A proportionate approach to E_pd

Dounreay Site Restoration Ltd. (DSRL) has historically taken a proportionate approach to dealing with the potential intrinsic bias in Monte Carlo calculations (E_PD), which is as follows: 'Any potential underestimation of k_eff from the intrinsic bias associated with the Monte Carlo code and neutron cross-section data library will be offset by the overestimation of k_eff due to the selection of conservative modelling parameters used in criticality safety assessments.' This paper quantifies E_PD and the minimum overestimation of k_eff from typical conservative modelling parameters for systems of relevance to DSRL. Justification for DSRL's historic approach is made by comparison of E_PD and the minimum overestimation effect for systems of relevance to DSRL. It has been shown that the minimum overestimation of k_eff is greater than or equal to E_PD for the MONK and JEF neutron cross-section data library combination when considering fissile systems of relevance to DSRL. It is therefore concluded that DSRL's historic position of not explicitly considering E_PD in criticality assessments has been justified for fissile materials considered within the scope of this study. One exception has been identified, which is when the isotopic composition of legacy MOX is modelled explicitly. In reality, the minimum overestimation of k_eff will be greater than considered here due to other conservative modelling assumptions e.g. geometry and moderation. However, these have not been explicitly quantified due to the highly varied nature of the Dounreay inventory