In-Drift Accumulation of Fissile Material From Waste Packages Containing Plutonium Disposition Waste Form

The objective of this calculation is to provide estimates of the amount of fissile material flowing out of the waste package (source term) and the accumulation of fissile elements (U and Pu) in a crushed-tuff invert. These calculations provide input for the analysis of repository impacts of the Pu-ceramic waste forms. In particular, the source term results are used as input to the far-field accumulation calculation reported in Ref. 51, and the in-drift accumulation results are used as inputs for the criticality calculations reported in Ref. 2. The results are also summarized and interpreted in Ref. 52. The scope of this calculation is the waste package (WP) Viability Assessment (VA) design, which consists of an outer corrosion-allowance material (CAM) and an inner corrosion-resistant material (CRM). This design is used in this calculation in order to be consistent with earlier Pu-ceramic degradation calculations (Ref. 15). The impact of the new Enhanced Design Alternative-I1 (EDA-11) design on the results will be addressed in a subsequent report. The design of the invert (a leveling foundation, which creates a level surface of the drift floor and supports the WP mounting structure) is consistent with the EDA-I1 design. The invert will be composed of crushed stone and a steel support structure (Ref. 17). The scope of this calculation is also defined by the nominal degradation scenario, which involves the breach of the WP (Section 10.5.1.2, Ref. 48), followed by the influx of water. Water in the WP may, in time, gradually leach the fissile components and neutron absorbers out of the ceramic waste forms. Thus, the water in the WP may become laden with dissolved actinides (e.g., Pu and U), and may eventually overflow or leak from the WP. Once the water leaves the WP, it may encounter the invert, in which the actinides may reprecipitate. Several factors could induce reprecipitation; these factors include: the high surface area of the crushed stone, and the presence of reactive components in the stone (such as calcium and silica); the contrasting chemistry of water trapped in the pores of the invert; and the possible presence of reducing materials in the support structure. This calculation estimates the amounts of Pu and U that may accumulate in the invert as a consequence of chemical precipitation. The degradation scenario is consistent with the overall degradation analysis methodology outlined in Section 3.1 of the Disposal Criticality Analysis Methodology Topical Report (Ref. 47). Specifically, the scenario NF-1 b of Figure 3-2a of that document prescribes the processes analyzed in this calculation. The only difference is that the present calculation extends the possibilities of in-drift interacting materials to include incompletely oxidized iron, which was not a major constituent of the drift at the time the Topical Report was written