Finding the right salt combination for a fluoride reactor isn’t too difficult. If you want a salt that has a relatively low melting temperature, low neutron absorption, and good temperature performance then you use lithium-fluoride/beryllium-fluoride, being careful to use the lithium-7 isotope instead of just the natural stuff.
But when looking for a moderator, things are a lot more difficult. It turns out that the inside of the reactor is a pretty tough place for most moderators, and there’s really good reasons most of the conventional moderators don’t cut it.
First of all, why do we want a moderator? Well, the moderator is the “stuff” in the reactor that slows neutrons down from the really high energies that they are “born” at to the slow energies that are best for fission. Why are slow neutrons better for fission? I don’t know–maybe one of you could tell me. But neutrons that have been slowed down have been shown by four out of five dentists to be more likely to cause fission.
The other dentist didn’t pay attention in class.
There’s several attractive features you’re looking for in a moderator–first of all, you want a substance that’s really good at moderating! Mr. Neutron is cooking along and when he hits your moderator, you want him to really get the wind sucked out of his sails. The best moderator turns out to be the nucleus most like the neutron–a single proton, most often found called a “hydrogen nucleus”. Don’t be fooled by this fancy name; the hydrogen nucleus is just a proton, which is just about the same size as a neutron.
For those of us that used to shoot marbles as a kid, or play pool as teenagers, the reason protons work so well as moderators can be intuitively understood. When the neutron strikes the proton, depending on the angle of the collision, it can give up some to all of its energy in a single collision.
Anybody have a good animated gif of this?
Hydrogen seems perfect in this regard, and so it is, but hydrogen has a tragic, hidden flaw–it really likes to eat neutrons. You’d think it would just be happy being a single proton, but no. Hydrogen every now and then wants to end its solitary existence and take up with that visiting neutron, forming a nucleus that has a single proton and a single neutron. Chemically, it’s still hydrogen, but now it has a new name: deuterium.
Now it turns out deuterium is EVEN better than hydrogen as a moderator. Despite the fact that the neutron won’t lose nearly as much energy in a collision with a nucleus that is twice as big as itself, the deuteron practically never wants to gobble up the neutron. Perfectly content with its single neutron, the happily monogamous deuteron skillfully resists the advances of any and all other neutrons that seek a union.
So there we have it–the perfect moderator is deuterium, and we simply have to find a substance that contains deuterium in a form that will be stable inside the reactor. No problem right?
No, there’s the problem itself. It’s really hard to find a substance that contains hydrogen and is chemically stable at the high temperatures of the fluoride reactor. The basic problem is that when those high-speed neutrons hit hydrogen or deuterium, they knock it clean out of the park, or at least out of whatever it’s chemically bound to. Things would be okay if the hydrogen found its way back to where it came from, but typically the hydrogen finds another hydrogen and becomes a gas, which comes right out of the moderator material.
Lithium hydride, beryllium hydride–they all have this problem. Hydrogen oxide (aka water) also isn’t stable at the elevated temperatures of the reactor. For a time in the 1950s, hydroxides (and deuteroxides) of lithium and sodium were considered as high-temperature moderators, but they had extreme corrosion problems.
So alas, in all likelihood we must abandon hydrogen and deuterium and keep marching along the periodic table, with the unfortunate knowledge that every step we take leads to heavier nuclei that are poorer and poorer moderators.
Next stop is helium. Not too big (with an atomic mass of 4 atomic mass units) and has practically no tendency to absorb neutrons. In fact, to the best of my knowledge helium-4 is the only nucleus that has NO neutron absorption cross section at all! That’s because the helium-4 nucleus, with two protons and two neutrons, is an exceptionally stable form of nuclear material. The alpha particle itself is simply a helium-4 nucleus.
BUT–helium doesn’t chemically bind to anything! So the densest form of helium around is liquid helium, which has the small problem of being only a few degrees above absolute zero. So no helium.
How about lithium and beryllium? They’re not too bad, but even lithium-7 tends to like the occasional neutron every now and then. Beryllium is better as a moderator because of very low neutron absorption, but if you want to use a lithium/beryllium fluoride salt around your lithium/beryllium moderator, then the moderator better be a fluoride salt too. Otherwise, it will tend to scavenge fluoride ions from other materials in the salt such as uranium or thorium, reducing them to metallic form and causing them to precipitate in any old place. Not a good situation.
Carbon is the oasis of opportunity. With 12 atomic mass units (6 protons and 6 neutrons) it’s not an especially good material at slowing down neutrons. But it is a super-stable atomic nucleus, since its a multiple of that basic helium-4 unit. So almost no neutron absorption. It’s abundant, and even better for us, it ACTUALLY IS STABLE IN SALT AT HIGH TEMPERATURE!
(I know, you wanted to shout it on the housetops too, so I did it for you.)
Graphite is the form of carbon we’ll typically want to use in the reactor, being decidedly less expensive than diamond. But graphite also has some real drawbacks, some of which I’ll discuss in the next post on this topic.