Determination of radial and axial neutron flux distribution in irradiation channel of NIRR-1 using foil activation technique

Abstract

Recently, one of the outer irradiation channels of the Nigeria Research Reactor-1 (NIRR-1), which is Miniature Neutron Source Reactor (MNSR) was cadmium-lined. In order to ascertain level of flux disturbance on the inner and the remaining outer channels and also to establish flux profiles in sample container as experienced by samples for Neutron Activation Analysis (NAA), a foil activation technique was used to determine the radial and axial neutron flux distribution of the reactor. Measurements were carried out with copper wire flux monitors arranged in radial and axial directions in a typical irradiation container used for routine NAA in NIRR-1. Our results show that for a preset neutron flux of 5.0×1011cm−2s−1 the axial and radial neutron flux of NIRR-1 ranges from 4.47×1011 to 5.16×1011cm−2s−1 and 4.80×1011 to 5.55×1011cm−2s−1 respectively, with mean value of 4.79×1011cm−2s−1 for axial and 4.99×1011cm−2s−1 for radial. The mean value for each position was compared with our previously measured values after commissioning of NIRR-1 (Jonah et al., 2005) and the one done recently at the Ghana MNSR (Abrefah et al., 2010). The values all pointed towards a level of consistency in variation despite the recent installation of cadmium-lined irradiation channel in NIRR-1. This is an indication that the installed cadmium line did not affect NIRR-1 flux stability. However, our individual foils shows slight flux variation from one foil position to another in the same irradiation container as against the claims of MNSR manufacturers and some users for perfect flux stability across the container. There is therefore the need for flux corrections to be made by MNSR users during NAA particularly for samples in the axial position. This can be achieved by sandwiching flux monitors between samples during irradiations.