Neutron measurements using the SSNTD technique during prototype fast breeder reactor bulk shielding experiments

Abstract

The in-vessel radial shield configurations for a Prototype Fast Breeder Reactor (PFBR) consist of layers of steel and borated graphite/boron carbide with sodium as coolant. This in-vessel shield is provided to reduce the neutron flux at the intermediate heat exchanger so that secondary sodium activation in the heat exchanger can be kept to an acceptable level. It is necessary to optimise this in-vessel shield because an over design will increase the cost of the reactor and under design will increase secondary sodium activation. Mock up experiments to study neutron transport through various combinations of shield models consisting of layers of steel, sodium, borated graphite and boron carbide were carried out. In these shield model combinations, along with activation detectors, SSNT detectors were also kept to measure the reaction rate by the ( n, f) technique. For this purpose, Lexan ® polycarbonate track detector were used in combination with various fissionable nuclides such as natural uranium, thorium, neptunium, depleted uranium, etc., either in deposition form or foil form. The fission reaction rates per atom in various neutron energy regions were evaluated by counting the induced fission track density. These reaction rates are being interpreted using an appropriate computer code for unfolding the neutron spectrum. The paper describes the results obtained from the fission track data in the various sets of the experiments.