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America's ageing nuclear plant

A great chunk of America's nuclear power fleet is celebrating its 40th birthday right about now. Is this aged stock a danger to society? Or is it just getting warmed up?

In June this year the news 'US nuke regulators weaken safety rules' made headlines worldwide. Released by US-based news network Associated Press (AP), the article asserted the US Nuclear Regulatory Commission (NRC) was consistently lowering its standards to keep the nation's ageing reactors operating within safety margins.

The so-called exposé followed a year-long investigation from AP journalists who report trawling through tens of thousands of pages of government and industry documents, test results, inspection reports and policy statements filed over four decades. They cited example after example of corroded pipes, cracked wires, leaking valves and brittle reactor vessels, but perhaps most damning, revealed that radioactive isotope tritium had leaked from at least 48 of 65 nuclear sites.

In an industry still reeling from the Fukushima Daiichi disaster, this spelt bad news. The NRC's rapid response – released less than 24 hours later – disagreed with many of the observations, stating: 'The AP article fails to recognise that the NRC's inspection and maintenance requirements have led plants to detect, repair or replace the equipment, systems or other issues that were described in the article or in other instances not highlighted'.

A swift response from the US Nuclear Energy Institute (NEI), described the AP article as 'selective and misleading'. At the same time, the industry lobby group's newsletter editor wrote in his organisation's blog: 'It's reasonable for journalists to beaver around the nuclear energy industry to find evidence that the industry is a nest of vipers plotting disaster and misery. That's what journalists do'.

Health and safety

Weeks later, debate has subsided but the problems surrounding ageing nuclear plants are more conspicuous than ever before. As UK-based independent nuclear engineering consultant and former Brunel University academic, John Large puts it: 'What [the AP article] has indicated is that the inspection regime is failing to keep abreast of the degradation that is going on. The regime is determined by the state and condition of a plant, but in some cases, components start to degrade at a rate that wasn't expected, so problems are being picked up too late to guarantee absolute safety.'

Certainly the tritium leaks at US plant are evidence of this. While the NEI emphasises the leaks posed no health or safety risk to the public and commends the industry and NRC for launching initiatives to 'bolster oversight in this area', a recent report from the US government Accountability Office reveals how only limited methods exist to monitor underground pipes for leaks. And according to Large, issues such as this are only the beginning.

As of July 2011, 70 of the 104 operating reactors in the US had their initial 40-year licences renewed by the NRC for an additional 20 years. Pointing to the now infamous adage that operating these plants is akin to driving a 1950s car on a motorway, Large asserts the licence renewals were instigated following international pressure to lower carbon dioxide emissions. 'Politicians were convinced by environmental groups to give ambitious carbon dioxide targets but renewable generation hasn't come online as hoped,' he says. 'The stop-gap measure is to extend the lifetime of existing nuclear plant and there are risks with that.'

Allen Hiser, senior adviser of licence renewals at the NRC is straightforward in his response. He points out how the original 40-year licences were based on financial considerations, not technical limitations.

He also says the industry has a better understanding of how the materials in a nuclear power plant behave as well as more knowledge on how the properties of these materials change over time. Importantly, researchers and engineers can better model nuclear components. 'Forty years ago [industry] used conservative factors to account for certain behaviours of components. Today, we can do 3D modelling of components to more accurately calculate what an effect is,' Hiser explains.

Large is not convinced. When asked if he thinks the nuclear industry can reliably model component behaviour he says: 'You think we're so advanced now everything can be predicted? Of course it can't. Yes our knowledge of materials is better, as is our knowledge of predicting materials behaviour, but there will still be some whoppers out there waiting to trip us up.'

Whoppers or not, almost every US operator that has requested a 20-year plant renewal licence has received it. Hiser maintains the NRC and industry is aware that ageing effects could become more severe as nuclear plant edge toward 60 years of service but insists inspections and analysis will ensure ageing 'doesn't compromise safety'.

