FULLY CERAMIC MICROENCAPSULATED FUEL FABRICATED WITH BURNABLE POISON AS SINTERING AID

Abstract

A methodology is disclosed for compaction of a ceramic matrix of certain nuclear fuels incorporating neutron poisons, whereby those poisons aid in reactor control while aiding in fuel fabrication. Neutronic poisons are rare-earth oxides that readily form eutectics suppressing fuel fabrication temperature, of particular importance to the fully ceramic microencapsulated fuel form and fuel forms with volatile species.

Claims

1) A method comprising: providing a plurality of fuel particles; mixing the fuel particles with ceramic powder and rare earth oxide neutronic poisons to form a precursor mixture; and compacting the precursor mixture at a predetermined pressure and temperature to form a fuel element.

2) The method according to claim 1, wherein the fuel particles are tristructural-isotropic fuel particles (TRISO).

3) The method according to claim 1, wherein the rare-earth oxide neutronic poisons include rare-earth oxides having a large neutron capture cross-section and the ability to suppress the sintering temperature of the ceramic powder below the critical damage temperature of the fuel particles.

4) The method according to claim 1, wherein the rare-earth oxide neutronic poisons are selected from the group consisting of Gd2O3, Er2O3, Dy2O3, and Eu2O3, and combinations thereof.

5) The method according to claim 1, further comprising mixing additional sintering additives to the precursor mixture of ceramic powder and rare earth oxide neutronic poisons.

6) The method according to claim 5, wherein the additional sintering additives include alumina, yttria, or other rare earth oxides, or combinations thereof.

7) The method according to claim 1, wherein one or more of the rare earth oxide neutronic poisons are the only oxide sintering additives in the precursor mixture.

8) The method according to claim 1, wherein the precursor mixture consists essentially of ceramic powder and rare earth oxide neutronic poisons.

9) The method according to claim 1, wherein the ceramic powder comprises silicon carbide (SiC).

10) The method according to claim 1, wherein the precursor mixture includes the rare earth oxide neutronic poisons in an amount up to 10 weight percent of the total weight of the precursor mixture.

11) The method according to claim 1, wherein the combination of the rare earth oxide neutronic poisons and any additional sintering additives is in an amount up to 10 weight percent of the total weight of the precursor mixture.

12) The method according to claim 5, wherein the rare earth oxide neutronic poisons are included in an amount of 0.5 to 6 weight percent of the total weight of the precursor mixture.

13) The method according to claim 1, wherein the predetermined temperature is less than 1900° C.

14) A nuclear fuel comprising a fuel element comprising a plurality of fuel particles intermixed in a silicon carbide matrix, wherein the silicon carbide matrix separates at least one of the plurality of fuel particles embedded in the silicon carbide matrix from the other fuel particles embedded in the silicon carbide matrix, wherein the silicon carbide matrix is near-stoichiometic and has pockets of porosity of not more than 4%, and wherein the pockets include rare earth oxide neutronic poisons.

15) The nuclear fuel according to claim 14, wherein the pockets include only rare earth oxide neutronic poisons and tramp elements.

16) The nuclear fuel according to claim 14, wherein the pockets include only rare earth oxide neutronic poisons, additional sintering additives, and tramp elements.

17) The nuclear fuel according to claim 14, wherein the rare-earth oxide neutronic poisons are selected from the group consisting of Gd2O3, Er2O3, Dy2O3, and Eu2O3, and combinations thereof.

18) The nuclear fuel according to claim 14, wherein the fuel particles are tristructural-isotropic fuel particles.

19) The nuclear fuel according to claim 14, wherein the plurality of fuel particles comprises transuranic elements extracted from a spent fuel of a light water reactor.

20) The nuclear fuel according to claim 14, wherein the plurality of fuel particles comprises transuranic elements extracted from a nuclear weapon.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, of which:

[0035] FIG. 1 is a schematic diagram illustrating a precursor mixture according to embodiments of the invention prior to sintering to Rum a fuel element.

[0036] FIG. 2 is a pair of graphs illustrating the eutectic temperature and neutron poison cross section of certain rare earth oxides.

