Compact nuclear fuel form

Abstract

A method for producing a compact nuclear fuel form comprises applying at least one coating layer to a plurality of kernels arranged within a container 206 such that the kernels become bound together i

Description

Field of the Invention

[01] The present disclosure relates to nuclear fuel. In particular, the present disclosure is concerned with techniques for producing nuclear fuel forms. Background

[02] A current approach to nuclear reactor fuel is to encase millimetre sized uranium particles in ceramic layers (a TRISO particle), thereby forming a pressure vessel around each individual fuel particle. Advantages of this approach are an enhancement in the fuels ability to contain fission products even at extremely high temperature, while reducing the overall fissionable mass.

[03] However, such techniques require a high degree of enriched uranium to ensure sufficient fissile density (which carries significant risk), and also involves a complex manufacturing process.

[04] It is therefore desired to develop an alternative approach to manufacturing nuclear fuel as an alternative to previously available techniques. Summary

[05] The present invention is defined according to the independent claims. Additional features will be appreciated from the dependent claims and the description herein. Any embodiments which are described but which do not fall within the scope of the claims are to be interpreted merely as examples useful for a better understanding of the invention.

[06] The example embodiments have been provided with a view to addressing at least some of the difficulties that are encountered with current approaches to nuclear fuel manufacture, whether those difficulties have been specifically mentioned above or will otherwise be appreciated from the discussion herein.

[07] Suitably, in one aspect of the invention there is provided a method for producing a compact nuclear fuel form. The method comprises firstly obtaining a plurality of kernels which are substantially spherical in nature (sphericity 0.79 to 1). The plurality of kernels comprise a first set of kernels each comprising nuclear fuel and, optionally, one or more sets of unfuelled kernels (e.g., a second set of kernels and a third set of kernels). The plurality of kernels are arranged within a container, and then at least one coating layer is applied to the plurality of kernels within the container. Having applied the coating(s) to the kernels, the plurality of kernels become bound together in a rigid structure forming the final fuel form. In this way, a fuel form with higher fissile density than existing fuel forms may be provided.

[08] In an example, applying the at least one coating layer to the plurality of kernels comprises coating the plurality of kernels with a first composition to provide a first coating layer, and subsequently coating the plurality of kernels within the container with a second composition, different to the first, to provide a second coating layer. The first coating layer may comprise silicon carbide and the second coating layer may comprise graphite. Further coating layers may be applied comprising at least one or a combination of zirconium, molybdenum, niobium, Tungsten, zirconium carbide, zirconium nitride, hafnium, ZrO2, ZrB2, carbon, silicon carbide.

[09] Each set of kernels in the plurality of kernels may be provided to perform a different function. Sets of kernels after the first set may be unfuelled. Unfuelled kernels may be selectively positioned within the container with respect to the first set of kernels.

[10] In an example, a set of unfuelled kernels (a second set of kernels) configured to change an operating characteristic of the compact fuel form. These may comprise at least one of Gadolinium, Boron, or Erbium compound.

[11] In an example, a set of unfuelled kernels may be configured to provide voids within the fuel form structure. Such kernels may comprise a polymer. Suitably, the method may comprise, after arranging the plurality of kernels within the container and prior to applying the at least one coating layer, burning away the unfuelled kernels configured to provide voids, thereby leaving voids in the pre-coated structure.

[12] Suitably, the nuclear fuel may comprise a fissile compound, preferably uranium, further preferably one of UO2, U3O8, a uranium nitrogen compound, a uranium carbon compound, UCO, UCN, U-B-N, U-B-C, a U-Si compound, and a Uranium alloy compound (e.g.U-AI or U-Nb). A percentage of enriched uranium within each kernel may be less than or equal to 20% of total uranium weight within each kernel. Alternatively fuel kernels comprising plutonium or thorium compounds or a mixture thereof may be utilised.

