The UK is unique in using advanced gas-cooled (AGRs) reactors to supply electricity.
The AGR succeeded the UK’s first-generation Magnox reactors, of which 26 were built between the 1950s and early 1970s. By the end of 2015 all the Magnox reactors will be out of service and in various stages of decommissioning. The UK is currently reliant on its sole PWR, Sizewell B, plus the ageing AGR fleet, to provide the nuclear power that makes up around a fifth of its supply.
The AGR fleet consists of seven twin-unit stations, which started generating between 1976 and 1989. The fleet was born after two reactor types – one based on the UKAEA’s advanced gas-cooled reactor (AGR) and the other on US water-cooled reactors – competed in the 1960s to become the design choice to follow the Magnox stations. The government chose the AGR, based on UKAEA’s 30MW Windscale AGR prototype (WAGR).
In May 1965 Dungeness, on the Kent coast, was announced as the first AGR site. The Atomic Power Construction Ltd (APC) consortium was awarded the contract to design and build two 660MW AGRs, subsequently downrated to 600MW.
Dungeness B encountered problems in scaling up the design. In 1969 APC went into administration and a succession of delays and financing difficulties held up the project. The first reactor finally started generating in 1983, with the second a year later. In the meantime, AGRs at Hunterston B and Hinkley Point B started up in 1976, Hartlepool and Heysham 1 in 1983, and Heysham 2 and Torness later that decade. The fleet has a full load capacity of 7720MW Although the AGR design was intended to be the UK standard reactor there are significant design variations.
The AGRs have undergone a number of changes of ownership. The most recent was in 2009, when EDF Energy acquired them from British Energy. The original AGR designers calculated that their reactors would operate for at least 30 years, with an accounting life of 25 years. The 14 reactors are all still in operation and EDF Energy has made investment in the fleet with a view to extending their service life out past 40 years.
The choice: plant life extension or decommissioning
EDF Energy aims to extend the lives of its nuclear stations until at least 2023. In 2012 it announced seven-year extensions at Hinkley Point B and Hunterston B, extending their life until at least 2023.
UK nuclear operators are required to give the Nuclear Decommissioning Authority (NDA) three years notice of plans to shut down power plant. As Dungeness B was due to cease generation in 2018, EDF Energy had to decide by 2015 whether to shut down or seek to carry on generating.
Station director Martin Pearson explains what is involved in taking the decision.
“We enter into a well-used process across the nuclear industry, which takes roughly two years. The simple questions we try and answer are: is it technically feasible to extend the life of the station; is it commercially viable; and can the supply chain support it?"
In 2015 the company announced plans to extend the life of Dungeness B beyond its projected shutdown date of 2018. The plans for the 10 year life extension are buttressed by investment of £150 million.
After more than 30 years of operation components must be replaced to maintain and improve reliability. Key engineering factors include technical considerations of the condition of the nuclear island including the graphite core, boilers, reactor internals, and pre-stressed concrete pressure vessel.
EDF Energy’s life extension team, based at Barnwood in Gloucestershire, coordinates life cycle programmes. It has now turned its attention to life extension at Hartlepool and Heysham 1.
Fukushima: heightened flood risk awareness
Following the tsunami in March 2011, which knocked out the reactors at Japan’s Fukushima Daiichi, UK nuclear regulators carried out a review. Chief nuclear inspector Mike Weightman found that UK facilities had "no fundamental weaknesses" but made a number of recommendations to enhance safety and resilience. EDF Energy initiated a £180 million plan to meet these recommendations across the fleet, including a programme of flood defence reinforcement.
The land around Dungeness is at risk from flooding and EDF Energy set out to upgrade Dungeness B’s sea defences to a level where they could protect against a one in 10,000 year weather event. There is a new 1.7m high concrete wall along the 1.5km site perimeter and its main access points, providing protection against tidal or storm surge flooding.
As well as the new sea wall, the £8 million spent on coastal flooding protection over the last three years includes internal upgrades – fitting dam boards to building entrances, rendering buildings, sealing trenches, and raising batteries to a metre above floor level. Points have been installed around the site to connect power and water to cool the boilers in the event of an emergency.
The flood plans are based on pessimistic climate change assumptions and are designed to keep the site safe until after decommissioning has been completed – possibly into the next century.
Operators also made contingency plans for an emergency where a site loses all facilities. EDF Energy has set up north and south off-site emergency equipment stores. The stores contain command centres, electrical generators, high pressure pumps and other essential equipment which could be brought onto any of its sites within 24 hours.
Ongoing investment required
Modern digital power plant control centres offer better understanding of plant processes, improving operation and asset maintenance.
But for old nuclear fleets, remaining with the original design intent may be less problematic than retrofitting new technology.
It is nearly 50 years since construction at Dungeness B started, but EDF Energy decided, in advance of planning life extension, to invest £75 million to replace the main data processing and control system.
The new control centre has completely replaced the old system but it replicates the functionality of the previous system. Station director Martin Pearson notes with some pride that the Dungeness B now has the most modern computer system in the UK fleet, which will last the station beyond the life extension.
