I’ve never heard of them being federal property.
The Federal Energy Regulatory Commission (FERC) requires that all power plants submit the cost of construction broken down in a code of accounts for depreciation purposes. That’s for nukes and non nukes.
That Makes no sense if the federal government owned them.
I think that you are right about the ownership.
I believe that over a certain period of time, the stainless steel tubing and parts become brittle,
after exposure to radiation and high temps,
and must be replaced periodically over time.
It's all part of the routine maintenance required by the NRC.