The nuclear R&D roadmap - critical materials challenges

Materials World magazine
1 Sep 2013
Materials modelling and simulation will play a critical role in advancing new nuclear technologies

In March 2013, the UK Government confirmed nuclear as a key future low-carbon energy source when it announced a new, longterm nuclear strategy. Professor Andrew Sherry of The University of Manchester’s Dalton Nuclear Institute and Professor Robin Grimes of Imperial College London, both UK, discuss the critical materials challenges involved.

Nuclear energy has received renewed attention in recent years as an attractive and potentially major contributor to the UK's future low-carbon energy requirements, alongside renewables and carbon capture and storage. Materials play a vital role in nuclear energy, from the fuel and structural materials of the reactor to the legacy and immobilising materials that constitute radioactive waste.

In March 2013, the UK Government published its Nuclear Industrial Strategy, aimed at enhancing the UK’s supply chain contribution to its civil nuclear programmes, and increasing international engagement in nuclear via export and inward investment. To help realise these objectives, the strategy includes priorities on the development of a new generation of nuclear skills and renewed commitment to nuclear R&D.

Neither the level of new nuclear generating capacity that will be delivered in the UK nor the associated fuel cycle is fully established. As such, the UK Government’s 2013 Nuclear Energy R&D Roadmap sets out a broad scope of priorities to enable up to 75GWe new nuclear generating capacity to be installed by 2050, which would make a significant contribution to driving down carbon emissions.

These priorities are set out across a number of high-level programme areas and can be summarised under four key headings:

  • Current operations – operation and life extension of the advanced gas-cooled reactor (AGR) fleet and the pressurised water reactor (PWR), as well as nuclear chemical facilities such as the thermal oxide reprocessing plant (THORP) at Sellafield
  • Nuclear new build – the next generation of light water reactors (LWRs), future small modular reactors (SMRs), generation IV reactors and fusion systems
  • Waste and decommissioning – clean-up of nuclear sites, management of spent fuel and geological disposal of nuclear waste
  • Fuel cycle services – current fuel processing and advanced reprocessing to enable efficient recycling of nuclear materials

Current reactor operations

There are several materials R&D challenges critical to the delivery of the priorities the Roadmap sets out. The safe operational life of nuclear plant, as with any other engineering plant, is largely dependent upon the ageing and degradation of its structural materials. For the UK’s current fleet of AGRs, the major materials challenges relate to degradation of the graphite core and stainless steel welds that operate at high temperatures within the boilers. For the graphite core, the combination of high temperature, CO2 coolant, radiation field and stress results in weight loss and embrittlement of the material, causing the evolution of complex cracks. For the boiler welds, which are not stress relieved, creep relaxation at high temperatures leads to the formation of reheat cracks.

Research programmes are addressing these materials challenges by developing an improved mechanistic understanding of the degradation processes and drivers. This is enabling the generation of predictive computational models that not only provide insight into forward behaviour, but also inform workers on the safe operating parameters for the plant.

New nuclear build

The development of a future generation of reactors in the UK has opened up an opportunity for the manufacturing supply chain to provide a significant proportion of the new reactors, including pumps, valves, pipework and containment buildings. Establishing a leading edge in an internationally competitive market requires UK companies to adopt leading edge manufacturing technologies that have been accredited for nuclear application. This calls for the development of advanced welding, machining and near net shape manufacturing technologies that have been proven to perform at least as well as existing technologies in the challenging environments experienced in a nuclear power station.

Future reactor systems including SMRs, generation IV and fusion systems will push materials into more challenging operational environments and make R&D into materials ageing, including the study of higher temperature performance alongside radiation damage, increasingly important. For some systems, the UK has unique experience gained in the operation of the AGR fleet. This includes graphite and high-temperature welds, which would enable us to make a significant contribution to the development of future high temperature gas-cooled reactor systems. In addition, synergies between fission and fusion materials, particularly in understanding and modelling the detailed mechanisms of radiation damage in structural materials less common in conventional fission systems, could be hugely beneficial.

