Nuclear reactors, which started commercial operations in the 1960s are wearing out faster than expected. As the plants age, numerous unexpected problems become inevitable. In particular, the reactor vessels become brittle because of year of bombardment by subatomic particles. The potential for a disaster increase with each year.
Embrittlement depends mostly on plant design, metallurgy and years of operation.
In 1992, the Nuclear Regulatory Commission found 15 nuclear reactors around the USA with reactor vessels so weakened by radiation that they would required careful analysis to determine if they are safe.