Numerical simulation and analysis of the pulsed neutron source method measuring the prompt neutron decay constant of a Godiva-like subcritical facility

The theory and method of the pulsed neutron source (PNS) method measuring the prompt neutron decay constant alpha(0) of a subcritical system was introduced. Using the Monte Carlo code JMCT, the neutron transport process in a Godiva-like subcritical facility was simulated. The alpha(0) value of the facility was obtained with PNS method, and was compared with the alpha-k power iteration result. To investigate the factors affecting the calculated alpha(0) value of the PNS method, the system response, the fitting residual distributions, and the neutron energy spectra of different time intervals were analyzed. It is observed that the statistical fluctuation is the key factor leading to the inaccuracy of the alpha(0) calculation value of the facility. Furthermore, to obtain an effective alpha(0) value with the PNS method, the reasonable width of fitting time window was discussed by analyzing the relationship of the calculated alpha(0) values and the fitting time width in this paper.