To reveal the melting behaviors of core internals mechanistically and to reduce the uncertainties of existing severe accident analysis codes, a numerical simulation code for melt relocation and accumulation behaviors based on computational fluid dynamics named JUPITER has been developed by JAEA. In this paper, we performed a simulation of the accumulation and spreading of a melt to the pedestal region of a typical BWR containment vessel by JUPITER to consider a method for estimating the fuel debris composition. We performed recriticality analysis by the continuous energy neutron transport Monte Carlo code MVP using detailed fuel debris composition data obtained by JUPITER to evaluate recriticality for fuel debris. It was revealed that JUPITER has the potential to obtain a complicated fuel debris distribution mechanistically. Also, in effective multiplication factor analyses, we investigated the effect of parameters (uranium enrichment, water content ratio and partitioning resolution in MVP analysis) on the effective multiplication factor. It was also revealed that the partitioning resolution is one of the most important factors in JUPITER-MVP coupled analysis, and an appropriate partitioning according to the inhomogeneity of the fuel debris distribution obtained by JUPITER will be very important.

　Following the accident at the Fukushima Daiichi Nuclear Power Plant in 2011, radioactive nuclides caused serious contamination. In particular, cesium (Cs) and iodine (I) are the most important fission products (FPs) because they are easily released due to their high vapor pressure. However, their release behavior from nuclear fuels during the accident has not been clarified. Understanding such behavior can contribute to improving the accuracy of the source term evaluation. Simulated nuclear fuels containing non-radioactive FPs can be used in laboratory experiments to understand such behavior. However, simulated nuclear fuels containing Cs and I are difficult to synthesize because of their high volatility. Here, cerium dioxide (CeO₂)-based simulated fuels containing cesium iodide (CsI) are synthesized by spark plasma sintering, allowing us to obtain bulk samples rapidly at low temperatures compared with those of conventional sintering methods. CeO₂ is used to simulate uranium dioxide (UO₂) owing to its similar chemical and physical properties to those of UO₂. The obtained simulated fuels are characterized by X-ray diffraction and scanning electron microscopy/energy dispersive X-ray spectrometry. CsI is confirmed to exist as small precipitates almost uniformly distributed throughout the CeO₂ matrix. The optimized conditions to synthesize the simulated fuels are proposed.

　Tokyo City University Atomic Energy Research Laboratory was established in 1960 and operated the research reactor “Musashi Reactor” (TRIGA-II type, 100 kW) for 25 years from January 1964 until December 1989. At present, this reactor is being decommissioned. In 2013 the institute decided to introduce a 1.7 MV pelletron tandem accelerator (TCU-tandem). In this paper, we describe the construction of this accelerator system. Among the work, we emphasize the conditioning of the accelerating tube, which had been kept for a long time without operation. The construction was implemented together with students of a nuclear engineering course and was utilized as education and training for them. As applied research using this tandem accelerator, we are planning to extract MeV proton beams and carry out elemental analysis by particle induced X-ray emission(PIXE)/Rutherford backscattering spectroscopy(RBS) methods. Taking advantage of the feature that TCU-tandem is located in a research reactor facility being decommissioned, material analysis related to the decommissioning process will be carried out.