with high nickel alloys in fast reactors.

[0022] In the current fuel rods of water reactors, helium is mixed with UO 2 It fills the gap between the pellet stack and the zirconium-based cladding. Early in the life of the reactor fuel, about 35% of the thermal resistance is through the helium-filled gap. 2 The low thermal conductivity of UO2 combined with the low thermal conductivity of the helium in the gaps causes thermal expansion of the pellets. In addition, fission products generated from the fuel are 2 The pellets expand and close the size or depth of the gap when the fuel is used in the reactor.

[0023] The SiC coating is 3 S 2 Alternative fuels such as UN and CN have been proposed. However, all of these fuels have poor resistance to water corrosion. If a leak occurs, U 3 S 2 UO 2 / U 3 O 8 The volume increase caused by oxidation to 0.1% can cause the fuel rod to bulge in the case of a metal cladding or fracture in the case of a ceramic cladding (e.g., SiC), blocking the coolant path.

[0024] SiC cladding is a very promising accident-resistant cladding for UO2 as a nuclear reactor fuel. 2 To cooperate with the U-type fuel, high thermal conductivity and low stored energy in the fuel are required. 3 S 2 or to cooperate with the UN, during a leak event 3 S 2 A protective coating over the fuel pellets is necessary to prevent, or at least delay, contact with the fuel or UN fuel water vapor or coolant.

[0025] A solution to the aforementioned problems can be provided by allowing a self-repair mechanism to take place within the cladding during operation to prevent coolant and / or steam from reacting with the pellets, which may be introduced into the cladding by microcracks that form in the cladding. 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 gaps between the fuel pellets and the cladding, the liquid material will flow into any cracks, microcracks, crevices, 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 schematic diagram of a cross section of a fuel rod 10 shows a fuel pellet 12 centrally disposed within a cladding tube 16. A radial gap G separates an inner surface 20 of the cladding tube 16 and an outer surface 22 of the fuel pellet 12. An exemplary crack 18 is shown extending from an opening in the inner surface 20 into the body of the cladding tube 16. A liquid material 14 fills the gap G, enters an opening in the inner cladding 20, and fills the crack 18. A protective layer 24 may be formed covering the outside of the fuel pellet 12. Although not shown for simplicity, one skilled in the art will appreciate that there may be multiple cracks 18 that form in the cladding 16 during use. The liquid material 14 will flow into and fill each such crack that forms in the inner surface 20 of the cladding. As the cracks form from the outside of the cladding 16 and eventually reach the inside of the cladding, the liquid material reacts with the water vapor to form a solid protective oxide layer 24 that covers the outer surface of the pellet 12. Layer 24 is shown in the figures for illustrative purposes, however, one of ordinary skill in the art will appreciate that in various embodiments, layer 24 will not form unless a reaction with a coolant occurs.

[0027] Various embodiments of the cladding tube 16 may be made of SiC composite material. SiC composite material often takes the form of wrapping or braiding of SiC fibers on a core tube or SiC tube to form a fuel rod cladding. Continuous SiC fiber reinforced SiC matrix composite materials 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] The pellets 12 in various embodiments are UO 2 Alternatively, the fuel pellets 12 may be UV or U uranium pellets. 3 S 2 may be also possible.

[0029] In some embodiments, the liquid material is liquid in a temperature range of 315° C. to 1600° C., and in some embodiments, in 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 to form a protective layer 24 over the pellets 12 and to penetrate into cracks 18 in the cladding 16, if any are present.

[0030] In various embodiments described herein, a liquid material 14 having a high thermal conductivity is used to fill the gap G between the exterior 22 of the pellets 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 reactor having fuel rods. When the cladding 16 is made of SiC or a SiC composite, the liquid material 14 filling the gap reduces the thermal resistance in the gap, which in turn reduces the thermal expansion of the fuel and reduces the amount of UO2 during the thermal cycle. 2 By reducing the chance of core melting of the fuel, SiC is 2 and conform it to

[0031] In various embodiments, the liquid material 14 acts as a protective layer such that in the event of a leakage, the liquid 14 reacts with the steam to form a solid protective oxide layer 24 that covers the exterior 22 of the pellets 12 and prevents 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 leakage, 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 contacted by the coolant.

