Development of the fuel debris criticality characteristics measurement system

The measurement method for quantification of fissile material in fuel debris at TEPCO's Fukushima Daiichi Nuclear power plants is essential for efficient decommissioning. We propose a new measurement system which combines the techniques of the Active Well Coincidence Counter and the Neutron Moisture Meter. The detector has a two-layer structure surrounding a canister containing fuel debris fragments. The first layer consists of two spherical ³He proportional counters with different energy sensitivity characteristics, and the second layer consists of a moderator and two cylindrical ³He proportional counters. In addition, B₄C is inserted between the first and second layers as a thermal neutron shield. The fuel debris is irradiated with neutrons from an external ²⁴¹AmBe neutron source and the scattered, moderated, and induced fission neutrons are counted by the respective proportional counters. We demonstrated the feasibility of measuring the amount of fissile material and water content for the assumed fuel debris composition by Monte Carlo neutron transport calculations for the ²⁵²Cf spontaneous fission neutron source and the ²⁴¹AmBe neutron source. (author)