Use of one delayed-neutron precursor group in transient analysis. [PWR; BWR]

Abstract

In most reactor dynamics calculations six groups of delayed-neutron precursors are usually accounted for. However, under certain circumstances it may be advantageous to simplify the calculation and utilize a single delayed-neutron group. The motivation for going to one precursor group is economy. For LWR transient codes that use point kinetics the equations are solved very rapidly and six precursor groups should always be used. However, codes with spatially dependent neutron kinetics are very long running and the use of one precursor group may save computer costs and not impair the accuracy of the results significantly. Furthermore, in some codes, the elimation of five presursor groups makes additional memory available which may be used to give a net increase in the accuracy of the calculations, e.g., by allowing for an increase in mesh density. In order to use one delayed neutron precursor group it is necessary to derive a single decay constant, /sub 6/lambda-, which, along with the total (or one group) delayed neutron fraction ..beta.. = ..sigma../sub i = 1/..beta../sub i/, will adequately describe the transeint precursor behavior. The present summary explains how a recommendation for lambda- was derived.