Building up strength

Materials World magazine
1 Jun 2009

William Herbert and Sam Humphry-Baker, students at Oxford University, UK, consider the country’s strengths in new nuclear build.

The UK needs to replace around 35GW of electricity generating capacity due to be decommissioned by 2020. It must secure dependable sources of fuel, safeguard against the intermittent nature of renewables and work towards meeting optimistic emissions targets. The only option that can deliver on all fronts is nuclear power.

The latest generation III+ reactors on the market are pressurised water reactor designs. Two manufacturers are undergoing the generic design assessment in Britain – Areva, with its EPR and Westinghouse's AP1000. These comparable reactors present similar challenges to the materials community. With the cost of nuclear in real terms approaching four times that of an equivalent gas plant, it will remain cost competitive only if the materials used can withstand high temperatures and prolonged exposure to irradiation over an extended lifetime of 60 to 100 years.

Predicting properties

The ability to run the reactors safely over this timescale with minimum maintenance will not require novel materials but the reliable prediction of the service properties of existing ones. Research in this field will have to play to the UK’s strengths – materials characterisation, modelling and understanding failure processes.

In any new build, online monitoring of structural degradation will be key, and more work is needed in this field. This will involve in-situ, non-destructive detection for early warning signs of failure. The critical structural components can be split roughly into two groups – core materials around the pressure vessel (zirconium-based alloys, graphite, etc) and out-of-core alloys that are subject to high temperatures, such as in the steam generators and primary cooling circuit (duplex and austenitic stainless steels).

Understanding irradiation embrittlement requires complex atomistic modelling and experimental work, such as 3D atom probe tomography, to map local changes in nanostructure. This will enable multiscale models for better life prediction. In the primary cooling circuit, for example, the ferritic phase in the duplex stainless steel is thermodynamically metastable, preferring to separate into iron- and chromium-rich regions via spinodal decomposition. This can reduce the alloy’s fracture toughness by up to 90%, with drastic consequences.

Experimentation over 50 years is not an option, so thermal ageing tests at various temperatures are carried out to record early warning signs of phase separation. This allows better understanding of the process’ kinetics needed to make more reliable long-term extrapolations. Predicting these developments in alloys will allow them to be brought out of service long before mechanical failure is possible. Similar models are required for stress-corrosion cracking in irradiated or non-sensitised stainless steels with increased susceptibility due to cold work.

To supplement these prognostics, the UK is in a perfect position to study materials forensically from its decommissioned plants, which have been in service for up to 50 years. There is no substitute for samples that have been subjected to the extreme environment inside a nuclear reactor over many years, and the Nuclear Decommissioning Agency must build materials harvesting into its decommissioning programme.

Supply and demand

Looking at the supply chain, the UK’s long experience in the nuclear industry means the manufacturing infrastructure can cope with 70% of new build. Large forgings present a considerable bottleneck in supply and may affect the UK’s ability to deliver new reactors. These crucial 500t single-piece steel forgings, used in steam generators and pressure vessels, have extremely long lead times. With over 80 new reactors planned globally, demand is expected to outstrip supply until at least 2020.

Looking forward, if Britain is to re-establish a world-leading primary manufacturing sector in the nuclear industry, fourth generation technologies become the only competitive option. Heralded the safest reactor ever to be designed, China’s High Temperature Reactor Pebble Bed Module (HTR-PM) is immune to structural failure or operational error. With only one other design in competition (the South African PBMR) pebble-bed technology could be the perfect export for a restored UK industry.

Further information:William Herbert and Sam Humphry-Baker