Patent Number: 041349415
Section: summary

The invention concerns a new type of pressed spherical fuel element made of graphite for high temperature reactors consisting of a graphite nucleus (or core) containing only fertile (breeder) particles, a graphite shell containing only fuel particles and a further outer shell of pure graphite and an especially advantageous process for reprocessing this fuel element after the irradiation in the reactor. The three layers of the fuel element are concentric. Spherical graphite fuel elements are necessary for gas cooled high temperature reactors. They usually consist of a fuel and fertile material containing spherical nucleus which is surrounded by a fuel free shell (Hrovat German Offenlegungsschrift 1,646,783). The graphite matrix, i.e., the graphite material of the nucleus and shell is identical. The fuel element diameter generally is 60 mm and the thickness of the shell 5 mm. In the known spherical graphite fuel elements the spherical nucleus contains in homogeneous distribution the fuel or fertile material in the form of spherical particles. To retain fission products the particles are surrounded by a multiple layer of pyrolytic carbon, in a given case with an intermediate layer of silicon carbide. There is added as fuel Uranium 235 and as fertile material Thorium 232 in the form of the carbide or oxide. Thereby in the so-called THTR-element, the standard spherical fuel element of the thorium high temperature reactor, fuel and fertile material jointly are provided for in the same particles, in the so-called feed-breed-element, however, they are separated in discrete particles which are distributed mixed together in the nucleus of the sphere. In the uranium-thorium cycle there is sought to be obtained from the thorium the especially valuable Uranium 233 in as pure as possible condition and without being admixed with other uranium isotopes because of its high fission neutron yield. For this reason there has been tried the separation from each other of the fuel and fertile material particles in the reprocessing of the irradiated fuel elements. As the best suited process for reprocessing graphite HTR-fuel elements there has proven conbustion (Atomwirtschaft Vol. 18 (1973) page 294 and Kerntechnik Vol. 15 (1973) page 249.) According to the state of the art today there cannot be satisfactorily attained a separation of fuel and fertile material. The processes which depend upon a sieve separation of the smaller fuel particles from the larger fertile material particles after burning off of the pyrolytic carbon coatings still have the danger of a contact contamination of the different particles with each other. Furthermore a part of the material is lost for the separation because weakened by the burning off and irradiation are broken. Also the possibility of protecting one of the types of particle (preferably the burned off particles) by an unburnable SiC intermediate layer still includes a number of disadvantages, namely, the increase in expense of fabricating the fuel element through additional coating costs, the deterioration of the neutron economy in the reactor, the danger of the Uranium 233 contamination with Uranium 235 in the particle breaking and the increase of the radioactive waste. Especially there cannot be avoided in the burning that a part of the irradiation weakened, SiC-coated particles disintegrate after burning off the outer pyrolytic carbon layer and thereby there occurs a mixing of Uranium 235 and Uranium 233. These disadvantages are avoided by the spherical fuel element of graphite of the invention which is characterized by a nucleus (core) which only contains fertile material particles, surrounded by a graphite zone which only contains fuel particles and an outer shell of pure graphite.