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 plutonium pits is described. The MCNP-4A code was used as the major analysis tool. An MCNP-4A model of an ALR8(SI) container with a “hypothetical” ex-weapons pit inside was formulated. The hypothetical pit consisted of a hollow sphere of plutonium encased in stainless steel. For the photon shielding analysis, data for the gamma rays emitted during the decay of the plutonium isotopes were taken from the LANL website, “Map of the Nuclides.” Dose rates due to the photons emitted by the hypothetical pit’s decay 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 also considered. The neutron dose rates exterior to the ALR8(SI) container (with a hypothetical pit inside) were determined using two MCNP-4A calculations. In the first, the effective neutron multiplication factor (keff) of the container was calculated. In the second, the results from the keff calculation were used to define a surface neutron source at the pit’s outer edge, and the neutron dose rates external to the container were then computed based on this neutron source. The effective neutron multiplication factor (keff) of the container was evaluated both for dry conditions and for when the container was flooded with water.