Graphite issues
AGRs have a graphite core acting both as a moderator and as part of the structure of the reactor. During normal operations the thousands of interconnected graphite bricks lose mass. Graphite weight loss is a key concern when extending the life of AGRs and understanding the way that the graphite is ageing is critical in determining AGR lifespan. To understand how the graphite changes over time, reactor cores are routinely inspected, graphite samples taken and modelling carried out.
The station’s life extension lead Richard Gill explains: "We remotely inspect the core with cameras, and then carry out ‘trepanning’ operations where we take physical samples from the core. During the outage last summer we took 48 samples. This helps build up the modelling and the safety case. The modelling improves with time, and we have high confidence in this."
Gill details the company’s £150 million investment in Dungeness B’s life extension. He says: "The majority of the investment will be used to address some of the aged conventional plant, however one major safety case modification is required: the boiler tube leak consequence modification.
“The current safety case demonstrates that when a boiler tube leak occurs, the extra water in the core may increase the moderation. The additional moderation allows more of the stray neutrons to get moderated and continue the fission reaction, thus more rod absorption is required to hold down the reaction, although our hold-down margins are very conservative. Through time the graphite weight loss and the threat of boiler tube leaks get worse. In the next few years we will need to modify the plant to reduce the ingress of water to the boiler in the event of a boiler tube leak."
The main modification will reduce the steam pressure to the same level as that of the CO2 coolant to prevent water getting into the reactor vessel.
Similar modifications to prevent over-feeding of the boiler have been carried out at other AGRs. The modifications do not affect output and will only be initiated if the system identifies a tube leak (in which case the reactor trips), when it automatically reduces the pressure. Implementing the boiler tube leak modifications is a three to four year programme from design, through safety case, ordering and physical installation.
Modifications will also be made to the refuelling machine control systems, some of which are original. If the plant is to operate until 2028 the choice is between replacing like with like or upgrading to a more modern system.
Gill explains, "We have an option of stabilisation, which is largely preferable if we can do it. It is less intrusive and reduces the complexity of rewriting the safety case. The alternative is renewal and replacement with new system. In some instances the preferred option is to stabilise; in other instances renewal with high integrity PLC is more appropriate. In the case of the fuel machine control systems, optioneering identified renewal as appropriate for two systems and stabilisation for a third. It is most important to keep things simple to ensure reliability and maintain the safety case. We are not looking for bells and whistles."
Pros and cons of changing technology
The AGR safety case requires that a nitrogen plant be available in case of a reactor shutdown where a small number of control rods do not drop into the core. Nitrogen, a neutron absorber, will hold down the nuclear reaction. Dungeness B’s existing nitrogen plant dates back to the original construction.
Life extension at Hinkley B and Hunterston B required seismically qualified new nitrogen plant to be installed, but a difference in reactor design means that Dungeness B has the option of refurbishing.
Gill explains, "We are currently going through optioneering as to which is preferable. It costs less for a refurbishment, but…implementation will be harder. A new plant could be built in isolation on a currently unused area off the site and connected to the system once completed and tested. A new plant is likely to be around a third more expensive than the refurbishment."
The team generally tries to replace ageing plant with newer versions of the original equipment. However, it is sometimes appropriate to move to more modern designs. An example is the electro-chlorination plant.
Gill explains, "We use low concentrations of sodium hypochlorite to prevent marine growth in our seawater systems. We produce that on site by electrolysing seawater and because the plant is fed by seawater it degrades very quickly, faster than any other piece of plant on this station. We have carried out three refurbishments in 15 years."
The team is considering three solutions: a modular system that can be taken off site for overhaul; replacing the seawater inlet system with a system using table salt as its raw material; or a tanker delivery system.
Investment underway
The decision taken by EDF Energy to invest in life extension for Dungeness B is quite separate from the Office for Nuclear Regulation’s decisions about the site. The UK regulator requires nuclear operators to present 10-yearly Periodic Safety Reviews (PSRs) to demonstrate the health of the plant and its safety case. Dungeness B is due for its next PSR in 2018 and preparation started in 2014.
Station director Martin Pearson says: "PSR is largely a stand-back review of how we operate, our processes and our safety cases against modern standards. It looks across the whole of the power station to ensure we are operating appropriately."
The Dungeness B PSR documentation suite to demonstrate the health of the plant and its safety case will be presented to the regulator in January 2017 and the regulator will formally respond in January 2018. Pearson describes this as "an incredibly robust process."
He is upbeat about the life extension work programme, which was approved by EDF Energy earlier in 2015. He says: "Many of these modifications can be done during normal statutory outages. Others can be done while the station is on power. We are looking to do most of it in the next seven year period and have plenty of opportunity to schedule them in. We are not anticipating any major shutdowns.
“We recognise that this work would be a distraction from the normal business of the station and we are setting up a sophisticated project structure and project control structure to implement this in a way that causes the least disruption to the normal work of the station."
