Sensitivity methods for effective delayed neutron fraction and neutron generation time with summon

Abstract

Accurate and reliable tools for sensitivity analysis and uncertainty quantification are needed to estimate the uncertainties in reactor key parameters (keff, βeff, Λeff, safety coefficients, …) and to identify possible nuclear data weaknesses. A Sensitivity and Uncertainty Methodology for MONte carlo codes (SUMMON) has been developed to calculate the sensitivities of the criticality safety parameters as well as their uncertainties due to uncertainties in nuclear data, addressing limitations of existing tools. The methodology is currently based on the use of the KSEN card of MCNP6 code to perform the eigenvalue sensitivity calculations, although any Monte Carlo code that can provide sensitivity coefficients can be used. The sensitivity coefficients of a reactivity response are calculated using the eigenvalue definition of reactivity, which is equivalent to applying the Equivalent Generalized Perturbation Theory. Moreover, the effective delayed neutron fraction sensitivity coefficients are derived from Bretscher’s approximation, whereas the sensitivity coefficients of the effective neutron generation time are obtained using the 1/v insertion method and the Equivalent Generalized Perturbation Theory. Uncertainties are propagated using the “Sandwich Rule” of the “Propagation of Moments” method employing state-of-the-art covariance libraries. The implementation in SUMMON of Bretscher’s approximation and the 1/v insertion method has been validated using integral experiments from ICSBEP and IRPhE databases, while the methods for calculating the sensitivity coefficients have been verified against consolidated codes, such as SCALE, SUSD3D, SERPENT and XSUSA and good agreement has been obtained.