A. Filip’s research while affiliated with Institute of Geophysics, China Earthquake Administration and other places

Today there is no well-established theoretical model to predict the fission delayed neutron yield vd with the required accuracy. In this field the recommended data result from the rare experimental data analysis or from purely phenomenological or semi-phenomenological models. There is another source of valuable information: the related integral data or βeff- data. In this report we demonstrate, via a careful analysis of the experimental methods leading to revisited experimental βeff values and associated uncertainties, that for the major nuclei the vd evaluated data are of acceptable quality. For U-235 U-238 and Pu-239 we recommend vd values for the thermal and the fast reactor ranges which have been obtained from a statistical consistent adjustment to the βeff data. In the course of this study we show that the energy dependence of vd, suspected from a physics point of view, probably exists with a different magnitude according to the nucleus. Concerning the major nuclei it is of negligible importance for the applications. The improvement of the higher Pu isotopes and minor actinides data is the main motivation for developing the theoretical investigations of the delayed neutron generation mechanism at the same level as the necessary experimental activity.

This report has been prepared by WPEC Subgroup 6, which was set up in 1990 with the aim of reducing the discrepancies between calculated and measured values of the reactivity scale based on reactor kinetics. These discrepancies were resulting in undesirable conservatism in the design and operation of reactor control systems. A collaborative effort was initiated to reduce the uncertainties in the delayed neutron data used in these calculations. This effort included international benchmark measurements of the effective delayed neutron fraction, made on fast critical assemblies, which aimed to provide high-quality experimental information for 235U, 238U and 239Pu. A study was also made of the representation of the time dependence of delayed neutron emission. Based on this work new recommendations have been made concerning the total delayed neutron yields for 235U, 238U and 239Pu, and the time dependence and energy spectra for delayed neutron emission in the fission of a comprehensive set of isotopes.

The experimental results on delayed-neutron yields from thermal-neutron-induced fission of some actinides in the IBR-2 pulsed reactor are presented. A method of periodic irradiation without displacement of the sample was used. The measurements of delayed-neutron total yields in thermal-neutron-induced fission of 239Pu, 233U, and 237Np and in cold-neutron-induced fission of 235U, 233U, and 239Pu were carried out. All values were obtained with the use of the value of β0 for (n th+235U) as a reference. Precise measurements of decay curves in the time interval 5–350 ms for 235U and 239Pu were performed.