6 ms·

Can someone explain to me how this reactor is meltdown proof?

DennisL123 2y ago
It’s meltdown proof in principle since the process goes into an equilibrium rather than into a runaway process.
cosmotic 2y ago
Wikipedia has an explanation: https://en.wikipedia.org/wiki/Pebble-bed_reactor#Safety https://en.wikipedia.org/wiki/Pebble-bed_reactor#Safety Summary: As the temperature rises, neutron absorption increases, reducing fission and thus temperature.
- chickenbig 2y ago
  > As the temperature rises, neutron absorption increases, reducing fission and thus temperature. Negative fuel temperature coefficient is not an unusual feature. The real question is whether the heat removal system of the reactor as a whole is sufficient to remove the decay heat to keep the fuel within the limits.
  - Gibbon1 2y ago
    I remember talking to an engineer at the old GE nuclear research facility in San Jose. He said you can design reactors to be cooled by natural convection.
    - pfdietz 2y ago
      It just makes them larger. And it makes the building containing them larger. And this makes them more expensive. NuScale's reactor was originally motivated by the desire to make it safer by using natural convection. But it ends up requiring 1/3rd more labor hours to build a NPP using their reactors than it does to build a conventional large reactor power plant.
    - chickenbig 2y ago
      > He said you can design reactors to be cooled by natural convection. That's the point of the Westinghouse AP1000; the containment (steel liner) and protection from the outside world (concrete wall) are separated, allowing the liner to cool by convection and water dripping from above. Admittedly you need to top up the water tank at the top, but that is less of a task than trying to push water into the containment.
euroderf 2y ago
IIRC there is a question about graphite fires.
AtlasBarfed 2y ago
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.
cyberax 2y ago
The reactor vessel is humongous, so the natural convective cooling can carry away the decay heat. The pebbles themselves can tolerate extremely high temperatures (literally glowing white-hot) without burning.
kevin_thibedeau 2y ago
It's not, just statistically unlikely assuming no fuel pebbles crack and coalesce their fragments.

It’s meltdown proof in principle since the process goes into an equilibrium rather than into a runaway process.

Wikipedia has an explanation: https://en.wikipedia.org/wiki/Pebble-bed_reactor#Safety https://en.wikipedia.org/wiki/Pebble-bed_reactor#Safety Summary: As the temperature rises, neutron absorption increases, reducing fission and thus temperature.

chickenbig 2y ago
> As the temperature rises, neutron absorption increases, reducing fission and thus temperature. Negative fuel temperature coefficient is not an unusual feature. The real question is whether the heat removal system of the reactor as a whole is sufficient to remove the decay heat to keep the fuel within the limits.
- Gibbon1 2y ago
  I remember talking to an engineer at the old GE nuclear research facility in San Jose. He said you can design reactors to be cooled by natural convection.
  - pfdietz 2y ago
    It just makes them larger. And it makes the building containing them larger. And this makes them more expensive. NuScale's reactor was originally motivated by the desire to make it safer by using natural convection. But it ends up requiring 1/3rd more labor hours to build a NPP using their reactors than it does to build a conventional large reactor power plant.
  - chickenbig 2y ago
    > He said you can design reactors to be cooled by natural convection. That's the point of the Westinghouse AP1000; the containment (steel liner) and protection from the outside world (concrete wall) are separated, allowing the liner to cool by convection and water dripping from above. Admittedly you need to top up the water tank at the top, but that is less of a task than trying to push water into the containment.

euroderf 2y ago

IIRC there is a question about graphite fires.

AtlasBarfed 2y ago

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.

cyberax 2y ago

The reactor vessel is humongous, so the natural convective cooling can carry away the decay heat. The pebbles themselves can tolerate extremely high temperatures (literally glowing white-hot) without burning.

kevin_thibedeau 2y ago

It's not, just statistically unlikely assuming no fuel pebbles crack and coalesce their fragments.