What's in Spent Nuclear Fuel? (after 20 yrs)
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.