Thermal neutron diffusion cooling in two-region small systems

Abstract

The thermal neutron diffusion cooling effect is a result of the perturbation of the thermal neutron energy distribution owing to the leakage of neutrons outside a finite volume of the medium. In multi-zone systems an additional perturbation appears because of diffusion of neutrons between regions which have different diffusion cooling properties. A study of the thermal neutron diffusion cooling in two-region small cylindrical systems is presented in this paper. A Plexiglas shell (hydrogenous medium) completely surrounds the inner cylinder filled with an absorbing aqueous solution (hydrogenous medium) or with a mixture of hydrogenous and non-hydrogenous substances. The pulsed neutron technique has been used for the experimental investigation of the problem. The time decay constant of the thermal neutron flux in the two-zone systems has been measured. It has also been calculated as a function of the system geometry and neutron dynamic parameters, one of these being the diffusion cooling coefficient of the outer Plexiglas shell. From a combination of results of the calculation and the experiments, it has been found that the obtained function for the particular diffusion cooling coefficient of the Plexiglas shell is controlled by the scattering properties of the inner zone of the system.