Shielding and criticality characterization of ALR8(SI) plutonium storage containers

Abstract

An examination of the shielding and criticality safety performance of the Pantex-designed ALR8(SI) container for storage of ex-weapons pits is described. Both experimental and calculational studies were performed. The MCNP-4A code was used as the major analysis tool. The nuclear data libraries and usage of MCNP-4A was verified by comparison to photon dose rates from experiments with isotopic photon sources placed inside an ALR8(SI) container. An MCNP-4A model of an ALR8(SI) container with a "hypothetical" ex-weapons pit inside was also formulated. The hypothetical pit consisted of a hollow sphere of plutonium encased in stainless steel. Data for the gamma ray intensities from the decay of the plutonium isotopes were taken from the LANL web site, "Map of the Nuclides.'' All of the energy lines from the isotopes in weapons-grade plutonium were used for the photon source description. Dose rates due to the photons emitted by the hypothetical pit were calculated at various positions external to the ALR8(SI) container. Since Pu-241 may undergo significant decay during the possible period of storage, the effect of varying the amount of Am-241 in the hypothetical pit's plutonium was considered. The criticality and neutron dose rates around the ALR8(SI) container with a hypothetical pit inside were also calculated using MCNP-4A. To increase the computational efficiency, the neutron dose rate calculations were carried out using the neutron source distribution at the pit surface that was generated during the criticality calculations. The effective neutron multiplication factor (k[]) for the ALR8(SI) container including the hypothetical pit was calculated both for normal conditions and when the container was flooded with water.