5 ms·

TIL about liquid metal cooled nuclear reactors. Wikipedia makes lead-cooled reactors sound pretty amazing, and yet lead-bismuth seems to be preferred, presumabl

TIL about liquid metal cooled nuclear reactors. Wikipedia makes lead-cooled reactors sound pretty amazing, and yet lead-bismuth seems to be preferred, presumably simply because of the lower melting point, despite the disadvantages of bismuth (polonium byproduct, high cost). I'd be interested to hear if there are other reasons pure lead cooling isn't being more actively pursued. Especially for small, single-use reactors, it seems like it would be perfect.

mechhacker 7y ago
Lead has some nice benefits: Provides shielding or reflection (depends on the reactor spectrum) High boiling point Doesn't have the same chemical reactivity as other liquid metals like sodium or NaK But, it needs to be balanced with the drawbacks: High density. Liquid sodium is less than a tenth of the density of liquid lead. This becomes more of a problem the larger the reactor gets due to seismic loading. Poorer heat transfer qualities than sodium (which is why liquid metals have mostly stuck to sodium) Must be designed to flow with low velocities due to corrosion/erosion* *This can drastically affect the size of the piping or flow passages required for cooling. Flowing above a certain velocity will erode and destroy pumps, piping, etc. A low performance reactor won't be as highly impacted as a high performance reactor that needs to flow as much coolant as fast as possible. The Russians have done a lot of work in this area.
- marcosdumay 7y ago
  Hum... All those drawbacks are things that improve at least linearly when you reduce the reactor size and power. Looks like lead would be a great option for those small reactors.
minikites 7y ago
The reactor is difficult to turn off: https://en.wikipedia.org/wiki/Alfa-class_submarine https://en.wikipedia.org/wiki/Alfa-class_submarine >The issue was that the lead/bismuth eutectic solution solidifies at 125 °C (257 °F). If it ever hardened, it would be impossible to restart the reactor, since the fuel assemblies would be frozen in the solidified coolant. Thus, whenever the reactor is shut down, the liquid coolant must be heated externally with superheated steam.
- Florin_Andrei 7y ago
  Well, sounds like it's difficult to restart actually.
  - corporate_shi11 7y ago
    Which makes it difficult to turn off if you would like to restart it.
    - postingFourFast 7y ago
      Which doesn't matter in the most important scenario: a crisis.
- makomk 7y ago
  Yeah, those Soviet lead-bismuth reactors were basically single-use devices - once they ran out of fuel, they were useless and had to be replaced with a new reactor. I don't think Russia had any plans on how to safely decommission and dispose of them at the end of their life either.
  - mechhacker 7y ago
    If you're interested in a more modern Russian lead reactor, check out: https://inis.iaea.org/collection/NCLCollectionStore/_Public/49/085/49085788.pdf https://inis.iaea.org/collection/NCLCollectionStore/_Public/... They also have a lot of recent experience with sodium fast reactors, of which the BN-600 is still running, BN-800 is running (and was started within the past 2 years), but the BN-1200 is delayed: http://fissilematerials.org/blog/2019/08/the_construction_of_the_b.html http://fissilematerials.org/blog/2019/08/the_construction_of...
- pfdietz 7y ago
  And the presence of bismuth means the coolant becomes contaminated with polonium, a very nasty alpha emitter.
jabl 7y ago
Hot molten lead eats structural (stainless) steels for breakfast. If you get the redox chemistry just right, a thin protective oxide layer forms on the steel surface, but it's tricky to maintain in a vessel with flowing liquid. The Russians in their Alfa submarine IIRC had some mechanism for dissolving lead oxide into the coolant under way. The Swedish company Leadcold has a process to produce a protective alumina layer on the surface, remains to be seen whether it works out.

mechhacker 7y ago

Lead has some nice benefits: Provides shielding or reflection (depends on the reactor spectrum) High boiling point Doesn't have the same chemical reactivity as other liquid metals like sodium or NaK But, it needs to be balanced with the drawbacks: High density. Liquid sodium is less than a tenth of the density of liquid lead. This becomes more of a problem the larger the reactor gets due to seismic loading. Poorer heat transfer qualities than sodium (which is why liquid metals have mostly stuck to sodium) Must be designed to flow with low velocities due to corrosion/erosion* *This can drastically affect the size of the piping or flow passages required for cooling. Flowing above a certain velocity will erode and destroy pumps, piping, etc. A low performance reactor won't be as highly impacted as a high performance reactor that needs to flow as much coolant as fast as possible. The Russians have done a lot of work in this area.

marcosdumay 7y ago
Hum... All those drawbacks are things that improve at least linearly when you reduce the reactor size and power. Looks like lead would be a great option for those small reactors.

minikites 7y ago

The reactor is difficult to turn off: https://en.wikipedia.org/wiki/Alfa-class_submarine https://en.wikipedia.org/wiki/Alfa-class_submarine >The issue was that the lead/bismuth eutectic solution solidifies at 125 °C (257 °F). If it ever hardened, it would be impossible to restart the reactor, since the fuel assemblies would be frozen in the solidified coolant. Thus, whenever the reactor is shut down, the liquid coolant must be heated externally with superheated steam.

Florin_Andrei 7y ago
Well, sounds like it's difficult to restart actually.
- corporate_shi11 7y ago
  Which makes it difficult to turn off if you would like to restart it.
  - postingFourFast 7y ago
    Which doesn't matter in the most important scenario: a crisis.
makomk 7y ago
Yeah, those Soviet lead-bismuth reactors were basically single-use devices - once they ran out of fuel, they were useless and had to be replaced with a new reactor. I don't think Russia had any plans on how to safely decommission and dispose of them at the end of their life either.
- mechhacker 7y ago
  If you're interested in a more modern Russian lead reactor, check out: https://inis.iaea.org/collection/NCLCollectionStore/_Public/49/085/49085788.pdf https://inis.iaea.org/collection/NCLCollectionStore/_Public/... They also have a lot of recent experience with sodium fast reactors, of which the BN-600 is still running, BN-800 is running (and was started within the past 2 years), but the BN-1200 is delayed: http://fissilematerials.org/blog/2019/08/the_construction_of_the_b.html http://fissilematerials.org/blog/2019/08/the_construction_of...
pfdietz 7y ago
And the presence of bismuth means the coolant becomes contaminated with polonium, a very nasty alpha emitter.

jabl 7y ago

Hot molten lead eats structural (stainless) steels for breakfast. If you get the redox chemistry just right, a thin protective oxide layer forms on the steel surface, but it's tricky to maintain in a vessel with flowing liquid. The Russians in their Alfa submarine IIRC had some mechanism for dissolving lead oxide into the coolant under way. The Swedish company Leadcold has a process to produce a protective alumina layer on the surface, remains to be seen whether it works out.