Simulation study on the design of nondestructive measurement system using fast neutron direct interrogation method to nuclear materials in fuel debris

In order to measure the amount of nuclear materials in the fuel debris produced in the Fukushima Daiichi Nuclear Power Plant accident, we have designed a measurement system based on a Fast Neutron Direct Interrogation (FNDI) method. In particular, we have developed a fast response detector bank for fast neutron measurements by Monte Carlo simulations. The new bank has more than one order of magnitude faster response compared to the standard ones. We have also simulated the nondestructive measurements of the nuclear materials in homogeneously mixed fuel debris with various matrices which contain stainless steel (JIS SUS304), concrete, and various control-rod (CR) contents in the designed system. The results show that if the debris contains less than 1.0 vol% of CR component and more than 2.5 vol% of spent fuel component, the fissile materials in the debris can be measured by using the designed system.