The approach taken by Karen Gott to the elicitation was to first consider how her experience from the Swedish nuclear fleet was applicable to the US fleet of nuclear power stations. In this respect she took into account the known histories of the various degradation mechanisms which have troubled the two fleets as well as the mitigation methods which have been developed. This led her to amongst other things to the conclusion that the likelihood of an unexpected mechanism leading to failure is probably larger than the likelihood of a known mechanism resulting in failure in a region which is inspected on a regular basis. In general a new area of concern with regard to component degradation has arisen on a seven to ten year cycle over the lifetime of commercial nuclear plants.

To produce the numbers she used her database of failures and degradation in mechanical components in Swedish plants. The degradation mechanisms are the same, but the numerical figures are different because of differences in design and construction. She based her elicitation figures on the number of leaks in proportion to the number of reported cases (many were detected early) and took these to be the current figures for a good safety culture situation. The database includes other mechanical components than pipes, but does not cover steam generator tubes, so she was able to generate figures for pump and valve housings, for example. She then considered the differences in the philosophies concerning qualification and application of inspection programmes between the two countries. This she incorporated into her thinking about the safety culture aspects, both for the current time and for the extrapolation to 40 and 60 years.