So – thus far we’ve gone over a little of the history of the Yucca Mountain project and how both geology and hydrogeology can affect waste disposal. What I thought could be interesting today would be to talk a little about how the spent reactor fuel is packaged – both for transport and for disposal – because this is a third factor that has a profound impact on how well the waste can be isolated from the environment. Then, for the last installment in this series (next week) I’ll try to examine some of the claims both for and against the site to see how well they hold up.
To recap a little bit – fissioning a uranium atom splits it into 2 radioactive fission products. These accumulate as the reactor operates – adding more radioactivity as time goes on. As the reactor operates, though, the fuel “burns” up the uranium – after a few years the concentration of fissionable atoms drops to the point where it’s time to swap out the spent fuel rods for new ones. The spent rods are intensely radioactive so they’re normally stashed in spent fuel pools until they can cool off a bit – and in this case, “cooling off” means thermally as well as radiologically since the energy given off by the decaying fission products causes the spent fuel to heat up. But after a long enough time the fuel will cool off to the point at which it can be removed from the water and placed into huge casks that are placed in storage yards at the reactor sites – this is called dry cask storage.
At some point – if Yucca Mountain or some other high-level waste repository opens up – either the dry casks will be used for transport or the spent fuel will be transferred to transport casks that will be loaded onto rail cars or trucks and relocated to their final resting place. It’s these casks that will also be the penultimate barrier between the radioactivity within and the environment so they warrant a description.
First of all the things are huge. I saw some in Lithuania about a decade ago and they looked to be at least 10 feet tall and 5 feet in diameter. And since the physics of uranium fission are the same around the world (reactor design changes somewhat from place to place, but not enough to make a huge difference for commercial reactors) the characteristics of spent reactor fuel are reasonably similar as are the characteristics of the casks. In other words, the spent fuel casks in the US are huge as well.
In addition to providing protection to the spent fuel they are also designed to reduce radiation dose rates to an acceptable level – low enough to pose no risk to those sharing the road with the casks if they are transported by truck. But there’s a lot more to safely shipping waste than keeping rad levels down – the spent fuel casks must also be able to protect the waste while it’s in transit to the final disposal site, not to mention protecting it during its long millennia in storage. We’ll tackle these one at a time.
Spent fuel casks have to meet some stringent requirements to ensure that they don’t release highly radioactive fission products while they’re in transit to the final disposal site. Casks must be able to pass these tests without suffering a failure:
- A 9-meter (30 foot) fall onto a hard surface
- Puncture test where the container falls 1 meter onto a 6” steel rod
- 30 minutes of being engulfed in an 800 degree C (1475 degree F) fire
- 8 hours of immersion beneath 3 feet of water
- 1 hour of immersion beneath 200 meters (655 feet) of water
These requirements are more than theoretical – in the 1970s Sandia National Laboratories tested some spent fuel containers with full-scale crashes to confirm that what looked good on paper and in the laboratory would work in real life. The most dramatic test was running a locomotive engine into a flatbed truck carrying a cask on it – the locomotive was pretty much destroyed while the cask, while damaged, survived and would not have leaked radioactivity into the environment. There’s a nice video on YouTube showing the locomotive test and others – these videos alone ought to allay any doubts about the ability of these casks to protect spent fuel while it’s en route to the disposal site.
Physical ruggedness is nice, but there’s more to keeping radioactive waste safe than protecting it from collisions – once delivered to the site the casks have to help keep the waste isolated from the environment for up to a million years and that takes a lot more than strength. Rust and corrosion will attack the strongest container – all they need are the right conditions and enough time to work. Not only that, but metals behave differently (and chemical reactions proceed more quickly) at higher temperatures – such as those produced by the decay of fission products. So the thermal effects also have to be factored in when designing the things.
So here’s the bad news about long-term disposal of spent reactor fuel – and the containers meant to hold it. Nobody knows how a container is going to hold up over even 100,000 years, let alone a million years (the time span required by EPA). We can do our best to design something with the lowest possible corrosion rate and we can do our best to design in a high level of structural strength – but no matter how we try to artificially age these materials in the lab we can only guess at their long-term performance. Let’s face it – all of human history is only about 5000 years and the Pyramids are younger than that. We can assert the longevity of our designed structures all we want, but we have no direct experience with anything so long-lived. Of course we can put other barriers in place as well – and likely will – but anything artificial suffers the same drawback, that all of human history is far shorter than the period of time for which we’re hoping to isolate the waste.
On the other hand, the engineered packages aren’t the only barrier between the radioactive waste and the environment – and we actually do have one data point about the ability of rock to hold radioactive waste for prolonged periods of time. In fact, what we have is the remnants of a natural nuclear reactor that achieved criticality in what is now the nation of Gabon (in Western Africa) about two billion years ago. The details of how the reactor (called the Oklo reactor) formed and operated are fascinating, but there’s not enough room in this posting to go into the details. For the purposes of this, let it suffice to say that in two billion years, virtually all of the fission products have remained in place. This is in spite of the reactor zone being located in fractured and porous sandstone that was below the water table more often than not – in fact, if the reactor zone were not completely saturated with water the reactor could never have operated. So – remembering the last two posts – porous and water-saturated rock are not well-suited for waste disposal. But in spite of this, the fission products have remained in place for two billion years. This bodes well for the ability of Yucca Mountain (or whatever location ends up with the spent fuel repository) to safely isolate the waste until it decays to stability.
So here’s the bottom line with regards to the waste containers. First, they certainly seem capable of safely storing spent reactor fuel for the length of time that they’re stored at the reactor plants and multiple tests have shown they can protect the waste while it’s en route to wherever it will be disposed of. But no matter how well we design the containers – no matter how convincing our computer models and calculations might be, there’s no guarantee that they’ll last the million years that is the current standard for the waste site. But that doesn’t mean that Yucca Mountain is incapable of storing radioactive waste safely for that length of time – the natural nuclear reactor in Oklo shows that even radioactive waste that’s stored in porous and water-saturated sandstone can remain in place for the eons. This bodes well for the Yucca Mountain site’s ability to retain our radioactive waste for a paltry million years or so.