LAUSR

Abstract The effective delayed neutron fraction ( β eff ) is an important characteristic of nuclear reactors since it affects transients significantly. It is therefore important to characterise it correctly. The use of best-estimate codes and data together with an evaluation of the uncertainties is required not only for their use in safety studies but also to assess reactivity effects which are being measured relative to the effective delayed neutron fraction (βeff) in $. The use of the Iterated Fission Probability method in the Monte Carlo code Tripoli4 ® confirms results obtained with deterministic codes such as ERANOS for calculating β eff . The asset of Tripoli4 ® is the possibility to get a better representation of experimental cores. Its use for evaluating the calculated parts of the “experimental” β eff of the BERENICE experimental programme has led to significant improvements of the C/E ratios, especially the R2 experimental core. This work is in support of the ASTRID project which developed the concept of a Sodium cooled Fast Reactor (SFR). In order to get a reduced uncertainty it is recommended to repeat the β eff measurement within the future GENESIS experimental programme in the refurbished zero power reactor (ZPR) MASURCA with an improved noise measurement technique. The nuclear data uncertainty propagation has led to a 2.8% uncertainty for U-Pu core and 2.6% for enriched uranium cores (with JEFF-3.2) with main contributors being the delayed neutron fission yield and the fission cross section of U238.