Patent Number: 054597676
Section: summary

BACKGROUND OF THE INVENTION The present invention generally relates to a testing method for nuclear fuel materials, and more particular to a testing method for multi-component nuclear fuel particles which is characterized by a high level of accuracy and efficiency. Recent developments in nuclear reactor technology have created a corresponding need for improved fuel materials having a high level of structural integrity. In particular (as discussed in further detail below), fuel particles have been developed for high temperature gas reactor systems (hereinafter known as "HTGR" systems) which involve small, substantially spherical particles (microspheres) having an average diameter of about 300-900 .mu.m. Each of these particles includes a central core or center portion comprised of a fissionable radioactive material. In a preferred embodiment, this material will consist of .sup.235 UCO (uranium-235 carbonate). The center portion or central core of each particle is entirely covered/encapsulated by multiple protective layers preferably consisting of pyrolytic carbon, as well as at least one barrier layer preferably consisting of silicon carbide (SIC). The pyrolytic carbon layers are optimally applied by chemical vapor deposition using a conventional fluidized bed system. The SiC layer is preferably derived by the thermal decomposition of methyltrichlorosilane. The completed particles which incorporate the foregoing chemical compositions are often called "TRISO" particles, and are further discussed in the following references (incorporated herein by reference) which likewise discuss HTGR technology: Tennery, V. J., et al., "Structural Characterization of HTGR Pyrocarbon Fuel Particle Coatings", J. Am. Ceram. Soc., 60(5-6):268-274(1977); Stinton, D. P., et al., "Effect of Deposition Conditions on the Properties of Pyrolytic SiC Coatings for HTGR Fuel Particles", Ceramic Bulletin, 57(6):568-573(1978); Krautwasser, P., et al., "Raman Spectral Characterization of Silicon Carbide Nuclear Fuel Coatings", J. Am. Ceram. Soc., 66(6):424-433(1983); Smith, C. L., "SiC-Fission Product Reactions in HTGR TRISO UC.sub.2 and UC.sub.x O.sub.v Fissile Fuel: I., Kinetics of Reactions in a Thermal Gradient", J. Am. Ceram. Soc., 62(11-12):600-606(1979); and Allen, P. L., et al., "Nuclear Fuel Coated Particle Development in the Reactor Fuel Element Laboratories of the U.K. Atomic Energy Authority", Nucl. Technol., 35:246-253(1977). Furthermore, while the present invention shall be described herein with reference to a nuclear fuel particle containing a .sup.235 UCO center region with multiple pyrolytic carbon protective layers and at least one SiC barrier layer, the present invention may likewise be used in connection with nuclear fuel particles of comparable physical character/dimensions which contain other materials aside from those listed above. Further information regarding the physical, chemical, and structural character of nuclear fuel materials suitable for testing in accordance with the present invention shall be discussed in greater detail below. Of particular importance regarding the use of nuclear fuel particles (e.g. particles having a radioactive core/center portion surrounded by at least one protective layer and at least one barrier layer) is the physical strength and integrity of each particle with emphasis on the barrier layer. As indicated above, a preferred barrier layer associated with HTGR fuel particles of the type described herein is comprised of SiC. This material is chemically characterized as a moderately brittle ceramic composition. The barrier layer is of particular importance since a significant amount of the strength and structural integrity of each fuel particle is directly attributable to the barrier layer associated therewith. In addition, the barrier layer is designed to retain fission products (e.g. xenon, krypton, carbon monoxide, cerium, cesium, and palladium) within each particle unit during use in an HTGR system. The presence of a weak and ineffective barrier layer in a nuclear fuel particle will diminish the strength/durability of the particle, and will also permit the leakage of fission products outwardly from the particle. For this reason, it is desirable to test the structural integrity of a particle sample before using a particular batch or supply of fuel particles within a selected reactor system. In this regard, the present invention involves a new and unique method for testing nuclear fuel particles as discussed in further detail below. When nuclear fuel particles and brittle ceramic materials therein (e.g. SiC) are tested for mechanical strength, they exhibit a wide sample-to-sample variation in measured strength values. Strength distribution and stress analysis results are also affected by the selected test method. Many prior testing methods have been used to test the strength and structural integrity of "TRISO"-type nuclear fuel particles. For example, strength tests have been conducted using diametrical compression involving rings of SiC barrier layers removed from TRISO particles containing a center region comprised of .sup.235 UCO as discussed in Bongartz, K., et al., "The Brittle Ring Test: A Method for Measuring Strength and Young's Modulus