The experimental determination of the effective delayed neutron parameters: βeff, βeff, /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 βeff, βeff, /Λ. In the measurement of the βeff 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 βeff, βeff, /Λ show very good agreement with independent measurements. The theory/experiment comparison shows deviations as large as 6.8% for βeff when the ENDF/B-VI.8 library and its revised version performed at LANL are employed. For the βeff, /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 βeff. This result fully supports the reduction of the 235U thermal yield as proposed by Okajima and Sakurai. The C/E ratio of βeff, /Λ is relatively higher, 11.1%, but this is due to the underestimation of the calculated prompt neutron generation time, Λ. (authors)