Patent Number: 046876006
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a treating process for the separation of coated nuclear fuel particles from a graphitic matrix in which the nuclear fuel particles are embedded, as well as to an installation for the conditioning of graphitic fuel elements of high-temperature nuclear reactors. Nuclear fuel particles are worked out of a graphitic matrix during the reconditioning of spent fuel elements of high-temperature nuclear reactors for the reconditioning of the nuclear fuels. For this purpose, there is required a breaking open of the fuel elements with the subsequent separation of the exposed nuclear fuel particles which contain the nuclear fuels, prior to the particles being chemically dissolved and cleaned. For graphitic fuel elements there must be taken special care that the graphite, which constitutes about 90 to 95% of the quantity of the fuel element, must be removed and the coated nuclear fuel particles which are embedded in the graphitic matrix are isolated. The comminuting of the fuel elements, the separation of the nuclear fuel fraction and graphite fraction, the removal of the graphite mass and a chemical dissolution of the nuclear fuel particles are consequently the process steps which characterize the initial stage of a reconditioning installation for fuel elements. The initial stage is designated as the "head-end" of a reconditioning installation. 2. Discussion of the Prior Art During the breaking open of the graphitic fuel elements there is aimed at conducting the comminuting step so that subsequently thereafter there becomes possible an extensive quantitative separation between graphite and nuclear fuel particles. The utilization of known comminuting aggregates, such as a hammer mill, jaw crusher, and roll crusher, does, however, deliver an extremely heterogeneous ground product from which only about 60 to 70% of the ground material can be sifted off as nuclear fuel-free graphite; referring to the report GULF-GA-A No. 10 784,1971. Upon the exposure of the graphitic matrix with the integrated nuclear fuel particles in roll crushers, during the subsequent sifting of the ground product, transmitted in an undesired manner are also fragments of the nuclear fuel particles within the separated graphite fraction, which further reduces the economics of the already extremely complex reconditioning process. The effectuation of a controlled comminuting step in which the graphite is essentially destroyed, and wherein the coated nuclear fuel particles, insofar as they include hard coatings, such as, for example, silicon carbide(SIC) shells, or the nuclear fuel particles themselves are to remain substantially preserved, is also a goal for a head-end stage in which the graphite is burned in a high-frequency vortex furnace, before the nuclear fuel particles are chemically dissolved. However, independently thereof, that during such process step, besides the emission of volatile fission products into the environment, there are encountered additional problems due to the therewith connected emission of radioactive carbon(carbon isotope C-14), through the employment of jaw crushers and subsequently used roll crushers, there is also only produced a ground product which consists of up to 87% of intact nuclear fuel particles, up to 5% of fragments of nuclear fuel particles, and up to 8% of nuclear fuel particles which are still coated with SiC; refer to H. M. Muller, "Head-End Processes for HTGRBE", NUKEM-160, 1973. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a novel treating process for the breaking open of the fuel elements and the separation of the nuclear fuel particles, wherein the nuclear fuel particles are removable undestroyed from the graphitic matrix and are separable from the graphite. The foregoing object is achieved pursuant a process according to the above-mentioned type and in accordance with the invention wherein the graphite which encompasses the nuclear fuel particles is conducted away through the action of a brush which isolates the nuclear fuel particles together with their coatings or the nuclear particles themselves, and wherein the nuclear fuel particles which are contained in the brushed product are then separated from the comminuted graphite. The nuclear fuel particles are worked out of the graphitic matrix through the action of a brush whose bristles conduct away the graphite, however, in which the coated nuclear fuel particles insofar as they contain, for example, hard coatings such as SiC shells, or the nuclear fuel particles themselves, remain preserved. Proven as being suitable therefor has been, for instance, a brush with bristles formed of steel. In accordance with the particular requirements, other materials come also into consideration for the bristles as long as during their application the surface of the nuclear fuel particles are not excessively worn off. The worked out nuclear fuel particles allow themselves to be separated in a simple manner from the comminuted graphite fraction, for example, through the delivery of the brushed product into a sifter which is passed through by a separating gas, in which the fine-grained fraction is conveyed away together with the gas stream. In a further embodiment of the inventive working process, the nuclear fuel particles which are isolated during sifting are exposed in a crusher and/or in a mill. Depending upon conditions, the graphite residues which have not yet been conveyed away during brushing and in which there are still contained nuclear fuel particles, are concurrently conveyed into the crusher and/or the mill. After the sifting of the ground product, the fragments of the nuclear fuel particles are chemically dissolved, the separated graphite masses conditioned and stored. Preferably, the inventive process is utilized in an installation for the preparation of the graphitic fuel elements of high-temperature nuclear reactors in a head-end stage of a reconditioning installation. The installation includes a brush device with a rotably driven brush and pressure device for the fuel elements to be reconditioned which is arranged so as be movable against the bristles of the brush, in which the nuclear fuel particles are embedded in a graphitic matrix. The bristles of the brush are hereby so constructed that during rotation of the brush, the graphite which encompasses the nuclear fuel particles is worn off without damaging, or at most only slightly damaging the coated nuclear fuel particles, insofar as they evidence SiC shells, or the nuclear fuel particles themselves. Preferred is a brush having bristles constituted of steel. During the rotation of the brush, the steel bristles disintegrate the graphite and work the nuclear fuel particles, whose surface is harder than the graphite, out of the graphitic matrix. The coated nuclear fuel particles are hereby not exposed to either a sudden striking or impact effect or to a high pressure load, as in crushers or mills. In an advantageous manner, also for SiC coated nuclear fuel particles the hull or shell fracture rate remains low. At the outlet end of the brush arrangement there is connected a pneumatic aspirating arrangement for the brushed product. In order to maintain the graphite quantity which is to be worked out in the brush device as low as possible during the reconditioning of fuel elements, the installation includes a device located ahead of the brush installation for the conducting away of the outer, nuclear fuel-free graphite regions of the fuel elements. Connected downstream of the brush device is a sifter by means which the nuclear fuel particles are separated from the comminuted graphite in a separating gas stream; as well as a jaw crusher and/or a roll crusher for the exposure of the nuclear fuel particles. Leading to the jaw crusher and/or to the roll crusher is also a transport conduit for the graphite residues which have not been carried away during the brushing step, and which contain nuclear fuel particles. Following the above-mentioned aggregate of comminuting sequences is again a sifting of the ground product. The thusly recovered nuclear fuel particle fraction is thereafter conducted to a chemical solution, the separated graphite mass together with the remaining previously separated graphite is conditioned and stored.