Effects of Spatial Harmonics on Thermal Neutron Spectra Measured by Time-of-Flight Method

Abstract

Spatial effect-the variations in space of transient neutron flux noted in pulsed neutron experiments, and which is ascribable to the occurrence in moderator of spatial harmonics-is studied on the measured flight time spectra of neutrons reentering the atmosphere from parametrically varied depths of a graphite moderator. Measurements were made using moderator slabs of two different axial lengths, to examine the influence of differences in moderator size on the spatial effect. The master equation representing the scalar neutron flux as function of time and energy is solved by a semi-analytic method that takes account of spatial harmonics. Simulation of the solution thus obtained of the master equation proved to reproduce the measured flight time spectra with good accuracy. It was indicated that the spatial effect on the thermal neutron spectrum is stronger with a larger than with a smaller moderator slab. The neutron temperature in graphite moderator was calculated by formula fitted in Maxwellian distribution to the peaks of the thermal neutron spectra. It was indicated that in a graphite moderator of 120 em axial length, spatial harmonics caused a variation of more than 40 K in neutron temperature between depths in moderator differing by 30 em.

Effect of Higher-Order Harmonics on the Steady-State Neutron Flux in Accelerator-Driven Subcritical Reactors

“…In Algorithm 1, since the vector v is determined, Equation (19) can be transformed into a fixed source problem (FSP):…”

“…For example, the prompt neutron decay constant is theoretically a lumped parameter that should be spatially independent [18]. Nevertheless, when the external neutron source of an ADSR is changed in practical measurements (e.g., in pulsed neutron source experiments), the spatial effects cause the prompt neutron decay constant to be correlated with detector's spatial locations [19]. Thus, it is significant for the spatial effect research of an ADSR to analyze the effect of higher-order harmonics on the steadystate neutron flux via the spatial calculation of the highorder modes.…”