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The Curious Wavefunction

The Curious Wavefunction

Musings on chemistry and the history and philosophy of science

Who’s afraid of nuclear waste?: WIPPing transuranics into shape

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Waste arriving at the WIPP from all over the country's non-commercial nuclear reactor sites (Image: PBS Nova)

About 50 miles from the Texas border in southeastern New Mexico sits the town of Carlsbad, home of the renowned Carlsbad Caverns. Its lesser-known claim to fame which actually might have a disproportionately long-lasting impact on the future of energy and the human species is as a site for the Department of Energy's Waste Isolation Pilot Plant (WIPP), the only official waste repository in the US currently accepting high-level nuclear waste. Jessica Morrison from PBS has an excellent article on the workings of the WIPP and its importance for nuclear power (hat tip: Bora). With the disaster of Yucca Mountain still beckoning in people's memory, the WIPP offers a welcome and unique possibility for the future:

The Waste Isolation Pilot Plant, known locally as WIPP (pronounced “whip”), opened in 1999 after decades of back and forth between state and federal regulators. Today, it holds more than 85,000 cubic meters of radioactive waste arriving from as far away as South Carolina. Currently, WIPP is only authorized to handle waste containing elements with atomic numbers higher than 92—primarily plutonium—that originated from the development and manufacture of nuclear weapons. Between 1944 and 1988, the U.S. produced about 100 metric tons of plutonium, most of which was used to develop nuclear weapons.

What I find pleasing on a deep level about the WIPP is that it relies on entirely natural mechanisms for sequestering the waste from the outside world. The basic principle is to dig a hole deep into a salt bed. Salt has the unique property of displaying "creep", the tendency to flow into and around cracks and naturally form seals; seals that can be as tight as those formed by the hardest rock when they are laboring under pressures operating at 2100 ft underground. When you bury waste in salt, you are basically letting geology do its job and create a seamless tomb for the waste.

WIPP’s operators stack waste containers in rooms dug into the salt formation and then let geology do the rest. Under pressure from the ground above, salt formations flow into cracks and open spaces. Over several dozen years, salt will settle around the containers, forming a rocky seal. That self-sealing ability also protects the site from cracks caused by earthquakes—any that open will quickly close. So far, the site has been successful in containing radiation from the waste.

Working in WIPP is therefore a job with a time stamp; in some sense the mine itself is urging the workers to do their job quickly and get out the hell out of there, so that the earth can close around the waste and clasp it in its tight embrace.

WIPP feels like it’s in constant motion—the continuous care needed to control the salt, the movement of the electric carts within the mine’s pathways, the loading of waste first into the walls and then the room, back to front. It all serves as a reminder that the place really is moving, just at a slower, inexorable pace. WIPP depends “on salt and the behavior of salt,” Elkins says. Salt flows under pressure, and it’s under a great deal of pressure this far underground. On a geologic time scale, it presses down with surprising speed, crushing and then encapsulating whatever is placed inside.

What this also partly means is that the sooner the repository fills up with waste, the better it would be to close it and let the salt do its job. This is a good incentive for carting high-level waste from the nation's myriad nuclear sites to WIPP as soon as possible. It's not as if there is a shortage of waste waiting to be disposed:

While WIPP has been accepting nuclear waste from weapons programs, no central repository currently exists in the U.S. for spent nuclear fuel and related waste from commercial reactors. Until one opens, waste has been sitting in interim storage at or near each of the nation’s 65 nuclear power plants. At the end of 2011, these sites and others held more than 67,000 metric tons of spent nuclear fuel, according to a report issued by the Congressional Research Service.

Waste buried deep underground in the right kind of geological formation is extremely safe and many people who criticize the problem of nuclear waste don't realize that good technical solutions based on burying waste have already been at hand for decades; the problem is mainly a political one. It's worth appreciating the basic fact that there are two kinds of waste, short-lived intensely radioactive and long-lived mildly radioactive. The inverse relationship is a basic law of physics and plays to our advantage. Thus, short-lived isotopes like strontium-90 and cesium-137 might be biologically dangerous, but they also reach safe levels rather quickly (half-life about 30 years for both). On the other hand, long-lived isotopes like plutonium-239 (half-life 24,000 years) are less dangerous because of their lower activity. Typically nuclear waste contains both kinds of elements, and one of the bad decisions taken by the government in this country based on rather flimsy grounds was to halt reprocessing, a process that would have separated plutonium and other valuable and proliferation-prone elements from the short-lived waste and which is routinely done in Europe, Russia and Japan. Burying plutonium is thus both an unnecessary invitation to potential proliferation as well as a waste of valuable fuel for civilian nuclear reactors.

It's hard to think of proliferation though when the plutonium is lying 2100 ft below ground covered by salt and earth as hard as kryptonite. Even when Yucca mountain was being discussed in the 70s and 80s, there were sound techniques for enclosing waste in borosilicate glass surrounded by layers of tamper-proof materials like copper and clay. The following illustration from a 1991 article on nuclear power by physicist Hans Bethe displays the multiple barriers separating transuranic waste from the environment:

Multilayered cylindrical design for the isolation of transuranic waste (Image: Engineering and Science, 1991)

When this kind of waste burial was being discussed, one of the cogent problems was that of groundwater seepage which might potentially transport the waste over great distances. But this problem is not as serious as it sounds at all. To begin with, waste repositories are already located away from both residential areas and groundwater sources. But even if groundwater were to come in contact with the waste, it would be several hundred thousand years before it ever reached the surface. As Bethe clarifies it in the same article:

"Groundwater doesn't flow like a river; it creeps. At a disposal site in Nevada called Yucca Mountain the Department of Energy has measured the flow of groundwater at 1 millimeter per day. And it has to flow a distance of about 50 kilometers before it comes to the surface, because it generally flows horizontally. With this alone, it takes more than 100,000 years (italics mine) to come to the surface. In addition to that, at Yucca Mountain the waste can be placed about 400 meters below ground, and the groundwater is 600 meters below ground, so the waste won't even touch it. This might change due to geological upheavals, but to start with it's a very good disposal site.

And even if the groundwater is flowing 1 millimeter per day, experiments have shown that most dissolved elements take 100 times longer to flow than groundwater; they are constantly adsorbed by the surrounding rock and then put back into solution again. And plutonium, which is the element people are so afraid of, takes 10,000 times longer again to migrate than most elements. In other words, during plutonium's half-life of 20,000 years, you are insured 100,000 times over."

Yucca Mountain is now abandoned, but these general principles of waste storage still stand and plutonium can still be considered to be confidently isolated from the environment over multiple half lives when buried this way. With short-lived elements the solution is easier. It's a pity that political inaction and public opinion has not allowed us to cart most of existing waste to sites like the WIPP. The waste is relatively small in amount to begin with - the annual waste from the 100 odd reactors in the US would only fill a football field to a depth of one foot - and storing it around creates unnecessary safety issues. Dry cask storage is a good solution but since the casks are often stored on land is far from a permanent one.

The public, government officials and experts should take the lessons of the WIPP and of existing techniques for disposing waste to heart. As with many things nuclear, one of the major problems is that of education; many members of the public think that all nuclear waste is alike, that all of it will kill you even on slight exposure, and that there is no way at all of disposing it. Stories like that of the WIPP should hopefully change their minds and demonstrate that the problem of nuclear waste is not a technical problem, it is one of psychology and politics.

Note: As Twitter user @AtomikRabbit pointed out to me, WIPP is only a repository for non-commerical nuclear reactors. It's waste from such sources that's displayed in the photo above.

The views expressed are those of the author and are not necessarily those of Scientific American.

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