A proposal of a benchmark for β_eff, β_eff/Λ, and Λ of thermal reactors fueled with slightly enriched uranium

A reactor noise approach has been successfully performed at the IPEN/MB-01 research reactor facility for the experimental determination of the delayed neutron parameters β_eff, β_eff/Λ, and Λ. In the measurement of the β_eff parameter, the reactor power, which is of fundamental importance, was obtained with a very high level of accuracy by a fuel rod scanning technique and a subsequent irradiation of a highly enriched ²³⁵U foil for the fission density normalization. The final measured values of β_eff and β_eff/Λ show very good agreement with independent measurements and can be recommended as benchmark values for thermal reactor applications because their uncertainties are much lower than the target accuracy recommended for β_eff calculations (|C-E|/E less than 3%). The theory/experiment comparisons reveal that only JENDL3.3 attends the target accuracy for β_eff calculations. This result fully supports the reduction of the ²³⁵U thermal yield as proposed by Okajima and Sakurai. The ENDF/B-VI.8 library and its revised version performed at LANL overpredict β_eff by as much as 7.2%. The newly released JEFF-3.1 library shows a discrepancy of 4.8% for β_eff. For β_eff/Λ, the deviations are relatively larger (more than 10%) for all libraries due to the underprediction of the prompt neutron generation time (Λ).