Nuclide inventory calculation based on modeled fuel assembly specifications and burnup histories for Fukushima Dai-ichi NPP Units 1–3

Abstract

ABSTRACT In order to evaluate heterogeneity in nuclear inventories in the fuel debris of Units 1–3 of the Fukushima Dai-ichi Nuclear Power Plant (NPP), detailed burnup calculations of fuel assemblies were performed. Applying publicly available information, specifications were modeled for the five different types of fuel assemblies loaded in the cores before the shutdown for the accident. The assembly burnup histories reported by Nishihara et al. were used to develop power histories at 24 axial nodes of the fuel assemblies adopting the measured data on the axial burnup profiles of similar fuel assemblies. A correlation between axial node power and an in-channel void fraction was derived from the typical BWR core simulation analysis results and applied to the above-mentioned axial node power histories to obtain the in-channel void fraction histories. Then the burnup and cooling calculations were performed with CASMO5 and ORIGEN2.2 for each batch of fuel assemblies with the same irradiation periods. The calculated results were used to estimate the locally maximum contents of the nuclides such as 235U, 238Pu, 239Pu, 240Pu, 241Pu, 241Am, 242Cm, 243Cm, 244Cm, 90Sr, 134Cs, 137Cs, and 154Eu, which are important in the safety evaluation of the process of the disposal of the fuel debris.