The next nuclear model

Materials World magazine
,
1 Sep 2017

Andrew Wisbey looks at the latest R&D into the next generation of nuclear reactors. 

Generation IV reactors are a set of nuclear reactor designs being researched for commercial applications by the Generation IV International Forum. Over the past ten years, there has been a resurgence of interest in advanced nuclear reactors, which are of various types and environments but operate at higher temperatures than the current light water-based reactors.

Generation IV includes reactors with ‘once through’ fuel usage and fast reactors, which make use of fast neutrons to achieve fuel transmutation and the production of additional useable fuel material. Fast reactors are attractive for their greater fuel usage efficiency and because they assist in anti-proliferation efforts by processing material that would otherwise be suited to weapons of mass destruction. At the same time, these systems also generate useful levels of power. 

The UK’s current fleet of advanced gas-cooled reactors (AGRs) demonstrate some of the benefits associated with Generation IV-type reactors such as a relatively high operating temperature (500–650°C), good thermal efficiency (~45%) and the use of gas (CO2) as the primary coolant. However, some of these reactors have been in operation for more than 40 years. This AGR fleet represents leading international experience in the practical operation of high-temperature gas-cooled reactors, where creep is an important damage mechanism as well as fatigue and environmental interactions. There is much that the UK and the AGR operators (EDF Energy) can contribute to Generation IV developments.

R&D

Looking beyond the AGRs, there are several Generation IV-type systems being proposed for future deployment in the UK including: 

U-Battery – a micro-reactor concept (~4MWe) based around a high-temperature (~750°C) helium-cooled gas primary circuit, which has the potential to provide electrical power and local heating up to 10MWt. This may be suited to isolated communities or facilities. Amec Foster Wheeler is a partner in the U-Battery alongside Urenco, Cammell Laird and Laing O’Rourke.

Moltex Energy – a molten salt reactor, using chloride salts for the fuel and fluoride salts at high temperatures (~650°C) for the coolant. This requires no pressurisation of the primary coolant, thus simplifying some structural aspects of the design.

Both of these UK-based concepts are at relatively early stages of development and there is much detailed work required to formulate a safety case that will be acceptable to national nuclear regulators. There is also interest within the UK in the USA-based nuclear energy company GE-Hitachi’s sodium-cooled power reactive innovative small modular (PRISM) fast reactor system, which would enable the UK plutonium stockpile and used nuclear fuel to be reused safely for energy generation. This design is reasonably well developed and a number of international opportunities are being pursued to deploy this system.

Within Europe, there are a number of Generation IV reactor systems under consideration. The French-led advanced sodium technological reactor for industrial demonstration (ASTRID), a sodium-cooled fast reactor, is one of the most advanced designs. Sodium is used for the reactor core cooling in both ASTRID and PRISM because of its excellent heat transfer properties. Current plans have a primary sodium circuit circulating within the reactor core region but with an intermediate heat exchanger heating a secondary sodium circuit, which exits the reactor core and interfaces with a nitrogen circuit (under consideration to avoid liquid sodium/water interfaces), powering conventional turbines for electrical power generation. The 600MW ASTRID system operates at similar temperatures to the UK AGRs (500–600°C) with similar structural materials such as austenitic stainless steels being proposed, giving opportunities for the UK experience to contribute. A decision on whether to build this system is expected in 2019–2020. 

The multi-purpose hybrid research reactor for high-end applications (MYRRHA), is a Belgium-based, lead-bismuth eutectic cooled reactor and is the next most developed system. But this is an R&D tool, rather than a power generation system and operates at lower temperatures (core outlet temperature of 400°C) than the sodium cooled systems. MYRRHA could provide a materials irradiation facility, with a fast neutron spectrum that will enable key structural integrity issues such as brittle fracture transition temperature shifts due to irradiation to be evaluated. 

An Italian-based lead-cooled fast reactor system is also being developed – the advanced lead fast reactor European demonstrator (ALFRED), offering the efficiencies and benefits of the fast reactors. The use of liquid lead as the primary coolant is aimed at eliminating the risks posed by sodium/water interactions, while retaining heat transfer benefits. Neither the lead-or sodium-cooled systems require pressurisation of the primary circuit region, reducing structural loads, although the high density of lead will require structural considerations. ALFRED is expected to operate at a core outlet temperature of 480°C, meaning that conditions necessitating creep considerations with austenitic stainless steels can be evaluated. 

