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

Highlights: • Sensitivity analyses were performed to determine an optimized source intensity for a numerical reactor model. • Sensitivity analyses were performed to determine an optimized time bin size for a numerical reactor model. • When the source intensity is greater than 10⁵ s⁻¹, the Rossi-alpha method has errors of 200 pcm or less. • When the source intensity is smaller than 10⁵ s⁻¹, the change in accuracy was shown tobe greater. • When the time bins were smaller than 10⁻⁵ s, the Rossi-alpha method showed errors than 200 pcm. • When the time bins were greater than 10⁻⁵ s, the change in accuracy was shown to be greater. • In this study, setting the neutron source intensity to 10⁵ s⁻¹ or higher and the time bin unit to 10⁻⁵ s or less provides converged accuracy and standard deviation. - Abstract: 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 $k_{\mathrm{eff}}$ values were compared to the reference $k_{\mathrm{eff}}$ 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 10⁵ s⁻¹. It is shown that the standard deviations of 50 analyses are converged when the source intensity is greater than 10⁵ 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 10⁵ 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.