Estimation of Radiation Source Distribution in RPV Based on Prior Knowledge of Fuel Debris Spreading

This study proposes a novel method for estimating radiation source distribution in a reactor pressure vessel (RPV), especially focusing on the fuel debris retrieval at the Fukushima Daiichi Nuclear Power Plant. Before the fuel debris retrieval, it is necessary to grasp the internal situation of a primary contain- ment vessel (PCV). Our research group is planning to insert an investigation unit from the top of the PCV and measure radia- tion using non-directional gamma-ray detectors. The proposed method defines the bottom of the RPV as two-dimensional grids and estimates the radiation source distribution. The proposed method consists of four steps: database construction, choosing candidates from the dataset, comparing the candidates with the prior knowledge, and additional measurements. Simulation experiments demonstrate that the proposed method is possible to accurately estimate the radiation source distribution in the RPV with a limited number of measurements.