> It looks to me like they just reflect operating costs on assets that are already paid for, or mostly paid for.

In Ontario there used to be a government-owned utility, Ontario Hydro. They built all the nuclear plants (and everything else).

But at some point things were re-jigged so parts of Ontario Hydro could be privatized (the government still owns shares), but because of how it was broken up its debt could not be split up or given to any of the resulting components.

* https://www.oefc.on.ca/debtmanage.html

So a separate legal entity was created where the debt is now held, and rate payers give money to that corporation to pay off old debt (both nuclear and non-nuclear) as part of their rate.

Also worth noting that the nuclear plants in Ontario are going through a refit currently, so it's not like the plants were built and nothing has done with them except for OpEx; there's a whole bunch of CapEx happening right now (and has been for the last few years):

* https://www.brucepower.com/life-extension-program-mcr-projec...

* https://www.ans.org/news/article-6280/bruce-power-refurbishm...

* https://www.opg.com/projects-services/projects/nuclear/darli...