Feasibility Study for Detection of Reactor State Changes during Severe Accidents via External Gamma Radiation Measurements

The gamma radiation field outside of a nuclear reactor carries information about the coolant inventory and the nuclear fuel distribution inside the reactor pressure vessel. Hence, it may serve as an indicator for changes in the reactor internal structures, e.g. in the course of a severe accident. To study the feasibility of using external gamma radiation measurements for the detection of reactor state changes, three-dimensional Monte-Carlo simulations were performed to evaluate the vertical gamma flux distribution outside of a generic pressurized water reactor pressure vessel. The gamma flux was calculated for a reactor with different decreased coolant levels and different core melt states. The results indicate that the gamma flux is very sensitive to the reactor states. The shape and magnitude of the gamma flux distribution are unequivocally subject to the coolant levels and to the relocation of corium into the reactor lower head. A simple state detection algorithm was tested to infer predefined reactor states. It yielded an accuracy of 0.983(2) using bootstrapped test data.