IN-CORE MEASUREMENT OF DELAYED-NEUTRON PARAMETERS.

Abstract

An in-core experiment is proposed to measure all delayed-neutron parameters (λᵢ and βᵢ for each delayed group) and the neutron generation time applicable for a given reactor system. The method uses a least-squares fitting technique to simultaneously fit a series of transient responses produced by step changes (or rapid ramps) in reactivity of arbitrary size. The function which is least-squares fit is the exact analytic solution describing a reactor response following a step change in reactivity as given by the standard multi-group point-reactor model. The method does not require any knowledge of the absolute reactivity of the system. The results are based solely upon the measurable quantities of relative power, time, and asymptotic inverse period. Unlike out-of-core delayed neutron experiments, the results are independent of the total number of fissions that have occurred in the fissioning isotope that produced the delayed neutrons, and are independent of the efficiency of the detector used to monitor the transients. Modal contamination is presumed not to be a serious problem in small or intermediate size cores as long as small reactivity changes are used (i.e. +$.10 to -$.25), which is consistent with the best method of performing this experiment. The method is equally applicable to both homogeneous and heterogeneous reactors and may be used in either thermal or fast systems. The number of delayed-neutron groups is not a constraint.