Yesterday I posted on the issues that surround finding a good moderator. Today I’ll walk you through the math that will let you do the calculations yourself. At the end I’ll link a spreadsheet that already has the calculations done that you can download and monkey around with if you want.
First of all, you want a moderator that’s good at moderating. That means that when a neutron hits it, the neutron loses a lot of energy, specifically kinetic energy. Just like in marbles or pool balls, the bigger the moderator nucleus, the less energy the neutron will lose when it hits it.
In nuclear engineering, the fancy term for this feature is the average logarithmic energy decrement per collision. And if that’s not fancy enough, they use a funny Greek letter that looks like a squiggle to represent it. I think the letter is actually called “xi”, but I prefer to call it “squiggle” since that’s what it looks like.
Calculating squiggle for each nucleus is really easy. All you have to know is “A”, the atomic mass of the nucleus. For hydrogen-1, A is 1, for hydrogen-2 (deuterium) A is 2, for helium-4 it’s 4. You can see this is very difficult. For carbon-12, you’ll never guess what A is. Okay, stop guessing, it’s 12.
After hours of deductive logic to compute A for a nucleus, you’re ready to calculate “alpha”. Alpha is simply:
So for hydrogen-1 (protium), it’s
For helium-4 it’s
For carbon-12 it’s
You can see the trend–for heavier and heavier nuclei, alpha gets closer and closer to one. For hydrogen it’s zero, and everything else is between zero and one. We’ll find that moderation goes better when alpha’s more like zero than like one.
Armed with our calculation of alpha, we can calculate the logarithmic energy decremental operation of collisionary …. um, squiggle. Yeah, with alpha we can find squiggle.
And here’s the equation for squiggle:
Let’s use a spreadsheet to spare us some of the complicated math and look at the results for a bunch of different nuclei. You can do this at home and catch all my math errors:
Thanks to the spreadsheet and it’s patience in doing tedious calculations like alpha and squiggle, I was able to quickly find out these values after only going to the trouble of typing in the name and deducing the atomic mass. Now some of you may be wondering why I typed in 1.000004 for the mass of hydrogen rather than just one, and I won’t deprive you of the fun you’ll have later on if you try to build your own squiggle-calculating spreadsheet and DON’T do this, so I won’t tell you that all your equations that use hydrogen won’t work if alpha is EXACTLY zero, so go ahead and use one for hydrogen’s mass and see what happens.
What can you do with a calculation of squiggle? Well, a few different things. One of the things you can do is that you can figure out how many collisions it takes (on average) to slow a neutron down from the high-energies at which it is born to the slow energies that improve the probability of fission.
For instance, if we wanted to find out how many collisions it took between a neutron and a moderator nucleus to slow the neutron down from 2 million electron-volts to 1 electron volt, then the equation would simply be the natural log of the ratio of the energies divided by squiggle.
# of collisions = ln (2,000,000 eV/1 eV)/squiggle
ln (2,000,000/1) = 14.51
So for hydrogen, it would take 14.51/1 = 14.5 collisions, on average, to slow a neutron down from 2 MeV to 1 eV. If your squiggle’s less than one, it will obviously take more collisions. Here’s a quick list:
|A||alpha||squiggle||2 MeV > 1 eV|
You can see that it takes a lot of collisions to slow down neutrons when you’ve got a lot of heavy stuff in the core, and a lot less when you’ve got light stuff like hydrogen or deuterium.
Now we still don’t know if a neutron can survive 20 or 30 collisions with a moderator and not get gobbled up! We also don’t know how to look at combinations of nuclei, such as compounds like water or beryllium fluoride or things like that.
OK, so now we want to take on moderating compounds. I mean, knowing these element numbers is nice and all, but let’s say we want to compare heavy water to beryllium hydride…how do we know which one is better/worse/whatever?
To take the next step, we’re going to need some of the “microscopic cross-sections” for each of the moderator nuclei. Conveniently, we can find these cross-sections in a variety of different places. I happened to get these from the “Map of the Nuclides” on the LANL Nuclear Information Service site. These cross-sections are the room-temperature (2200 m/s) cross-sections from the JENDL 3-2 library.
I include lousy moderators like chlorine and uranium in here for comparison purposes. Each of the cross-sections is presented in the unit of “barns”. A barn is a useful little definition–one barn is one-septillionth of a square centimeter, or 10^-24 cm^2.
We use “barns” a lot in nuclear engineering–I think the name came from something like “hitting the side of a barn”. At any rate, we can look at these barn numbers and see a few different things. There are two cross-sections shown here. One is the scattering cross-section. That’s the “size” of nucleus in the sense of colliding with a neutron and “scattering” it, in other words, slowing it down and not absorbing it. This scattering cross-section is called “sigma sub s” for the way you see it written in the books: a Greek letter sigma (lower-case) with a little “s” as a subscript, indicating that it’s a scattering cross section.
The other cross-section is the “absorption cross-section”, also called “sigma sub a”. This is written as a lower-case Greek letter sigma with a little “a” as a subscript indicating that it’s for absorption, not scattering.
As you might have guessed, for a moderator, a big scattering cross section is GOOD, and a big absorption cross section is BAD. You don’t want moderators to absorb neutrons, you want absorbers to slow them down! So we want material that presents a big target for slowing down, and presents a small target for absorption.
But what about our old friend squiggle? Well, squiggle is how much energy is lost in one of those collisions in the first place. So we really need to think about how big of a scattering target the nucleus is (sigma sub s), how much energy it loses in the collision (squiggle), and how improbable an absorption might be (sigma sub a).
But what is squiggle for a compound? What is it for something like beryllium fluoride, where you’ve got both beryllium and fluorine nuclei in the compound? How do you find squiggle?
Well, in the case of compounds, squiggle becomes a weighted average, and the weighting factor is the microscopic scattering cross-section (sigma sub s). For instance, for something like heavy water (D2O), where you have two deuterium nuclei and one oxygen nuclei, to calculate squiggle for this compound you would multiply the squiggle for deuterium by the sigma sub s for deuterium, times two (because you have two deuterium nuclei in the compound) and add to that the squiggle for oxygen multiplied by the sigma sub s for oxygen. Times one because there’s only one oxygen in heavy water. Then divide the whole bunch by two times the scattering cross section of deuterium added to the scattering cross section of oxygen.
Here’s how it would look:
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.