Absolute measurement of β_eff based on Rossi-α experiments and the two-region model in the IPEN/MB-01 research reactor

A new method for absolute measurement of the effective delayed neutron fraction β_eff based on Rossi-α experiments and the two-region model was developed at the IPEN/MB-01 Research Reactor facility. In contrast with other techniques such as the slope method, the Nelson-number method, and the ²⁵²Cf-source method, the main advantage of this new methodology is to obtain the effective delayed neutron parameters in a purely experimental way, eliminating all parameters that are difficult to measure or calculate. In this way, Rossi-a experiments for validation of this method were performed at the IPEN/MB-01 facility, and with the use of the present approach, β_eff was measured with a 1.46% uncertainty. In addition, the prompt neutron generation time A and other parameters were also obtained in an absolute experimental way. In general, the final results agree well with values from frequency analysis experiments. Comparison of theory and experiment reveals that JENDL-3.3 shows deviation for β_eff lower than 1%, which meets the desired accuracy for the theoretical determination of this parameter. This work supports the reduction of the ²³⁵U thermal yield, as proposed by Okajima and Sakurai.