3 ms·

With the traditional flibe salt, you have to enrich the Li to reduce tritium production. And Be is a pretty nasty material to handle as well. But in the end th

by jabl 8y ago

With the traditional flibe salt, you have to enrich the Li to reduce tritium production. And Be is a pretty nasty material to handle as well. But in the end the decisive factor is the neutronics. Turns out that in the fast spectrum chloride salt are better, in the thermal spectrum flourides.

JumpCrisscross 8y ago
> in the fast spectrum chloride salt are better, in the thermal spectrum flourides Would you mind expanding on this? Why?
- pfdietz 8y ago
  He's talking about FLiBe. Both 7Li and 9Be are fairly light isotopes, so they moderate neutrons, better than carbon in graphite does. Unfortunately both are expensive. The fluoride cooling salt used by Moltex does not use lithium or beryllium. Instead, it's sodium-zirconium fluoride or sodium-potassium-zirconium fluoride, with no isotope separation required. The atomic mass of these isotopes is high enough that the neutron spectrum remains hard. In fact, Moltex's design adds a bit of hafnium (a strong thermal neutron absorber that is chemically very similar to zirconium) to the coolant salt to shield the walls of the reactor from thermal neutrons. One issue with fluoride salts in homogenous reactors is that the salt must be kept a bit oxidized, so uranium does not disproportionate into UF3 and U metal. The metal would plate out on surfaces, which is a no-no. Unfortunately, this means the salt must be oxidizing enough that it will dissolve chromium, so ordinary stainless steels cannot be used. By using a barren coolant salt, Moltex can make the salt reducing enough that chromium is stable (they do this by using a sacrificial anode of zirconium metal.) Their design can use stainless steels that have already been tested and proven for use with the neutron exposures of fast reactors. The chloride fuel salt in Moltex (and Terrapower) don't have that chemistry issue; uranium is more chemically stable there.
- jabl 8y ago
  In addition to what the sibling poster said, https://info.ornl.gov/sites/publications/files/Pub29596.pdf https://info.ornl.gov/sites/publications/files/Pub29596.pdf and http://moltensalt.org/references/static/downloads/pdf/MCFR_BATR.pdf http://moltensalt.org/references/static/downloads/pdf/MCFR_B... contain some discussion of the salt options, particularly for fast reactors. For thermal reactors, particularly if you want to breed, then you have to be very careful wrt losing neutrons (since you're barely with your nose above water to begin with), so there parasitic neutron capture becomes a very important parameter. And yes, in that regime FLiBe turns out to be a pretty good choice. For fast reactors, you have more excess neutrons so you have a bit more freedom. Cl, being heavier, allows a slightly harder spectrum, which is good, though not that critical. That being said, there are many fast spectrum paper reactors using flouride salts as well (not FLiBe, but FLiNaK, or some ZrF salts etc.).

by jabl 8y ago

JumpCrisscross 8y ago

> in the fast spectrum chloride salt are better, in the thermal spectrum flourides Would you mind expanding on this? Why?

pfdietz 8y ago
He's talking about FLiBe. Both 7Li and 9Be are fairly light isotopes, so they moderate neutrons, better than carbon in graphite does. Unfortunately both are expensive. The fluoride cooling salt used by Moltex does not use lithium or beryllium. Instead, it's sodium-zirconium fluoride or sodium-potassium-zirconium fluoride, with no isotope separation required. The atomic mass of these isotopes is high enough that the neutron spectrum remains hard. In fact, Moltex's design adds a bit of hafnium (a strong thermal neutron absorber that is chemically very similar to zirconium) to the coolant salt to shield the walls of the reactor from thermal neutrons. One issue with fluoride salts in homogenous reactors is that the salt must be kept a bit oxidized, so uranium does not disproportionate into UF3 and U metal. The metal would plate out on surfaces, which is a no-no. Unfortunately, this means the salt must be oxidizing enough that it will dissolve chromium, so ordinary stainless steels cannot be used. By using a barren coolant salt, Moltex can make the salt reducing enough that chromium is stable (they do this by using a sacrificial anode of zirconium metal.) Their design can use stainless steels that have already been tested and proven for use with the neutron exposures of fast reactors. The chloride fuel salt in Moltex (and Terrapower) don't have that chemistry issue; uranium is more chemically stable there.
jabl 8y ago
In addition to what the sibling poster said, https://info.ornl.gov/sites/publications/files/Pub29596.pdf https://info.ornl.gov/sites/publications/files/Pub29596.pdf and http://moltensalt.org/references/static/downloads/pdf/MCFR_BATR.pdf http://moltensalt.org/references/static/downloads/pdf/MCFR_B... contain some discussion of the salt options, particularly for fast reactors. For thermal reactors, particularly if you want to breed, then you have to be very careful wrt losing neutrons (since you're barely with your nose above water to begin with), so there parasitic neutron capture becomes a very important parameter. And yes, in that regime FLiBe turns out to be a pretty good choice. For fast reactors, you have more excess neutrons so you have a bit more freedom. Cl, being heavier, allows a slightly harder spectrum, which is good, though not that critical. That being said, there are many fast spectrum paper reactors using flouride salts as well (not FLiBe, but FLiNaK, or some ZrF salts etc.).