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

A static subcritical assembly (system under test) of highly enriched uranium with a low atomic number (low-Z material) reflector is constructed. Based on the Neutron Diagnosed Subcritical Experiment (NDSE) method, the effective multiplication factor k_eff of the assembly is measured. A subcritical model consisting of a solid uranium sphere with uniform density distribution and a low-Z reflector tightly surrounding it is constructed as the calculation model. The Monte Carlo code is used to calculate the neutron and γ transport processes in the system under test and the calculation model initiated by a pulsed neutron source. The parameters of the calculation model are adjusted (keeping the radii of the fissile region and the reflector consistent with those of the system under test and adjusting the mass density of the fissile region; keeping the masses of the fissile region and the reflector consistent with those of the system under test and adjusting their radii and the mass density of the fissile region) to make the leakage γ count rate versus time curve consistent with that of the system under test within the specific time range. Then the calculation model can be regarded as an equivalent model of the system under test and its effective multiplication factor k_eff can be taken as the calculated value of k_eff of the system under test. By comparing the calculated value with the actual value of k_eff, the measurement accuracy of the NDSE method for the k_eff value of a static subcritical system is given. The effectiveness of the NDSE method in measuring the k_eff value of a subcritical system, as well as the reasons for the deviation of the measured k_eff value from the actual value, are analyzed.