[0032] The liquid material 14 may 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 an increase in internal pressure 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 releases fission products but also U. 3 S 2 and UN from corrosion by water and steam. 5. In some embodiments, it can solidify at the coolant temperature: Because the melting temperature is slightly higher than typical coolant temperatures (~315°C), liquid metal or alloy can fill the 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 so that it penetrates 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 a pressurized water reactor environment, lead-based alloys are not the best choice. The limit for lead in the coolant is 10 ppb, per Electric Power Research Institute guidelines, because lead can 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, the fuel of choice may be UO 2If , the use of liquid material 14 to fill gap G reduces the temperature of the fuel, reducing the thermal expansion of the fuel and the risk of center melting, and the liquid material 14 has a higher thermal conductivity than that of helium, therefore allowing the design of larger gaps to avoid contact between the pellets 12 and the cladding 16, reducing the fuel temperature and reducing the release of fission gases. The reduction in the release of fission gases will allow for higher uranium-235 enrichment and longer fuel life, since the internal pressure of the rods is reduced and kept lower than the coolant pressure for longer fuel burnup.

[0036] The fuel of choice is U 3 S 2 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 compatible with water reactors, and filling internal cladding cracks, allowing the cladding to "self-heal." If a crack does occur on the outside of the cladding, liquid material 14 can fill crack 18 and fill the crack after reaction with water. This "self-heal" mechanism prevents further leakage events.

[0037] The improved fuel rod design described herein uses a UO2 SiC clad. 2 , UV, and U 3 S 2 This increases compatibility with SiC composites, 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. Allows for a wide gap (e.g., up to 0.1 mm) that can be used to ensure that 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 reduce the UO2 Allowing the fuel to work within the SiC-based cladding; 3. Reduced release of fission gases due to lower fuel temperatures; 4.U 3 S 2 and UN fuels from water vapor and allow them to be used directly 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 materials, or portions thereof, that are said to be incorporated herein by reference are incorporated herein only to the extent that the incorporated materials do not contradict existing definitions, explanations, or other disclosure materials described in this disclosure.Therefore, to the extent necessary, the disclosures described herein supersede any conflicting materials incorporated herein by reference and the disclosures that are expressly described 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 may 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. Thus, it will be understood by those skilled in the art 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. Thus, the present invention is not limited by the description of the various embodiments, but rather by the scope of the claims.

Claims

1. A cladding tube; a plurality of fuel pellets stacked within the cladding tube; 1. A nuclear fuel rod comprising: 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.

2. 10. The nuclear fuel rod of claim 1, wherein said liquid material is selected from the group consisting of lead-based and tin-based alloys, and the metals tin, zinc, lead, and bismuth.

3. 10. The nuclear fuel rod of claim 1, wherein said nuclear fuel rod is used in a light water reactor.

4. The cladding is a silicon carbide composite cladding and the fuel is UO ２ , U.V., U. ３ S ２ 10. The nuclear fuel rod of claim 1, wherein the Cr is selected from the group consisting of:

5. 10. The nuclear fuel rod of claim 1, wherein said nuclear fuel rod is for use in a boiling water nuclear reactor and said liquid material is a lead-based alloy.

6. 6. The nuclear fuel rod of claim 5, wherein the lead-based alloy is selected from the group consisting of Pb--Sn--Bi and Pb--Bi alloys.

7. 2. The nuclear fuel rod of claim 1, wherein the liquid material is a metal alloy selected from the group consisting of Pb-Sn-Bi, Pb-Bi, Li-Be fluoride, and eutectic alloys containing 10-18 atomic % Zn with residual Sn.

8. 2. The nuclear fuel rod of claim 1, wherein the tin-based alloy is selected from the group consisting of Sn-Zn or Sn.

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