Patent Number: 039883973
Section: summary

For high temperature power reactors a uranium-thorium fuel cycle is used with the object of recovering the uranium 233 bred from the thorium as well as the thorium which was not exhausted to gain it in the reprocessing plant and to use it again as a fuel after refabrication. At the same time the recovered uranium 233 replaces a part of the otherwise necessary uranium 235. The atom ratio of uranium to thorium lies between 1:5 to 1:20 in the reactor core in the case of known fuel element types. The arrangement in the fuel element is selected in such a way that the coated fuel particles are available from pure uranium 235 or uranium 233 as well as the coated particles from thorium which contain only a few percent by weight of uranium. Since the valuable uranium 233 is recovered practically only from particles containing thorium, one must have already strived during the head-end process for reprocessing to separate the coated particles containing uranium from those containing thorium, in order to obtain an uranium 233 as pure as possible, i.e., an uranium free of other uranium isotopes. This object can be achieved only in an unsatisfactory manner in the known block fuel element concepts. Since the coated particles containing uranium and thorium in the case of the concepts which have been known hitherto, are used as a mixture, a simple separation is not possible particularly because they are connected together in the fuel particles by the graphite matrix. Suggestions of producing the coated particles containing uranium and thorium in variable sizes, in order to be able to separate them mechanically, have not led to any satisfactory result either, because such block fuel elements must be mechanically comminuted. In the certainly necessary mechanical comminution of the blocks, and in the burning of the graphite in the head-end step, a high percentage of the particles is destroyed so that even in the case of a variable size of the particles no satisfactory separation of the uranium containing particles from the thorium containing particles is possible. The mechanical comminution of the block fuel elements and the subsequent separation of the graphite from the coated particles therefrom make the reprocessing considerably more difficult. The present invention by-passes the above mentioned difficulties. In published German Offenlegungsschrift 1,902,994, Sept. 24, 1970, and Hrovat U.S. application Ser. No. 218,244, filed Jan. 17, 1972 now abandoned, (corresponding to published German application P 21 04 431.5) entitled "Process For The Production Of Block Fuel Elements For Gas Cooled High Temperature Power Reactor", there is described a monolithic block fuel element with pressed in cooling channels. The entire disclosures of said German applications and said U.S. application Ser. No. 218,244 are hereby incorporated by reference. As described in those applications the block fuel element is a compact prism consisting only of a homogeneous graphite matrix. The coated particles are embedded in the graphite matrix and constitute the fuel zones whereby the fuel material in the coated particles either can be a homogeneous mixture of uranium oxide and thorium oxide (or carbide), same in each particle, or otherwise each fuel zone contains a mixture of uranium particles as feed material and thorium particles as breed materials. At the same time it is essential that these zones be connected with the remaining graphite matrix without transition and that they produce the actual fuel element structure. The fuel zones themselves are the integrated components of the block element and they contribute substantially to the strength of the fuel element. From this the possibility results of making the fuel zones substantially larger and designing them with more flexibility. The high mechanical integrity of the pressed block element achieved thereby permits attachment of the uranium containing coated particles separate from the thorium containing particles in the form of fuel columns located side by side in a so-called heterogeneous arrangement (according to FIGS. 1 - 3). In the previously known fuel element types made of prefabricated and drilled electrographite blocks, such an arrangement is not possible, since such a block has been too greatly weakened in its strength as a result of the large number of bores. Therefore the object of the invention is the arrangement of feed and breed zones in a block of a fuel element, in which these zones contribute to the mechanical strength of said block, arranging said zones in such a way that they can be selectively drilled out of this block. Thereby the thorium containing breed particles and the uranium containing feed particles can be taken separately from the structural graphite. As a result, the burning of the graphite, which raises a difficult exhaust gas problem, is essentially avoided. Therefore, the thorium containing particles can be supplied free of feed particles for the recovery of uranium 233. The feed zones and breed zones can be separated independently from the structural graphite prior to the processing operation. In the reprocessing, uranium 233 is produced from the breed particles and uranium 235 and fission products are produced from the feed particles. The feed and breed arrangement can be selected differently, dependent upon the power density of the core and the useful life of the fuel element, as well as on the conversion rate striven for.