Testing the long term effects of radiation exposure on material properties

Materials for nuclear applications have a certain mystique. Like the quantum world, the normal rules don’t quite apply and those that do are, as yet, not exactly known. Categorising properties and defining behaviour is vital to both next generation nuclear fission power plants and future nuclear fusion reactors. This is why the UK Atomic Energy Authority (UKAEA) has decided to bring about fresh focus on materials for nuclear applications in its all-new £10 million Culham Materials Research Facility (MRF), part of the Culham Science Centre.

“If you want to do thousands of experiments on pieces the size of your finger, that is an enormous amount of irradiated material,” says Martin O’Brien from the UKAEA. “The cost and practicality becomes very prohibitive.

“Here, the idea is to bridge the gap between university laboratories and larger nuclear facilities like Sellafield, so UK scientists can do thousands of experiments on very small samples, and so at a much lower cost.”

The rolling Oxfordshire hills is perhaps not the likeliest place for world leading research in to materials for nuclear applications, but Culham has a reputation for nuclear innovation. It became famous in 1983 for housing JET, the Joint European Torus, the world’s first, and still largest, operational tokamak – a device capable of producing thermonuclear fusion reactions that mimic our own sun.

“We put a star in a box,” explains Nick Holloway, media manager from UKAEA. “But, we are still working out what the box should be made out of. One of the biggest challenges in getting fusion power in to the electricity grid is finding materials that can withstand the high heat fluxes created and cope with the effect of irradiation.”

Culham is therefore the obvious site for such a bridging facility, with renowned experts including Professor Steve Cowley, former chief executive of the UKAEA, advocating the move. He says: “With new nuclear builds on the horizon, and the prospect of Generation IV and fusion entering the market later this century, it is vital for the UK to develop a first-class range of research facilities to meet the challenge.

“The MRF provides academic and industrial users – in both fission and fusion – with equipment for the processing and characterisation of radioactive materials, for onsite analysis or for taking back to the researcher’s own lab.”

The MRF aims to bring industry and academia much closer together by offering the tools and testing equipment needed to advance current understanding. It is part of the larger National Nuclear User Facility (NNUF) and will complement companion facilities at the National Nuclear Laboratory in Sellafield and the University of Manchester’s Dalton Nuclear Laboratory, Cumbria – enabling all three facilities to do research on materials with varying levels of radiation damage.

The need for such a facility has long been known to industry. However, the Government responded to a House of Lords Science Committee report in 2011, and now failings on nuclear R&D strategy look to be finally to be addressed.

“That Committee looked in to the status of fission nuclear power research and development in the UK, and was basically very critical that it got to such a low ebb,” says O’Brien. “There is a massive lack of skills in nuclear that have been lost. But, with facilities like the MRF, the aim is to put that right.”

The MRF will provide small irradiated sample materials to universities and industry laboratories to be tested and analysed. To date, the issue has been the obvious problem of cutting up radioactive materials, which are not only expensive and time consuming to create, but difficult to transport and handle. It’s meant that research has been restricted to nuclear licensed sites like Sellafield, and though this will still be necessary for some experiments, the MRF is on an unlicensed site and so can offer a quicker, cheaper service.

The MRF will enable neutron-irradiated materials currently stored at Sellafield and other sites to be analysed by many other labs, by making them low risk and capable of normal transport. The trick is not to reduce the potency of the irradiated material, but the size.

“One of our main roles is to do experiments at the microscale on irradiated material and compare it with non-irradiated material,” explains O’Brien. “Generally, radiation makes metals harder and therefore more brittle, so you have to factor that in during the design. Knowledge of the underlying reasons is presently limited, but modern analysis techniques offer the prospect of much deeper understanding.”

The irradiated bulk materials are delivered to the MRF in protective barrels and lifted by crane into a receiving ‘hot cell’. The soon to be commissioned hot cells are made of steel. Externally the cells are around 4m high by 3m by 3m. Inside sits a work area lined with 350mm steel and a 500mm lead glass screen for operators to look though while operating manipulator arms as samples are prepared.

