21st Century Chemistry: Disposing of our Nuclear Legacy

Meeting report by Tom Sizmur
University of Reading
t.sizmur@reading.ac.uk
ECG Bulletin July 2019

The role that nuclear power will play in our future global energy mix is still being debated. Emitting less carbon dioxide is often considered a safer form of energy production than using fossil fuels. It is also more reliable than renewable energies such as solar or wind, which only generate power under specific climatic conditions. Nevertheless, the generation of nuclear power has left a legacy of radioactive waste requiring safe disposal.

The technical challenges and potential threats associated with disposing of our nuclear legacy formed the focus of the Environmental Chemistry Group’s 2019 Distinguished Guest Lecture, delivered on 27th March by Professor Melissa Denecke (International Atomic Energy Agency) and supported by talks from Dr Joanna Renshaw (Strathclyde University), Professor Mike Wood (University of Salford), and Dr Juliet Long (Environment Agency).

Dr Joanna Renshaw opened the symposium with an introduction to the UK’s nuclear legacy. She explained the basis of nuclear fission and then went on to provide an overview of the nuclear fuel cycle, emphasising that spent nuclear fuel, largely comprising enriched or depleted uranium, could be reprocessed (closing the cycle) or directly disposed of as radioactive waste. Radioactive wastes can be classified according to their levels of radioactivity: Low (LLW) or Very Low Level Wastes (VLLW), with less than 4 GBq (gigabecquerels) per tonne of α, or 12 GBq per tonne of β/γ activity, largely emanating from slightly contaminated materials produced in the decommissioning of nuclear sites.  These require careful handling, but do not require shielding. These wastes can be disposed of in permitted landfill facilities. Intermediate Level Wastes (ILW) do require shielding, but are not heat generating, and so do not require cooling. High Level Wastes (HLW), however, require shielding and cooling and arise primarily from the reprocessing of nuclear fuel. Dr Renshaw explained that the Nuclear Decommissioning Authority categorise radioactive waste into 24 different streams according to their radioactivity and chemical and physical forms, ranging  from inert solids to liquids and sludges. Each waste stream requires separate disposal arrangements.

As the 2002 UK Government White Paper on ‘Managing the Nuclear Legacy’ states, this is “…one of the most important and demanding managerial, technical and environmental challenges facing the UK over the next century...” This White Paper led to the formation of the Nuclear Decommissioning Authority, and a budget of £121 billion to clean up the UK’s nuclear legacy.

Next, Dr Renshaw focussed on the challenge facing the Nuclear Decommissioning Authority in the 21st century to decommission nuclear facilities, using Sellafield as an example. There are 15 reactors currently operating in the UK, and all are due to be shut down by 2035. The three primary strategies adopted to decommission nuclear facilities are either (i) dismantle the facility immediately, (ii) defer the dismantling for ~40-60 years, allowing the residual radioactivity to decay, or (iii) entomb of the facility with no intention of ever completely removing the radioactive waste.  Dr Renshaw concluded her talk by outlining the challenges faced in decommissioning Sellafield, particularly mentioning the 22 Magnox Swarf Storage Silos used to store ILW.  Each of these is the volume of six double decker buses; decommissioning these will take 30 years.

Professor Mike Wood spoke next, introducing us to his work on radioecology and environmental radiation protection for wildlife, with a particular focus on research undertaken in the Chernobyl Exclusion Zone. He explained that, in order to assess the risk to wildlife from radioactivity, we need to understand (i) how wildlife are exposed to radioactivity, and (ii) how radioactivity impacts the organism in question, leading to questions such as ‘How much radiation does it take to cause harm to wildlife?’ and ‘Is radiation good or bad for wildlife?’.

Professor Wood began his narrative by taking us back to the early 1980s and the town of Pripyat the Soviet Union (as was), where residents experienced a good quality of life. Then, on 26th April 1986 at 01:23, an explosion blew the roof off reactor No. 4 of the adjacent Chernobyl nuclear facility and, over the following 10 days, released radioactive material to the atmosphere. Changing meteorological conditions during this time resulted in a patchy distribution of radioactive contamination throughout the Chernobyl Exclusion Zone. Iconic scenes included the ‘Red Forest’ in which the pine trees died and turned red. Today, a deciduous forest has regenerated, and the Chernobyl Exclusion Zone, which includes around 100 Ukrainian villages, has been taken over by nature as the forest invades ruins.

However, there remains considerable uncertainty about the effect of radioactivity on wildlife, leading to conflicting reports in the media. One report highlighted by Professor Wood identifies major declines in insect numbers as radiation increases, yet the doses extrapolated to zero insect population are similar to those naturally present in Cornwall. Since mammals are generally more susceptible to radiation than insects, this raises the question as to whether Cornwall is safe or not! To address this uncertainty, independently verifiable experimental methods are used that reduce disturbances caused by observations and avoid unconscious bias, beginning from the null hypothesis that large mammal abundances and diversity are not significantly affected.

To test this hypothesis, Professor Wood set up motion-activated camera traps in three areas of the Chernobyl Exclusion Zone that represent areas of high, medium and low contamination. This resulted in 250,000 photographs  which feature wildlife such as red foxes, racoon dogs, red deer, grey wolves, Eurasian lynx, European bison, brown bears, and Przewalski’s horses (a particularly rare breed that is seemingly common within the exclusion zone). The dataset from these images presents a considerable processing challenge. However, there currently appears to be no evidence to suggest that abundance or diversity differ between  the low, medium and high contamination sites. Future work will study the age, structure and behaviour of wildlife in the exclusion zone and investigate whether radioactivity affects species interactions.