Waste and decommissioning

Research into the performance of nuclear waste is crucial, given the timescales involved in both the interim storage of spent fuel in ponds as well as the immobilised waste in a geological repository.

For spent fuel management, the long-term behaviour of the fuel assemblies is controlled by the microstructure of the fuel cladding, which will have been influenced during reactor operation by high temperatures, as well as gas or water coolant and the radiation field. In the storage pond, this microstructure is further influenced by the local chemistry and temperature of the pond water near the cladding, which may contain traces of chlorine and will also undergo radiolysis. For AGR fuel, the performance of the niobium-stabilised 20/24 stainless steel, and for PWR, the performance of Zircaloy, both of which are subject to such pond water conditions over long periods of time, requires further study. Many countries are also considering dry storage facilities, which offer different challenges. Either way, due to the long timescales involved, accelerated ageing studies are being developed, often in combination with modelling and simulation, so that the influence of altered timescales can be assessed.

Research is also required to characterise and quantify the long-term stability and integrity of:

  • the waste packages, including vitrified waste glass for high-level or cement for intermediatelevel waste
  • their containment material in stainless steel canisters
  • the materials comprising the multi-barrier system of proposed underground nuclear waste repositories, including the backfill clay, concrete and host rock

This research plays an integral part in delivering a database and process understanding for any reliable safety case, and is crucial for guiding implementers and decision makers. In addition, research into conditioning of waste and components prior to disposal is needed if their volume is to be reduced. One example is the strategy of partitioning and conditioning high-level, heat-producing waste from spent nuclear fuel, which can significantly reduce the volume of underground capacity required.

Fuel materials

The UK currently manufactures all the fuel for its AGR fleet and has the capability to manufacture PWR fuel. Materials research is underway to understand the performance of fuel, including the fuel cladding materials within the reactor under normal and off-normal operating conditions. For example, research into the oxidation and hydrogen pick-up behaviour of Zircaloy cladding is being undertaken within a broad academic collaboration with industry. The study employs an integrated modelling and experimental approach across length scales, so that defect processes predicted on the atomic scale are combined with nanoscale microscopy, yielding rationalised observations of microstructural evolution and hydrogen transport. Collaboration between the UK and the USA in the coating of fuel cladding to improve Zircaloy performance under a loss-of-coolant accident is also a priority.

Scientists are also looking into advanced fuels for future reactor concepts, including the development of tri-structural isotropic (TRISO) fuel for use in high-temperature reactors. TRISO fuel consists of two layers of pyrolytic carbon and one layer silicon carbide (SiC), which encapsulate the fuel material alongside all fission products. One challenge this poses is how best to ensure the integrity of the SiC layer during operation, so that fission products are reliably contained during operation at very high temperatures, or possibly replacing SiC with an alternative ceramic such as zirconium carbide.

Depending on fuel cycle and reprocessing decisions, researchers may look to develop advanced mixed actinide oxide fuels. Although these would greatly reduce the long-term species burden on waste forms, they will demand a significantly better mechanistic capability to predict fuel performance and evolution. For example, it is still not adequately understood how fission gas is released from oxide fuels despite its crucial impact on fuel swelling, and the interaction of fission products with cladding (PCI) is also a priority.

It is clear that materials research, using advanced experimental methods as well as state-of-the-art modeling and simulation, cuts through all areas of the Roadmap. To support the Roadmap, industry and Government (particularly through the UK Research Councils) are supporting the nuclear research crucial to enhancing the UK domestic nuclear programme.

Manufacturing is a clear area in which the UK can adopt leading-edge technologies to make a greater contribution to the global supply chain for structural components. It can also supply fuel components, a major opportunity given the global increase in nuclear energy. Another development is the establishment of the new National Nuclear User Facility, which is enhancing and connecting the UK’s major materials irradiation and examination capability. This collaborative research project undertaken at the National Nuclear Laboratory Central Laboratory, the Culham Centre for Fusion Energy and The University of Manchester’s Dalton Cumbrian Facility will provide a step-change in the UK’s ability to address materials research issues in the nuclear field.