on Coatings of HTR Fuel Particles", J. Nucl. Mater., 62:123-137(1976). Testing as described in the foregoing article involved the production of ring sections from each test particle using parallel cuts through the particle. Compressive force was thereafter applied to each ring section until it fractured in order to generate information regarding stress characteristics of the ring section under consideration. However, in many instances involving the use of this procedure, structural damage occurred to the ring sections during removal from each particle. Ring sections damaged during processing (e.g. cutting and polishing) were thereafter discarded since they could not be effectively tested. As a result, data was lost for many ring sections, especially those having inherent defects or weaknesses which could have generated valuable comparative information. Furthermore, when an individual ring section is tested using diametrical compression, only a small portion of the inner barrier layer (e.g. SiC) associated with each particle is exposed to maximum tensile compression. A particular ring section may represent only 10% of a particle's SiC surface area. In this regard, the area under maximum tensile stress may be about 10% (or less) of the ring section. For any particle being tested, use of the foregoing test procedure will therefore expose only about 1% of a given particle to maximum stress levels. In contrast, when the selected fuel particles are actually used in a reactor system, the entire surface and volume of the barrier layer (SIC) is exposed to maximum stress levels. Another testing technique is discussed in Gilchrist, K. E., et al., "A Technique for Measuring the Strength of High Temperature Reactor Fuel Particle Coatings", J. Nucl. Mater., 43:347-350(1972). This technique involved a probability-based method designed to test the surface and interior volume of the barrier layer (SIC) in each particle. To implement this test, various portions of each test particle were physically removed (e.g. by cutting and the like), ultimately resulting in the preparation of a hollow hemispherical section from the particle. The hemispherical section was then cemented over a small hole in a metal (copper) plate and internally pressurized to determine the amount of pressure necessary to fracture the section. Further information regarding this technique is disclosed in Allen, P. L., et al. "Nuclear Fuel Coated Particle Development in the Reactor Fuel Element Laboratories of the U.K. Atomic Energy Authority", Nuclear Technology, 35:246-253(1977). Finally, an additional method is disclosed in Minkato, K., et al., "Crushing Strength of Irradiated TRISO Coated Fuel Particles", J. Nucl. Mater., 119:326-332(1983). The method disclosed in this reference (hereinafter referred to as the "point load test") involved a crush test designed to determine the strength of selected fuel particles. Specifically, individual particles were positioned between flat platens of hardened steel and compressed between the platens. This method is particularly characterized by a process in which limited portions of the selected fuel particle (e.g. those portions or "points" touching each flat platen) are exposed to stress levels compared with the present invention which more broadly distributes compressive forces. The considerable benefits associated with the broad distribution of compressive forces, as well as further technical and substantive comparisons between both methods will be discussed below. The present invention involves a unique and highly efficient method which is characterized by numerous benefits compared with prior testing methods including but not limited to: (1) the avoidance of potentially-destructive process steps which involve the physical removal by cutting and the like of various sections of the selected fuel particles; (2) an absence of process steps involving the use of adhesive agents or other materials designed to retain various portions of test particles within the selected testing apparatus; and (3) the use of a process which more broadly distributes compressive forces over test particles, thereby resulting in more accurate, complete, and comprehensive data involving structural integrity, stress capability, and the like. In this regard, the present invention provides numerous advantages compared with prior methods in terms of effectiveness, accuracy, and simplicity. For this reason, the invention described herein represents an advance in the art of nuclear fuel testing as discussed in greater detail below. SUMMARY OF THE INVENTION It is an object of the present invention to a method for testing the strength and structural integrity of nuclear fuel particles which involves a minimal number of process steps and testing components/structures. It is another object of the invention to provide a method for testing the strength and structural integrity of nuclear fuel particles which is readily undertaken in a rapid and efficient manner so that large numbers of test particles may be accurately and effectively analyzed. It is another object of the invention to provide a method for testing the strength and structural integrity of nuclear fuel particles which is applicable to a wide variety of different nuclear fuel materials. It is a