SYVAC, Risk Assessment from Underground Radioactive Waste Disposal in UK

1 - Description of program or function: The SYVAC program simulates the ground water mediated movement of radionuclides from underground facilities for the disposal of low and intermediate level wastes to the accessible environment, and provides an estimate of the subsequent radiological risk to man. The simulated timescales are usually within the range 1.0 E+03 to 1.0 E+07 years. SYVAC is capable of modelling both shallow disposal facilities (located in argillaceous media and overlaying an aquifer) and deep disposal facilities (in a saturated environment). It is not suitable for considering high level wastes as it does not allow for the heat produced by such wastes. SYVAC has been developed for use within the DOE Radioactive Waste Management Programme, as one tool in the DOE Assessment Methodology. 2 - Method of solution: - The Physical System: A simplistic description of the physical simulated by SYVAC is given. This simulation requires data to describe the physical structure and transport characteristics of the vault and geosphere. The necessary data is supplied to SYVAC in the form of parameters that represent hydraulic conductivities, diffusion coefficients, porosities etc for the vault and geosphere. Thirteen basic vault parameters are used to model deep disposal sites, and fifteen for shallow disposals. For both types of site, an additional six parameters are needed for each geological layer modelled in the geosphere. The values of some of the parameters are not precisely known, and will vary over the timescale of the transport to the biosphere. SYVAC allows for this with a probabilistic approach; the parameter values are sampled from distributions chosen to cover the expected range of values that may be encountered during the post-closure phase of a vault. For each parameter, SYVAC allows the user to select one of five distributions: constant, uniform, log uniform, normal and log normal. Usually, for each parameter, the user will select the distribution that best fits the observed parameter values. The calculation of risk to man requires knowledge of both the dose from a particular run, and the probability of that run. Clearly, the dose to man calculated for a particular simulation run will depend on the parameter values chosen for that run. Two different sampling methods can be chosen for SYVAC; Random Sampling and Deterministic Generator Sampling. Random Sampling chooses values (randomly) from the entire range of the distribution chosen for each parameter. Deterministic Generator sampling selects one of 11 possible (discrete) sample points for each parameter for a run; the method of sampling is such that all eleven points are chosen in the course of 11 consecutive runs. The eleven points span the range of the distribution chosen for each parameter. The Executive is the framework that controls the simulation program as a whole, and specifically calls the Vault and Geosphere sub-models. The Vault sub-model simulates the release of radionuclides from non heat-producing radioactive waste placed within an engineered vault in either deep or shallow land disposal facilities. The model includes representations of the waste matrix, waste canister, vault liner, the vault backfill material and the region of host rock around the vault disturbed during construction. The migration model includes the processes of solubility-limited or mass-limited leaching of radionuclides from the waste matrix, linear equilibrium sorption of radionuclides onto the barrier materials, radioactive chain decay, dispersion and advection. The Geosphere sub-model is a 1-dimensional model that deals with the migration of radionuclides from the vault to the biosphere through the geosphere. This is through the calculation of a response function for each geosphere layer. Up to five distinct layers can be modelled. The biosphere code ECOS calculates the rate of accumulation of committed effective dose equivalent (dose) arising from the flux of radionuclides entering the biosphere. Dose to a maximally exposed individual and collective dose are calculated deterministically. 3 - Restrictions on the complexity of the problem: - for the Vault Sub-model, they concern the representations of the flow, the time invariant chemistry, the equations for flow and transport, the water table, the interaction between adjacent Vault structures, the limitations on number of canisters, nuclides per chain, etc.; for the Geosphere Sub-model, they concern the time-steps in the model, the number of nuclides and geosphere layers; for the Biosphere Model, they concern the estimates of activity and dose to man, the production or transport of gaseous radionuclide species and doses