Effects of Spatial Harmonics on Power Distribution in Accelerator Driven Subcritical Cores

Abstract

A method for solving the time-energy-dependent diffusion equation was devised to appreciate the effect of spatial harmonics on the power distribution in a subcritical graphite-moderated core with a pulsed fast neutron source. An instantaneous power peaking factor (IPF) was calculated using the thermal neutron flux thus obtained, to characterize the power distribution in the core. In case of a 50-cycles-per-second injections of fast neutron pulses of 3 ms pulse width, it was indicated that the value of IPF increased by 4.1 times with the decrease in the multiplication factor from 0.9951 to 0.9762, accompanying fuel burnup. To appreciate the dependence of the core power distribution on the spectrum hardening of the thermal neutron flux caused by injections of pulsed fast neutrons, comparison was made between the value of IPF and that derived from the time-dependent-one-group diffusion theory where the spectrum hardening of the thermal neutron flux was ignored. It was indicated that the disregard of the spectrum hardening of the thermal neutron flux resulted in the low estimate of IPF that changed from −1% to −14% with the decrease in the pulse width of pulsed fast neutrons from 3 ms to 0.15 ms.