[13] As used herein, unless otherwise expressly specified, all numbers such as those expressing values, ranges, amounts or percentages may be read as if prefaced by the word "about", even if the term does not expressly appear. The term “about”, or substantially, when used herein means +1- 5% of the stated value. Also, any numerical range recited herein is intended to include all sub-ranges subsumed therein, and the terms “from” and “to” a pair of values are intended to indicate such values are included in the range. Singular encompasses plural and vice versa. Additionally, although the present invention has been described in terms of “comprising”, the processes, materials, and coating compositions detailed herein may also be described as “consisting essentially of’ or “consisting of’. Brief Description of the Drawings

[14] For a better understanding of the present disclosure reference will now be made by way of example only to the accompanying drawings, in which:

[15] Fig. 1 shows a method of forming a nuclear fuel form;

[16] Fig. 2 shows stages of the nuclear fuel form according to the method of Fig. 1;

[17] Fig. 3 shows an optional technique for forming a nuclear fuel form;

[18] Fig. 4 shows a further optional technique for forming a nuclear fuel form;

[19] Fig. 5 shows another example method of forming a nuclear fuel form; and

[20] Fig. 6 shows example container cross-sections. Detailed Description

[21] With reference to Figures 1 &2, there is shown a technique for producing an improved nuclear fuel form. The example form provides high fissile density while being convenient to manufacture. Many other advantages and improvements will be discussed in more detail herein.

[22] The method 100 for producing a compact nuclear fuel form comprises, at step 102, obtaining a plurality of kernels to be used in the nuclear fuel form. The plurality of kernels are preferably spherical, having a sphericity between about 1 (ideal sphere) and about 0.79, although other shapes of kernel may also be utilised if desired. Assuming substantially spherical kernels, the plurality of kernels may have a diameter from about 0.05 mm to about 2 mm. In some embodiments, the kernels may each have substantially the same size. In other embodiments, it may be desirable to have kernels with different sizes, particular if they are of different types (types of kernel being discussed below).

[23] The plurality of kernels 200 may comprise a set of fuel kernels 202. That is, a set of kernels 202 which comprise a nuclear fuel. The fuel kernels may be taken as forming a first set, or subset, from amongst a number of different sets of kernels which together form the totality of the plurality of kernels 200. The different sets of kernels may be taken to be different types of kernels in terms of the functionality they provide to the final fuel form.

[24] For the purposes of the present disclosure it should be taken that the fuel kernels 202 comprise a fissile or fertile compound - that is, a compound comprising a fissile isotope - such as an isotope of uranium, plutonium, or a compound that breeds a fertile isotope such as thorium; although the disclosure is not limited thereto, and kernels comprising other forms of nuclear fuel, and combinations thereof, may be utilised as well. Preferably the fuel kernels comprise a uranium compound. In particular, such a compound may be selected from among UO2, U3O8, a uranium nitrogen compound, a uranium carbon compound, UCO, UCN, U-B-N, U-B-C, a U-Si compound, and a Uranium alloy compound such as U-AI or U-Nb. In some examples a fuel kernel may comprise more than one fissile compound. Notably, a percentage of enriched uranium within each kernel should be less than or equal to 20 wt. % of a total weight of uranium, for regulatory reasons.

[25] In an example, a subset 204 of the first set of kernels 202 - that is, a subset of the fuel kernels - may also comprise burnable absorber material. Put another way, one of more of the fuel kernels 202 may comprise burnable absorber material. The burnable absorber may be suitable for tailoring the neutron absorbing capabilities of the final fuel form. By way of example, the material may be selected from one or more of Gadolinium, Boron, or Erbium containing compounds (e.g. GdC2, ZrB2 or Er2O3).

[26] Fuel kernels of the appropriate size, shape, and fissile material may be readily manufactured using known techniques such as sol-gel, spray drying, and so on, as will be familiar to those in the art.

