Comment by pfdietz | Hacker News Reader

Comment by pfdietz | Hacker News Reader

Terrapower's reactor uses molten chloride salts, with fuel dissolved in the salt.

An issue with this sort of reactor is the entire primary loop becomes loaded with fission products, and becomes intensely radioactive.

I prefer the MSR (molten salt reactor) design by Moltex. http://www.moltexenergy.com/

Their design also uses molten salt, but contains the fuel-bearing chloride salt inside tubes. A separate, non-fueled salt circulates around the tubes.

For reasons of chemistry, this enables them to use a fluoride salt outside the tubes, and unlike homogenous fluoride-based MSRs the reactor structure can be made of stainless steel rather than more exotic (and expensive) nickel-based alloys. It also reduces the volume of chloride needed, which is an important cost savings if the chlorine has to be isotopically separated to reduce production of 36Cl (which has a halflife of 3x10^5 years.) The high cross section of chlorine for thermal neutrons is also why chloride MSRs are necessarily fast reactors.

Moltex's design retains the major advantage of MSRs, which is the absence of volatile materials in the containment building. It is the vaporization and pressurization of these materials in an accident which forces the containment to be large and strong.

Have you looked at the press release that yazr linked to? It sounds like this is a new design for Terrapower that is similar to the concept that you prefer. It would be great for someone that is familiar with the design issues to confim if this is the case.

Fair points. The challenge with fluid in tubes is fuel density. Very hard to get it high enough to be economical.

Good old solid nuclear fuel can be upwards of 15 g/cm3.

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.

> in the fast spectrum chloride salt are better, in the thermal spectrum flourides

Would you mind expanding on this? Why?

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.

In addition to what the sibling poster said, https://info.ornl.gov/sites/publications/files/Pub29596.pdf and 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.).

Isn't stainless steel not so resistant to chloride ?

Correct. But there's some fancy high-nickel and other stuff that can likely handle it if you control the chemistry carefully.

The challenge in a traditional MSR design is that you have half the periodic table swimming around in your primary loop at high temperature. From a corrosion perspective that's pretty challenging.

Salty water is a problem, but pure chloride salt is less corrosive to stainless steel than pure water, according to a presentation I saw by someone from another chloride reactor company.

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