I'm starting to use Nuclear Waste for coolant in my current game. I've set up a small loop for a sulfur geyser, and am working towards a nuclear reactor. However on the sulfur cooling loop, i keep seeing the nuclear waste "pop out" of the pipes every so often. There's no Cold Damage/state transition (i have the thermo sensor set to 42c), however slowly but surely a little pool of 10kg begins to for and grow over time, all without seeing a damaged pipe. Last time i mopped it up it was up to 110kg.
For the reactor, i am running petroleum for the turbines loop, and accumulating waste to eventually swap it out. However i noticed the Damage Overheated notification, and it was my reservoir having dumped all of its waste content into the ground (something like 500kg at the time). Not sure how that one happened.
Run Nuclear Waste through pipes/store in a reservoir. Its not fast but consistent.