Fuel debris detection system using in-situ measurements of gamma rays and neutrons for investigation of fuel debris distribution

This paper presents a compact, integrated system for in-situ detection of fuel debris via simultaneous gamma-ray and neutron measurements, developed for application within the primary containment vessel (PCV) of the Fukushima Daiichi Nuclear Power Station Unit 1. The system combines a miniaturized CdTe detector with 100-ns signal processing for gamma-ray spectroscopy and a compact B-10 proportional counter structually designed to suppress its gamma-ray response, both enclosed in a shielded container weighing less than 8 kg. Performance was validated through laboratory experiments with sealed sources, functional verification using a spent fuel assembly under simulated PCV conditions, and in-situ demonstration using a swimming remotely operated vehicle (ROV). The system reliably detected Eu-154 gamma rays and neutrons under dose rates exceeding 10 Gy/h, achieving an energy resolution of 41 keV at 1274 keV. Field deployment suggested the presence of fuel debris-derived material at most measurement locations near the sediment surface. These findings establish the system’s suitability for decommissioning support in severe radiation environments.