There are four cells in total, all interconnected, giving the ability to receive and then pass along materials so they can be cut in to smaller samples, mounted in to plastic polymer backing plates, before being grinded and polished. Once the samples are prepared, they can be tested onsite, in shielded rooms if necessary, or sent to various universities for further detailed analysis. To date, it has captured the attention of companies such as Rolls Royce, AMEC and EDF Energy as well as universities from Oxford, Bristol, Sheffield, Manchester and London.

While many may react with alarm at the prospect of radioactive materials being transported around the UK to university labs, it is worth remembering that most samples will end up less radioactive than a banana due to their size.

These samples are cut using reasonably traditional machining methods, but adaptations are made. Monica Jong, head of operations at the MRF says: “Although these are all relatively standard techniques that have matured, they are not standard within the hot cell or research environment.

“We are cutting off thin slithers using a slow cut saw, to minimise any dust. We are also working on using a spark erosion machine as well to do erosion cutting with a wire to get more complicated shapes like small disks or even dog-bone shaped samples for tensile testing.”

Material understanding

Once the samples are prepared, many of the tests are relatively standard in principle, carried out on hardness and tensile test machines. Part of the art is minimising the size of the sample, while still enabling meaningful data to be gathered.

Key to this is the use of material modelling software to help researchers take an understanding of the very fine scales, up to larger sizes.

“We need to be able to extrapolate from microns up to mm, and upwards again,” says O’Brien. “We need to be confident that what we are seeing on the microscale is representative of macroscale irradiated material behaviour, which is why both modelling and where possible experiments at different scales are so important.”

Onsite at the MRF, scientific instruments include a dual beam focused ion beam, scanning electron microscope, a nanoindenter, a 10kN universal testing machine, an atomic force microscope and thermal desorption spectroscopy.

“For mechanical testing, we are looking in to fatigue,” says Steven Van Boxel, the lead scientist at the Centre. “For thermophysical testing we do dilatometry, a thermo-analytical method for measuring the shrinkage or expansion of materials over a controlled temperature range. We also do laser flash analysis to measure thermal diffusivity and STA (simultaneous thermal analysis) to measure weight changes and thermal capacity while cycling through thermal ranges.”

Combining these techniques will allow the team to uncover what they describe as the ‘holy grail’ of thermal testing: determining the thermal conductivity of a material.

“It is interesting to see how irradiation effects the thermal properties of a material and how it changes,” adds Van Boxel. “So we want to measure these changes at different irradiation regimes.

“As materials become irradiated they become harder and more brittle and generate defects. This all has consequences on mechanical properties as well as thermal properties, and we want to develop a much better understanding of how these mechanisms work.”

Nuclear importance

Ultimately, the work done at the MRF will develop a better understanding of the effects that operating in a radioactive environment has on materials’ mechanical properties, so the design and engineering of future nuclear equipment can be improved and the power stations operating today using fission reaction can be lifed more accurately, and run more efficiently.

“Understanding the lifetime properties of materials in operation, in certain conditions, is what designers of future power plants need to know,” says Van Boxel. “Whether that is using fission or fusion, knowing how materials respond under neutron-irradiation means we can design to overcome those effects.”

And with a new generation of nuclear power stations set to enter service from 2025 with an expected life of 40 to 60 years, this work cannot come soon enough.

The Henry Royce Institute

The initial investment in the Culham Materials Research Facility was from the Government’s National Nuclear User Facility (NNUF). Now further funding is coming from the Henry Royce Institute, which is investing in new facilities across material science and engineering in the UK from 2D materials to advanced metals processing to the biomedical – and of course nuclear.

The Royce Institute is championing UK advanced materials research and innovation, with Royce supporting a variety of world-leading research and innovation in advanced-materials science. It has also established itself as a critical component of the Government’s Northern Powerhouse initiative.

The Royce brings together world-leading academics from across the UK, and works closely with industry to ensure commercialisation of fundamental research. The Institute will have its hub at The University of Manchester, with spokes at the founding partners, including universities in Sheffield, Leeds, Liverpool, Cambridge, Oxford and Imperial College London, as well as UKAEA and NNL.

It will focus on nine key areas of materials research, which are grouped into four themes – Energy, Engineering, Functional and Soft Materials – critical areas to underpin the Government’s industrial strategy.

Justin Cunningham

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