Experimental Studies of Passive Neutron Measurement for Fuel Debris at Fukushima Daiichi Nuclear Power Plants

This paper provides experimental studies of passive neutron measurements based on the combination of Differential Die-away Self-Interrogation (DDSI) and coincidence counting for fuel debris at the Fukushima Daiichi Nuclear Power Plants (1F). The applicability of the concept was evaluated by MCNP simulations and the results were presented at the last INMM annual meeting. As the next phase, we conducted small scale experimental tests to proof the concept. Because irradiated fuel cannot be handled at our facility due to the licensing limitation, small scale simulated fuel debris which consists of un-irradiated MOX samples, neutron absorbers and Cf- 252 sources were utilized and measured together to imitate the properties of the fuel debris. The correlation between DDSI response and the leakage multiplication for each simulated debris sample was evaluated. Test results agreed well with the trends of simulation results. This indicates that DDSI has an enough capability to evaluate the leakage multiplication of the sample which includes unknown amount of fissile material and neutron absorbers such as fuel debris at 1F. It was also confirmed in this test that the presence of B-10 degraded DDSI response. It indicates that the optimization of gate width would be required to apply DDSI to fuel debris measurement.