A proposal of a benchmark for βeff, βeff/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 βeff, βeff/Λ, and Λ. In the measurement of the βeff 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 βeff and βeff/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 βeff calculations (|C-E|/E less than 3%). The theory/experiment comparisons reveal that only JENDL3.3 attends the target accuracy for βeff 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 βeff by as much as 7.2%. The newly released JEFF-3.1 library shows a discrepancy of 4.8% for βeff. For βeff/Λ, the deviations are relatively larger (more than 10%) for all libraries due to the underprediction of the prompt neutron generation time (Λ).