Minimum enrichments of uranium for criticality in homogeneous and heterogeneous systems using MCNP4B

With the increase in the amount of highly enriched uranium available and numerous proposals for down-blending, the criticality safety community is again examining limits for safe operations with various enrichments of uranium. Most tabulated data on criticality limits does not provide information on minimum enrichment for various moderators. In a presently unpublished paper), the minimum enrichments for infinite homogeneous and heterogeneous systems moderated by water, heavy water, beryllium, carbon, Celotex^R, or polyethylene were analyzed using the discrete ordinates code DANTSYS, The homogeneous systems were modeled with a 100 cm-radius sphere with a reflective boundary. A dimensional search for the enrichment necessary for k_∞ = 1 was performed. The minimum enrichment for criticality was found to be 0.47 wt% using heavy water as the moderator. The heterogeneous systems are modeled by a series of slabs. The moderator thickness is varied for several different thicknesses of uranium to generate curves of the type shown. For this paper, it is desired to verify the results of deterministic calculations with 16 group Hansen-Roach cross-sections by using the Monte Carlo code MCNP4B and continuous cross-sections. Preliminary results show the minimum enrichment for criticality is 1.03 wt.% for the homogeneous case and 0.77 wt.% for the heterogeneous case using MCNP4B for water. The minimum enrichment required to achieve criticality in infinite homogeneous and heterogeneous systems of uranium metal using the moderators and models mentioned above will be found with MCNP4B and compared with the results obtained from DANTSYS. This will give greater confidence that the analyses using DANTSYS are valid or, if inconsistencies arise, point to areas of further analysis