Geochemistry Model Validation Report: External Accumulation Model

The purpose of this report is to document and validate the external accumulation model that predicts accumulation of fissile materials in the invert, fractures and lithophysae in the rock beneath a degrading waste package containing spent nuclear fuel (SNF) in the monitored geologic repository at Yucca Mountain. (Lithophysae are hollow, bubblelike structures in the rock composed of concentric shells of finely crystalline alkali feldspar, quartz, and other materials (Bates and Jackson 1984 [DIRS 128109], p. 299.) A secondary purpose is to predict the quantity of non-fissile minerals that accumulate alongside the fissile materials. The results of the model will be used in external criticality risk assessments to support the post-closure safety case. The scope of the report is to (1) describe the model and the parameters used to develop the model, (2) validate the model for use in criticality calculations, and (3) use the model to determine external accumulations from degrading waste packages. Commercial SNF (CSNF) waste packages and Department of Energy (DOE) SNF codisposed (CDSP) waste packages containing DOE SNF codisposed with high-level waste (HLW) glass are analyzed. The DOE SNF considered originates from Three Mile Island (TMI), the Fast Flux Test Facility (FFTF), and N-Reactor. The report considers (1) the nominal scenario, in which the drip shield is intact, and only diffusive losses of U and Pu can occur through a film of condensate, (2) the seismic-induced fault displacement scenario, in which the waste package and drip shield are damaged and pore water flows through the breached waste package, and (3) the igneous scenario, in which the drift is filled with basalt, the waste package and drip shield are damaged, and water flows through the basalt and through the breached waste package.