EXPERIMENTAL DETERMINATION OF THE THERMAL NEUTRON FLUX DISTRIBUTION PATTERN IN A POLYTHENE WALLED NEUTRON TANK THROUGH ACTIVATION OF IN-115 FOIL

Thermal neutron flux in a polythene walled neutron water tank at Schuster labouratory, University of Manchester have been measured through activation experiment using Indium-115 foils activated at different distances from the centred Pu/Be neutron source. The aim was to determine the way thermal neutrons diffuse and distribute themselves in a medium filled with nuclei, analogous to what happens in a critical nuclear reactor core at steady state. It was found out that thermal neutron flux besides being directly proportional to activity and indirectly proportional to the distance from the centre, have a unique pattern of distribution. The flux pattern was observed to have a definite maximum at the centre of the neutron tank and then decreasing downward towards the edge of the tank seemingly obeying the Fick’s law and the diffusion theory. In addition, there was noticed a very little rise in flux towards the edges of the tank before dropping off to zero finally. This may be because of the thermalizing effect of the pollythene wall of the tank. The significant of determining thermal flux distribution pattern from this study is therefore very important, in that thermal neutron flux pattern reveals power level distribution in the reactor core, helping maximum utilization of fuel by enriching fuel at the edges higher than those at the centre to balance thermal flux deficiency at the edges and sufficiency at the centre due to the discovered neutron diffusion pattern. In addition, using a reflector material around the core will also help in fuel utilization because of this observed effect.