Problems highlighted by operating experience

During the past 45 years of peaceful use of nuclear energy, no case of a nuclear reactor pressure vessel rupture has occurred. This hypothetical event is not included in the design basis accidents (DBAs) nor, according to the most recent trends, among the severe accidents to be reasonably considered. This is not, as it will be discussed more extensively later, the only possible choice it, however, has been considered acceptable and practicable. 

Other additional facts must be considered in the operating experience of nuclear pressure vessels. An example was the case of a nuclear reactor steam generator which was built twenty years ago according 

It has to be said, however, that in retrospect, following tests on material samples, the initial estimates resulted to be too pessimistic. 

Another example is the corrosion damage caused by boric acid of the vessel upper head of the US reactor at Davis Besse in 2002. A cavity as deep as the carbon steel wall and with similar dimensions in plan was produced, leaving the stainless steel internal liner as the only barrier against the massive efflux of primary fluid (see Chapter 20). 

Whereas the dangers of the above example never materialized, the behaviour of the reactor pressure vessel during the Three Mile Island accident is exceptional. It withstood the outpouring of about twenty tons of molten core on its bottom, in conditions of highly deteriorated internal cooling. This behaviour indicated to the technical experts the presence of a powerful and up to then neglected barrier in the Defence in Depth, which is now utilized in a planned way as a potential asset. 

---