The fact that a reactor may be failing after decades of use does not make it a poor design. One optimizes for longevity during design, as well as other factors

The fact that a reactor may be failing after decades of use does not make it a poor design. One optimizes for longevity during design, as well as other factors which cost money. It may have, for example, been cheaper to construct, with a strict lifetime after which it would be taken down."Good design" is almost always subjective. That politics drive nuclear operators to maintain plants past their lifetimes does not indicate poor engineering.