Biosphere modeling in waste disposal safety assessments - an example using the terrestrial-aquatic model of the environment

Geological disposal of radioactive wastes is intended to provide long-term isolation of potentially harmful radionuclides from the human environment and the biosphere. The long timescales involved pose unique problems for biosphere modeling because there are considerable uncertainties regarding the state of the biosphere into which releases might ultimately occur. The key to representing the biosphere in long-timescale assessments is the flexibility with which those aspects of the biosphere that are of relevance to dose calculations are represented, and this comes from the way in which key biosphere features, events, and processes are represented in model codes. How this is done in contemporary assessments is illustrated by the Terrestrial-Aquatic Model of the Environment (TAME), an advanced biosphere model for waste disposal assessments recently developed in Switzerland. A numerical example of the release of radionuclides from a subterranean source to an inland valley biosphere is used to illustrate how biosphere modeling is carried out and the practical ways in which meaningful quantitative results can be achieved. The results emphasize the potential for accumulation of radionuclides in the biosphere over long timescales and also illustrate the role of parameter values in such modeling.