A dangerous legacy
Sellafield reprocessing plant
15 April 2011 by Kath Morris and Jon Lloyd
The UK has a substantial amount of radioactive waste and understanding how to store it safely in the long term is a big challenge. But some of the answers lie at the smallest scale, as Kath Morris and Jon Lloyd explain.
More than 50 years of nuclear power generation and weapons production has left the UK with a significant problem - where to keep the leftover waste. While some of the nation's waste is safely encased in cement and awaiting disposal, a number of our old nuclear facilities still house a variety of other sources of radiation, including discarded fuel and fuel containers, old storage buildings, and contaminated soil and groundwater. For example, the nuclear facility at Sellafield in West Cumbria is the UK's largest, oldest and worst affected legacy site, and it is judged to be the most hazardous industrial site in Europe.
To decommission and manage our nuclear facilities safely we have to control and remove this contamination, but to do that we need to understand what happens to the radioactive elements, or radionuclides, once they're in the soil - their 'environmental behaviour'.
Soil systems naturally contain bacterial species that can directly affect the biogeochemistry (literally the biology, geology and chemistry) of their environment through what are known as 'redox' reactions. These bacteria live by taking electrons from organic matter in the soil (called the electron donor).
Our findings have important applications to the long-term storage for the UK's radioactive waste.
The transfer of electrons releases energy, effectively acting as 'food' for the bacteria, which then expel the used or 'terminal' electrons onto other elements in the soils or sediments (the electron acceptors). Electron acceptors in these environments include nitrate, manganese, iron or sulfate. These 'redox' processes (so named because they involve the 'oxidation' of the element losing the electron, and the 'reduction' of the element subsequently gaining the electron) are the basis of energy generation in life. These reactions alter the chemical behaviour or 'speciation' of the electron acceptors - how they interact with other chemicals - and in this way the bacteria can affect the chemistry of the whole system of soil and rock.
Up to now, our work has focused on understanding how radioactive elements might be affected by these geomicrobiological processes in the soil. Bacteria may interact directly with radionuclides in a redox reaction, or indirectly through the radionuclides' reactions with electron acceptors. But we don't know very much about either of these processes in the environment, so understanding under what conditions this might happen - and working out whether it's a good thing or a bad thing - has been a fundamental question for us.
We focused on the most problematic of these 'redox active' radionuclides, including technetium (Tc), uranium (U), and neptunium (Np) - problematic because they are long-lived, and neptunium in particular is extremely radiotoxic. Over the years we have worked with more than 20 scientists from a range of disciplines - radiochemists, geochemists, microbiologists, spectroscopists (who study chemical properties using electromagnetic radiation) and molecular ecologists, to understand radionuclide biogeochemistry.
Remote handling of radioactive waste
In our most recent work we studied how these radionuclides behave under different conditions, with different species of bacteria and different soils. We used soil, rock and groundwater to create model 'microcosms' of nuclear sites, then added in the radionuclides along with some food in the form of an organic electron donor. Under each combination of circumstances we watched to see what happened to the radionuclide, as well as to the biogeochemistry and microbial ecology of the microcosm itself.
To start with we focused on a form of technetium, Tc(VII), which is water soluble. Solubility is a problem in contaminated sites, because it potentially allows the radionuclide to be transported away from the source of radioactivity in groundwater. Clearly, movement of radioactive contaminants outside a nuclear facility is an issue, as it is uncontrolled and may eventually reach humans. In our experiments we found that, as the bacteria began to use iron as an electron acceptor, technetium was removed from the groundwater and became stuck to the soil instead.
Using synchrotron spectroscopy to see what was happening at the molecular level, we discovered that the reduced iron produced by the anaerobic soil bacteria was causing the Tc(VII) to change to its reduced form Tc(IV), which isn't water soluble. So the action of the iron-reducing bacteria was indirectly causing the soluble Tc to become insoluble - a good thing, because a radionuclide 'stuck' in the soil is much easier to manage than one moving in groundwater.
We also found that this process happens at extremely low, 'ultra-trace', concentrations of technetium, such as those found near Sellafield. Tc(VII) is mobile in the sub-surface at nuclear sites, so these processes could be very useful: if this bioreduction occurs naturally in contaminated land, or can be stimulated by pumping an electron donor (food) into the sub-surface, the technetium should become less soluble, and therefore less mobile.
Our experiments with uranium and neptunium produced similar evidence for bioreduction in natural soil systems. Neptunium is a man-made radionuclide which is 'transuranic' (beyond uranium in the periodic table), and the transuranics are some of the most significant long-lived radionuclides in our nuclear waste. As it is extremely radiotoxic, Neptunium is difficult to work with. The results have been worth the effort, though, because until we did this study no one had seen this bioreduction reaction take place with neptunium.
What does all this tell us? Well, to manage radioactive wastes effectively you really need to understand how microbes can affect the behaviour of radionuclides in contaminated environments. The bioreduction we've seen is good news as it typically causes radionuclides to get 'stuck' on sediments. It's also a process that we might be able to manipulate to make contaminated sites easier to manage. But these reactions are complex and each radionuclide behaves differently, so we need to keep experimenting on them despite the hazards involved.
Our findings have important applications to the long-term storage for the UK's radioactive waste, and this will be the focus of our next piece of work. The most hazardous wastes, which are highly radioactive and/or contain long-lived radioisotopes, have to be managed very carefully. Current UK government policy is that these should be disposed of in an engineered Geological Disposal Facility (GDF) between 200 and 1,000m below ground, but the site for this GDF has not yet been decided.
Some of the radionuclides present in the wastes, like Tc, U, Np and Pu (plutonium), will remain dangerous for hundreds of thousands of years, and the GDF will inevitably degrade over time, releasing some radioactivity into the surrounding environment. If the UK's radioactive waste is going to be stored safely we need to understand the key processes that will control the movement of radioactivity out of the GDF to the surrounding rock and beyond. As part of a new research consortium called BIGRAD (BIogeochemical Gradients & RADionuclide transport) we will spend the next four years looking at the GDF environment in detail, and our soil-dwelling microbes or their rocky relatives will be an important part of the story.
Professor Kath Morris and Professor Jon R Lloyd are based in the School of Earth, Atmospheric & Environmental Sciences at the University of Manchester.