[0037] FIG. 3 is a graph illustrating the neutron absorption cross section for matrices for exemplary fuel elements with and without presence of a rare earth oxide neutron poison.

[0038] FIG. 4 is a graph illustrating the neutronic impact of including percent-level neutron poisons on core reactivity.

[0039] FIG. 5 is a series of SEM images with the top left being a polished cross section of a ceramic matrix processed with a rare-earth oxide poison (Gd.sub.2O.sub.3). The top center being the characteristic x-ray map for Gd; the top right being the characteristic x-ray map for Al; the bottom left being the characteristic x-ray map for Si; the bottom center being the characteristic x-ray map for Y; and the bottom right being the characteristic x-ray map for O.

[0040] FIG. 6 is a schematic diagram illustrating precursor mixture according to embodiments of the invention to be processed within a multi-fuel die.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

[0041] The following detailed description can be read in connection with the accompanying drawings in which like numerals designate like elements.

[0042] FIG. 1 is a schematic diagram illustrating the formation and processing of nuclear fuel in accordance with the methods described above. In FIG. 1, an unprocessed fuel element 1 includes a plurality of micro-encapsulated fuel particles 10 mixed with a ceramic matrix 3 within a ceramic fuel sleeve 2. The plurality of micro-encapsulated fuel particles 10 may be tristructural-isotropic (TRISO) fuel particles. The term “TRISO fuel particle,” as used herein, refers to any type of micro fuel particle comprising a fuel kernel and one or more layers of isotropic materials surrounding the fuel kernel. By way of example only, the fuel particle 10 may have a diameter of about 1 millimeter.

[0043] In the embodiment shown in FIG. 1, the fuel particle 10 includes a fuel kernel 11 at its center. The fuel kernel may comprise fissile and/or fertile materials (e.g., uranium, plutonium, thorium, etc.) in an oxide, carbide, or oxycarbide form. In a particular embodiment, the fuel kernel 11 includes low enriched uranium (LEU) of any suitable enrichment level.

[0044] When the fuel element is used for waste mitigation and/or disposal purposes, the fuel kernel 11 may alternatively or additionally include transuranics (TRU) and/or fission products extracted or otherwise reprocessed from spent fuels.

[0045] For example, the fuel element may be used for destruction of transuranic waste generated from, for example, light water reactors or decommissioned nuclear weapons. For that purpose, the fuel element may include fuel kernels 11 formed of transuranic elements extracted from a spent fuel of a light water reactor and/or a core of a nuclear weapon. According to a particular embodiment, a fuel element formed in accordance with the described methods may be used as fuel for a light water reactor to destroy the transuranic waste while, at the same time, generating power from it.

[0046] The fuel particle 10 illustrated in FIG. 1 also includes four distinct layers coated over the fuel kernel 11, namely (1) a porous carbon buffer layer 15; (2) an inner pyrolytic carbon (PyC) layer 14; (3) a ceramic layer 13; and (4) an outer PyC layer 12.

[0047] The porous carbon buffer layer 15 surrounds the fuel kernel 11 and serves as a reservoir for accommodating buildup of fission gases diffusing out of the fuel kernel 11 and any mechanical deformation that the fuel kernel 11 may undergo during the fuel cycle.

[0048] The inner PyC layer 14 may be formed of relatively dense PyC and seals the carbon buffer layer 15.

[0049] The ceramic layer 13 may be formed of a SiC material and serves as a primary fission product barrier and a pressure vessel for the fuel kernel 11, retaining gaseous and metallic fission products therein. The ceramic layer 13 also provides overall structural integrity of the fuel particle 10.

[0050] In some embodiments, the SiC in the ceramic layer 13 may be replaced or supplemented with zirconium carbide (ZrC) or any other suitable material having similar properties as those of SiC and/or ZrC.

[0051] The outer PyC layer 12 protects the ceramic layer 13 from chemical attack during operation and acts as an additional diffusion boundary to the fission products. The outer PyC layer 12 may also serve as a substrate for bonding to the surrounding ceramic matrix 3.

