Nuclear reactors are licensed with a fixed service life in all countries. This is despite the fact that reactors have widely varying ages (some plants have taken decades to complete), varying use cycles (not all plants run at 90%+ capacity factor, some are even mothballed), and undergo power downrates/uprates. There are some features that represent a hard limit, but apart from situations as with the graphite blocks in Advanced Gas Cooled Reactors most system critical components can either be replaced or see decades of use. Given this, why do nuclear reactors have a time based service limit instead of one based on their real use, as with most other machinery? What are the service limits of a nuclear reactor from an engineering perspective?