In the decommissioning of the Fukushima Daiichi nuclear power plant, objects removed from the primary containment vessel included not only fuel debris containing nuclear materials but also structural materials. The process from debris removal to storage can be streamlined by sorting these objects based on their nuclear material content and optimizing storage facility dimensions and management methods. However, fuel debris may melt and mix with control rods, which are neutron absorbers, making it difficult to apply conventional nondestructive measurement methods using neutrons.

Through experiments and simulations, this study evaluates the performance of the fast fission neutron coincidence counting (FFCC) method, a nondestructive measurement technique for nuclear materials less affected by neutron absorbers. The FFCC method utilizes fast fission and fast coincidence detection on the nanosecond scale. The results obtained using a simple measurement system demonstrated that the FFCC method can detect 84.7 g of uranium, even in the presence of substantial amounts of neutron absorbers, and has the quantification capability for masses ranging from 28.2 g to 3.7 kg.

Furthermore, to validate the effectiveness of the FFCC method, simulations were conducted for several debris compositions. Consequently, the results indicated that there was no notable difference between the true uranium mass and the analytical values. The performance of the measurement system can be notably enhanced by optimization. Therefore, the FFCC method is promising for quantifying fuel debris containing neutron absorbers.