4 ms·

Can pebble beds have a cooldown pan similar to a LFTR, where a plug melts and the "pebbles" fall and spread into a pan where they won't stay critical because th

Can pebble beds have a cooldown pan similar to a LFTR, where a plug melts and the "pebbles" fall and spread into a pan where they won't stay critical because they are too separated / unconcentrated? Because the real problem with solid rods is that they ... are solid rods, and if they start "overreacting" you can't split up the rods, unlike a pile of pebbles/spheres.

cyberax 2y ago
The unique "melt plug" safety story of LFTRs is mostly a fairy tale. Modern PWRs also have this safety feature, if a core melts down, the molten mass will be contained in a core catcher. Where it'll be mixed with inert material that can provide enough surface area and thermal mass to prevent further fuel mass migration. The biggest problem in the core catcher design was to make sure that the molten fuel lava spreads out enough for the passive cooling to stop it from melting through concrete. Pebble bed reactors will have a similar problem. You can "drain" pebble beds somewhere, but then you need to make sure that this "somewhere" can conduct away the decay heat without melting.
- AtlasBarfed 2y ago
  ... why? You're saying LFTRs can't have a pool of cooling liquid that the overheating fuel liquid can't mix with and cool down even faster than solid rods? So how could LFTR safety plug and cooling pool be a fairy tale?
  - cyberax 2y ago
    A failure that results in the plug melting will mean that the reactor is beyond salvaging. It's essentially the same situation as with PWRs equipped with core catchers. > can't have a pool of cooling liquid What "liquid"? Water? Do you realize what's going to happen if molten salt drops into water? First, there's going to be a steam explosion that will atomize the fuel and spread it through the whole containment building (because the water vapor can't be contained in a reasonable volume). Then the water will boil away from decay heat, and the fuel lava will continue chewing through your reactor building. That's why LFTR reactors with "melt plugs" will essentially use the same approach as PWRs: spread the molten fuel across sacrificial concrete cladding.
    - AtlasBarfed 2y ago
      Gee, I dunno, maybe any of a thousand different materials/chemical reactions to absorb heat? Yeah, I didn't say water. Maybe you just have the dump pool be a bunch of molten thorium or even solidfied thorium salt and that also plummets the neutron economy as the neutrons get absorbed by thorium as the hot uranium salt melts the thorium salts. Maybe you keep the thorium salts liquid. But fundamentally since the nuclear fuel is a liquid, the cooling pool is very wide and shallow, separating the fuel well past sustained nuclear fission. This is the problem with solid rods, they go runaway, the fuel is a solid rod, you can't separate apart the fuel without shoving some moderator into it. Yeah I don't know the viscosity of Uranium tetrafluoride, maybe that's a problem, but I doubt it. Why does the plug only melt past some point of no return? The plug can melt at whatever temperature point is desired. A "meltdown" can just be part of the usual fuel flow and recirculation. The difference is that the liquid spread because, you know it is a liquid. In liquid form? Rather than a solid form or some semi-solid uranium lava. See the difference? What am I missing here? In the dump pool, it will cool into solid salts. Then you just need to reheat it to recover the fuel, and pass it back through the salt reprocessing systems. Plus I thought LFTRs have some mechanism for self-moderation by expansion of the fluid when it gets hot, separating the fuel apart and reducing neutron economy. You seem stuck in the limitations of solid fuel rods, solid fuel reactors, and their inherent inconvenience. Yeah, the liquid is 650 degrees or something like that, but it's still liquid and you can do things with liquids that you can't do with solid rods
      - cyberax 2y ago
        > Gee, I dunno, maybe any of a thousand different materials/chemical reactions to absorb heat? Well, I definitely don't know which reaction can absorb on the order of 2GWh of residual decay heat within the first 2 days. I'm assuming a 3GWt reactor, something that at least can be competitive with PWRs. To give you some perspective, this amount of energy is enough to vaporize more than 3000 tons of water. More than an Olympic swimming pool. > Maybe you just have the dump pool be a bunch of molten thorium or even solidfied thorium salt and that also plummets the neutron economy as the neutrons get absorbed by thorium as the hot uranium salt melts the thorium salts. Maybe you keep the thorium salts liquid. Sigh. It's not the fission that is a problem. Fission will be quenched by all the neutron poisons. Even in Chernobyl or Fukushima the fission stopped immediately after the accident. It's the decay heat that has to be conducted away. > Why does the plug only melt past some point of no return? The plug can melt at whatever temperature point is desired. A "meltdown" can just be part of the usual fuel flow and recirculation. If normal recirculation works, then there's no problem with supplying cooling water. The touted advantage of molten salt reactors is their passive safety, they are supposed to fail safe even if EVERYTHING fails. > You seem stuck in the limitations of solid fuel rods, solid fuel reactors, and their inherent inconvenience. Yeah, the liquid is 650 degrees or something like that, but it's still liquid and you can do things with liquids that you can't do with solid rods Yeah, because I actually worked in the nuclear power industry.

