THE RELATIVE IMPORTACE OF NEUTRON CAPTURE AND INELASTIC-SCATTERING INTERACTIONS TO THE SECONDARY GAMMA-RAY DOSE TRANSMITTED THROUGH LAMINATED TUNGSTEN AND LITHIUM HYDRIDE SNAP REACTOR SHIELDS

摘要

Calculations have been made of the spaclal and neutron energy dependence of the neutron captures and Inelastic scatterings that con-tribute to the secondary gamma-ray dose transmitted through a laminated tungsten (Hevimet) and lithium hydride shield adjacent to a SNAP reactor. Four shield configurations were consideredt a 5-cm-thick slab of Hevimet. alone;the Hevimet followed by a 30.5-cm thickness of LiH;the Hevimet preceded by the LiH;and the Hevimet preceded and followed by the LiH. For each configuration both a forward calculation and an adjoint calcula?tion were performed with the one-dimensional discrete ordinates code ANISN, the forward calculation yielding neutron fluences within the shield and the adjoint calculation yielding probabilities that a gamma ray of a given energy originating at a given position in the shield would con?tribute to the transmitted dose. The results from the two calculations were convoluted with gamma-ray production data to determine the spacial distribution of the neutron interactions contributing to the exit secondary gamma-ray dose. The calculations show, as was expected, that in each con?figuration essentially all of the exit secondary gamma-ray dose is due to neutron interactions in the Hevimet, with the contribution from the last half of the Hevimet being a factor of 3 to 5 higher than that from the first half. When LiH follows the Hevimet, 80% of the exit gamma-ray dose is produced by neutron captures, with nearly half of them occurring at neutron energies between 3.06 eV and 0.111 MeV. When LiH does not follow the Hevimet, the importance of neutron inelastic scatterings approaches that of the captures.