Record breaker, but for how long?

Working on the principle that no news is good news, nuclear utilities are generally happy for their power stations to stay out of the limelight and focus on the business of producing electricity.

However, there are occasions when headlines are welcome. One such cause for celebration occurred at Heysham in North West England, which recently set a new world record for uninterrupted generation by a nuclear reactor. Heysham 2’s Unit 8 achieved 940 days of continuous operation before coming offline for a planned maintenance outage in September 2016.

The previous record of 894 days had been set 22 years earlier by Pickering Nuclear which operates six Candu reactors on the shores of Lake Ontario in Canada. The 540MW Pickering 7 reactor had been operating for a decade when it set the world record for continuous generation.

Heysham 2 has been operating since 1988 and station director John Munro is proud of the increase in productivity which has seen the 28-year-old station claim the world record. He credits long-term investment in plant and people for the achievement.

“The investment in people has involved not just core skills but developing a culture focussed on improving error prevention,” he says, adding that ‘upskilling’ nuclear professionalism among staff has led to a mentality where avoiding making mistakes is at the front of people’s minds.

Heysham 2’s record may be eclipsed in February 2017 by its Scottish sister station Torness. If all goes well Torness 1 is due to reach 996 continuous days of operation before its next planned outage in April 2017. However, Munro is confident that Heysham 2’s continuing drive for improvement will eventually see it take back the record for continuous generation, pushing it beyond three years.

How to improve operational performance?

What has enabled the second generation AGR fleet to be in contention for world records years after they have exceeded their design life, at a time when they were originally predicted to have ceased generation?

Munro identifies the turning point as the 2009 takeover of the veteran reactors by French nuclear operator EDF. “The new owner brought a wealth of nuclear experience and a readiness to invest for the long-term.”

He is also very positive about the change in working culture that is permeating the nuclear utility, saying: “The ingredients are investment over and above what is required to maintain safety margins; excellent support from the fleet; good operational experience; and a learning organisation that shares experience.”

Investment in training encourages people to focus on how to improve error prevention as well as developing their core skills.

Munro gives an example of how this culture is manifested: one of his engineering staff proposed a simple modification for the existing hydraulic fire resistant fluid (FRF) system to eliminate a ‘work around’ that had been in place for decades. For 28 years an over-engineered and flawed design meant running with a cooling valve shut on the standby train. If the train in service failed, as happened occasionally, the team knew that they had to open the cooling valve on the now running, previously standby train within a few minutes. More recently, however, the culture of ‘ownership’ whereby all staff are encouraged to have an aligned operational focus, saw one of the staff design and install a completely new and modern FRF system, which has engineered out the workaround. A small but significant improvement, illustrating the way that staff are encouraged to actively engage with understanding and improving the way that the plant operates.

Privatising nuclear power generation

Heysham 2 is one of the UK fleet of seven advanced gas cooled reactor (AGR) stations acquired by EDF from British Energy in 2009, along with the UK’s sole PWR at Sizewell. British Energy had been formed in 1995 as part of the UK government’s restructuring and privatisation of the electricity industry – an initiative that has been subsequently followed elsewhere. Investors had no appetite for the first generation Magnox stations, but nuclear assets from the state-owned Central Electricity Generating Board and South of Scotland Electricity Board were bundled together to form a nuclear utility, British Energy. The new entity operated five English and two Scottish AGRs along with Sizewell B, which had commenced generation the year before.

The new privatised company initially flourished as the price of electricity rose. However deregulation led to new wholesale trading arrangements, which attracted new competitors. Within four years the price of wholesale electricity had dropped by 40% and British Energy was one of a number of generators who found themselves in trouble. A 2002 restructuring plan led to a government bailout, in exchange for a majority shareholding. This state ownership was always intended to be a temporary arrangement. In 2007 the government sold a tranche of British Energy shares to institutional investors, retaining a 35% share in the company. In September 2008 it agreed a £12.5 billion deal, which gave EDF control of British Energy.

At the time attention was focussed on the acquisition of sites suitable for new nuclear build. But in acquiring the bulk of Britain’s nuclear fleet as well as putting itself in pole position for UK nuclear new build, EDF was taking on the challenge of operating the aging AGRs.

AGRs: a unique British design

The genesis of the AGR was in the 1960s, when politicians determined that the UK should build on its experience with Magnox reactors and develop a fleet of high temperature, gas-cooled civil nuclear power stations. They use slightly enriched uranium fuel in a solid graphite moderator. The UK was alone in pursuing this reactor technology. The UK’s AGRs were built at seven sites over a 20-year period from the late 1960s. All 14 are high temperature, gas cooled (carbon dioxide), graphite-moderated nuclear reactors, but they were built by different companies and designs were modified, so that no two stations are identical.

