Analysis of Subcriticality Monitoring Results for First Loading of a PWR

An activated secondary neutron source (ASNS) is used to complete the first loading of a PWR. During the first loading, the response test results of the temporary neutron detector (TND) in the reactor are much higher than the simulation results and the counting rate drops significantly. In order to find out the causes of the above problems, the supervision working group analyzed the characteristics of secondary neutron sources and the radiation field established by ASNS, analyzed and verified the influence of subcritical multiplication neutrons of nuclear fuel on TND counting rate, and analyzed the subcritical monitoring data of reactor first loading using ASNS. The results show that the radiation field around TND is a mixed radiation field formed by γ-rays and neutrons. After installing the nuclear fuel assembly between the neutron source assembly and TND, the influence of subcritical breeder neutrons of nuclear fuel on TND counting rate is to increase it; The ASNS decay produced a large amount of γ rays, the peak accumulation of γ pulses from TND in the main amplifier leaded to pulse amplitude distortion. The reason why the TND response test results are much higher than the simulation results is that the pulse amplitude discriminator cannot effectively distinguish the distorted γ pulse and neutron pulse; The reason why the TND count rate drops significantly is that the uranium in the nuclear fuel shields most of the γ rays from ASNS to TND. The operation status of the source range channel and TND meets the requirements of the subcritical monitoring equipment for the first loading program.