Biggest issue with older nuclear plants is the material degradation as a consequence of radiation, after a some decades of constant irradiation steel can become as brittle as glass, which is not only not reversible, but it also means that the actual rebar and concrete surrounding the core becomes damaged and the only way to replace that is to rebuild the whole core I am rather against setting up new reactors, given t…
I think you are making a cofusion between thermal fission reactors and fast reactors where neutron radiation is indeed a problem. Rebar and concrete in fission plants ages due to thermal stress plus the usual culprits (humidity, oxygen), not necessarily neutron radiation.
Even when the reactors themselves, generally rest on light or heavy water, the steel, titanium and other metals and materials nearby the pellets or rods suffer greatly as a consequence of radiation, so it is not a matter of just neutron degradation, but a generalized problem of the physical interaction between radiation and the molecular structures of the materials
Sadly for my case, my materials science course on University was very, very weak, but last year I found this MITOpenCourse which covers this topic on detail, I would highly recommend it to absolutely everyone interested on the topic
