Nuclear physicists describe the propensity of a nuclide to absorb a neutron in terms of a very small area, called a “barn”. A barn is a trillionth of a trillionth of a square centimeter (10-24 cm2).
Now imagine if we could inflate the size of a couple of nuclides, brothers actually, by a factor of two-and-a-half trillion.
These two nuclides, despite being from exactly the same element, would look very very different in their “size” to a thermal neutron.
One would be really big, about the size of a 36″ beachball. We would accurately conclude that a neutron would have a very good chance of being absorbed by this big guy.
The other would be really small, about the size of a pea.
Now you might not believe that two brothers could be so different! Furthermore, you might be tempted to think that the beach-ball nuclide is the bigger one, and the pea-sized nuclide is the smaller one.
But you’d be wrong, because in the strange world of nuclear physics, the little one is big and the big one is little.
The nuclides I’m talking about are the two natural nuclides of lithium.
The little one, lithium-6, has three protons and three neutrons. It’s the beach ball.
The big one, lithium-7, has three protons and four neutrons. It’s the pea.
It’s the way they behave towards neutrons that’s the subject of this post. You see, they’re as different as Jekyll and Hyde. One wants the neutron, and the other one pretty much ignores it.
For the purposes of making energy from thorium, lithium turns out to be an very important element. That’s because lithium and beryllium fluorides make the finest mixture of salts into which we put our nuclear fuel as we operate the liquid-fluoride thorium reactor.
Beryllium only has one natural isotope. It doesn’t pay much attention to neutrons.
Fluorine also only has one natural isotope, which also doesn’t pay much attention to neutrons.
But lithium has two, and like the beach-ball and the pea, they look very different to the neutron. So it’s very important that we only use “peas” in the reactor instead of “beach-balls”. Fortunately for us, lithium is already mostly “peas”. Unfortunately for us, there’s still far too many “beach-balls”.
Natural lithium is about 92.5% lithium-7. But there’s enough lithium-6 in there that the mixture looks about like a “soccer ball” to a neutron. That’s much too absorptive.
If we were to enrich the lithium so that it was 99% lithium-7, things would be better, but still far too absorptive. Kind of like an apple.
At 99.9% we’d be down to the size of a strawberry, at 99.99% it would be about the size of a grape, and at 99.999% it wouldn’t be much larger than the pea.
But to get to that level of separation we’d have to do a lot of work. Separative work, to be more exact. We’d have to go from a lithium mixture that had 76,000 Li-6 per million down to a mixture that had only 10 per million. That’s a lot of separative work.
Leaving too much lithium-6 in the mixture could be a really bad idea too. Lithium-6 absorbs neutrons and turns into radioactive tritium gas, which is hard to capture and contain and if released into the environment will behave just like normal hydrogen. It becomes part of water (tritiated water) and depending on how much you ingest, it can give you an elevated dose of radiation.
In addition, it’s just plain wasteful of neutrons to leave much lithium-6 in the mixture.
So one of our key challenges in preparing the materials to make energy from thorium is learning how to separate lithium-6 from lithium-7. And despite what I’ve told you here, to a chemical separation process they don’t look like a pea and a beachball.
They look just the same.