Sorry American Nuclear Society, I’m hopping on your coat-tails for the title.
This is what I would probably consider the last of the basics of nuclear energy topics. If you’ve been following along with my blog so far I wold say that this is the last of the 100 level courses.
So straight to it. Burn up is the colloquial term for fuel utilization and it has this godawful unit of GWd/tHM. Seriously, what is that trying to say? If it was a person at a restaurant I would be concerned that they needed someone to give them the Heimlich maneuver. But like most things, when you break it down it makes a lot more sense. GWd is basically just a really big version of something you see on your energy bill every month, the humble kWh. 24,000,000 times bigger to be precise. But it’s the last part that really tells us something about what burn up really is. The tHM stands for tonne Heavy Metal.
So putting it all together GWd/tHM is a measure of energy per unit mass of heavy metals, of which it specifically means fissile and fertile materials like U235, U238, Pu239 and Th232. What it is mostly used for is measuring the fuel efficiency of a nuclear fuel cycle. Theoretically, uranium fuel has a total energy of 909 GWd/tHM. So by measuring the amount of energy produced by a mass of fuel over a certain time period, for most current nuclear designs other than CANDUs this is usually around 45 GWd/tHM, to determine the percent utilization aka “Burn Up”.
Burn up isn’t just used for reminding ourselves how terrible the fuel efficiency of nuclear reactors currently is, it is also a useful rule of thumb metric for determining how radioactive the spent fuel will be. I’ve spoken before about how neutron absorption works and how it can create isotopes that don’t normally exist naturally anymore like Plutonium. When you keep solid fuels in reactors for longer periods of time to get higher and higher burn up values you actually push even farther up the periodic table into things like Americium, Curium and other transuranic elements. These are called Actinides and they are some of the largest contributors to the long term radiation in spent fuel.
If you want to prevent actinide build up, either as a method of reducing risk in handling spent nuclear fuel or to reduce reprocessing costs, you have to accept a lower burn up of your fuel. Luckily, lower burn up means you can use a less enriched fuel in the first place because you don’t need the extra fissile material to overcome the build up of reaction poisons like Xenon. For example, CANDUs have almost embarrassingly low burn up values (~7.5 GWd/tHM) but that lets them use unenriched fuel. So they trade lower fuel efficiency for lower fuel cost.
Now there are some benefits to higher burn up rates for fuels. the thing that surprises most people is that high burn up values make nuclear fuel much more proliferation resistant. mostly because all the bomb material is randomly mixed in with other very similar mass crap that it is practically impossible to separate from. Also if your plant does not have the ability to be refueled during operation like a CANDU plant does, then high burn up fuels mean that your plant can operate longer between refueling, safely.
There are many reasons why burn up can vary so much, and it is something that needs to be taken into account with every reactor design and type. There is no one right answer for all situations, which makes it very easy for the issue to get clouded in all the possibilities. but as with everything I’ve written about on this blog, my hope is that a little knowledge can go a long way to clearing the air.