Year
2015
Abstract
JAEA has been developing various NDA techniques that can be applied for nuclear material accountancy of molten core material in Fukushima Daiichi Nuclear Power Station units 1, 2 and 3 (1F1, 1F2 and 1F3). One of these techniques is measuring high energy gamma rays emitted from low-volatile high-energy fission products (FPs) coexisting with nuclear material, which enables the measurement from outside of a canister that stores fuel debris. The correlation of actinides and FPs inventory in the cores of 1F1, 1F2 and 1F3 was studied considering the sensitivity of axial neutron spectrum, void, burnup, enrichment distribution unique to BWR fuel1234. Numerical simulation of leakage gamma-ray from molten core materials in a hypothetical canister was studied 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. In this paper, numerical simulation of detectability of leakage gamma-ray from molten core materials in hypothetical canister is dealt with for optimizing the detection system. The basic gamma measurement system consists of a detector (HPGe or LaBr3) and a lead collimator. System configuration such as collimator thickness, slit size, distance between detector and canister etc., is optimized for better detection of gamma-ray emitted from index FPs considering various distributions and filling fractions of fuel debris in the canister. Two measurement modes, which are spiral scanning using one detector and bundle scanning using several detectors, are considered and their characteristics are compared from a detectability view point. Expected system performance such as measurement time, uncertainty etc., is evaluated based on the results of optimization. Based on the optimization results and design conditions assuming actual equipment (canister properties, required operation of physical objects, and installation environment (submerged/radio-resistance)), overall system configuration including handling capability and stand of detector is also studied. In addition, technical challenges are identified for the planning of future development.