3 ms·

Fluoride salts by themselves are very radiation resistant, chemically more so than water. The biggest concern would be liberation of elemental fluorine, but if

Fluoride salts by themselves are very radiation resistant, chemically more so than water. The biggest concern would be liberation of elemental fluorine, but if the salt is kept slightly reducing (which one can do in a sterile salt without dissolved uranium) the fluorine instantly reacts back to fluoride. The slightly reduced salt is also preferred to limit corrosion, allowing the vessel and pipes to be made of stainless steel (FLiBe with dissolved uranium needs special more expensive alloys because chromium would dissolve.) Activation is also low. The two concerns would be traces of tritium from the two-step activation of beryllium (formation of 6He by (n,alpha) reaction, decay of 6He to 6Li, then (n,t) on 6Li), and also formation of 10Be (half life, 1.4 million years, but the thermal neutron capture cross section of 9Be is only 8.5 mb). The chemical toxicity of beryllium would considerably exceed its radiotoxicity, I imagine.

credit_guy 2y ago
> traces of tritium from the two-step activation of beryllium Also from the 6Li absorption of 1 neutron. The Lithium used is almost pure 7Li, at 99.995%. But there is still 50 ppm 6Li in it.
- pfdietz 2y ago
  Yes. The inescapable tritium production from Be sets the lower bound on how pure the 7Li has to be.
cyberax 2y ago
> The biggest concern would be liberation of elemental fluorine Yep. But I'm personally more worried about the pelletized fuel. Pellets are not a great form-factor, and they don't have cladding. Fluoride salts are also far more aggressive than water (mechanically and chemically), so this will limit the maximum specific power of the reactor. So I don't think that something like 1GW molten salt reactor is even possible. It might be OK if they want to continue using SMRs, though. > The slightly reduced salt is also preferred to limit corrosion Sidenote: that's actually not always a great idea. Steel is stainless because it's covered in a film of oxides, and without oxygen it might not be able to form. This is a problem for the Russian BREST-300 reactor that is cooled by molten lead, they had to do almost 10 years of research to perfect a system that controls the amount of dissolved oxygen in the molten lead. And it's still not clear if they succeeded until the full-scale reactor is built. In this case, though, I think that they can tune the reducers to react with fluorine preferentially, while still leaving enough oxygen.
- pfdietz 2y ago
  Lead is a rather different case, because liquid lead itself corrodes steel. The oxide layer is needed to protect the steel from the lead. FLiBe by itself does not corrode steel. https://www.sciencedirect.com/science/article/abs/pii/S0010938X09000936 https://www.sciencedirect.com/science/article/abs/pii/S00109...

credit_guy 2y ago

> traces of tritium from the two-step activation of beryllium Also from the 6Li absorption of 1 neutron. The Lithium used is almost pure 7Li, at 99.995%. But there is still 50 ppm 6Li in it.

pfdietz 2y ago
Yes. The inescapable tritium production from Be sets the lower bound on how pure the 7Li has to be.

cyberax 2y ago

> The biggest concern would be liberation of elemental fluorine Yep. But I'm personally more worried about the pelletized fuel. Pellets are not a great form-factor, and they don't have cladding. Fluoride salts are also far more aggressive than water (mechanically and chemically), so this will limit the maximum specific power of the reactor. So I don't think that something like 1GW molten salt reactor is even possible. It might be OK if they want to continue using SMRs, though. > The slightly reduced salt is also preferred to limit corrosion Sidenote: that's actually not always a great idea. Steel is stainless because it's covered in a film of oxides, and without oxygen it might not be able to form. This is a problem for the Russian BREST-300 reactor that is cooled by molten lead, they had to do almost 10 years of research to perfect a system that controls the amount of dissolved oxygen in the molten lead. And it's still not clear if they succeeded until the full-scale reactor is built. In this case, though, I think that they can tune the reducers to react with fluorine preferentially, while still leaving enough oxygen.

pfdietz 2y ago
Lead is a rather different case, because liquid lead itself corrodes steel. The oxide layer is needed to protect the steel from the lead. FLiBe by itself does not corrode steel. https://www.sciencedirect.com/science/article/abs/pii/S0010938X09000936 https://www.sciencedirect.com/science/article/abs/pii/S00109...