Delayed neutron study using ENDF/B-VI basic nuclear data

Abstract

The basic nuclear data of the latest releases of ENDF/B-VI were used in preliminary calculations with the CINDER'90 nuclide inventory code to simulate the activity of fission delayed-neutron precursors. Total delayed-neutron production was obtained at times during and following pulse (0.1-ms) and equilibrium (4-hr) fission histories for each of the sixty fission systems having fission-product yields in ENDF/B-VI. The equilibrium studies — at unit fission rate for constant fission periods sufficiently long that all precursors reached saturation inventories — yielded the v d value for each system. Delayed-neutron production rates at 54 decay times t, extending to 500 s following a fission pulse, were fit using the STEPIT code to the pulse function R( t) = ∑a iλ ie −λ i t . Results following equilibrium irradiations were fit to the equilibrium function R(∞, t) = ∑a iλ ie −λ i t . It was observed that functions from fits to pulse results did not well represent equilibrium results at long cooling times. Similarly, functions fit to the equilibrium results did not well represent pulse results at short cooling times. A comprehensive series of CINDER'90 calculations was then made for irradiation times T of 0.1 ms, 1 s, 10 s, 100 s, 1000 s, and 4 hours; results were obtained at 60 decay times t extending to 800 s following irradiation. Comprehensive calculations were made using both the 1989 Pn data of England and Brady and the new Pn data of Pfeiffer, Kratz and Möller described elsewhere in this issue. The body of results for each system was included in fits to obtain the neutron production rate R( T, t) = ∑a ie −λ i t (1 − e −λ i T ) for each system. Fits were made for the traditional sum of six exponentials, with all variables free to vary; additional fits were made for a sum of eight exponentials with decay constants set to values suggested by Piksaikin. The resulting pulse functions R( t), defined by the a i and λ i thus obtained, accurately represent calculated delayed-neutron production when integrated with any irradiation history. The pulse functions thus produced and other published pulse functions fit to past measurements and calculations are compared numerically at several times after fission. Reactivity effects of all functions from measurements and calculations for each of the sixty systems are indicated by the asymptotic periods following positive 10¢– 50¢ reactivity steps simulated in point-reactor kinetics calculations using the AIREK-10 code.