Solving the rock-hard problem of nuclear waste disposal

Finland avoided some of the mistakes made elsewhere and opened its waste repository.

A tunnel in Finland’s nuclear waste repository.

Posiva

Even if all nuclear power plants were shut down today, there’s a mountain of radioactive waste waiting to be disposed of. Yet only Finland has an approved solution for nuclear waste disposal, while projects in the US, UK, and Germany have failed for decades, and progress is also slow in other countries. With growing calls to extend the life of existing nuclear power stations and build new ones, that mountain of radioactive waste sitting in temporary, vulnerable, and expensive storage will keep growing.

The challenge is daunting. “High-level” nuclear waste, which includes spent nuclear fuel, stays radioactive for hundreds of millennia, so a waste facility must keep it safely away from aquifers, violent weather, war, plane crashes, sea level rise, future ice sheets, volcanic activity, and even curious future humans for a time span that dwarfs all of previous human history.

Ultimately, it’s the geology of a proposed disposal site that determines if it's a safe place to entrust nuclear waste for millennia. We talked to people involved in the Finnish, US, and UK programs about what investigations of the rock and groundwater at those sites revealed about their suitability—or lack thereof.

Burial is best

Worldwide, the consensus is that deep underground disposal is the best way to keep nuclear waste away from water sources and the food chain until its radioactivity has decayed to the normal background.

Radioactive garbage from medical, research, and nuclear facilities, classified as “low-level” or “intermediate-level” waste, is not the problem. This includes things like disposable shoe covers, rags, filters, swabs, syringes, medical radiotherapy sources, and debris from decommissioned nuclear plants, and it is routinely packed into special containers and buried in dedicated shallow landfills.

The real problem is spent nuclear fuel and nuclear reprocessing waste that is classified as "high-level." It is lethally radioactive and physically hot, and it takes several hundred thousand years for its radioactivity to decline to about the level of natural uranium ore.

The right stuff

Under current disposal plans, this radioactive waste will be sealed in a "multibarrier" system—a series of canisters within canisters, engineered to contain the waste for hundreds of millennia, even if some of the layers fail. The canisters will eventually leak, but if all goes to plan, that will only happen in the distant future, after the radioactive isotopes have mostly decayed.

The canisters will be entombed for millennia in tunnels deep underground, and the space around them will be packed with another barrier in the form of a clay called bentonite, which is impermeable and swells into gaps when it’s wet, sealing off the canisters and giving some support against any collapse of the tunnels.

The rock housing the repository acts as a final barrier.

Its job is to protect the waste canisters and the bentonite so they can do their job. It also serves as the last line of defense when canisters eventually leak or fail sooner than expected. As such, the rock must keep the flow of waste from reaching the biosphere for as long as possible—a million years in Finland’s design. To be the “right stuff” for that job, the rock must be strong enough for tunnels to stay intact, it must be effectively impermeable, and its groundwater must have minimal flow and cannot be corrosive.

Finland has a lot of hard crystalline bedrock and many places that are potentially suitable for a repository. The country eventually chose an island on the Baltic coast for its Onkalo repository, and it hopes to seal off the first tunnel of nuclear waste sometime around 2025.

“I'm not saying that there is something special in the Onkalo geology," said Antti Mustonen, research manager with Posiva, the organization in charge of the Finnish repository. "It meets the requirements.”

vuonna_2020_rollup_kuvakollaasi-980x565.

Artist’s rendering of Finland’s “Onkalo” nuclear waste repository. Inset shows a copper waste canister in a disposal shaft, with that shaft and tunnel packed with blocks of bentonite.

Posiva

In contrast, the rock at America’s doomed Yucca Mountain repository project is highly fractured, porous volcanic ash. In 1978, the US National Research Council published a set of geological criteria for a nuclear waste repository, and Yucca Mountain “violates every single one,” according to Dr. Jane Long, former associate director for energy and environment at Lawrence Livermore National Laboratory. Long worked on the project before her retirement.

Because it is situated in the Nevada desert in what’s termed the “vadose zone” above the water table, the Yucca Mountain was expected to be dry. In reality, the area gets seven inches of precipitation a year, and the water trickles through the rock and drips into the tunnels. This water would eventually corrode the waste canisters, according to Long, and would ultimately flush leaking waste into a regional aquifer. In fact, radioactive fallout from nearby atomic bomb tests was found deep inside Yucca Mountain, showing that rainwater had percolated to the depth of the repository in under 50 years. The project responded to this revelation by calling for eye-wateringly expensive titanium drip shields.

