The experimental determination of the effective delayed neutron parameters: β_eff, β_eff/Λ and Λ of the IPEN/MB-01 reactor

A reactor noise approach has been successfully performed at the IPEN/MB-01 research reactor facility in order to determine experimentally 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 degree of accuracy by a fuel rod scanning technique and a subsequent irradiation of highly enriched ²³⁵U foil for the fission density normalization. The final measured values of β_eff, β_eff/Λ show very good agreement with independent measurements. The theory/experiment comparison shows deviations as large as 6.8% for β_eff when the ENDF/B-VI.8 library and its revised version performed at LANL are employed. For the β_eff/Λ, these deviations are of the order of 15.6%. The best agreement is obtained for JENDL3.3 library, where it is found a deviation of only 1.9% in the C/E ratio of β_eff. This result fully supports the reduction of the ²³⁵U thermal yield as proposed by Okajima and Sakurai. The C/E ratio of β_eff/Λ is relatively higher, 11.1%, but this is due to the underestimation of the calculated prompt neutron generation time, Λ.