The impact of the correlation matrix of the measured effective delayed neutron effective parameters in the inferred reactivity

The impact of the correlation matrix of the fitted delayed neutron parameters for the determination of the reactivity of a thermal multiplying system fueled with enriched uranium where most of fissions occurs in ²³⁵U has been successfully performed in this work. The analyses have been considered in the IPEN/MB-01 benchmark for reactivity determination approved by IRPhE. The analyses reveal that the consideration of the correlation matrix in the reactivity error propagation is very important. Tn general, the introduction of the correlation matrix reduces the overall uncertainty by a reasonable amount. There are huge cancellations in the uncertainty analysis. The analyses also reveal that the uncertainty in the reactivity will depend the specific range where this quantity is considered. For most of the period range considered, the reactivity uncertainties for negative periods are around 3.5% while for positive periods they are nearly 2%. There are two extreme cases where the correlation matrix plays no role; very large negative periods and very small positive periods. In the first case the first decay constant plays a fundamental role while in the second the prompt neutron generation is of great importance. The experiments performed at the IPEN/MB-01 reactor and evaluated for inclusion in the IRPhE handbook can be considered extremely useful to validate methods and nuclear data related to the reactivity determination of thermal reactors fueled with Uranium.