[27] Once suitable kernels have been obtained, at step 104 the plurality of kernels are arranged in a container 206. In the present example, the container 206 is assumed to be cylindrical; that is, having a circular cross section when viewing along the container 206s major (z) axis. Other shapes of container may also be appropriate, as shown for example by Figure 6. That is, instead of having a circular cross section, an elongate container may be provided which has one of a square / rectangular, hexagonal, or triangular cross section (as viewed along the major (z) axis). The container 206 may be formed from a suitable metal, such as zirconium, molybdenum, and vanadium, or from a ceramic such as silicon carbide, zirconium carbide, or graphite.

[28] In some examples, the container 206 may be filled randomly with the plurality of kernels 200; for example, by pouring in the plurality of kernels. Preferable, however, the plurality of kernels are specifically arranged / placed within the container 206 in order to set the positions of each of the plurality of kernels. This may be achieved, for example, by a robotic aid. Setting positions of each of the kernels allows for the density of the kernels in the container 206 to be suitably controlled in order to achieve maximum packing into the container 206. Specific placement is also useful when different types of kernel may be utilised, to ensure a desired distribution of different types (including a suitable distribution of the subset of fuel kernels comprising burnable absorber). In this way the packing of the kernels may be made as efficient as possible for a given container size and shape, preferably approaching the theoretical spherical packing density of 74%.

[29] Once suitably arranged in the container 206, at step 106 the method comprises applying at least one coating 208 to the plurality of kernels 200. The coating layers provide benefits to the fuel operation, and also cause the fuel form to become bonded into its final structure.

[30] In the example shown, the method comprises applying a first coating 208, followed by a second coating 210. The first coating 208 may comprise a first composition which suitably provides a first coating layer, while the second coating 210 may comprise a second, different, composition which suitably provides a second coating layer. For example, the first layer 208 may be silicon carbide, while the second layer 210 may be graphite (i.e., carbon), or vice-versa.

[31] In general, any number of coating layers may be provided as desired for the particular style of fuel being provided. Suitably, the coating layers provide a degree of hermeticity to prevent fission products produced within the fuel kernels from escaping into the matrix, free volume of the fuel, and beyond.

[32] For example, in addition to the first and second layers 208, 210 above, other coatings may be applied which may comprise a selection of nitrides, metals, isotopes, burnable absorbers (like those mentioned above), and so on. The additional layers may be designed to aid accommodation of higher density, higher enrichment fuels that can be utilised for significantly longer residence times. These additional coatings may be applied before, after, or in-between the first and second layers 208, 210, as desired. By way of example, additional coating layers may comprise zirconium, molybdenum, niobium, Tungsten, zirconium carbide, zirconium nitride, hafnium, ZrO2, ZrB2, and combinations thereof. Coating compositions comprise selections from above and also comprising additional additives up to approximately 20% weight, may also be appropriate. Although the ultimate fuel form may comprise multiple layers of different compositions, it will also be appreciated that in some examples some layers (and the compositions used therein) may be repeated after intervening layers have been applied.

[33] Suitably, coating(s) may be applied until they have sufficient depth to bond the fuel structure together while retaining a degree of inter-kernel porosity (i.e., small spaces between kernels); this may allow for free flow of gasses through the final fuel form to aid in cooling. Alternatively, coatings may be applied until they are of sufficient thickness to close any inter-kernel gaps.

[34] Coating of the plurality of kernels may be accomplished by a variety of techniques known in the art, such as chemical vapour deposition. For example, SiC (first layer 208) coating may be achieved by chemical vapour infiltration (CVI) using trichlorosilane with hydrogen gas, either within a forced flow CVI or isobaric-forced flow CVI set up at 900-1100°C (pressures between 1-100 kPa depending on setup). Carbon (second layer 210) can be deposited via similar means with kerosene / methane at similar temperatures. Additional metallic layers can be deposited using fluorides (e.g. Mo from MoFe and W from WFe) with H2 gas (forming HF).

[35] Once coated with the desired layers, the fuel form (which now has a fixed structure and shape according to the shape of the container 206) is ready for use as a fuel. Suitably, in some examples the fuel form (which may also be termed a fuel compact) may be removed from the container 206 used during the coating process and used directly as a fuel. In another example the fuel compact may be inserted within a cladding material. In another example, the fuel compact may be held within the coating container which is then used as the final fuel form (for example, as a nuclear rod).

