U-Th system mixed oxide fuel

Thorium oxide has the results of actually used as nuclear reactor fuel in LWRs, LWBRs, HTGRs and so on as a part of the development of 233U-Th cycle. In the case of HTGRs, the development tests on large scale have been carried out in USA and Germany aiming at the full scale development of 233U-Th cycle. The LWR fuel is in the form of the mixture of UO2 and ThO2, while the HTGR fuel is in the form of the mixture in Germany, and in the form of two-particle separation of UCO and ThO2. Japan Atomic Energy Research Institute has carried out the researches on the production of the fine particles of thorium oxide, their characteristics, their irradiation behavior and the production of high density thorium oxide pellets. The results related to the irradiation of the recently carried out research are reported. When high energy iodine ions were implanted in a sample surface, the phenomena like high burnup occurred. The state of this surface is introduced. The irradiation test on the coated particle fuel of thorium oxide, the observation of the structure after the irradiation and the measurement of fission product distribution were carried out. The change of lattice constant due to burnup of thorium oxide with additives and others were examined. Cs diffusion coefficient was calculated from Cs-137 release measurement. (K.I.)