further object of the invention to provide a method for testing the strength and structural integrity of nuclear fuel particles which avoids the destructive physical treatment of test particles (e.g. the removal by cutting and the like of large sections from each fuel particle). It is a further object of the invention to provide a method for testing the strength and structural integrity of nuclear fuel particles which provides a controlled amount of compressive force to test particles in a highly accurate manner so that complete analytical results may be achieved. It is a still further object of the invention to provide a method for testing the strength and structural integrity of nuclear fuel particles which involves the application of compressive force in a manner wherein the applied forces are widely distributed over each fuel particle. As a result, a significantly greater degree of testing accuracy is achieved compared with other methods including those which involve the application of compressive force to each particle at single points thereon. It is an even further object of the invention to provide a method for testing the strength and structural integrity of nuclear fuel particles which avoids the use of complex testing equipment and chemical fixatives (e.g. adhesives) in the testing process. In accordance with the foregoing objects, the present invention involves a highly efficient and unique method for testing the strength and structural integrity of nuclear fuel particles which are substantially spherical in configuration. In particular, the method described herein is particularly designed to test spherical fuel particles which individually comprise a center region of fissionable nuclear/radioactive material (e.g. .sup.235 UCO) and at least one barrier layer surrounding the center region. The barrier layer provides the nuclear fuel particle with a significant and dominant part of its structural integrity, and also maintains fission products within the particle during use in a selected reactor. An exemplary and preferred composition suitable for manufacturing the barrier layer will consist of SiC. Each fuel particle comprises a hemispherical upper portion and a hemispherical lower portion, with the upper and lower portions being equal in size. In addition, each particle may likewise include at least one protective layer surrounding the barrier layer and/or beneath the barrier layer. Exemplary materials which may be used to construct each protective layer will consist of pyrolytic carbon and equivalent compositions. The protective layer or layers (especially those outside of the barrier layer) may be retained in position during testing of the selected particle or may optionally be removed as discussed below. Regarding the construction materials used to manufacture the fuel particles, the present invention shall not be limited to the testing of any particular fuel materials and compositions associated therewith. Instead, many different types of nuclear fuel particles using different components/materials may be tested with an equal degree of efficiency. In accordance with the invention, the fuel particle to be tested is first placed in a testing apparatus comprising an upper compression member and a lower compression member. In a preferred embodiment, each compression member will consist of a rigid and durable planar structure (e.g. constructed from stainless steel.) The upper compression member will preferably include a first pressure-exerting surface (optimally planar in construction) and at least one first depression therein beginning at the first pressure-exerting surface and extending inwardly into the upper compression member. As discussed in further detail below, the first depression is preferably circular in cross-section and sized to allow only part of the upper portion of the selected test particle therein while preventing entry of all of the upper portion into the first depression. Likewise, the lower compression member will include a second pressure-exerting surface (preferably planar in construction) and at least one second depression therein which is equal in size, shape, and configuration to the first depression. The second depression begins at the second pressure-exerting surface and extends inwardly into the lower compression member. The second depression is preferably circular in cross-section and sized to allow only part of the lower portion of the selected fuel particle therein while preventing entry of all of the lower portion into the second depression. The upper compression member and the lower compression member are positioned within the testing apparatus so that the first pressure-exerting surface faces the second pressure-exerting surface and is parallel thereto, with the first depression in the first pressure exerting-surface being directly above and in axial alignment with the second depression in the second pressure-exerting surface. Thereafter, the selected fuel particle is positioned between the upper compression member and the lower compression member within the testing apparatus, with at least part of the upper portion of the fuel particle being positioned within the first depression in the upper compression member, and at least part of the lower portion of the fuel