[36] Figures 3 and 4 show variations of the method 100 to achieve modified final fuel forms in which the fuel form is created using one or more sets of un-fuelled kernels in addition to first set of fuelled kernels.

[37] In Figure 3, the plurality of kernels 200 comprises a second set of kernels 212 which are unfuelled dopant kernels. That is, second set of kernels 212 comprises kernels which are present within the final fuel form, along side the first set of kernels 202 (but different to them), which alter the operating characteristics of the fuel form in some way. For example, the second set of kernels 212 may act as a non-heat generating part of the final structure (e.g. zirconium, molybdenum, niobium, tungsten, zirconium carbide, zirconium nitride, beryllium oxide), as a dedicated burnable absorber (i.e., an unfuelled burnable absorber, in contrast to the previously discussed fuelled burnable absorber), a neutron hardening kernel (e.g. AI2O3, ZrO2, Mo, Zr), and so on. The second set of kernels 212 may be formed from a dedicated type of dopant kernel, may comprise subsets of different types of dopant kernel, or may comprise a composition which achieves multiple characteristic modifications simultaneously.

[38] Moreover, the second set of kernels 212 may have lower density than the first set of kernels, which may allow the fuel form to better accommodate fission gas generation, which may reduce pressure of the kernel matrix to avoid fracture. Relatedly, the second set of kernels may be provided with a different size than the first set of kernels.

[39] Suitably, step 102 of method 100 may comprise obtaining both the first set and second set of kernels 202, 212, while the step 204 may comprise selectively arranging the second set 214 with respect to the second set 202. For example, the second set of kernels 214 may be arranged to be evenly distributed throughout the container 200. Put another way, each of the set of second kernels may be positioned equidistant to their nearest second kernel neighbour. In the preferred case where there are fewer second kernels 214 than first kernels, each of the second kernels 214 may be suitably surrounded by (i.e., in contact with) a first kernels 202, and not in contact with any other second kernels 214.

[40] In Figure 4, the plurality of kernels 200 comprises a third set of kernels 214, different to the first set of kernels 202 and, if present, different to the second set of kernels 212. Here the third set of kernels 214 are unfuelled spacer kernels. That is, the plurality of kernels 200 comprises a set of kernels 214 which are provided to generate spaces in the final fuel form in between other kernels of the fuel form. The spacer kernels 214 may be formed from a polymer, such as polyethylene, which can be burned away by exposing the fuel form to suitable heat.

[41] Figure 5 shows a modification of the method of Fig. 1 to accommodate this third set of kernels 214. Here, step 102 comprises obtaining the first set of fuelled kernels, the third set of (spacer) kernels 214, and optionally the second set of kernels 212. Step 104 comprises selectively arranging the third set of kernels 212 with respect to any other kernels within the container 206 such that, once removed, the remaining kernels 202, (optionally 212), retain a substantially freestanding lattice structure within the container 206. A new step 108 is introduced, between steps 104 and 106, comprising applying heat to the container 206 / plurality of kernels 200 such that the spacer kernels 214 burn away leaving voids 216 in their place. The voids 216 provide large spaces within the final fuel compact that provide enhanced cooling of the fuel form.

[42] Optionally, step 108 may comprise applying sufficient heat to not only burn away spacer kernels, but to also sinter the remaining plurality of kernels, in order to provide a structure that can be subsequently coated. That is, to ensure that the shape of the structure does not collapse due to the voids before being coated. In some cases it may also be desirable to sintering the kernels before coating even when spacer kernels 214 are not being used, in order to ensure a more rigid structure before coating. In an example, sintering the plurality of spheres may be carried out in an inert atmosphere or under vacuum, at temperatures from about 1000 °C to about 2200 °C, depending on the compound of the spheres and the desired reduction in porosity for the form before coatings that are applied.

