Reliability study of friction stir welded copper canisters containing Sweden's nuclear waste
Cederqvist, L., Öberg, T.
Reliability Engineering & System Safety 93, 1491-1499 (2008)
Abstract
The plan for the final disposal of spent fuel from Swedish nuclear power plants
is to place it in copper canisters that are sealed and stored in a deep
repository. The canisters will be sealed by friction stir welding (FSW) and the
reliability of this process has now been evaluated. The reliability study was
performed in three steps: first an optimization experiment to identify optimal
process settings and establish the process window; then a demonstration
experiment with welding under production-like conditions; and finally a
post-demonstration series to evaluate an adjustment in the welding equipment. A
process window was defined around the optimal process setting, i.e., the limits
within which the welding variables must lie in order for the process to produce
the desired result. In the demonstration experiment, a series of 20 sealing
welds was carried out under production-like conditions. The maximum
discontinuity in each weld — detected by non-destructive testing — was
fitted to a generalized extreme value distribution. The 95% confidence interval
for the maximum discontinuity in a production series of 4500 canisters was
estimated at 4.5–7.7 mm. The best estimate from the post-demonstration series
suggests that the maximum size of a discontinuity will not exceed 2.3 mm. A main
conclusion is that the FSW process produces reliable results, fulfilling the
predetermined requirements for minimum copper thickness by a very good margin.
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