Dr Juliet Long, the Head of Legacy and Waste Issues in Radioactive Substances Regulation at the Environment Agency (EA), provided an independent regulatory perspective on the disposal of the UK’s nuclear legacy. The EA has the responsibility to issue permits to nuclear sites to ensure all discharges are safe. This includes around 30 nuclear facilities, and ~2000 other sites permitted to discharge radioactive waste (including sites in the defence, oil and gas, manufacturing, hospitals and life sciences sectors).

Showing a photograph of the LLW Repository at Drigg in Cumbria from 2005, Dr Long demonstrated how limited our remaining storage capacity, is and explained that there was some concern over the safety case for the site, primarily coastal erosion from the Irish sea. Since anything buried in the ground eventually returns to the surface, the aim is to ensure that when it does reappear, it will not pose a threat. The policy, at the time Drigg was opened, required all LLW to go to this one site. However, this policy was changed in 2007, allowing radioactive wastes to be disposed at other permitted landfill sites around the country. Each site requires an individual safety case. As a result, we now have a diverse range of sites where LLW can be disposed of around the country.

Looking forward, considerable quantities of LLW are likely to be generated, including ~4.5 million m³ from civil nuclear decommissioning and up to 6 million m³ more from nuclear site clean-ups. This far exceeds the current permitted disposal capacity of ~1.2 million m³. Dr Long highlighted a potential problem whereby demand for disposal eventually outstrips capacity. Much of the waste that will be generated by the decommissioning of nuclear facilities includes large concrete and metal structures. Our limited waste disposal capacity raises the question of whether some of these structures should be disposed of on site or left in situ.

In the final part of her talk, Dr Long discussed the issue of a Geological Disposal Facility, outlining reasons why the UK currently does not have one and the difficulties involved in generating the political will or social capital to identify a site where one could be built. The new government policy, published in 2018, is to dispose of higher activity waste deep underground. A selection process is now under way to identify a local community with a suitable geological setting that is willing to house such a facility. The EA would then regulate such a facility.

The 2019 Distinguished Guest Lecture was given by Professor Melissa Denecke. Professor Denecke’s research focuses on what happens when there is a breach of a Geological Disposal Facility, or if there is water ingress. Until recently, she was a Professor of Chemistry at the University of Manchester, but now works for the International Atomic Energy Agency (IAEA) as part of a group spanning nuclear science, technology and its applications. She provided some examples of the work conducted by her new group using nuclear technologies to help achieve UN Sustainable Development Goals. This includes the use of isotopes to characterise groundwater and precipitation to support hydrological and climate studies. One example of this technology is the use of Kr-81, Cl-36 and I-129 to date groundwater and identify sites that contain very old groundwater suitable for a Geological Disposal Facility. However, in giving this talk, Professor Denecke represented herself and her personal career path, rather than the IAEA.

Providing an overview of the global nuclear waste inventory, Professor Denecke highlighted that waste volumes are small compared to those produced by non-radioactive power generation. A large proportion of the world’s LLW has already been disposed of. Globally, ILW and HLW only represents 2% of the volume of radioactive waste, but 98% of the radioactivity. The international consensus is that a Geological Disposal Facility is the most appropriate option for HLW because the geology provides a barrier between the material and the biosphere. A typical design incorporates multi-barrier containment. The first country to provide a safety case for a Geological Disposal Facility was Switzerland, approximately 20 years ago.

Professor Denecke’s research applies X-ray spectroscopy to investigate the barriers that may be employed in a Geological Disposal Facility and the waste itself. The techniques she uses help to determine the oxidation state of metals in a mixture without physically separating species. For example, it has been possible to determine that selenium is present as selenite (SeO₃^2-) and technetium is present as pertechnetate (TcO₄^-) in a glass fragment from the Karlsruhe Reprocessing Plant.

Her research included an investigation into uranium-rich tertiary sediment from Ruprechtov in the Czech Republic, a natural analogue of a waste disposal site. Confocal µ-XRF (X-Ray Fluorescence spectroscopy) was used to generate 3D maps of the sediments and determine the elemental composition beneath the sample surfaces, which are typically oxidised. This technique was combined with µ-XRD (X-Ray Diffraction) and µ-XANES (X-ray Absorption Near Edge Structure spectroscopy) to focus on a single element and, for example, identify the speciation of uranium in the sediments. U(IV) was identified, likely present as a phosphate or a sulphate. By identifying these species, it is possible to apply thermodynamic data and generate a model to track the modifications to uranium after expulsion from the reactor, strengthening a safety case for geological disposal.

Thereafter, Professor Denecke highlighted the importance of colloid-mediated transport of radioactive contaminants, since colloids are able to travel through geological materials faster than water (as demonstrated by the use of tritium in tracer studies). Research was presented on the role of colloids as carriers for plutonium and the role played by natural humic substances that act as surfactants in groundwater and facilitate colloidal dispersion. At the end of her talk, Professor Denecke identified some of the exciting developments that she believes will drive the field forward, including the use of  X-ray ptycho-tomographic imaging to create 3D images from 2D slices, and which can identify a layer of uranium in a solid object with a 13 nm resolution. She anticipated that future work would provide more opportunities for in situ investigations of materials at greater resolutions than those previously achieved, the combination of analytical techniques, the marriage of experimental and theoretical methods, and further development and use of models.

slides by juliet long

Cover image: Conceptual Image Of A Radioactive Nuclear Waste Barrel Or Drum Near Water In The Countryside / Shutterstock