Estimation of the Effective Multiplication Factor k_eff of a Static Subcritical Assembly Based on NDSE Method

To verify the accuracy and effectiveness of the Neutron Diagnosed Subcritical Experiment (NDSE) in estimating the effective multiplication factor k_eff of a static subcritical system, a subcritical assembly (referred to as the "system under test") was constructed using highly enriched uranium with a low atomic number (low-Z) material reflector, alongside a corresponding subcritical calculation model. While the calculation model shares a similar structural design with the system under test, it differs in terms of both size and density distribution. The Monte Carlo code was employed to simulate the neutron and γ-ray transport processes in both the system under test and the calculation model, with both systems initiated by a pulsed neutron source. Subsequently, the density and size parameters of the calculation model were adjusted to ensure that its leakage γ-ray count rate versus time curve aligned with that of the system under test within a specified time range. Once this alignment was achieved, the calculation model could be deemed an equivalent model of the system under test, and its effective multiplication factor k_eff was adopted as the estimated k_eff value of the system under test. The k_eff value estimated via the NDSE method exhibited good agreement with the reference value, with a relative deviation of approximately 0.15%. This result confirms the effectiveness of the NDSE method in estimating the k_eff value of static subcritical systems.