The reactor pressure vessel (RPV) is rated a highest priority key (Category 1) component in all national categorisation exercises because the RPV is considered irreplaceable or prohibitively expensive to replace. This in turn means that if it degrades sufficiently it could be the feature that limits the nuclear plant's operational life. The RPV houses the reactor core and because of its function it has a direct safety significance. The majority of early Westinghouse designed plants had a 'design basis life' (as distinct from physical life) of 30-40 years. The design basis life was based mostly on fatigue usage factors, not general technical studies of material degradation. Now that light water reactors are being operated for longer than originally intended, this has become relevant, as radiation embrittlement is the most important degradation mechanism limiting RPV life. Neutron irradiation degrades the mechanical properties of RPV steels and the extent of the degradation is determined by a number of factors such as neutron fluence, irradiation temperature, neutron flux and the concentration of deleterious elements in the steel.
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