Simulating the Behavior of Natural Geological Systems: Sources of Uncertainty and Bias

by D. Read, WS Atkins Science & Technology, Surrey, United Kingdom,
D. G. Bennett, WS Atkins Science & Technology, Surrey, United Kingdom,
J. B. Thomas, WS Atkins Science & Technology, Surrey, United Kingdom,

Abstract: Post-closure performance assessments are used, in conjunction with controlled laboratory experiments and observations on natural systems, to generate confidence that a radioactive waste disposal facility will not give rise to unacceptable risks over timescales of many thousands of years. A number of countries now invoke highly developed assessment methodologies employing a combination of deterministic and probabilistic techniques. The procedures vary from country to country but, in each case, a risk-based criterion is used to evaluate performance against regulatory targets. It is important to identify the sources of uncertainty in these numerical calculations and to demonstrate that they can be traced through to the final risk estimate. Most probabilistic studies include a formalized treatment of uncertainty arising from, inter alia: an incomplete understanding of the disposal system and the processes operating; the inherent variability of natural systems and the constraints of sampling; the need to extrapolate the effects of radionuclide release, transport and exposure pathways over protracted timescales; the largely indeterminate nature of climatic evolution. This paper examines the problem of identifying and dealing with uncertainty associated with geochemical systems. Examples from both the laboratory and field illustrate the often dominating effect of conceptual model uncertainty and the pitfalls which result from matching data to assumed or 'consensus' models.

Subject Headings: Uncertainty principles | Geology | Risk management | Probability | Data processing | Radioactive wastes | Waste disposal | Radioactive materials

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