In a typical AGR system, the reactor core, boilers and gas circulators are housed within the concrete pressure vessel. The reactor moderator is a 16-sided stack of graphite bricks. As well as acting as a moderator these form individual channels for fuel assemblies, control devices and coolant flow. AGR boiler (i.e. steam generator) pressure is much higher than that of a PWR and boiler tube failure can result in boiler water ingress into the AGR primary coolant. In addition AGRs have a much hotter steam and primary circuit gas than a PWR and the plant must be designed and qualified to tolerate these higher temperatures.

Two key factors affect AGR lifetime. One is weight loss or loss of structural integrity in the graphite moderator leading to impaired function. There has been a high level of uncertainty on the timing of this and understanding the way that the graphite is ageing is critical in determining the lifespan of such reactors. Reactor cores are routinely inspected, graphite samples taken and sophisticated modelling carried out, helping operators to build up a clearer picture of the graphite weight loss at each reactor.

The other is boiler tube degradation, which can lead to leaks into the core which may increase the moderation, meaning that more rod absorption is required to hold down the reaction.

Planning for life extension

Because it already operates 58 nuclear reactors in France, EDF brought a great deal of nuclear experience to its new acquisition. However, operating the AGR fleet represented a new challenge. The AGRs were notorious for over-design and a lack of standardisation. Investment in the fleet had been adequate to ensure safe operations, but long-term plans were lacking. Decisions on life extension had been short-term and incremental, with 17 separate life extensions approved mainly in five-year tranches.

Key engineering factors in determining life extensions to AGR lifetimes include technical considerations of the condition of the irreplaceable nuclear island, including the graphite core, boilers, reactor internals, and pre-stressed concrete pressure vessel. Two non-replaceable components – the graphite core and the boilers – are ultimately the life-limiting factors. Making predictions about the degradation mechanisms of these components is complex and relies on analysis of detailed data from the AGR fleet.

EDF brought a new approach and new resources. It sought a clear long-term forecast about the potential of the fleet so that realistic plans could be made for future investment and operations. In 2009 it set up a comprehensive programme to assess the lifetime potential of the AGR fleet, and it followed through with appropriate investment.

By 2012 EDF had developed a suite of comprehensive through-life plans covering system, station and fleet. The utility engaged with key supply chain partners with original equipment manufacture (OEM) knowledge to create realistic underpinning strategies and plans. It developed through- life management strategies (TLMSs) for 20 key systems. These summarise the strategy for lifetime management including investment requirements, personnel and supply chain issues.

EDF says that the TLMSs have enabled it to identify the key risks and investments required to reach the revised planning dates safely and reliably. A lifetime resource planning framework has also been developed in parallel, to help develop an understanding of what resources the business will need to operate to the revised planning dates.

EDF has made substantial investment in the AGR fleet in order to extend their operational lives. In 2012 it announced seven-year extensions to twin-AGR reactor stations Hinkley Point B and Hunterston B, extending their operational life until at least 2023. In 2015 the company announced plans for £150 million investment at Dungeness B which would enable a ten-year life extension beyond its projected shutdown date of 2018. In 2016 it announced that following extensive technical and safety reviews of the plants the scheduled closure dates for its Heysham 1 and Hartlepool plants had been extended by five years to 2024, while the operational lives of Heysham 2 and Torness had been extended by seven years to 2030.

The decision about life extension is based on the business case. The questions raised are: is it technically feasible to extend the life of the station; is it commercially viable; and can the supply chain support it? The operator does the analysis, works out whether it is safe and economical, and will test this assumption every year.

EDF Energy, EDF’s UK arm, says that since it took over the UK nuclear power stations in 2009, output has increased by 50%, safety performance has increased by 51% and lifespans have increased by 25%.

A breadth of engineering challenges

One of the challenges of keeping old plant in good running order is finding replacement parts for components that are no longer manufactured. If it is possible to replace like with like then regulatory approval or qualification is not required, whereas installing a component with a more modern design may be problematic. As the plants age and the rate of change in technology increases, procuring replacements for obsolete parts can become increasingly difficult.

One approach is to develop specialist groups whose role includes procurement. EDF Energy set up the Turbine Support Group 15 years ago to provides specialist turbine generator maintenance for Heysham 2, Hunterston B and Dungeness B. Based at Heysham, one of its roles is to procure and maintain strategic spares both refurbishment and new, as well as specialist labour for specific projects and breakdowns.

Generally plant life extension teams try to replace ageing equipment with newer versions of the original equipment that does not affect the safety case. However, it is sometimes appropriate to make design changes. Whatever decisions are taken, safety and security are paramount.

As the cost and complexity of nuclear new build increases, investment in the life extension of existing reactors can be an attractive proposition. Increasing the efficiency and reliability of the AGR fleet has required substantial investment. It is a notable achievement that these ‘silver foxes’ are achieving their best performance so late in life.