So if the moderator fails (like Fukushima and all meltdowns) and the active cooling fails, what does the passive cooler do to drop the neutron economy/chain reaction in the fuel rods? It just keeps the rods cool so they don't melt through the floor, and they do that until the rods finally drop the economy?

I still don't like it because nothing in the fuel rod safety does anything about the continued criticality. What are the passive cooling systems, are they big heat sinks and pipes? What happens if an earthquake or explosion disrupts the heat sinks or heat pipe connection to the solid rods? Makes the coolant leak out?

Speed of melt of the plug doesn't seem like a big deal, you simply use a thinner plug if you're worried about that. Dumping out of the core in a liquid fuel isn't a big deal, if the core is intact but a dump-out occurs with a plug you simply replace the plug and send the liquid fuel back into the reactor.

I mean, liquid fuel reprocessing obviously isn't simple, the materials around the liquid fuel and high temperature isn't simple.

The bottom line is that you can downvote me, but I'm basically an example the first tier of people you need to convince for politically viable nuclear. This is what LFTR really appeals to me on:

- total fuel use. Yes I know it won't be 0% waste, those fission products can be nasty, but ... still total fuel use is a big selling point to me

- ability to breed / consume spent fuel waste ... clean up the mistakes of the past

- modular : some hope to be economical

- contained on single facility: no transport of waste, no disruption of transportation infrastructure, no risk of terrorism/hijacking, no Yucca mountain

- closed loop economics: you see the full lifecycle. No hiding costs in reprocessing or transportation or storage, you have the facility, it's operating cost, and you know.

- safety: you didn't refute that liquid fuels are safer than solid rods. No one really knows what at-scale processing of MSR fission products involves, so I could be wrong, but "mobile" implies "processable" to me.

I've always wondered that even if LFTRs aren't economical, they might be an economical cleanup facility: let cheaper nuclear designs generate the power, then send the spent fuel to a LFTR facility that ... maybe ... melts the spent fuel and breeds/processes the products, and at least the processing cost is offset by the power you get from the LFTR and the useful/valuable products.

The real issue is that nuclear isn't cost competitive with solar/wind, and might not be competitive with solar/wind+storage. Solar/wind and especially batteries are going to go through a decade of nonlinear cost improvement in the next decade that ... probably ... drops their cost by half.

So nuclear will only be a load leveller tech, and needs to compete with hydro (and pumped hydro storage), geothermal, whatever comes out of synthfuels/"green" hydrogen.

I'm of the view we need to invest in nuclear research, but going all-in on nuclear plants? Nope, the nuclear industry should have gotten off its tush 20-30 years ago with a more compelling design that addresses full lifecycle and safety and economics.

Nuclear should have recognized the enormous opportunity global warming represented, but the nuclear industry seems full of "green" hostile (from the antinuke conflicts) and regulatory hostile people that it couldn't bring itself to align with left-wing environmentalism.