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Nuclear waste being stored underground

Time to bury the radioactive waste issue

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Radioactive waste is dangerous and is here to stay. How are we going to make it safe for thousands of years to come?

In terms of problems that won’t go away, this is the one that’s going to hang around for the longest. The nuclear waste we are generating will continue to emit dangerous levels of radiation for tens of thousands of years, so getting rid of this waste needs a solution built to stand the test of time.

That solution “will be one of the UK’s largest ever environmental projects,” according to Cherry Tweed, chief scientist at Radioactive Waste Management (RWM), the public body charged with delivering a geological disposal facility (GDF).

Under European law, all countries that create radioactive waste are obliged to find their own disposal solutions – shipping nuclear waste is not generally permitted except in some legacy agreements. However, when the first countries charged into nuclear energy generation (or nuclear weapons research), disposal of the radioactive waste was not a major consideration. For several of those countries, like the UK, that is now around 70 years ago, and the waste has been ‘stored’ rather than disposed of. It remains a problem.

Globally, only the Waste Isolation Pilot Plant (WIPP) in New Mexico is an operating permanent disposal site, while a new breed of GDFs are being planned by some countries, with Finland’s Onkalo repository likely to be the first to be commissioned in two to three years. Sweden and France are not far behind.

Developing such a site is a long process. Longer, for example, than many political lifetimes, so the issue in the UK has been championed several times before only to fall off the agenda when administrations change or it just became ‘too hard’.

Now it is back on the agenda, seen as a necessity, and cross-party buy-in appears to guarantee continuity for the project. Moreover, for all that it is a project that has not resolved its ultimate venue, it has defined much of its design and format. While this may sound fundamental, there have been many proposed options for disposing of nuclear waste, such as jettisoning it into space, burying it under subduction zones in the sea bed and continued surface storage.

The problem with the latter is the considerable time-span and the ongoing management and maintenance of the waste facilities. Such facilities are, and will continue to be, used for treating low and intermediate level waste. This includes waste from industry and medicine, including clothing and materials that have been in contact with radioactive substances. The Drigg Low-Level Waste Repository (LLWR) on the Cumbrian coast serves this purpose.

Back in 2014, the Environment Agency raised concerns that coastal erosion could result in leakage from the site within 100 to 1,000 years, although it was counter-claimed that the levels of radioactivity after such a time would be low enough to be harmless. This would definitely not be the case for high-level wastes, where radioactivity could remain a hazard into and beyond the next ice age, hence the need for longer-term disposal.

GDFs – a series of highly engineered secure tunnels and vaults constructed deep underground – are widely seen as the solution. They have three guiding principles, the first of which is isolation – moving the material away from the surface. “The kind of depth that we will plan to construct our facility will be somewhere between 200m and 1,000m underground,” says Tweed. To put that figure in context, the deepest parts of the London Underground are between 60m and 65m underground. She continues: “So we are going to put a very thick barrier of solid rock between the waste and the surface environment.”

The second principle is containment, ensuring it will remain contained for as long as it takes for natural radioactive decay to render the substance non-hazardous.

The third principle is one of passive safety, as Tweed describes: “Once a geological disposal facility has been filled and sealed, it needs no further ongoing active management. It remains safe. We call it a system of passive safety: it remains safe because of the inherent features rather than because of the active ongoing management. The way in which that’s achieved is a multi-barrier system, which puts multiple layers of engineering protection that work together with the natural barrier provided by the rock.”

Where will it be? The location of the site remains undecided, the key point being that a GDF needs to have the buy-in of the local community. “We can’t do anything without a community coming forward voluntarily willing to engage with us,” says RWM’s technical director, Mohammed Sammur.

Tweed adds: “We will only move at the rate with which the community is comfortable, so when we talk about timescales, anything that we say is an estimate. We will work with the community, helping them shape the project, looking to see whether there are areas with potential, both for siting of the surface facility and being able to access a suitable underground environment. That very first stage will probably be about five years.”

Obviously a key factor will be the geology, and one aspect is that it should be a geology that is unappealing for any other use. It has to be assumed that records may be lost in coming millennia, or surface indicators may be changed, so choosing a geology of no commercial interest will help ensure it is not disturbed.

Additionally, the facility should be located under the water table. Tweed says: “That water tends to be extremely old, and tests that have been done on its chemical composition often show that deep underground groundwater has moved very little, if at all, for thousands or tens of thousands of years. And the kind of rocks that are suitable to host a geological disposal facility are ones in which we know that fluids move only very slowly.”

Such rocks fall into three categories: high-strength rock (e.g. granite), lower-strength sedimentary rock (e.g. clay), and rock salt. The GDFs in Finland and Sweden are in granite, French and Swiss projects are in clay, while the American WIPP is in rock salt.

There are many places around the UK where these geologies exist, but while geological maps looked detailed, and there is much information about the surface, deep-down composition is less precise. “Many of the models that you see of the deep underground are an interpolation between the individual borehole points, which can be anything up to 50 miles apart,” Tweed says.

“All three rock types are potentially suitable,” she explains. “The principle is tailoring engineered solutions so they complement the natural properties of the rock so that the two work together to provide safety. There are combinations of engineering materials which are very effective with the different kinds of potential host rock and where the support and understanding has been well developed and documented and shared with the nuclear [and environmental] regulators. There’s a high level of confidence in the safety of those engineered and natural combinations.”

