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

A new methodology for absolute measurement of the effective delayed neutron fraction βeff based on Feynman-α experiments and the two-region model was developed. This method made use of Feynman-α experiments and the two-region model. To examine the present methodology, a series of Feynman-α experiments were conducted at the IPEN/MB-01 research reactor facility. In contrast with other techniques like the slope method, Nelson-number method and 252Cf-source method, the main advantage of this new methodology is to obtain βeff with the required accuracy and without knowledge of any other parameter. By adopting the present approach, βeff was measured with a 0.67% uncertainty. In addition, the prompt neutron generation time, Λ, and other parameters, was also obtained in an absolute experimental way. In general, the measured parameters are in good agreement with the values found from frequency analysis experiments. The theory–experiment comparison for the βeff measured in this work shows that JENDL3.3 presented the best agreement (within 1%). The reduction of the 235U thermal yield as proposed by Okajima and Sakurai is completely justified according to the βeff measurements performed in this work.