Strategy for verification and demonstration of the sealing process for canisters with spent nuclear fuel 
Öberg, T.
Presentation vid 2004 Annual Meeting of the Society for Risk Analysis, December 5-8, Palm Springs, California.

Abstract
The Swedish Nuclear Fuel and Waste Management Co (Svensk Kärnbränslehantering AB) is responsible for the management and disposal of spent nuclear fuel in Sweden. A research, development and demonstration program is now being implemented, aiming for final disposal of the radioactive waste in a deep repository.

Electron beam welding and friction stir welding are the two processes now being considered for sealing copper canisters with Sweden's radioactive waste. This paper outlines a strategy for verification and demonstration of the encapsulation process which here is considered to consist of the sealing of the canister by welding followed by quality control of the weld by non-destructive testing.

The initial verification experiments of the two welding technologies considered are suggested to proceed by experimental plans that can be accomplished with only four complete lid welds each. Similar experimental arrangements can be used to evaluate process 'robustness' and optimization of the process window.

Unusual events, like failures in the lid welds, are best described with extreme value statistics and the extreme value paradigm give a rationale for extrapolation. Models based on block maxima (the generalized extreme value distribution) and peaks over threshold (the generalized Pareto distribution) are considered. Experiences from other fields of the materials sciences suggest that both of these approaches are useful.

Two series of twenty demonstration trials each, mimicking assembly-line production, are suggested as a final evaluation before the selection of welding technology. This demonstration is also expected to provide a data base suitable for a baseline estimate of future performance.

The final estimation of failure ratios depend on the performance of the canister production, the welding technology and the non-destructive testing. These estimates will provide input for probabilistic modeling in the overall safety assessment for the deep repository.

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