To run a large-scale grid of the sort currently in use, you need to be able to produce huge amounts of power every second of every day. It’s very difficult to get that much power that reliably by any means other than burning a lot of fossil fuels, either directly – say, in a coal- or gas-fired power plant – or indirectly. Tot up the total energy content of the fossil fuels needed to mine and refine uranium and urn it into fuel rods,
Our hypothetical post-collapse warlord wouldn't need to do any of these things.
All you need to do is feed a mass of throrium-flouride salts into a steel vessel. Use a synchrotron or cyclotron to fire neutrons into a spallation target just inside the vessel (a technique developed in the 1930s) and after a few days, a self-sustaining, self-regulating chain reaction will begin.
to build, maintain, and decommission a nuclear reactor, to deal with the short-term and long-term waste, and to account for a share of the energy cost of the inevitable accidents, for example, and you’ll have a sense of the scale of the energy subsidies from fossil fuels that prop up nuclear power, and a similar realization is in store. Lacking these subsidies, it’s probably a safe bet that nuclear reactors can’t be built or maintained at all.
Our hypothetical post-collapse warlord won't need to do any of these things. Most of the high costs involved with nuclear energy are entirely artificial. Imposed by regulatory agencies and the hoops that nuclear operators must jump through.
Our post-collapse warlord won't have the NRC, (or the CNSC), the DoE or the IAEA looking over his shoulder so he will be able to basically do whatever the @#$% he wants.
This is in fact, the biggest flaw in your collapse fantasy. You assume that our society and its institutions will fall apart from lack of energy. Upon hearing the scientifically proven fact that we have literally millions of years of nuclear fuel available, you hand wave it away chanting
"It's too expensive, can't be done. It's too late, we're all going to die, blah blah blah blah..."
It's as though you think that all of our institutions except for the regulatory bodies overseeing nuclear energy will fall apart. That the government will divert its dwindling resources towards making sure that no one does something to save it.
In a liquid fueled thorium reactor, the reaction in the fuel mass is self-regulating. Since the fuel exists in a liquid state, if the temperature increases, the fuel will expand, increasing the distance between atoms in the fuel and reducing the speed, in turn reducing the temperature. This is what makes LFTRs meltdown proof. Your collapse fantasy regulates itself in a similar fashion. As social institutions fail, the regulatory controls that keep people from exploiting resources that were always there go with them.