[43] In summary, exemplary embodiments of an improved nuclear fuel form have been described. The described exemplary embodiments are convenient to manufacture and straightforward to use. The fuel may be used in a number of reactor concepts and has a higher fissile density compared to traditional particle compact fuels. Reactors of interest for the fuel herein include: pressurised and boiling water reactors; high temperature gas reactors; liquid metal cooled fast reactors; micro-reactors (e.g. those used for space or forward operating bases); fusion systems (where the fuel form may act as a neutron multiplier)

[44] The process may be used industrially. An industrial application of the example embodiments will be clear from the discussion herein.

[45] Although preferred embodiment(s) of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes may be made without departing from the scope of the invention as defined in the claims.

[46] Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

[47] All of the features disclosed in this specification, and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive.

[48] Each feature disclosed in this specification may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[49] The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification, or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

1. A method for producing a compact nuclear fuel form, comprising:obtaining a plurality of kernels, the plurality of kernels comprising a first set of kernels each comprising nuclear fuel;arranging the plurality of kernels within a container; andapplying at least one coating layer to the plurality of kernels within the container wherein, after applying the at least one coating layer, the plurality of kernels are bound together, andwherein applying the at least one coating layer to the plurality of kernels comprises coating the plurality of kernels with a first composition to provide a first coating layer, and subsequently coating the plurality of kernels within the container with a second composition, different to the first, to provide a second coating layer.

2. The method of claim 1, wherein the first coating layer comprises silicon carbide, and the second coating layer comprises graphite.

3. The method of any of claims 1 or 2, wherein applying the at least one coating layer comprises applying a coating comprising at least one or a combination of zirconium, molybdenum, niobium, tungsten, zirconium carbide, zirconium nitride, vanadium, ZrO2, ZrB2.

4. The method of any preceding claim, wherein a subset of the first set of kernels comprises burnable absorber material selected from at least one of gadolinium, boron, or erbium containing compounds.

5. The method of any preceding claim, wherein the plurality of kernels comprise a set of unfuelled kernels configured to change an operating characteristic of the compact fuel form.

6. The method of claim 5, wherein the unfuelled kernels comprise at least one of gadolinium, boron, erbium, zirconium, molybdenum, niobium, tungsten, zirconium carbide, zirconium nitride, beryllium oxide, AI2O3, ZrO2.01 05 257. The method of any preceding claim, wherein the plurality of kernels comprises a set of unfuelled kernels configured to provide voids within the fuel form structure.

8. The method of claim 7, wherein each of the set of unfuelled kernels configured to provide voids comprises a polymer.

9. The method of claim 7 or 8, wherein the method further comprises, after arranging the plurality of kernels within the container and prior to applying the at least one coating layer, burning away the unfuelled kernels configured to provide voids, thereby leaving voids in the pre-coated structure.

10. The method of claim 9, wherein burning away the unfuelled kernels comprises simultaneously sintering the plurality of kernels together.

11. The method of any of claims 5 to 10, wherein arranging the plurality of kernels within the container comprises selectively arranging positions of the unfuelled the kernels with respect to the first set of kernels.

12. The method of any preceding claim, wherein the nuclear fuel comprises a fissile compound.

13. The method of claim 12, wherein the fissile compound comprises uranium.

14. The method of claim 13, wherein the fissile compound is selected from at least one of UO2, U3O8, a uranium nitrogen compound, a uranium carbon compound, UCO, UCN, U-B-N, U-B-C, a U-Si compound, and a U-alloy compound.

15. The method of claims 13 or 14, wherein a percentage of enriched uranium-235 within each kernel is less than 20 wt. % of total uranium weight within each kernel.

16. The method of any of claims 12 to 15, wherein the fissile compound comprises plutonium.

17. The method of any of claims 12 to 16, wherein the fissile compound comprises thorium.

18. The method of any preceding claim, wherein each of fuel kernels comprises a sphericity between about 0.79 and 1.LDCM

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