Passive Gamma Spectrometry of Low-Volatile FPs for Accountancy of Special Nuclear Material in Molten Core Material of Fukushima Daiichi Nuclear Power Plant -Detectability of leakage gamma-ray and fundamental characteristics of U-Ce/Eu compound-

For nuclear material accountancy of molten core material in Fukushima Daiichi Nuclear Power Station units 1, 2 and 3 (1F1, 1F2 and 1F3), a variety of technologies are being evaluated from the viewpoint of applicability for in-situ measurement. For one of the technologies, a feasibility study of passive gamma spectroscopy of low-volatile fission products (FPs) for nuclear material accountancy in molten core material has been performed by reviewing Three Mile Island Unit 2 (TMI-2) experience, and studying the correlation of actinides and FPs inventory in the cores of 1F1, 1F2 and 1F3, considering the sensitivity of axial neutron spectrum, void, burnup, enrichment distribution unique to BWR fuel, and leakage gamma-ray from molten core materials sphere model [1,2,3,4]. In this paper, numerical simulation of leakage gamma-ray from molten core materials in a hypothetical canister is dealt with for determination of radioactivity of low-volatile high-energy emission FPs, which could be utilized for special nuclear material (SNM) quantity estimation coupled with SNM/FPs ratio derived from core inventory calculations. The model of canister is assumed based on the fuel type one in used TMI-2 for gamma-ray leakage calculation, with 3 main geometrical regions; canister, surrounding water/air and shielding/collimator for detectors. Homogeneous loading model of molten core material and water/air is taken as a reference model as same as TMI-2 core bore cases, and patterns of loading model are also evaluated, and the detector applicability is compared. In addition to the sensitivity analysis of leakage gamma-ray, the coexistence of SNM and index FP is essential condition in case of using this methodology. As a consideration, even with their low-volatility of, for example, lanthanides such as cerium and europium, small but non-zero volatility, local migration inside debris and dissolution to cooling water must be considered. This paper also presents the summary of survey results of high-temperature chemical stability of lanthanides coexisting with nuclear material in a certain atmospheres and temperatures simulating severe accident.