Nuclear longevity

And while others in the industry remain tight-lipped on ageing – the US Electric Power Research Institute (EPRI), US utility First Energy Corp and Sandia National Laboratories all declined to comment – government-funded research is rife. EPRI alone runs project after project on cable, concrete, reactor pressure vessel and metals ageing and a recent US government-sponsored workshop, 'Life beyond 60', concluded near-term research should address ageing issues.

Gary Was is a key materials scientist leading research in this field. For more then 30 years, the Professor of Nuclear Engineering and Radiological Sciences at the University of Michigan, US has looked at 'materials degradation in nuclear power plants. He has scooped many awards including a Doctor of Science from Massachusetts Institute of Technology for his research contributions. He is certain today's US nuclear plant can at least operate beyond the latest 20-year licence extensions.

'The original 40-year licensing is a misnomer. People now think 'oh, these plants can't be safe any more', but the licences were not set by the physical condition of components,' he says. 'Today we are asking can these plant go to 80 or 100 years? We are talking about $10bn a plant, if we keep them going this is a tremendous cost-saving and will provide cheap electricity for the public.'

However, as Was points out, the future will bring some tough challenges. As he explains, every operator struggles with the question: can the plant still economically generate power?

Take, for example, the ageing of bolts within a reactor core. While these relatively benign core components do not pose a direct safety issue, if they crack and fall off the operator must still shut down the plant and replace them if the plant is to operate efficiently.

'So now the operator is faced with the questions: What is the cost of replacement? Can I afford it and keep the plant running? Or must I shut down?,' says Was. 'To some extent, if it's still economically viable you simply replace the component and that's how you deal with ageing.'

Clearly, however, not all components are benign when broken. Right now, nuclear operators worldwide are keeping a very close eye on their reactor pressure vessels. These components are relentlessly subjected to neutron radiation, which causes embrittlement and cracking. If the vessel was to fracture, radioactive materials would be released.

'US reactor pressure vessels haven't yet reached critical safety levels but when they get there, it will be a major issue,' says Was. 'Scientists are currently doing accelerated irradiation ageing experiments to see when [reactor vessels] will reach this critical point, and what actually happens.'

Replace and repair

But while the researcher cannot yet answer these crucial questions, he is confident that replacement does not necessarily mean plant shut-down. 'Every year we are replacing bigger components, and I'm no longer 100 per cent convinced that replacing a pressure vessel is uneconomical,' he says, 'In the 1980s, plants were shut down as they needed a new steam generator, but today [replacement] is routine. These older plants are totally amortised and so cheap to operate it may become economical to replace the pressure vessel.'

But while component replacement becomes affordable and commonplace, hasn't one of the nuclear industry's most disastrous and recent events been overlooked? The lion-share of research projects and licence renewals were granted before the six boiling water reactors at Fukushima Daiichi succumbed to Japan's 9.0 magnitude Tohoku earthquake and tsunami on 11 March 2011.

As John Large points out: 'Fukushima sets question marks over the way in which we define and demonstrate nuclear safety. There are fundamental design aspects of plants, designed 30 or 40 years ago that can't be upgraded. Take the seismic design of plant, you can put a few brackets here and there but you can't change it. Similarly, older plants would be very difficult to adapt for a long station blackout, as took place at Fukushima.'

Was, however, remains pragmatic. As he points out, when any major nuclear incident takes place the global industry looks closely at what happened and applies it's findings to its own plant. Indeed the NRC has just filed its 96-page report on lessons learned from Fukushima. But as he also highlights, by all probabilities, the disaster was a 'once in a millennium event'.

'I can speak for the US and have a feeling for the pulse here... Three Mile Island sent a shock-wave through the public as nuclear power was fairly new, but in the end not a single death is attributed to [the event],' he says. 'Today people are saying Fukushima is a terrible disaster but I have not heard undue concern from the public or government.

'Fukushima will slow things down but I'm confident that major nuclear countries will not abandon nuclear generation.' *

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