The soil submodel, SCEMR1, for the assessment of Canada's nuclear fuel waste management concept

Radionuclides from nuclear fuel wastes immobilized in a vault in stable plutonic rock of the Canadian Shield may eventually dissolve in groundwater. A biosphere model, BIOTRAC, has been developed to model the migration of radionuclides and document the consequences in an environmental impact statement. This report describes the soil submodel of BIOTRAC., SCEMR1. The SCEMR1 soil model is a mechanistic time-dependent transport model. This report presents full descriptions of its major subroutines for water and nuclide transport, its input parameter values, its major assumptions, and quality assurance procedures used in development. SCEMR1 performed adequately during independently testing. The SCEMR1 code uses too much computer time for direct inclusion in BIOTRAC. A sensitivity analysis indicated that the important parameters to accurately predict water and nuclide transport were the soil solid/liquid partition coefficient Kd, annual precipitation, soil texture, and soil depth. Using ranges of these four parameters relevant to the Shield and the disposal vault nuclide inventory, a database of steady-state soil concentrations was generated by running SCEMR1. This database covered a range of annual precipitation from 0.3 to 1.0 m, soil depth from 1.0 to 2.0 m, and soil Kd values from 10-2 to 102 L/kg for each of four major soil textures considered (sand, loam, clay and organic). Concentrations for contamination by way of groundwater and by irrigation were computed. A regression analysis of this database from SCEMR1 was coupled with a response function in BIOTRAC. This response function recreates the time dependence and incorporates radiological decay and ingrowth of daughters and losses of radionuclides from soil through degassing and cropping. The coupling of SCEMR1 and BIOTRAC is described and a sample calculation is shown. (Author)