cyberax 2y ago

The unique "melt plug" safety story of LFTRs is mostly a fairy tale. Modern PWRs also have this safety feature, if a core melts down, the molten mass will be contained in a core catcher. Where it'll be mixed with inert material that can provide enough surface area and thermal mass to prevent further fuel mass migration. The biggest problem in the core catcher design was to make sure that the molten fuel lava spreads out enough for the passive cooling to stop it from melting through concrete. Pebble bed reactors will have a similar problem. You can "drain" pebble beds somewhere, but then you need to make sure that this "somewhere" can conduct away the decay heat without melting.

AtlasBarfed 2y ago
... why? You're saying LFTRs can't have a pool of cooling liquid that the overheating fuel liquid can't mix with and cool down even faster than solid rods? So how could LFTR safety plug and cooling pool be a fairy tale?
- cyberax 2y ago
  A failure that results in the plug melting will mean that the reactor is beyond salvaging. It's essentially the same situation as with PWRs equipped with core catchers. > can't have a pool of cooling liquid What "liquid"? Water? Do you realize what's going to happen if molten salt drops into water? First, there's going to be a steam explosion that will atomize the fuel and spread it through the whole containment building (because the water vapor can't be contained in a reasonable volume). Then the water will boil away from decay heat, and the fuel lava will continue chewing through your reactor building. That's why LFTR reactors with "melt plugs" will essentially use the same approach as PWRs: spread the molten fuel across sacrificial concrete cladding.
  - AtlasBarfed 2y ago
    Gee, I dunno, maybe any of a thousand different materials/chemical reactions to absorb heat? Yeah, I didn't say water. Maybe you just have the dump pool be a bunch of molten thorium or even solidfied thorium salt and that also plummets the neutron economy as the neutrons get absorbed by thorium as the hot uranium salt melts the thorium salts. Maybe you keep the thorium salts liquid. But fundamentally since the nuclear fuel is a liquid, the cooling pool is very wide and shallow, separating the fuel well past sustained nuclear fission. This is the problem with solid rods, they go runaway, the fuel is a solid rod, you can't separate apart the fuel without shoving some moderator into it. Yeah I don't know the viscosity of Uranium tetrafluoride, maybe that's a problem, but I doubt it. Why does the plug only melt past some point of no return? The plug can melt at whatever temperature point is desired. A "meltdown" can just be part of the usual fuel flow and recirculation. The difference is that the liquid spread because, you know it is a liquid. In liquid form? Rather than a solid form or some semi-solid uranium lava. See the difference? What am I missing here? In the dump pool, it will cool into solid salts. Then you just need to reheat it to recover the fuel, and pass it back through the salt reprocessing systems. Plus I thought LFTRs have some mechanism for self-moderation by expansion of the fluid when it gets hot, separating the fuel apart and reducing neutron economy. You seem stuck in the limitations of solid fuel rods, solid fuel reactors, and their inherent inconvenience. Yeah, the liquid is 650 degrees or something like that, but it's still liquid and you can do things with liquids that you can't do with solid rods
    - cyberax 2y ago
      > Gee, I dunno, maybe any of a thousand different materials/chemical reactions to absorb heat? Well, I definitely don't know which reaction can absorb on the order of 2GWh of residual decay heat within the first 2 days. I'm assuming a 3GWt reactor, something that at least can be competitive with PWRs. To give you some perspective, this amount of energy is enough to vaporize more than 3000 tons of water. More than an Olympic swimming pool. > Maybe you just have the dump pool be a bunch of molten thorium or even solidfied thorium salt and that also plummets the neutron economy as the neutrons get absorbed by thorium as the hot uranium salt melts the thorium salts. Maybe you keep the thorium salts liquid. Sigh. It's not the fission that is a problem. Fission will be quenched by all the neutron poisons. Even in Chernobyl or Fukushima the fission stopped immediately after the accident. It's the decay heat that has to be conducted away. > Why does the plug only melt past some point of no return? The plug can melt at whatever temperature point is desired. A "meltdown" can just be part of the usual fuel flow and recirculation. If normal recirculation works, then there's no problem with supplying cooling water. The touted advantage of molten salt reactors is their passive safety, they are supposed to fail safe even if EVERYTHING fails. > You seem stuck in the limitations of solid fuel rods, solid fuel reactors, and their inherent inconvenience. Yeah, the liquid is 650 degrees or something like that, but it's still liquid and you can do things with liquids that you can't do with solid rods Yeah, because I actually worked in the nuclear power industry.