Effect of Boundary/Wall Material of the Shape of Thermal Neutron Flux Pattern

Abstract

An In-115 foils activated at various distances from the centred Pu/Be neutron source  in a polythene walled neutron water tank at Manchester labouratory; and an MCNP code simulation with a polythene wall and a stainless steel wall has been used to study neutron flux distribution pattern and the effect of the wall  material on this flux pattern. Four he-3 detectors was modelled using MCNP codes to have been inserted; first into a polythene walled neutron tank at different distances from the source to study how thermal  neutrons distribute themselves across the diameter of the tank. Again it was remodelled changing the wall from polythene to a stainless steel to observe  the wall effect on the flux pattern. It was discovered that thermal flux in both walls generally have a dume  shaped curve with definite maximum at the center and tending to zero at the edge seemingly obeying the Fick’s law and the diffusion theory. The polythene walled neutron tank experiment and it’s simulation exhibited same flux pattern but with little rise in flux towards the polythene wall before finally droping off to zero, while there was no such rise at the edges of the stainless steel walled tank. These two different behavious could be likened to reflected and bare core reactor, where the polythene because of its property thermalzed and reflect back some fast escaping neutron leading to such little rise in flux at the edges unlike the stainless steel which does not. This thermal flux pattern reveals power level distribution in the reactor core and has led to the theory of diffenrential enrichment, enriching fuel at the edges higher than those at the centre to balance thermal flux deficiency at the edges and sufficiency at the centre. Core with reflector will ensure maximum utilization of fuel.