[0052] The configuration and/or composition of the fuel particle 10 are not limited to the embodiments described above. Instead, it should be understood that a fuel particle consistent with the present disclosure may include one or more additional layers, or omit one or more layers, depending on the desired properties of the fuel particle. For example, in certain embodiments, the fuel particle is overcoated with an additional ceramic layer (i.e., SiC layer) prior to being mixed with the ceramic matrix material.

[0053] In particular embodiments, the ceramic matrix 3 includes SiC powder mixed with rare earth oxide neutronic poisons alone or in combination with additional sintering additives and may be in a form of a powder-based slurry, a ceramic slurry for tape casting, or any other mixture type known in the art. Prior to the mixing, the fuel particles 10 may be coated with a suitable surface protection material. The SiC powder may have an average size of less than 1 μm and/or a specific surface area greater than 20 m.sup.2/g. By way of example, the size of the SiC powder may range from about 15 nm to about 51 nm with the mean particle size being about 35 nm.

[0054] During or prior to mixing, rare earth oxide neutronic poisons are added, individually or in combination, to the SiC powder and/or coated onto the SiC powder surface. In certain embodiments, the amount of rare earth oxide neutronic poisons is up to 10 weight %, or, in more certain embodiments, from 1 to 10 weight %, or, in yet more certain embodiments, from 6 to 10 weight % based on the total weight of the precursor mixture.

[0055] The rare earth oxide neutronic poisons are selected based on a combination of the effectiveness of the element in capturing thermal neutrons, as well as, its compatibility with, and ability to aid in, the fabrication process. FIG. 2 presents an array of potential rare-earth oxides along with the important parameters such as eutectic reaction temperature with alumina, and thermal neutron absorption cross section in barns. In the upper graph of FIG. 2, the shaded box represents an upper limit for the processing temperature as represented by the eutectic temperature with alumina. This upper limit is approximately 1800° C. Suppressing the processing temperature may also prove beneficial to processing of inert matrix fuels that include volatile species, thus potentially reducing species loss during processing. In the lower graph of FIG. 2, the shaded box represents a lower limit for the neutron poison cross section. This lower limit is approximately 500 barns. As seen by the compounds in bold in FIG. 2, suitable rare-earths include Eu.sub.2O.sub.3, Gd.sub.2O.sub.3, Dy.sub.2O.sub.3, and Er.sub.2O.sub.3.

[0056] Also during or prior to mixing, in addition to the rare earth oxide neutronic poisons, additional sintering additives may be added. Acceptable additional sintering additives include, for example, alumina and other rare earth oxides, such as Y.sub.2O.sub.3. The additional sintering additives may be added individually or in combination, to the SiC powder and/or coated onto the SiC powder surface. In certain embodiments, the total amount of rare earth oxide neutronic poisons and sintering additives is up to 10 weight %, or, in more certain embodiments, from 6 to 10 weight % of the total weight of the precursor mixture. In certain embodiments in which additional sintering additives are present, the rare earth oxide neutronic poisons are included in an amount of 0.5 to 6 weight percent, or, in more certain embodiments, 1 to 5 weight percent, or, in even more certain embodiments, 1 to 3 weight percent, or, in yet even more certain embodiments, 1 to 2 weight percent of the total weight of the precursor mixture.

[0057] The ceramic fuel sleeve 2 may be fabricated from, as example, SiC of similar pedigree to the ceramic matrix or from nuclear grade graphite. Alternatively, the ceramic fuel sleeve may include SiC fibers or intermediate density green-bodies of nano-powder SiC. Where the ceramic fuel sleeve is an intermediate density green-body of nano-powder SiC, the nano-powder constituents would contain similar amounts of rare earth oxide neutronic poisons and additional sintering elements as the ceramic matrix. In certain embodiments of the nano-powder SiC of the ceramic fuel sleeve, the SiC powder is somewhat larger than the SiC powder of the ceramic matrix to retard flow during sintering and thereby inhibiting movement of the TRISO through this outer wall.

[0058] The wall thickness of the ceramic fuel sleeve is determined from fuel structural and reactor neutronic considerations. In certain embodiments, the wall thickness is 0.5 mm or greater. Where more rigid structures are desired, the wall thickness may be increased up to as much as 2 mm. The use of the ceramic fuel sleeve helps eliminate the need for final machining.