particle being positioned within the second depression in the lower compression member. As a result, the fuel particle is securely positioned between the upper and lower compression members in a manner which prevents the axial, rotational, and lateral movement thereof without the use of any auxiliary structural components or chemical fixatives (adhesives). Compressive forces may then be applied to the fuel particle in a highly efficient manner wherein the forces are broadly distributed relative to the test particle. As discussed below, the use of a system which allows the broad distribution of forces provides more accurate results compared with systems which distribute forces at discrete and singular points on the test particle. After engagement of the selected particle within the testing apparatus and between the upper and lower compression members, the fuel particle is compressed between the compression members until it fractures. Compression within the testing apparatus is accomplished by movement of at least one of the upper and lower compression members toward and against the fuel particle at a preferred rate of about 0.002-0.004 inches per minute. Finally, the amount of compressive force which was needed by the testing apparatus to fracture the fuel particle is measured and recorded. This value provides important and accurate information regarding the strength and structural integrity of the fuel particle being tested. In addition, the force value necessary to fracture the particle may thereafter be mathematically converted into a tensile strength value which can then be plotted, characterized, and/or interpreted using one of many different standard statistical approaches. An important aspect of the foregoing process involves proper formation of the first and second depressions within the upper and lower compression members. In addition, the testing apparatus should be configured to achieve the precise axial alignment and overhead orientation of the first depression relative to the second depression. The first and second depressions may be formed within the upper and lower compression members by manual processes including but not limited to machining and/or drilling of the upper and lower compression members as desired. Orientation of the first and second depressions may also be accomplished manually by selective manipulation of the upper and lower compression members within the testing apparatus. In an alternative embodiment, the first depression and second depression are formed in precise axial alignment by initially providing a depression-forming spherical member having a hardness level which exceeds that of the upper compression member and the lower compression member. As a result, deformation and/or fracturing of the spherical member is prevented when the spherical member is compressed between the upper and lower compression members as discussed below. For example, if the upper and lower compression members are constructed from stainless steel, an exemplary and preferred composition suitable for producing the depression-forming spherical member will consist of zirconia. To properly form the first and second depressions, the spherical member is positioned within the testing apparatus and placed between the upper and lower compression members. Thereafter, the spherical member is compressed between the upper compression member and the lower compression member until it is pressed inwardly into the upper and lower compression members. As a result, the first depression is formed within the upper compression member and the second depression is formed within the lower compression member in a manner wherein the first depression is directly above and in precise axial alignment with the second depression. The spherical member is then removed from the testing apparatus, followed by insertion and testing of the selected fuel particle using the steps described above. This alternative method ensures that the first and second depressions are formed in a highly exact and accurate manner without the need for precise pre-testing alignment procedures. The present invention enables highly accurate results to be achieved while avoiding the use of complex and intricate testing processes. In addition, implementation of the invention enables a broader distribution of compressive forces to each test particle so that widely-distributed flaws within the particle have a greater chance of detection and characterization compared with narrow-distribution compression systems. These benefits are achieved without removing portions of the test particle by cutting or other disruptive physical processes which can introduce additional flaws into the particle prior to testing. Finally, the present invention enables precise and secure immobilization of the fuel particle being tested without the use of extensive equipment and chemical fixative materials (e.g. adhesives). In this regard, the invention represents an advance in the art of nuclear fuel technology, and enables the testing of nuclear materials in a highly advanced and efficient manner. These and other objects, features, and advantages of the present invention will be described below in the following Brief Description of the Drawings and Detailed Description of Preferred Embodiments.