Sensitivity analysis of source intensity and time bin size for the Rossi-alpha method in a numerical reactor model

Until now, various studies focusing on subcriticality measurements have been conducted using the Rossi-alpha method. However, no guidelines have been provided for the source intensity and time bin size of the Rossi-alpha analysis. In this study, sensitivity analyses were performed to determine an optimized source intensity and time bin size for a numerical reactor model. Using the MCNP6 Monte Carlo code, fission count signals in the fuel region were tallied 50 times with different random seeds in real-time mode. These signals were applied to the Rossi-alpha method as detector signals. The estimated k eff values were compared to the reference k eff values generated by the MCNP6 criticality calculation. As a result of the sensitivity test for the neutron source intensity, the Rossi-alpha method has errors of 200 pcm or less when the source intensity is greater than 10 5 s ???1 . It is shown that the standard deviations of 50 analyses are converged when the source intensity is greater than 10 5 s ???1 . At the same time, as a result of the sensitivity test for the time bin size, the Rossi-alpha method shows errors less than 200 pcm when the time bin size is smaller than 10 ???5 s. It is shown that the standard deviations of 50 analyses are converged when the time bins are smaller than 10 ???5 s. For the numerical reactor model in this study, the threshold time bin size was set to 10 ???5 s. However, it generally depends on the neutron generation time of the problem. Thus, it is recommended to set the neutron source intensity to 10 5 s ???1 or higher and the time bins to the time of the neutron generation or less when the Rossi-alpha method is applied to subcriticality measurements in nuclear reactor cores