“Disposal in a fractured vadose zone presents serious challenges to safety compared to disposal in crystalline rock below the water table,” Long told me.

Yucca Mountain is also in an earthquake zone near geologically recent volcanoes, and there are mines nearby, all of which violate the 1978 National Research Council criteria. Nevada calls it “a singularly bad site,” and the project was canceled in 2010. Despite that, the US Nuclear Regulatory Commission still concluded in 2015 that Yucca Mountain met regulatory safety requirements.

While Yucca Mountain’s fractured volcanic deposits aren’t ideal, other types of rock may work for disposal. Salt is another rock type favored for nuclear waste disposal; the USA uses salt caverns in New Mexico to dispose of transuranic waste (similar to intermediate-level waste) from military sources. Salt is attractive because it flows over time—salt caverns in New Mexico get smaller at a rate of three to five inches per year. This closes gaps and keeps water out, but it also means that heat-producing high-level waste (excluded from the New Mexico site) could cause the salt to churn over time, jumbling and potentially damaging the waste canisters. Not all salt sites are workable, though; Germany spent decades investigating one, only to conclude that its groundwater wasn’t favorable, so the country is now looking for new locations.

France and Belgium are developing repositories in clay. Like salt, clay deforms under pressure, so holes and cracks self-heal, but it also means the facility can’t be very deep, or the clay would slump into the tunnels while work is in progress. The French Cigéo site, which sits in a thick layer of Jurassic clay, is scheduled to open in 2035.

Keep it simple

Whatever the rock type, it helps if its geology and chemistry are relatively simple. This makes it easier to demonstrate its safety to regulators and the public, and it makes the analyses less complex—and consequently more robust. The 1978 criteria recommends choosing a site “that can be expected to present the fewest surprises.”

In that respect, the UK’s waste disposal project—aborted in 1997—was up against it. When it came to the rocks at the site, “The more we drilled, the more we realized we didn't know,” said Titus Murray, a structural geologist who worked on the project in the 1990s.

Murray was tasked with putting together a geological and groundwater model of the proposed UK site, which features hardened volcanic ash from a series of violent ancient eruptions. “We had this triangular model for the shaft, and I started putting that together and people got really angry very quickly because it was just so bloody complicated!” he told me.

Complexity raises a dilemma: It requires more drillholes to understand the geology, but you want to limit drilling to keep the host rock and groundwater as pristine as possible.

First, do no harm

Constructing a nuclear waste repository is like a Miranda warning—anything you do to harm the integrity of the repository can count against you. But nondestructive techniques like geophysics and surface geology can only give you a broad picture. You still have to drill into the rock to inspect what’s really down there and to test the groundwater. These holes must be kept to a minimum, and they must later be sealed to prevent them from becoming pipes for future groundwater flow.

It’s also impossible to excavate tunnels without creating a zone of damage around them called the “Excavation Damaged Zone” or “EDZ.” This is because rock at depth is under high pressure, so it “relaxes” into the empty space of a tunnel, opening up cracks behind the rock face that could conduct any leaked waste to the surface.

So that’s another dilemma—how do you blast open a tunnel without damaging the rock face?

At Onkalo, excavators blast carefully. They perforate the rock face to be removed with a precise design of 114 drillholes—way more than would normally be used to blast a regular mine or road tunnel—using a computer-guided machine resembling something from the movie Aliens. They then charge the holes with cartridges of ammonium nitrate explosive rather than a more commonly used liquid emulsion explosive, as cartridges are more precise.

“The nuclear authorities are looking at this very closely,” Kimmo Lehtola, site manager at the Onkalo repository, told me. “It really needs to be done as we have designed. All the amounts of the explosives and what kind of detonators we have used are checked by the authorities, so it's very strict work.”

Drilling-for-blasting-a-deposition-tunne

Preparing to blast: A computer-guided drill machine at Onkalo drilling holes for explosives in three places simultaneously. The lower holes have been charged with explosives and are protected from falling debris by pipes.

Posiva

The “blast” is actually a complex sequence of detonations timed to the millisecond and staggered over six seconds. Three empty holes in the center of the rock face make space for rubble created by the initial explosion, then another detonation blasts rock into the space created by the prior detonation, and so on until the sequence reaches the tunnel perimeter. Here, the final detonations just split the rock between holes that are only 25 centimeters (10 inches) apart. “It doesn't break the rock; it creates a crack between the holes,” said Lehtola.

