Month

Lately I’ve been looking a lot at spent nuclear fuel, particularly what’s in it and what’s radioactive after a while.

I’ve seen graphs of fission product distributions before, but they’re always of fission products by atomic mass, and they’re usually showing the distribution right after the fission event.

I wanted to know what was in there from an elemental perspective, because if you’re going to do chemical processes on the spent nuclear fuel, you’ll be removing elements, not isotopes. So how do they all rack-and-stack overall? Well, here’s the results for a typical light-water-reactor fuel element, irradiated for a modest 400 days and then allowed to cool down for 20 years. Everything is scaled to one metric tonne of uranium before irradiation, and values for masses are given in grams. So xenon, for instance, constitutes 0.13% of the mass of the original fuel, per tonne of uranium.

Some of the smaller concentrations of fission products are easier to see in a log-scale graph of the same data:



Here’s another image that might help, that shows how the fission products “grow in” to the fuel as it is irradiated. Note that this is a 3 year irradiation, so there’s more of everything in this distribution.



As you can see from the graph, there’s not ALL that many significant (from a perspective of mass) fission products in the spent fuel. There’s xenon (#54) and neodymium (#60). Then there’s zirconium (#40) and molybdenum (#42) and ruthenium (#44). Cesium (#55), barium (#56), lanthanum (#57), cerium (#58), and praseodymium (#59) all figure in at varying levels of importance. And there’s samarium (#62) in there to make things difficult.

But it’s a smaller list than I would have thought, and the xenon, neodymium, molybdenum, and lanthanum are all recoverable at this stage. Something to think about–even the fission products of spent nuclear fuel probably aren’t really “waste” either.

This is hilarious and heart-breaking at the same time:

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In nuclear reactor design, we describe the cross-section of different nuclides in a unit called a “barn”. It has units of area. So what does that mean?

Well, take a look at this picture. This shows five important nuclides plotted against each other, with their size determined by their “barns”. You can see that one of them is absolutely HUGE.

That is xenon-135, as far as I know, the nuclide with the largest cross-section to absorb thermal neutrons.

Next on the list of trouble is samarium-149, which is really big, but not nearly as big as xenon-135. Again, to the best of my knowledge, samarium-149 is number #2 on the list of trouble.

For comparison purposes, I show the relative cross-sections of three fissile nuclides, uranium-233, uranium-235, and plutonium-239. These three are fuel in a nuclear reactor, and the bigger these are the better when it comes to making reactors small. By most descriptions of cross-section, U-233, U-235, and Pu-239 have big cross-sections, but you can see that they’re pretty small compared to Xe-135 and Sm-149. Like comparing the inner planets to Jupiter and Uranus.

Why does this matter? Because if we had it our way, we wouldn’t want any Xe-135 or Sm-149 gobbling up neutrons in our reactor. Neutrons they eat are neutrons that we can’t use to make energy by splitting fissile nuclides like Pu-239 or U-233.

Xe-135 has two major differences from Sm-149. The first is that it is radioactive. It goes away if you leave it for awhile. It has a half-life of about nine hours. Sm-149 doesn’t go away. It is not radioactive. It is stable.

The other difference is that xenon is a noble gas, and is easy to remove from a fluid fuel like the salts that we want to use in liquid-fluoride reactors. So getting Xe-135 out of the mix isn’t terribly difficult. Samarium on the other hand is pretty challenging to extract from the salt mixture. It’s one of a family of elements called the “lanthanides“, and they all have very similar chemical properties to each other, because their outermost electron layer (the one that does all the chemical bonding) is the same while they fill up the inner electron layers as you progress up the list of lanthanides. So it’s hard to come up with a chemical process that is particularly good at picking off samarium without picking off all of the other lanthanides at the same time.

I would really love to have someone figure out a nifty way to remove samarium while a fluoride reactor is running. Maybe this guy knows how to do it. I wish I did.