Study on the Fast Reactor System Characteristics in the Transition Period from LWR to FR

In fast reactor (FR), the amount of nuclear fuel is increased by conversion. The conversion is caused by the absorption of neutrons, and therefore Pu isotopes are used as fuel because they generate more neutrons by fission reaction than other fissile such as uranium-235. The spent fuel from the FR mainly contains a relatively large amount of TRU because FR uses Pu as fuel.Since TRU has a long-term heat generation, it is assumed that all TRU in the spent fuel of FR are reused. This is called the closed nuclearfuel cycle. In the closed nuclear fuel cycle, all the actinoids are extracted in reprocessing spent fuel, and only FP is disposed as radioactive waste.When shifting from using light water reactor (LWR) to FR, it is supposed that TRU extracted from spent fuel from LWR will be loaded into initial stage of FR operation. This is called the transition period. The TRU composition of the fuel loaded in the transition period can be significantly different from that in the equilibrium period, and this difference can affect both core and waste characteristics of FR. In this study, we will quantify the difference in core and waste characteristics between these two periods.In the transition period of FR, it is also envisioned that MOX fuel will be loaded in LWR. Thus, in this study, we will quantify the differences in the core and waste characteristics of FR when using TRU originating from UO2 fuel and MOX fuel in LWR during the transition period. In addition, when considering the difference between UO2 and MOX during the transition period, it should be also noted that TRU composition in the fuel loaded in FR can be greatly dependent on the operating conditions of LWR and reprocessing conditions of LWR fuel. Therefore, in this study, for each of UO2 and MOX, we will quantify how much TRU composition can be varied due to the difference in various conditions of LWR, and how much the core and waste characteristics of FR are affected by it. (author)