A source release model was developed to quantify time dependent liquid phase releases of radioactive material to the vadose zone from a disposal site. The model has been implemented to evaluate the source terms in the Performance Assessment for the Los Alamos National Laboratory (LANL) Low Level Radioactive Waste (LLRW) Disposal Facility at Area G, an analysis of the long term post-closure impact of disposal operations required by the USDOE orders.
Analytic solutions describe transport through the model compartments: the solid phase waste package, the liquid phase within each waste package and the liquid phase within the disposal unit. The model accounts for elemental solubility limits and retardation coefficients (R) separately in the waste package and in the disposal unit. Several parameters define the site specific aspects of the disposal unit. In our case for example, the disposal unit is waste buried with crushed volcanic tuff, and a small net infiltration rate is determined from independent numerical studies. The analytic solution allows efficient explorations of the sensitivity to the input parameters. Numerical solutions extend the model to account for decay product ingrowth which may have different transport properties than the parent nuclides.
Results quantify the decrease of source release rates with package or disposal unit Kd, and the effects of solubility limits. At the LANL LLRW disposal site, thorium and uranium are the only significant contributors -to dose which are solubility limited. Otherwise, a ‘rapid rinse’ waste package category dominates the peak disposal unit efflux. This occurs for the LANL disposal site at about 100 years for nuclides with Kd= 0 (R= 1) and for proportionately longer times as Kd or R increases. Additional simple analytic models or detailed numerical codes can be coupled to these results to predict groundwater concentrations.