Northwestern researchers have found a material that filters the radioactive cesium ions out of nuclear waste. Stefan Kühn
In our Future of the Environment issue, we mentioned one visionary’s suggestions: self-sinking tungsten spheres that stash spent nuclear fuel deep beneath the Earth’s surface. That idea is a long way from reality, but in our green-energy-starved present, it may be worth considering all options, no matter how wacky. Here are a few other pie-in-the-sky ideas.
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Nuclear reactors create high-level nuclear waste, composed of spent fuel rods loaded with the still-radioactive isotopes created when uranium-235 fissions. Some of those isotopes, like cesium-137 and strontium-90, have half-lives of 30 years or so — but high-level waste also includes plutonium-239, which has a half-life of 24,000 years. Thanks to the fission process, fuel rods are actually more radioactive when they come out of the reactor than when they go in. But at the moment, using the spent rods as a source of fuel just isn’t cost effective. And 24,000-year storage solutions are hard to come by, it turns out.
In our gallery, an overview of some of the options being considered today.
Additional reporting by John Bradley
The universe giveth radioactivity, the universe taketh radioactivity away. Or at least, maybe the universe would be willing to accept it, right? It’s true that nuclear waste would be hard-pressed to do much damage to people if it were sailing through the solar system or, say, falling into the sun. The trouble is getting it there. Lift-offs fail occasionally — catching fire on the launch pad, falling into the ocean, or exploding in the upper atmosphere. It’s unlikely the failure fraction will ever go to zero. And until it does, shooting spent fuel rods into space remains a very dangerous proposition indeed. Even if space launches were routine and safe enough to pitch our plutonium at the heavens, it may one day be worthwhile to recover this radioactive stuff. Plutonium, cesium and strontium are also limited resources, and if fission reactor technology advances enough, they could become fuel sources themselves. So maybe we want to keep nuclear waste reasonably handy.
Burying nuclear waste underground is the emerging favorite among disposal options. How exactly it’s buried, however, is still a matter of some debate. The deep borehole solution is still in its planning phases, but at least on paper, it involves wrapping spent fuel rods in steel and burying them miles below the Earth’s surface. Deep boreholes offer the advantage that they could be drilled very near the nuclear reactors themselves, reducing distance the high-level waste would need to be transported before disposal. Plutonium recovery, though, would still be a challenge — shoving nuclear waste three miles down is one thing, pulling it back up safely is another thing altogether.
In large parts of the ocean, the seafloor is made of thick, heavy clay, perfect for absorbing radioactive decay products. First proposed in 1973 by Charles Hollister, an oceanographer at Woods Hole Oceanographic Institute, sub-seabed storage was seriously considered by the U.S. through 1986. However, sub-seabed storage would require drilling underwater bores, something that the Deepwater Horizon disaster could make a touchy subject for quite some time. Plus, it’s a violation of international convention to dispose of nuclear waste at sea — so a sub-seabed solution would require revising international agreements.
Hypothetically, burying nuclear waste in subduction zones would carry the spent fuel rods along the conveyor belt of the Earth’s tectonic plates and into the mantle. Advocates for this method exist, but it’s not something the Department of Energy is considering. The international treaty violation would come into play here, just as in sub-seabed disposal. Never mind that magma made from subducted seafloors has a habit of welling up in volcanoes.
Nuclear waste is hot stuff. As mentioned in our Future of the Environment issue, Jessie Ausubel’s tungsten spheres filled with nuclear waste could produce enough heat to bury themselves into rock (though the stability of the rock below would need to be known very well before such an option could be considered). Another idea from the “rejected back in the 1970s” annals: nuclear waste that burrows into glaciers. Place a sphere of nuclear waste in a stable ice sheet and it could melt its way down, the ice re-solidifying behind it. There are plenty of reasons this idea was rejected early — not the least of which is that ice sheets move, a lot, so radioactive material could end up in floating in the sea as icebergs.
The most realistic current option, burying radioactive waste underground, raises its own set of problems, including how the waste is shielded to keep it from contaminating the surrounding rock and water. One possibility is to keep the radioactive waste isolated in synthetic rock. Synthetic rock, or “synroc” (pictured), was developed in the 1970s as a storage material for high-level nuclear waste. It’s designed to absorb specific waste products — there are different formulations for light-water reactors and plutonium fission products. It’s a ceramic that traps the nuclear material in its crystalline lattice, made to imitate geologically stable minerals.
Underground repositories of nuclear waste are particularly dangerous if they can seep into groundwater. If a cage of water — sort of like a three-dimensional moat — is built around the sealed-off nuclear waste, the groundwater is given an alternate path. Then, ideally, it won’t seep through radioactive material. Future nuclear waste disposals should be leak-proof, but the hydraulic cage could provide something of a worst-case backstop against groundwater contamination. A hydraulic cage was designed for the Richard Repository, in Litoměřice, Czech Republic, pictured here.