There is much work to do on developing materials that are resistant to erosion from the flowing lead coolant. Beyond Europe, there are numerous Generation IV reactors, of all types, under consideration, with more than ten different systems proposed in China alone.

Extending design life

Alongside the reactor concepts, there is also a drive within Europe to enable a 60-year design life in future nuclear power plants. This indicates a significant need for a robust understanding of the endurance and deformation behaviour of high-temperature materials. Creep, fatigue and environmental interactions should all be expected and may have synergistic effects. 

The long-term prediction of creep behaviour, as required by a 60-year design life, remains a somewhat distant goal, with current phenomenological models requiring similarly long-term creep data. The European Creep Collaborative Committee recommends that endurance extrapolation should not extend beyond a factor of three of the longest experimental data-point available. This suggests a need for 20-year creep tests. 

Part of the difficulty in predicting long-term creep behaviour with existing models, which are based on curve fits to experimental creep strain vs time data, is the microstructural changes that occur with prolonged thermal ageing, which often lead to changes in creep behaviour. However, alternative approaches to modelling creep deformation behaviour may be those models that relate the deformation to the underlying microstructure and change as it does. However, these models require further understanding of the fundamentals of creep deformation and extensive validation before they become accepted for such safety-critical structures as nuclear reactor systems.

The interaction of creep with fatigue cycling is another area where there is much work to do on damage accumulation modelling. There are also some issues specific to certain types of reactors, for example high cycle fatigue damage with liquid metal cooled reactor systems (thermal striping). On top of these mechanical damage mechanisms, the ambient environment in reactors may also be important, as has been demonstrated with the UK AGR fleet. The CO2 primary circuit coolant interacts with the austenitic stainless steels, resulting in carburisation, which occurs over a relatively narrow temperature range. This results in a significantly enhanced surface hardness and a corresponding reduction in creep ductility, which becomes a potential source of crack initiation. The importance of this interaction became evident after 30 years of AGR service life and highlights the need for fundamental understanding and long-term mechanical testing in relevant environments for the Generation IV reactors. The higher the operating temperature, the greater the potential for unexpected reactions in the long run.

This requirement for high-temperature mechanical testing capability comes after a reduction in the number of organisations undertaking such work – with two significant UK creep laboratories closing in Rotherham and Glasgow – or relocating from the UK over the past two years. Recognising this future requirement and growing shortage, the UK Government held a competition for funding to construct a new high temperature mechanical testing facility to support UK industrial involvement in Generation IV reactor technologies. Amec Foster Wheeler was awarded the prize to construct a new temperature-controlled laboratory for creep and fatigue testing at its Technology and Innovation Centre in Warrington, UK. The High Temperature Facility (HTF) opened in September 2016 and is beginning to undertake some initial evaluations. The laboratory is expected to mechanically test in relevant environments and some initial capability has been included in the HTF. Due to the wide range of environments envisaged for the various Generation IV reactors and the lack of focus on any particular Generation IV system in the UK, the HTF has a tool-kit of generic capabilities to simulate the different environments. Additional developments are already planned for some environments such as pressurised gas for helium-cooled reactors.

The HTF has also been equipped with crack detection and strain measurement systems, such as digital image correlation and high frequency AC-electrical potential difference, which enables accurate data to be generated. Although Amec Foster Wheeler hosts the HTF, the facility is open-access, allowing researchers from other organisations, such as universities, to use the facility, once external staff are suitably trained and can operate the equipment safely, hence an alliance of universities, research organisations and industrial partners has been formed under the HTF Alliance banner to help exploit the HTF. As interest in the UK grows in this area of technology, the potential benefits of the HTF will become clearer to industry and academia alike. Further investment, R&D and government support will assist UK manufacturing and technology involvement in the next generation of nuclear reactors.

Dr Andrew Wisbey is Principal Technologist at the Materials and Structural Integrity at Amec Foster Wheeler, UK, providing technical leadership and input across a wide range of programmes, particularly work associated with the new High Temperature Facility in Warrington, UK.