Going deeper underground

Whether you are pro- or anti-nuclear energy, the chances are your main concerns about atomic power centre around one thing: safety. 

As we’ve seen all too clearly from incidents such as Chernobyl and Fukushima, nuclear fuel, following use known as nuclear waste, is an extremely harmful substance, and its radioactivity (especially in intermediate- and high-level waste) can cause grievous damage to both the environment and human health if not properly managed.

Currently, most of the UK’s low-level waste (which accounts for 90 per cent of the total) is stored at Drigg in Cumbria in thousands of steel containers. But, as yet, there is no UK solution for the long-term storage of intermediate- or high-level radioactive waste (which is currently held in drums in interim storage at nuclear sites across the UK). However, the UK government has ruled that ‘geological disposal’ – treating nuclear waste in solid form before burying it deep underground in purpose-designed concrete containers – is the safest way to manage intermediate-level waste (ILW). 

But how safe is it? In theory, a geological disposal facility (GDF) should be capable of containing radioactive waste within multiple protective barriers deep underground to ensure that no harmful quantities of radioactivity reach the surface. A key challenge here is to design a system that can achieve this high level of containment for very long periods of time (hundreds of thousands of years), while the radioactivity in the waste decays away. Eventually, groundwaters will penetrate the protective barriers, though, and understanding what happens then is key to predicting the safety of the disposal process. 

Jonathan Lloyd, Professor of Geomicrobiology at the Williamson Research Centre for Molecular Environmental Science (part of the University of Manchester’s School of Earth, Atmospheric and Environmental Sciences), tells me: “When water in the ground interacts with materials like concrete, the environment becomes alkaline, which can lead to chemical degradation of some of the cellulose-based materials in radioactive waste to form something quite worrying. This compound is called isosaccharinic acid (ISA) and is of concern because it binds very, very strongly to radionuclides like uranium, and when it binds to them, it can make them much more mobile.” This is concerning, as it means that some of the toxic elements of nuclear waste could migrate away from storage areas, and, potentially, flow to surface environments, where they could eventually enter drinking water or the food chain.

Scientific breakthrough

However, Lloyd recently led a research project that made a new scientific breakthrough: they discovered a new single-cell organism that could help contain the nuclear waste in GDFs. 

He explains: “When you look at the ISA compound, it just looks like a carbon compound that microbes would like to use as an energy source. So we decided to research whether it could be. We collected soils from a site near Buxton in the Peak District where lime waste had been disposed, making the surrounding environment very, very alkaline – potentially leading to ISA formation. Despite most people assuming that there would not be much microbial activity in this sort of low-oxygen, highly-alkaline environment (because the pH levels are equivalent to those found in household bleach, which you’d use at home to kill bacteria such as this), we found the area to be teeming with microbial life, with millions of bacteria per gramme of sediment.”

The first ever discovery of this type of bacillus organism – yet to be formally named – was surprising not just because it was surviving in extreme environments (thus gaining itself the rather wonderful tag of ‘extremophile’), but also because of what it could do. After collecting samples, Lloyd’s team took them to a lab, and began trying to understand how the alkaliphilic microorganisms were surviving. “In these experiments, we put some of the ISA in as a carbon source, with some nitrate, or oxygen, or iron as ‘electron acceptors’ to support respiration of the bacteria, plus a few other trace elements, and left them in sealed bottles to stew for a few weeks. When we went back to look at what we had, we discovered that the microbes had been able to breakdown this ISA compound completely very quickly.

“Interestingly, the microbes are really flexible: they can switch their metabolism one way or another depending on the environment they are living in. So, they were happily using things like nitrate and iron oxide to survive when oxygen levels were low. We’re still trying to understand how they do this. But we know that they use the ISA carbon as an energy source and, through that process, break it down – eventually all the way down to CO2.”

Helping to assess the safety of GDFs

The field site in Buxton where the bacteria were found was 'teeming with microbial life' despite the alkaline environment

It’s this work that Lloyd thinks could help those tasked with developing the safety cases for the UK’s future GDF sites. “The lime site where we found these microorganisms has been contaminating the Peak District for decades, maybe up to 100 years, and the microbiology seems to have evolved really rapidly to feed off the ISA. These conditions are similar to those expected in and around intermediate-level radwaste disposal sites, so it’s not reaching to think they could be active there too. In relation to the lifetime of a GDF, the time it would take for the microbiology to kick in is a blink of an eye really.”

When I ask Lloyd if the radioactivity of the GDFs would be a problem, he reveals another remarkable trait of the bacteria that the Manchester team has been studying: many of them thrive under radiation. Indeed, after irradiating the soil samples, Lloyd’s team found that rather than killing off the biology, the radiation sometimes even stimulated it, as it helped to break down some of the nutrients in the soil to make it more ‘bioavailable’ for the microbes. 

This article was taken from Issue 79

With central government desperately seeking a UK community to volunteer to host a GDF, Lloyd outlines that his team’s research could help allay many safety fears (however, there has been no research undertaken yet as to whether these microbes could deal with the volume of ISA produced by these massive facilities). He concludes: “We’re very interested in helping the people that have to design and build these GDFs understand whether these sorts of activities are important and [in helping] make the safety case much more accurate. For us, if someone were to design a safety case that did not factor in the biological processes, they’d be overly conservative. For example, there might be concerns (such as ISA build up) that biology could actually take care of naturally. 

“We think these natural organisms that have been overlooked thus far could actually help stabilise the whole system and give you a biological barrier that could stabilise the GDF for long periods of time. In essence, our feeling is that microbes surrounding these facilities may actually help to get rid of the ISA and stop radioactive material from becoming mobile. The message that there is another, ‘natural’ protective barrier in the system is worth emphasising.”

Lloyd’s colleague, PhD student Naji Bassil conducted the initial work with the ISA-degrading bacteria and is now sequencing the genome of this isolated bacteria to try and understand the biochemistry of the bacteria, and the team will soon be researching how the bacteria directly impacts on radioactive materials.

It’s hoped that, eventually, the team’s research could be used for other applications – not just GDF safety case development, but also for cleaning up contaminated land, perhaps by treating heavy metals and breaking down toxic organics. “There are all sorts of things we could do. It’s just about understanding how to activate them and how to get them to do what you want them to do”, Lloyd says. We wait with baited breath for the results…