The Finnish repository design requires a level floor so that workers can bore the 8-meter-deep by 1.75-meter-wide (26- by 6-feet) disposal shafts to receive the waste canisters. To get that, excavators grind the floor flat using a mining tool with rotating tungsten-carbide picks, which also removes most of the floor’s zone of damaged rock, Mustonen told me.

Once the tunnels are ready to be sealed permanently, excavators will grind slots through the zone of damage in the tunnel walls and plug them, sealing the bentonite inside and blocking any flow between sections of the excavation.

Proving ground

Scientists must gather vast amounts of data on a proposed repository’s geology and groundwater to demonstrate the site's safety to regulators. They do this using techniques developed and refined in underground research labs in Sweden, Switzerland, Belgium, and elsewhere since the 1980s. Posiva’s safety assessment for the Onkalo repository has over 250 different analyses of data from over four decades of investigations at the repository. “We are doing geophysics. We are doing hydrogeological studies there—how that water is flowing and what kind of connections water has from point to point, and what kind of effects flows have to groundwater chemistry,” explained Mustonen.

Technicians in Finland conducting a water injection test to measure how much, and in what directions, the water will flow.

Posiva

This data also allows scientists to make sure disposal tunnels are located away from any problem areas like fracture zones. Those can conduct a lot of water, as Mustonen experienced during the construction of a tunnel leading into the repository. “We intersected [a] large fracture zone with a pilot hole. We tried to measure how much water was coming from the hole… it was a little bit less than 200 liters [53 gallons] per minute,” Mustonen told me.

It's critical for the long-term safety of the repository that such leaks are permanently sealed. In a mine or a road tunnel, workers would typically seal a fracture zone with cement-based grout. But cement alters the groundwater pH, so the Finns instead injected that fracture zone, and others like it, with a silica sealant described by Mustonen as “like liquid quartz.” These injections have been remarkably effective; today, the entire network of tunnels produces only about 20 percent of the water that once flowed from that single pilot hole, Mustonen told me.

Scientists keep an eye on the zone damaged by excavation by scanning the walls with ground-penetrating radar, and they use sound waves to detect cracks deeper in the rock. They also test the strength and mechanics of the excavation, monitor the tiny seismic waves produced by cracking, and watch for surface movement above the excavation using GPS and by surveying the ground level. “These kinds of tests, they can tell us… is this system developing as expected?” said Mustonen. “And so far, so good!”

At Onkalo, scientists were able to show how immobile the groundwater is by using helium. Naturally occurring uranium and thorium in rock produce helium as they decay radioactively, and the helium builds up in groundwater while it remains in contact with rock. Water at the surface has little helium. The helium levels in the groundwater at the repository level indicate that it has been sitting there for around 13 million years.

Another safety concern is the effect of heat generated by the nuclear waste. Even after it has cooled in ponds for decades, spent nuclear fuel gives out heat by radioactive decay, raising the temperature near the waste canisters. This heat could potentially corrode the canisters, compromise the bentonite, or even crack the rock face. Therefore, the Finnish and Swedish designs separate individual waste canisters in their own disposal shafts to avoid excessive heat buildup. “The temperature is pretty much saying how close the canisters can be [to] each other,” said Mustonen.

Posiva is currently conducting a long-term, full-sized demonstration using heaters in dummy canisters surrounded by bentonite and temperature probes. After three years, the temperature at the canister boundary is about 70° C, Mustonen said. A similar test in Switzerland lasted 18 years and found that bentonite “remains suitable as a sealing material” up to at least 100° C.

Modeling the future

The data from the excavation and monitoring is added to three-dimensional computer models, which are in turn used to predict what excavators expect to find next. As new data confirms predictions, it builds confidence in the reliability of how the scientists' models represent the rock and water of the repository today. But to project how the rock and groundwater will affect humans living near the site in future millennia, the scientists must model that numerically using the tests and data as the starting point. “We have modeled to that million years... with different scenarios and what the likely releases [are], and it seems that the releases are acceptable,” Mustonen told me.

To work with rock in which groundwater moves through cracks and fractures, like at Onkalo, scientists make computer models called “discrete fracture networks,” or “DFNs.” Since it’s impossible to know how every little fracture connects to every other fracture, the models use a range of possible permutations for how the cracks connect and their distribution through the rock volume, with each permutation tied to observations. That way, many different possible flows can be simulated to build a range of probable outcomes. “The DFN model is our main model to understand groundwater movements,” said Mustonen.

