Measuring the Photoneutrons Originating from D(γ, n)H Reaction after the Shutdown of an Operational BWR

The purpose of this study is to measure the photoneutron emission rate after the shutdown of an operational BWR. The photoneutrons originate from the D(γ, n)H reaction in the moderator region, while the high-energy gamma rays are generated from 140Ba-140La transient equilibrium of fission products in irradiated fuel. The photoneutron emission rate is measured by means of the photoneutron signal ratio of the Start-up Range Neutron Monitor (SRNM). The ratio is defined as a ratio of the photoneutrons to neutrons originating from spontaneous fission and the oxygen (γ, n) reaction of actinides (242Cm, 244Cm, etc.) in irradiated fuel. The principle of the measurement of the photoneutron signal ratio is the large difference between the decay constants of the 140Ba-140La transient equilibrium and those of the actinides. The measurement of the SRNM signal was continuously carried out over several months, and the photoneutron signal ratio was evaluated by using the least-squares method to fit a theoretical model to...