Uncertainty quantification of delayed neutron fraction of u235 based fuel cores

International audienceThe analysis of nuclear reactors behavior in transients depends among others on the effective delayed neutron fraction ($\beta _{eff}$). The present paper aims at describing how to provide uncertainties for the effective delayed neutron fraction ($\beta_{eff}$) to be used for safety studies.The use of the Iterated Fission Probability method in the Monte Carlo code TRIPOLI4 gives credit to deterministic codes such as ERANOS for calculating $\beta _{eff}$. The use of the Monte Carlo code TRIPOLI4 enables a better representation of experimental cores, especially the R2 experimental core which exhibit more heterogeneities for hosting experimental devices. Its use for evaluating the calculated parts of the $\beta _{eff}$ has been found essential. The nuclear data uncertainty propagation has been leading to a 3.5% uncertainty. This 3.5% uncertainty is confirmed by the $\beta _{eff}$ C-E bias for BERENICE R2 cores