[0059] In an alternative process, the mixture of fuel particles 10 and ceramic matrix 3 with or without the ceramic fuel sleeve may be placed within a die 4 and then a current can be applied to the die so as to sinter the mixture by direct current sintering into a fuel element. The die can include more than one parallel opening and the method can include placing a mixture of fuel particles 10 and ceramic matrix 3 in each of the openings. The die can comprise graphite.

[0060] The mixture of fuel particles 10 and ceramic matrix 3 may be uniform throughout or as a layered structure where the top and bottom layers of the mixture are free of fuel particles. An example of this layered structure is illustrated in FIG. 1 by reference number 3A referring to the central region of the green body or unprocessed fuel element 1 that contains fuel particles along with the ceramic matrix powder constituents and reference number 3B referring to top and bottom areas, which do not contain fuel particles. In certain embodiments, the nominal final thickness of the 3B layers is equal to or similar to the thickness of the wall thickness of the ceramic fuel sleeve. For example, the nominal thickness of the 3B layers is from 0.5 to 2 mm.

[0061] In certain embodiments, the 3B layers, if present, would function to be a layer having variable and likely reduced levels of poison and non-poison sintering aid oxide additives for reactor coolant compatibility issues. The level of sintering aid in this layer may be as low as zero. In certain embodiments, the 3B layers, if present, function to provide added safety to the fuel by increasing the path length for migrating fission products to reach the free surface of the fuel.

[0062] FIG. 3 is an example of the neutron absorption cross section or neutron poison cross section for the ceramic matrix with and without presence of Gd.sub.2O.sub.3, which is an example of a rare earth oxide neutronic poison identified above. It is shown that upon addition of 1 weight percent gadolinia to the ceramic matrix, the neutron absorption probability of this medium increases by more than 100-fold in the thermal region of the spectrum (neutron energy ˜0.025 ev).

[0063] FIG. 4 presents the impact of incorporating rare earth oxide neutronic poisons within the ceramic fuel on neutronic performance of a representative reactor core. In these examples, a high-temperature-gas-cooled reactor (HTGR) core is presented. Similar performance occurs in other FCM-fueled platforms such as light and heavy water cooled reactors. A comparison of the large initial reactivity (upper curve of FIG. 4: legend; FCM, U235=5.0w/0, No BP) with that of a standard UO.sub.2-fueled HTGR core (curve just above the unity line of FIG. 1: legend; Solid UO.sub.2, U235=0.712w/o) is clearly seen. Through inclusion of varying amounts of burnable poison, chosen in this example as combinations of Gd.sub.2O.sub.3 and Er.sub.2O.sub.3 in the range of 1.57 to 2.07 total weight percent, the reactivity curves are clearly flattened, approaching the neutronic behavior of the non-poisoned UO.sub.2.

[0064] FIG. 5 shows a backscattered electron microscopy image of a polished section of ceramic matrix fabricated with Gd.sub.2O.sub.3. In this example, 1 wt % of this poison replaces Al.sub.2O.sub.3 and Y.sub.2O.sub.3 in the SiC powder for a total oxide addition of 6 percent. As seen from FIG. 5, the matrix is comprised of large crystallites with low porosity typical of the FCM consolidation process. The image of the figure is qualitatively indistinguishable from an image of an FCM fuel processed with Al.sub.2O.sub.3 and Y.sub.2O.sub.3. As with the typical FCM matrix formed with Al.sub.2O.sub.3 and Y.sub.2O.sub.3, the Gd.sub.2O.sub.3 resides at the triple junctions (bright pockets in micrographs) rather than as a continuous layer along the SiC grain boundaries, assuring irradiation stability. This is also shown by mapping the characteristic x-ray signal associated with Gd and other constituents of the FCM matrix in the same figure.

[0065] Although illustrated in separate figures, any features illustrated and described within one figure or embodiment could be substituted or added to any of the other embodiments described above.

[0066] Although described in connection with preferred embodiments thereof, it will be appreciated by those skilled in the art that additions, deletions, modifications, and substitutions not specifically described may be made without departure from the scope of the invention as defined in the appended claims.