5 ms·

Nuclear engineer here. That's not quite how it all works. Solid vs. liquid fuel is not tied directly to reactivity stability, as quantified in the power coeffi

Nuclear engineer here. That's not quite how it all works. Solid vs. liquid fuel is not tied directly to reactivity stability, as quantified in the power coefficient of reactivity. If it goes up in power, you want the chain reaction to naturally go down. In solid fueled reactors, this is usually accomplished via the moderator. If the moderator temperature goes up, it reduced in density, thereby reducing the overall neutron moderation in the core. Thus, fewer neutrons make it to to the energy range that causes fission. Thus, reactivity goes down and the reaction stops. This is inherently stable, just like fuel density in a fluid fuel reactor going down and reducing the overall fission rate. LFTRs are pre-melted. You melt 100% of the core and then bring it critical. That's a lot of pretty mobile fission products! As for the melt plug, that's also a false solution. Given that achieving subcriticality is trivial in modern solid and fluid fuel reactors, the challenge in an accident is afterglow heat removal. As you may know, the plants at Fukushima had all rods in and were fully subcritical an hour before the tsunami hit). But when they lost afterglow heat removal, it still melted some containment barriers. Same can happen with fluid fuel, regardless of whether or not you've moved it from one tank to another. Fluid fuel is not the panacea many people want to think it is. Passive afterglow heat removal is the thing that lets reactors of any fuel form be safer than today's typical reactors, which generally require backup power to run the cooling systems. If you use certain molten salt, liquid sodium metal, liquid lead metal, etc. cooling configurations you can achieve indefinite heat removal without any external power. That reduces core damage frequencies by about 100x from modern large LWRs. Again, regardless of fuel form. That said, 100x safer than how safe current nuclear is is kind of just playing with very small numbers. Fossil and biofuel combustion kills 8 million per year from particulate emissions, according to the WHO, and also cause climate change. So we should just be building hundreds of regular large water cooled reactors now and then switch over to fancier cooling ones later, and also breeders that are ~infinitely sustainable for the long term.

Symmetry 4y ago
For people who don't know this stuff, like myself a few years ago: When a uranium atom is hit by a neutron some of the will split immediately releasing new neutrons but some of them will go into an unstable state and then split some period of time later. If the instantaneous splits are enough to keep the reaction going that's called a "prompt critical" configuration and usually seen only in atomic weapons. If the neutrons released by both the immediate reactions and the delayed reactions are enough to keep the reaction going that's only "critical." Because there are many atoms in the reactor that have been hit by neutrons and are unstable but haven't split yet a reactor continues to release a lot of heat even when it's no long critical, somewhere on the order of 10% as much power as when it was fully on.
pfdietz 4y ago
> That's a lot of pretty mobile fission products! https://gain.inl.gov/SiteAssets/MoltenSaltReactor/Module2-OverviewofMSRTechnologyandConceptsML17331B114.pdf https://gain.inl.gov/SiteAssets/MoltenSaltReactor/Module2-Ov... (slide 23) "Gaseous Fission Products Inherently Evolve from Fuel Salt" "Many FPs have Xe or Kr precursors – Over 40% of FPs leave core – Large fraction of cesium, strontium and iodine end up in offgas"
panick21_ 4y ago
I agree freeze plug isn't magic, passive heat removal can be much easier if you can move the fuel into a different contaiment with different geometry and different passive heat removal features. However it has to be noted that most of the companies that work on molten salt reactors dont use that method. Terrestrial Energy, Moltex Energy for example.
- acidburnNSA 4y ago
  Meh, I dunno I think freeze plugs are really falling out of favor in general. They take a long time to melt, aren't that reliable or predictable, and can spuriously actuate, dumping the whole core. Why make two vessels when you already need one? Just add passive afterglow heat removal to the one and you're done. Moving stuff around for no reason doesn't add anything.
  - panick21_ 4y ago
    If you have a breeding reactor that has a mantle and you have a graphite core that you might want to replace then being able to easily remove material from the core might be a good thing. People currently doing commercial have moved away from this and mostly just do fast reactors and mostly don't want to do breeding or do core replacements.
AtlasBarfed 4y ago
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.
p1mrx 4y ago
> Passive afterglow heat removal is the thing that lets reactors of any fuel form be safer than today's typical reactors, which generally require backup power to run the cooling systems. NuScale does not require backup power. They solve the afterglow heat removal problem by running the reactors under millions of gallons of water. By the time it all boils away (about a month), passive air cooling is sufficient.

Symmetry 4y ago

For people who don't know this stuff, like myself a few years ago: When a uranium atom is hit by a neutron some of the will split immediately releasing new neutrons but some of them will go into an unstable state and then split some period of time later. If the instantaneous splits are enough to keep the reaction going that's called a "prompt critical" configuration and usually seen only in atomic weapons. If the neutrons released by both the immediate reactions and the delayed reactions are enough to keep the reaction going that's only "critical." Because there are many atoms in the reactor that have been hit by neutrons and are unstable but haven't split yet a reactor continues to release a lot of heat even when it's no long critical, somewhere on the order of 10% as much power as when it was fully on.

pfdietz 4y ago

> That's a lot of pretty mobile fission products! https://gain.inl.gov/SiteAssets/MoltenSaltReactor/Module2-OverviewofMSRTechnologyandConceptsML17331B114.pdf https://gain.inl.gov/SiteAssets/MoltenSaltReactor/Module2-Ov... (slide 23) "Gaseous Fission Products Inherently Evolve from Fuel Salt" "Many FPs have Xe or Kr precursors – Over 40% of FPs leave core – Large fraction of cesium, strontium and iodine end up in offgas"

panick21_ 4y ago

I agree freeze plug isn't magic, passive heat removal can be much easier if you can move the fuel into a different contaiment with different geometry and different passive heat removal features. However it has to be noted that most of the companies that work on molten salt reactors dont use that method. Terrestrial Energy, Moltex Energy for example.

acidburnNSA 4y ago
Meh, I dunno I think freeze plugs are really falling out of favor in general. They take a long time to melt, aren't that reliable or predictable, and can spuriously actuate, dumping the whole core. Why make two vessels when you already need one? Just add passive afterglow heat removal to the one and you're done. Moving stuff around for no reason doesn't add anything.
- panick21_ 4y ago
  If you have a breeding reactor that has a mantle and you have a graphite core that you might want to replace then being able to easily remove material from the core might be a good thing. People currently doing commercial have moved away from this and mostly just do fast reactors and mostly don't want to do breeding or do core replacements.

AtlasBarfed 4y ago

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.

p1mrx 4y ago

> Passive afterglow heat removal is the thing that lets reactors of any fuel form be safer than today's typical reactors, which generally require backup power to run the cooling systems. NuScale does not require backup power. They solve the afterglow heat removal problem by running the reactors under millions of gallons of water. By the time it all boils away (about a month), passive air cooling is sufficient.