Data Assimilation Using Subcritical Measurement of Prompt Neutron Decay Constant

Abstract

The prompt neutron decay constant $\alpha$ in a steady-state subcritical system can be directly measured using a reactor noise analysis method such as the Feynman-$\alpha$ method. To reduce the nuclear data–induced uncertainty of $k_{\mathrm{e}\mathrm{f}\mathrm{f}}$ for a target system, this study investigates the applicability of data assimilation techniques, i.e., the bias factor method and the cross-section adjustment method, based on a subcritical measurement of $\alpha$ conducted at Kyoto University Critical Assembly (KUCA). The sensitivity coefficients of $k_{\mathrm{e}\mathrm{f}\mathrm{f}}$ and $\alpha$ with respect to the nuclear data were efficiently estimated using a deterministic S_N transport code with first-order perturbation theory. As a result, the a priori relative uncertainty of $k_{\mathrm{e}\mathrm{f}\mathrm{f}}$ due to the 56-group SCALE covariance data can be reduced if there is strong correlation between the measured $\alpha$ and the target $k_{\mathrm{e}\mathrm{f}\mathrm{f}}$. The experimental value of $\alpha$ contributes to improving the nuclear data of total fission spectrum $\chi$ and total fission neutron number $\nu$ via strong correlations between $\chi$ and prompt ${\chi }_p$ and between $\nu$ and prompt ${\nu }_p$, by utilizing the sensitivity coefficients of $\alpha$ with respect to prompt ${\chi }_p$ and ${\nu }_p$.

Keywords:

• Prompt neutron decay constant
• Feynman-α method
• perturbation theory
• bias factor method
• cross-section adjustment method

Acknowledgments

This work has been carried out in part under the visiting researcher’s program of the Kyoto University Institute for Integrated Radiation and Nuclear Science. The authors are grateful to all staff members of KUCA for their assistance during the experiment. This work was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant-in-Aidfor Young Scientists (B) and Scientific Research (C) [grant numbers 17K14909 and 19K05328].