Patent Number: 042960746
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention provides a method for the treatment of an assembly comprising a cladding material and a core of uranium, thorium or mixtures thereof to separately recover the cladding material and the core. The method is particularly applicable to the treatment of a nuclear fuel element comprising a cladding material containing a metallic fuel such as fissile or fertile uranium, thorium and combinations thereof. The cladding material generally comprises a stainless steel which consists principally of iron alloyed with chromium and containing minor amounts of other metal additives. The present invention also is applicable to zirconium or zirconium alloy cladding materials. The zirconium alloy generally consists principally of zirconium and contains minor amounts of one or more alloying materials such as nickel, chromium, tin or iron. For convenience, the present invention will be described with respect to its particularly preferred application, namely, the treatment of a nuclear fuel assembly. Referring to the sole FIGURE, the fuel assembly first is introduced into a cladding piercing zone 10 where at least a portion of the cladding material is pierced, perforated, scored, sheared or the like to at least partially expose the thorium or uranium core. The precise mechanical means used to accomplish the exposure of the core is not particularly critical. However, it generally is preferred to expose at least a portion of the surface of the core at intervals of about 1/2 to 11/2 inches throughout the length of the fuel assembly. The perforated or scored fuel assembly is next introduced into a hydriding-dehydriding zone 12. It will be appreciated that both the piercing of the cladding and the hydriding-dehydriding could be accomplished in a single zone; however, in accordance with the particularly preferred embodiment set forth herein, each operation is performed in separate zones. In hydriding-dehydriding zone 12, the assembly is reacted with hydrogen at a hydrogen pressure of from about 0.5 to 2 atmospheres (360 to 1400 torr) and preferably at about one atmosphere (760 torr). Lower pressures substantially reduce the hydriding reaction rate and at higher pressures the reaction rate is not significantly increased. The temperature during the hydriding reaction is maintained within a range of from about 400.degree. C. to 650.degree. C. When the fuel is uranium, the temperature is optimally maintained within a range of about 450.degree. C. to 600.degree. C. and for thorium, a temperature range of from 500.degree. C. to 650.degree. C. is preferred. In accordance with the particularly preferred embodiment, when the core of the assembly comprises uranium, thorium or mixtures thereof, the temperature is cycled during the hydriding process such that the core is exposed to both an optimum hydriding temperature and an optimum hydrogen diffusion temperature. The time required to achieve substantially complete reaction will vary, of course, depending upon the size and shape of the metallic thorium or uranium core as well as the amount of surface area of the core exposed in the cladding and piercing operation. Generally, when the assembly is treated in accordance with the preferred conditions set forth herein, it is found that the reaction is substantially complete in a time of from about 15 to 90 minutes. Following the hydriding reaction, the temperature in zone 12 is increased to from about 700.degree. C. to 900.degree. C. to decompose the hydride to elemental metal and release the hydrogen which is withdrawn via a pump 14. When the fuel assembly is one which has been irradiated and contains gaseous fission products, the hydrogen withdrawn preferably is introduced into a volatile removal zone 16 where the gaseous stream is treated, for example by condensation, to remove a major portion of the volatile fission products. It is a particular advantage of the present invention that forming the hydride and then dehydriding the product so formed, releases substantially all of the volatile fission products. In addition, during the hydriding step, the hydride is formed as discrete particles of substantially increased volume. These particles of increased volume tend to split or rupture the cladding material and increase the size of the openings therethrough such that after the subsequent dehydriding, the core material is left in the form of small, friable discrete particles which are readily recoverable from the assembly by simple mechanical means such as sieving or mechanical agitation to separate the particulate core material from the larger substantially intact pieces of cladding material. Advantageously, the hydriding-dehydriding step is repeated at least once and preferably twice to ensure complete release of any volatile fission products present as well as to ensure that substantially all of the elemental core material has been exposed to and reacted with hydrogen at least once. The resulting particulate fuel material is readily processible to produce new fuel assemblies by enrichment, if necessary, and sintering or arc casting to reform pellets of the fuel. Thus, it is seen that the present invention provides a method for treating such assemblies without the necessity of complex and expensive gaseous or liquid phase processing. Further, in accordance with the present invention, any plutonium which may be present is never isolated but remains with the fuel in such a dilute form, however, as to substantially negate the possibility of it being used in the production of a nuclear weapon. The following example is set forth to more clearly illustrate a specific embodiment of the present invention as applied to the decladding of a nuclear fuel element and recovery and separation of the valuable constituents of the core from the undesirable radioactive gaseous fission products. EXAMPLE The purpose of this example is to demonstrate the method of the present invention to (1) declad the fuel, (2) cominute the fuel so it will fall free from the cladding, (3) release the volatile fission product, and (4) restore the fuel to its initial chemical form (i.e., metal). To determine the