UNCERTAINTY / SENSITIVITY METHODOLOGIES FOR SAFETY ASSESSMENTS OF LOW-LEVEL WASTE DISPOSAL FACILITIES

Safety assessment for low-level waste facilities requires the interaction of a large number of disciplines in order to model environmental phenomena necessary to evaluate safety of disposal. The physical systems involved can often be very complex. The initial purpose of the safety analysis is to better understand the system under study. Eventually, as the system behavior becomes understood more fully, the assessment is used to support regulatory decisions. Corresponding to the specific goals of the project the objectives for uncertainty and sensitivity analysis will also vary, depending on the stage of the analysis. Typically, the safety analyst has to simplify the physical system into a conceptual model that can be modeled mathematically. The first step in this process involves defining an exposure scenario and this is often a significant source of uncertainty (future climate assumptions or individual habits). Simplification of the physical system to a mathematical model is another source of uncertainty, commonly called model uncertainty. Other sources of uncertainties include parameter estimation and random variability in parameters measurements. In assessments the analyst may need to rely on expert judgment due to lack of data, lack of knowledge concerning future conditions and parameter values (and distributions), or any aspects of the system under study that are not well understood by current science. This generates another kind of uncertainty, “subjective uncertainty”. This paper is part of the work being developed by the IAEA sponsored project