Flow modeled in fractures. Flow speed (left) and pressure (right) in part of a discrete fracture network model.

Barsch & Nagel, Technische Universität Bergakademie Freiberg (CC BY SA)

Scientists then project what will happen to the waste over the next million years, assuming everything works as planned. They also model for several “what if” scenarios. This projection includes looking at the stresses and groundwater pressures caused by possibilities like being buried deep under a future ice sheet and then having that ice sheet melt away, sea level changes, changes in groundwater chemistry, and failures of canisters. At Onkalo, even in the worst case, scientists calculate that the maximum dose released to humans would be one-tenth of the regulatory limit, which itself is about a hundredth of the normal dose that Finns receive every year.

Nature has done this before

I asked Mustonen how long he expects the Finnish repository to keep the waste contained. “If everything works as we planned so that the groundwater stays suitable for the engineered barriers, it should be millions of years,” he said.

That might seem like hubris, but it’s supported by examples in nature.

In a quirk of deep time, a series of nuclear reactors started spontaneously in Gabon, West Africa, about 2 billion years ago, creating natural “spent fuel.” Remarkably, the uranium and its fission products haven’t moved much since then, thanks to a combination of stable geology, a shroud of clay around the “reactors,” and a lack of oxygen, reducing groundwater chemistry. Clay has also kept the Cigar Lake uranium deposit in Canada sealed off from groundwater for 100 million years—there’s no trace of uranium at the surface today, 450 meters (1,500 feet) above that uranium deposit.

Peña Blanca in Mexico is a uranium deposit that is similar to Yucca Mountain in both climate and geology, including being in the vadose zone above the water table. Uranium, thorium, and protactinium there have been mainly immobile for the past 200,000 years, but in an echo of the complexities at Yucca Mountain, radium has recently flowed out through fractures, and one section of the deposit has recently been open to uranium movement.

Consent, the even harder problem

With the apparent success of the Finnish efforts, will it be easier for other countries to revisit their waste disposal efforts? The prospects depend on learning political lessons. “More than the geology, I think it's socio-economic aspects" that determine if a project can go ahead, Mustonen told me.

A key lesson is that the top-down designation of sites for nuclear waste disposal has generally failed. The UK failed in 1987, 1997, and 2013. In the US, politicians campaigned against the Yucca Mountain project, characterizing its authorization as the “Screw Nevada Bill.”

Yucca_Mountain_Exploratory_Studies_Facil

A tunnel in Yucca Mountain.

US DOE

Now, most countries have a consent-based approach, where the local community and local government agree to host a repository. But if that approach selects sites without sufficiently weighing the geological, engineering, and transport suitability, countries risk another long and costly lead-up to failure.

Both Long and Murray told me that they think a clash of cultures contributed to project failures in the US and the UK. If all you have is a hammer, the saying goes, everything you see looks like a nail. Murray characterized the initial engineering mindset at the UK project as “just like doing a big grouting job.” At Yucca Mountain, the engineering mindset saw everything as “system analysis and risk assessment,” whereas the geoscience mindset had “limited appreciation for the impact of new knowledge on the main issue—the safety of the repository,” Long wrote in a 2004 paper.

When Congress ended the investigation of alternative sites in 1987, Yucca Mountain’s engineering mindset was forced to defend the project from whatever “hair-brained” issue anyone came up with, Long told me, as opposed to seeing if it met preset geological requirements. “The whole approach was backward,” Long said. “They should have set requirements for an inherently safe site and then investigated whether the site met the requirements instead of choosing the site for political reasons and then trying to show the site was suitable.”

Bizarrely, US law still designates Yucca Mountain as the American repository, and current regulations contain no “prescriptive requirements for the host rock,” David McIntyre, a spokesperson for the US Nuclear Regulatory Commission, told me. The agency has called for a revision of those regulations.

Yucca Mountain’s wasted $15 billion pales in comparison to the roughly $50 billion in damages that American taxpayers have had to pay to nuclear utilities because the government was unable to honor its commitment to receive nuclear waste by 1998.

Meanwhile, more waste is piling up. “What I find truly depressing is that we haven't had the bollocks to get on and do something about it,” said Murray.

Solving the rock-hard problem of nuclear waste disposal

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Solving the rock-hard problem of nuclear waste disposal

Finland avoided some of the mistakes made elsewhere and opened its waste repository.

Burial is best

The right stuff

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