In situ time-dependent characterisation of corrosion processes in nuclear waste storage and GDF environments

The UK Government is committed to managing radioactive nuclear waste through long-term geological disposal with safe interim storage above ground. The waste will be stored in metal canisters, and corrosion is a key potential threat to their integrity throughout the process of above-ground storage, operation of the geological repository during emplacement of the canisters, and following its final closure. We plan to investigate the mechanisms and rates of a number of the likely corrosion processes, developing characterisation methods that will be generically useful for studying similar processes in the future.

Localised corrosion of metals takes place in wet environments, often in cavities under the metal surface. The commonest method for evaluating the total rate of corrosion or depth of penetration is to make a cross-section of the metal at the end of a corrosion process, so that any information on the time evolution of the shape or chemistry is lost. However, by using highly-intense X-rays from a synchrotron source, it is now possible to study these processes in situ in real time, since the X-rays can easily penetrate the water and metal surface. The 3D shape of the corrosion site can be determined with X-ray microtomography, and chemistry can be assessed with diffraction and spectroscopy. All of these techniques are available at Diamond, the UK's synchrotron facility. When these techniques are combined with advanced lab-based techniques, a full picture of the mechanisms and rates of corrosion processes will emerge, enabling the development and validation of corrosion prediction models underpinned with sound science that are necessary for underpinning policy decisions on nuclear waste storage.

Our project is a collaboration between researchers at the Universities of Birmingham, Manchester and Bristol and Diamond. A Research Fellow based at the Research Complex at Harwell (next to Diamond) will lead the research effort, co-ordinating the X-ray experiments of the PhD students based at each University, who will also use lab-based research techniques. Each will have an individual research project: these include the atmospheric corrosion and cracking behaviour of stainless steels in above-ground storage conditions, the corrosion of different stainless steels in cement containing sulfur species, and the corrosion behaviour of uranium and Magnox in cement wasteforms (all related to intermediate-level waste), and the behaviour of carbon steel in clay, which may be a candidate for storage of spent fuel. Towards the end of the programme we will explore the influence of radiation damage on some of these corrosion problems.

We will carry out the research in close collaboration with industrial and international experts in the field, who have committed to giving us informal advice in return for information on our findings at our three-monthly meetings. As our research progresses, the X-ray techniques that we are developing will become more routine, to the point where they will be taken up by industrial users who carry out contract work for both UK and international waste management programmes. The methods are also likely to be beneficial to other applications such as corrosion of reinforcing bars in concrete and corrosion of oil pipelines

We plan to hold an international workshop at Diamond on corrosion issues in nuclear waste storage to share our findings and discuss the possibility of establishing a programme of "legacy" corrosion samples that will allow monitoring of the development of corrosion in typical environments over years or even decades.

There is a continuing need for skilled researchers with knowledge in this area: we will train a research fellow and three PhD students in issues surrounding nuclear waste storage, mechanisms and modelling corrosion processes, and the use of X-ray techniques who will be able to continue this work as the country's nuclear waste strategy evolves.

The UK Government's strategy for nuclear waste storage involves safe interim storage followed by eventual underground disposal. Wastes will be contained in metal canisters, and corrosion is a key threat to their integrity during above ground storage, operation of the underground storage facility, and following its closure. It is important to be able to assess the risk of corrosion at different stages. However, the timescales involved are far longer than feasible laboratory measurements, so we must develop robust, validated corrosion models based on fundamental scientific understanding in order to underpin public policy. It is important to build public confidence based on transparency and sound science since the safety of nuclear waste could impact the health and quality of life of future generations. Because this, we plan to reach out to young people through the STEM network (one investigator is trained for this), and two of us have discussed our work on BBC Breakfast and plan to continue similar communication in the media.

Understanding corrosion processes is difficult as they often take place in wet environments under the metal surface, and the main methods currently used to study them involve cutting up the metal after it has corroded. However, with highly intense synchrotron X-rays, we can probe inside the sites where corrosion is happening and monitor its evolution with time. We plan to develop these methods to understand corrosion processes that pose a risk to the integrity of nuclear waste canisters and use our data to validate corrosion models being developed by our collaborators. We will work very closely with Diamond, the UK's synchrotron: the Research Fellow leading the experimental work will be based there, working with Diamond's Physical Sciences Director, who is a co-investigator and expert in the techniques that we require.

As we develop our techniques, they will become sufficiently routine that they can be used by industrial research organisations who carry out nuclear waste-related contract research for the Nuclear Decommissioning Authority. We have a number of collaborators from such organisations who will advise us on our experimental strategy, and learn about our progress at our three-monthly meetings. They will then be able to become industrial users of Diamond, carrying out world-leading measurements, and enhancing their ability to compete for contracts from international nuclear waste management organisations, with a net benefit to the UK economy.

We have support from the Environmental Sustainability Knowledge Transfer Network to establish a web-based community group for exchanging information with the national and international research communities in this research area. This will give us a direct route for reaching out to the industrial and academic communities beyond our immediate network of collaborators. Our work will also be of value to other industries such as construction and the oil and gas sector, and to academic researchers working on modelling and experimental measurement of corrosion processes.

We plan to have an International Workshop at Diamond to inform corrosion researchers working with international waste management organisations of our new methods. As part of this workshop, we will discuss how in the future we could set up a series of "legacy" corrosion samples that could be monitored over future decades in order to validate models of some of the very slow processes involved with nuclear waste storage canisters.

It is crucial to develop skilled people to tackle these problems in future. We will train a Research Fellow and three PhD students in corrosion processes, synchrotron experiments, and underlying issues in nuclear waste storage, and the students will spend a month overseas with an international expert. Two investigators are early career academics, who will also develop skills for future success in this area.