ability of the present method to release volatile fission products without the necessity of using radioactive materials, it was determined to monitor the radon evolved during the tests. Radon is a decay daughter of thorium, uranium and plutonium that is produced in situ within the fuel, just as xenon and krypton is produced during fission. Calculations indicate that the radon contained in one gram of metallic thorium that had decayed for a year since it was arc cast was sufficient to produce several hundred disintegrations per minute. Therefore, monitoring the radon radioactivity when thorium is pulverized in accordance with the present method would provide an excellent measure of the amount of volatile fission products which would be released during the treatment of the irradiated thorium fuel. A simulated fuel assembly was built which comprised 1/4.times.1/4.times.3-inch square strips of thorium which were rounded and cut into 1/2-inch lengths to simulate fuel pellets. The pellets were loaded into a 4-inch long.times.1/4-inch O.D. piece of stainless steel tubing which had a wall thickness of 0.112 inches. After the pellets were loaded, the tubing was crimped on each end and a 1/8-inch diameter hole was punched in the tubing at 1-inch intervals along one side. The simulated clad fuel assembly was then treated at various hydriding-dehydriding conditions in accordance with the present invention. After treatment, the assembly was removed from the reaction chamber and examined. The conditions and results are set forth in the following table. For the tests in the table, it was found that the thorium hydriding and dehydriding temperatures appear to be most rapid around 600.degree. C. and 900.degree. C., respectively. Based on the differential hydrogen pressure in the system, it appears that maximum hydrogen absorption reaction occurred around 600.degree. C. Hydrogen pressure in the closed system increased as a result of the initial heatup of thorium pellets to 350.degree. C. At 350.degree. C., the pressure leveled off and then decreased slowly with continued heating. The decrease in pressure became more pronounced in the 600.degree. C. range and continued to decrease (indicating continued hydrogen absorption and reaction) until a temperature of about 680.degree. C. was reached. Sharp pressure increases were observed when the temperature was increased to above 700.degree. C. Maximum pressure increase was obtained at about 900.degree. C. Thus, hydriding occurs between 350.degree. C. and 680.degree. C. for thorium and is most rapid around 680.degree. C. while dehydriding occurs above 700.degree. C. and is most rapid around 900.degree. C. Complete pulverization of the thorium metal by repeated hydriding and dehydriding (three cycles) was readily achieved. Comminution of the metal to less than 400-mesh without mechanical treatment was not achieved. However, the dehydrided metal is extremely friable and readily comminuted to a size of less than 400-mesh by ball milling, pressure screening or the like. TABLE __________________________________________________________________________ PROCESSING OF CLAD THORIUM PELLETS Radon Released* Pressure (Torr) Hydriding Dehydriding % of Accum Series Test No. H.sub.2 Total# Temp .degree.C. Time (hr) Temp .degree.C. Time (hr) Cts Total % __________________________________________________________________________ 1 a 380 760 520 0.25 850 0.33 45 1 -- b 380 760 600 1 800 1 20 Nil -- c 380 760 560 0.5 800 0.25 4 Nil 1 d 380 760 700 0.25 1000 0.25 Nil -- 1 e 380 760 -- -- -- -- 80 1 2 f 380 760 500 0.5 700 0.25 30 Nil 2 __________________________________________________________________________ 2 a 760 760 525-600 2 800 0.25 60 1 3 b 550 550 -- -- -- -- 170 2 5 c 550 550 500-660 3 700-800 1 2350 24 29 d 550 550 570 0.5 810 0.25 Nil -- 29 e 550 550 660 0.5 710 0.25 Nil -- 29 f 450 450 400-700 3 950 1.0 1500 15 44 g 500 500 -- -- 800 0.5 Nil -- 44 h 150 150 -- -- -- -- Nil -- 44 i 900 900 400-650 2 -- -- 750 7 51 j 400 400 -- -- -- -- -- -- -- Cooled to Room Temperature, Disassembled, Cladding __________________________________________________________________________ Inspected 3 a 900 900 550 2 -- -- 700 7 58 b 900 900 400-600 2 900 0.3 1600 16 74 c 900 900 500 1.5 800-900 1.0 900 9 83 d 900 900 650 0.25 870 0.25 Nil -- 83 e 1000 1000 500 0.5 800-900 1.00 400 4 87 f 640-1000 640-1000 500 0.25 900 0.25 1200 12 99 g 1000 1000 460-560 0.5 750 0.25 Nil -- 99 h 1000 1000 -- -- -- -- -- -- 99 Cooled to Room Temperature and Disassembled __________________________________________________________________________ *Total counting rate of radon if completely released = 9850. #Balance of gas was argon. Substantial amounts of radon were involved during the hydriding-dehydriding of the fuel. The radon counting rate in the hydrogen increased rapidly above 400.degree. C. to a maximum at a temperature of about 900.degree. C. The radon appeared to evolve during both the hydride and dehydride portion of the cycle. When the foregoing example is repeated, using uranium clad in stainless steel, zirconium or a zirconium alloy, or a mixture of uranium and thorium clad in such alloys, substantially the same results are obtained. Specifically, the uranium, thorium or mixture thereof is reduced to a fine friable particulate form and the cladding material is sufficiently ruptured by the hydride form, so that on subsequent dehydriding, the particles are readily removable from the cladding by mechanical means. It is readily apparent that the present invention provides an economical, safe and easy to operate method for the recovery and separation of uranium, thorium or mixtures thereof from a cladding material. While the foregoing example and description exemplify what are presently considered to be the preferred embodiments of the invention, it will be appreciated that many changes might be made in the embodiments described. The application of the method of the present invention to other elements clad or sheathed in various metals also will be readily apparent. Thus, the foregoing description is to be construed and interpreted as illustrative only and not in a limiting sense; reference being had to the claims for such latter purpose.