‘Once a geological disposal facility has been sealed, it needs no ongoing active management. It remains safe. We call it a system of passive safety.’

Cherry Tweed, RWM

Much work has already been done on how to package up radioactive wastes. Although the waste includes the naturally radioactive uranium and to a far lesser extent plutonium, much of it comprises materials that have become radioactive, such as the graphite cores of the Magnox reactors or the concrete and steel of the advanced cooled reactors. What this waste all has in common is that it is in solid format, which is essential for underground disposal. Waste in fluid forms, such as that created by fuel reprocessing, is turned into glass, concrete or epoxy material to make it a stable format for disposal.

The flasks containing the waste are typically made of stainless steel, copper or cast iron, and with a price tag of £5,000 for a top-end container they are well-engineered products in their own right, designed to allow safe sealing, handling and transportation in automated environments. The most radioactive waste requires much thicker-walled cannisters as the process of radioactive decay creates enough heat to cause damage.

Highly radioactive waste flasks will then be disposed of singly within the GDF and immediately backfilled, while less radioactive waste will be stacked in bigger vaults that are backfilled when full.

That backfill can be bentonite-based cement. The advantage of bentonite, a clay-based material, is that it can flex and it therefore cushions the container against any small movements of the host rock. It also provides a non-corrosive environment for the canisters, but even when they do finally fail the clay is very good at trapping radioactive particles that get caught in gas or groundwater.

Some interesting research has been going on at the Diamond Light Source synchrotron, one of RWM’s neighbours at Harwell. The synchrotron is a huge ring that accelerates electrons to nearly the speed of light, at which they provide light 10 billion times brighter than that of the sun, and this is very good for studying things at sub-atomic level.

Given the longevity of the GDF project and the consequences of not getting it right, the behaviour of ‘disposed of’ radioactive materials is not being taken for granted, and constant research is under way.

In one example, RWM helped frame a project conducted by the University of Manchester at the synchrotron looking at how uranium responds to simulated GDF conditions. A recent project even uncovered a new ‘species’ of uranium, albeit a temporary one.

Manchester’s Professor Katherine Morris explains: “To be able to predict the behaviour of uranium during geological disposal, we need to take into account that it may have interacted with other processes taking place in the ground. These biogeochemical reactions are often a complex set of interactions between dissolved chemical species, mineral surfaces and microorganisms.”

This study showed that a uranium-sulfide complex – a new form of uranium – can form under conditions representative of a deep underground environment. Professor Sam Shaw says: “What is really needed for the GDF safety case is a fundamental understanding of how the chemistries behave under these conditions and how that then links to their mobility etc.” As it turns out, the uranium forms at both the start and the end of the process are non-​soluble and immobile and therefore have the advantage of having predictable behaviour.

It is envisaged that such research will continue in parallel with setting up the GDF, though combined with the knowledge gained from the equivalent projects abroad, RWM believes there is enough expertise to start on a safe facility now.

An imminent start is unlikely. Having progressed the idea at community level, there is a long phase of testing the geology. “We need to have a full understanding of it,” says Sammur. “We need to guarantee all the requirements will be met and the data that we extract from it, we analyse correctly to provide the under­pinning arguments or safety engineering.  So it’s not as straightforward as just choosing a rock type. It depends on lots of factors.

“Then you’ve got that big decision to proceed to construct and go through the construction period, which could be up to 10 years,” adds Sammur. “Then you start the actual waste placement. And then the facility can run for over a hundred years, continuously constructing while disposing of the waste packages from all of the 20, 30 sites around the UK that have waste packages ready for disposal.”

So, for all that it is an ongoing multi-billion-pound project that will last well into the 22nd century, it looks like a realistic operational life might not start until around 2040.

The GDF will have to take in about 750,000m3 of packaged radioactive waste, which would fill about two-thirds of Wembley Stadium. Interestingly, if all nuclear activity stopped now and was decommissioned, the UK would already have created about 90 per cent of the volume that the GDF is expected to cater for.

Nuclear power generation raises split opinions from environmentalists – it’s low-carbon but with obvious potential hazards and legacy issues – so would having a GDF change perceptions? Tweed concludes: “I think that demonstrating that we have an operating safe disposal facility would do a lot to reassure people about continuing to use nuclear as part of a low-carbon energy mix.”

Global

Existing disposal sites

Two repositories in Germany have been used for permanent disposal of radioactive waste. One is at the rock salt mine Bartensleben in the Saxony-Anhalt region. Having been used to store low and intermediate grade waste from Germany’s nuclear power stations, it was closed in 1998 amid fears of the stability of the underground structures, and it has consequently been pumped with salt concrete to stabilise the old mine, although this is not a long-term solution.

Another salt mine, in the Asse Mountains of Lower Saxony, was used as a repository for low and intermediate waste during the 70s, but again use was discontinued because of concerns about stability and contaminated ground water.

This leaves the Waste Isolation Power Plant (WIPP) in New Mexico, USA. It not only has the distinction of being currently operational, it is also a permanent solution – it is not storing nuclear waste for future management, it is tying up that waste safely forever. Dug into rock salt, the site has been accepting legacy nuclear waste since 1999, although there was a three-year hiatus following an underground radioactive leak in 2014.

Also of interest?

The IET will be hosting the Nuclear engineering for safety, control and security conference in March 2020. Find out more about the programme and speakers here.

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