What would cause a nuclear power plant to break down after 2000 years, but not sooner?

What would cause a nuclear power plant to break down after 2000 years, but not sooner?

The setting is a society in a dark age. Their ancestors were considerably more advanced than earth currently is. The society has been stagnate at close to the tech levels of modern earth since. For the past two millennia, they have maintained the existing power plants, but cannot create new ones.

Schematics for everything in the plant mostly still exist, but are written in dead languages spread across ancient computer systems that no one understands well.

Now one of the most important plants has broken down.

What part of a power plant could work for 2000 years, but stop at that point?

My initial thought was that the uranium was depleted. But Uranium 238 and 235 have half-lives far too long. Uranium 234 only lasts ~200K years, but that still is too long.
Then I considered some mechanical or electronic failure, but it can't be something which would happen in the first few decades, or the plant managers would have a recent record of how to fix it.

Specifics about Power Plant:

The underground plant uses a super material 'durium' that is near indestructible. The ancestors used durium for any non-flexible permanent solid in the plant, such as:

-The walls, ceiling, and main structure

-The tube holding water that gets boiled and turbine sticking into it

-The water cooling tubes

The fuel is of some kind or quantity that lasts longer than 2000 years.

Bonus points if you can think of a reason why all the power plants would break down over the same century.

posted over 5 years ago

Vincent‭

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