At the time of a reactor scram, the control rods are inserted into the high-temperature reactor core, where they are exposed to a thermal load. When a high-temperature engineering test reactor (HTTR) is constructed, the control rod temperature is evaluated conservatively because there is no way to accurately measure the actual temperature. In this study, to measure the temperature of the control rods in an HTTR, we installed a “melt wire” made of alloys with various melting points in the tips of the control rods. After experiencing a reactor scram at 100% power, the melt wires were inspected visually. The melt wires clearly showed a molten state. The result of the visual inspection claritied that the highest temperature reached at the tip of the control rods was in the range of 505 to 651℃. The temperature measurement technique established by this study will lead to the improved accuracy of temperature estimation by design calculation and structural integrity assessment of the components in an HTTR.

　A study is performed on a molten salt fast reactor (MSFR) of 1.5 GWe output. The reactor is started up by using transuranium elements reprocessed from spent fuel of a BWR. The fuel salt of the reactor is the mixed fluoride salt NaF–KF–UF₄–TRUF₃, which is reprocessed almost continuously by an oxide-precipitation process during the reactor operation. By performing calculations using the nuclear analysis code PIJ–BURN in SRAC2006 and the nuclear data file of JENDL–3.3, the following results are obtained. (1) The burn-up characteristics of the reactor are mainly determined by the uranium inventory (U_inv) in the reactor and the reprocessing cycle (L_rep), which is the time interval necessary to reprocess all the fuel salt in the primary loop. (2) A large U_inv and short L_rep enhance the breeding performance of the reactor. (3) The period necessary to keep the radioactive waste under control will be about 400 years in the case of L_rep longer than 400 efpd. (4) Power stations consisting of 20 MSFRs (total output of 30 GWe) can operate for 600 years by utilizing 14,000 t of uranium obtained from the spent fuel of LWRs in Japan.

　X-ray absorption fine structure (XAFS) and imaging XAFS analyses were performed to elucidate the chemical state of rhodium in simulated waste glass. The chemical forms of Rh in the glass were evaluated to be 84% RhO₂ and 16% metal/alloy as the result of linear combination analysis of EXAFS data. According to the imaging XAFS analysis, the chemical form of Rh that was located together with Ru was mainly oxide (RhO₂). This suggests that a stable (Ru,Rh)O₂ solid solution exists in the simulated glass. On the other hand, that of Rh whose distribution did not coincide with Ru in the glass was mainly metallic. The metallic Rh in the glass tended to become an aggregate form. It can be concluded that the chemical state of Rh was strongly affected by the existence and distribution of Ru.

　In TEPCO’s Fukushima Dai-ichi Nuclear Power Station accident on March 11, 2011, a massive number of radionuclides were released from Unit–1, Unit–2, and Unit–3 to the environment. After the accident, the effects of multiple sources became one of the major topics of discussion in safety assessment. However, these discussions mainly focused on core damage frequencies. In the case of simultaneous accidents at multiple units, it is important to consider not only core damage frequencies but also risks because radionuclides are released from multiple units to the environment. In this study, we analyzed dominant parameters that affect health in the case of multiple sources with a simplified configuration of units. The analytical results show that the probabilities of early health effects near the site boundary depend on the locations of units and the wind direction at the site. They also show that the probabilities of early health effects due to releases from multiple units cannot be calculated by the arithmetic summation of probabilities of early health effects due to releases from single units because, for example, of the nonlinearity due to the threshold of the occurrence probability of early health effects.