Growing old gracefully: the UK's fleet of AGR nuclear power plants may be ageing, but constant testing and monitoring keeps them in good shape.
Inspection and testing keep the lights on in Britain. And are set increasingly to do so. That may sound melodramatic, but it's a fact that a substantial and pretty much constant proportion of the UK's energy supplies, just under 20% in recent years, comes from the existing fleet of nuclear power stations, ranging in age from 18 to 37 years. And with older gas and coal fired power stations set for decommissioning in the next few years, and not much mourned because of their contribution to greenhouse gas emissions, keeping the current fleet of nuclear stations going for as long as possible is likely to get even more important.Life extension for the older AGR (advanced gas-cooled reactor) power stations demands a complex and continuous programme of nondestructive testing and inspection. But life extension, though important, is the second consideration for operator EDF Energy: the first, and always the first, says Nigel Houlton, head of the lifetime programme, is safety.
It's the safety considerations too that guide much of the actual inspection and NDT work done by the teams led by Bev Parry-Mills, inspection group head. The work goes towards the assembly of a "safety case", which is what EDF Energy presents to the Office of Nuclear Regulation to win continuation of its individual site licences and is also the main internal working document for Parry-Mills' teams and for internal regulators.
Safety in the nuclear industry is absolutely a given but Houlton says it won't, in the end, be directly the factor that determines the eventual end of life for the AGR stations. "In order to be happy that we are still in line with the safety case the number of inspections will increase, and we're planning for that," he says. "But it will eventually reach a point where that number of inspections is just not viable any more."
It is, under this scenario, unlikely to be the regulator that will pull the plug; rather it will be EDF's own decision. "The volume of inspections that we are required to do, probably on the graphite moderator, will reach such a point that we'll think it's now time to take the station offline. We wouldn't be able to generate enough to maintain commercial viability," Houlton says.
Predicting when that moment will be reached for any of the 14 AGR reactors is one of the great unknowns of the UK energy industry The difficulty is that EDF is operating in uncharted territory: there are no other stations of this kind worldwide, and anyway the UK was leading the world in nuclear 40 years ago, so our stations would have been the oldest in any case.
This "uniqueness" is part of the challenge for Parry-Mills, and there are other aspects to it as well. "In terms of inspections and NDT, there isn't a lot of operating experience from other sectors that we can apply, and even Sizewell is based on different material and operates at lower temperatures, so the degradation mechanisms are quite different from the AGRs."
Industries such as petrochemicals and the oil and gas sector may have superficial resemblance, with lots of pipes and heat flows, but, she says, there are again differences in types of degradation and distinct differences in terms of accessibility and the potential for replacing parts. "In some of the inspections we do there's nothing like it in the world."
This brings a self-reliance that sits fairly comfortably alongside the emphasis in the regulatory regime on self-assessment: the safety case that drives the work has both an internal and an external function. Constructing the safety case determines the day to day work of measurement and test as well as the long-term programmes which are carried out largely during the regular outages when the individual reactors are taken off line for more detailed inspection and maintenance, and those programmes are part of the portfolio that goes forward to the regulators.
"The onus is on us to prove our compliance with the licence conditions," Houlton says. But it is wider than that. "We have integrated teams that are responsible not just for managing the day-to-day construction of the safety cases, but they also have to stretch out to look at research and development, and they have to judge whether the results of our inspections are what we expected long term and short term, how does it interface with the safety case assumptions.
"These aren't isolated, disparate pieces of work; we integrate them into an overall engineering programme view of our status and our forward look at the irreplaceable systems in particular. The work that Bev Parry-Mills' teams do and the inspection results are crucial in the build-up of evidence to support the safety case and that is both ongoing operational input to help us continue to say we are operating in line with the safety case and also part of the confidence that we can maintain the safety case and develop it over time to support our lifetime ambitions too."
The "irreplaceable" parts are just that, elements and components that cannot be replaced. Some of this is to do with difficulty of access: mostly in the reactor's graphite core and to the boilers. There is the other "unique" ingredient as well: the radiation profile of the stations, which can also interfere with some of the monitoring and inspection equipment.
"We undertake NDT to monitor the in-service degradations on both the replaceable and the irreplaceable parts," says Parry-Mills. "I have a staff of about 20 people and we have contractors with about 200 staff with suppliers who physically undertake the inspections, with my group doing the supervision and working with them to design the inspection routines. We'll do manual NDT, automated NDT and remote operations. The tests vary from very basic thickness tests which are done for both day to day operations and to support the lifetime work all the way through to more complex remotely deployed NDT, and we've got virtually everything in between."
Manual inspections will use tools such as magnetic particle tracing and dye penetrants to detect places where corrosion is breaking surfaces. Some of the materials used in the stations are very specialised; quality in areas such as welds is critical. Ultrasonics and radiography techniques are used for other kinds of "volumetric defects" of corrosion and cracking, and the ultrasonics is both manual and automated.
