The uranium combustion scale is always full, there is no waste of resource, literally infinite energy if there is at least one uranium fuel cell.
PS. in multiplayer, in single not tested

It only looks that way because of how much energy nuclear fuel has. For example, in a reactor running at full power, a uranium fuel cell will last for about 8.1 real-world days.

Does that seem overpowered? Well, that's the description, "realistically overpowered." That's really how much energy nuclear fuel contains!

If you want to make it harder, you can enable the nuclear cores option, which requires providing a realistic amount of upfront fuel loading and enrichment to start up a reactor with the properties seen in Factorio. These last even longer (2.2 real-world years at full power), but the upfront cost makes it a much heavier investment per reactor, which still only provide 160 MW each.

On second thought, I realized that I might have misunderstood you, and that the reactors might actually be consuming zero fuel, if the fuel values are so large that single-tick power consumption is smaller than the floating-point precision at that magnitude, which could cause all power consumption to round down to zero.

So, I added a debug script that would print out the burner fuel remaining in reactors and tested it. And I can now confirm that fuel IS being consumed at the correct rate, even with a 11.2 PJ nuclear core powering a single, idle yellow inserter.

Shout out to the devs for making their game robust against such extreme values.

Thank you for response!