"For geometry and repeatability we would want to do automated NDT, but the automated technologies are more expensive," she says. "But the most important thing is safety, so in areas where there might be a higher radiological dose we would want to limit the amount of dose any one person receives. Sometimes it's actually quicker to put a person hi to do the inspection than to set up an automated test; the sort of dose we're talking about is probably less than the general public gets just walking around on the streets, though."
Patently, though, there are more difficult areas where specialised and automated techniques are the only option. "Remotely deployed systems include putting a video probe on a long cable and lowering it through tubes, and we also have remotely controlled submarines that we put into our fuel ponds to do inspections there. We have things that look like a child's Tonka truck with a camera on the front to drive down pipelines, which is the kind of thing that the oil and gas and the water industries have as well."
The high temperatures of the AGR mean that creep cracking is a big issue for the inspections as well as problems with corrosion and weld degradations, and mean also that there is a continuing balance to be struck between the day-to-day inspection routines while the plant is under load and the longer-term issues that can be tackled during the outages, which are planned for every three years.
There is work every day on every site, Parry-Mills says, but then there will also be the preparation of a list of maybe 700 to 800 different items that will be thoroughly inspected during an outage by a team of between 40 and 60 qualified and approved people. Some tasks can be scheduled and planned for across the whole fleet of AGRs, but not all of them: engagingly, the seven sites differ marginally in the configuration, shape and size of equipment and of outputs, and of course they have had different lifetime experiences.
"The basic technical aspects of physical inspection may not be much different from other industries," says Parry-Mills. "But we do have the added difficulty of access, and sometimes we're 100m away from where we're actually deploying the equipment, or maybe 10m away but the equipment is down an annular gap of just 6mm. It's why we're so reliant on our knowledge and that of our contractors. In addition, some of the equipment we use can't then be used anywhere else because it is contaminated, so we need to consider a disposal route for it or to keep it in that particular location for ever."
"For ever" is not, of course, the life extension that EDF is hoping for from its current nuclear power stations. The current target is for a seven-year extension over and above the original projected closure dates, which would mean the first of the AGRs going out of service in 2018 and the others by around 2023.
But "life extension" is not the same as "lifetime ambitions". "If you ask me that," says Houlton, "then I'd say we would hope still to be operating at least part of the AGR fleet out in 2030. We're obviously not there today, and there's a lot of uncertainty around that. But if we're successful, what it means is that you take the UK security of supply, the low carbon economy, and some of the current issues around nuclear new-build, and actually the way the government sees life extension of the AGR fleet is pretty crucial. It's not just for our business but the whole UK context."
So the inspection and testing routines will go on for as long as is reasonably practicable, and the AGRs will keep contributing. And it's important for all of us that they do.
RELATED ARTICLE: The nuclear line-up
The UK has 16 commercial-sized currently-operating nuclear reactors, and 14 of them are of the advanced gas-cooled reactor (AGR) type, with reactors arranged in pairs at seven sites, two of them next door to each other in the Lancashire seaport of Heysham. The AGRs, plus the single pressurised water reactor (PWR) at Sizewell in Suffolk, are all operated by EDF Energy, the French-owned electricity supply group which took over the previously state-owned and then part-privatised British Energy in 2009.
The only other UK commercial nuclear is the last remaining previous generation plant, the Magnox reactor at Wylfa in North Wales, which is due to go out of service in 2014. A smaller reactor at Calder Hall in the West Cumbrian Sellafield complex is not really counted in the figures, though it is connected to the grid.
Belying their current venerable status as the ageing but reliable workhorses of the UK energy supply scene, the AGRs were hugely controversial when built. The controversy stemmed from the original design choices: the UK industry, supported by successive governments in the 1960s and 1970s, was determined to have its own reactor design with which it aimed to compete worldwide. Various options were considered and heavily canvassed, and the AGR, pretty much a direct descendant of the earlier Magnox reactors, was picked. But the rest of the world didn't buy, and the UK's AGRs were the only ones of their kind ever built.
Controversy didn't end with the reactor selection. The first of the AGR fleet was intended to be the Dungeness B station, sat next to an existing Magnox A station on the south-east corner of Kent. The construction of Dungeness B was beset by the problems of cost overrun and industrial relations that were a feature of 1970s living, and the power station name became a watchword at the time for construction inefficiency. It eventually started producing electricity in 1983, 15 years after work had begun, and seven years after two later AGR starts, Hunterston B and Hinkley Point B, had come on line.
By the time the last two AGRs, Heysham 2 and Torness, came into operation in 1988, the UK's pretensions to total nuclear design autonomy had been abandoned, and Sizewell B, the first of what was planned to be a fleet of the American-origin PWRs, was being built. But nuclear then went off the menu, so Sizewell remains as a one-off: the next crop of nuclear stations will use a technology that is different again, though related more to Sizewell than to the AGRs.
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| Title Annotation: | NDT and inspection |
|---|---|
| Author: | Pullin, John |
| Publication: | Environmental Engineering |
| Geographic Code: | 4EUUK |
| Date: | Oct 1, 2013 |
| Words: | 2128 |
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