Recovering the safety margin of nuclear reactors

Age is no barrier to prolonging the operation of nuclear power plants thanks to technological advances that could not have been predicted by reactor designers working decades ago. That was the conclusion of nuclear industry leaders at a conference hosted recently by EDF Energy and the World Association of Nuclear Operators (WANO) in London.

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Commonly referred to as ‘life extension’ – the practice of upgrading existing units to add years to their productivity – is therefore better described as “recovering safety margin”, said Duncan Hawthorne, president and CEO of Bruce Power. Hawthorne, who has led the Canadian nuclear power plant operator since its formation in 2001, said: “Every one of us that works in this industry today is kind of standing on the shoulders of giants. The people who designed the Generation II reactors did so without the benefit of very much operational experience; less than 100 years of reactor opex was available for these plants. So I never talk about life extension, but rather about recovery of safety margin.”

Those “stalwarts of our industry” were working without computers, animation and 3-D stress analysis, he said. “Not surprisingly, they chose to apply conservatisms to their designs to acknowledge what they didn’t know and to recognise that some of the material properties were covered by assumptions.”

Today, the industry has more than 12,000 reactor years of experience to reply upon, along with a “massive quantum of analysis”, he said. “Intergranular stress corrosion cracking wasn’t even talked about when these plants were built and yet every one of us has to deal with it now in some form or other. As an industry we’ve learned how important it is to do good preventative maintenance programs, to have an asset-led management plan and to think of all those components.”

Nuclear plant operators today can also inspect reactors using multiple phased array ultrasonic detectors. “We can use so many more data calculations to come up with proper analysis of what really is the life-limiting factor,” said Hawthorne, who served as chair of WANO’s Atlanta Centre and, until recently, was president of WANO’s governing board. On 1 May, he will become CEO of Horizon Nuclear Power, the wholly owned UK subsidiary of Japan’s Hitachi.

UK Challenge

Jerry Haller, EDF Energy Generation’s programs director, said that EDF Energy is investing £600 million ($868 million) on its “long standing strategy to extend the lives of its nuclear power plants where it is safely and economically viable to do so”.

Since it bought British Energy in 2009, the French state-owned company “has been deploying a lifetime approach” that has resulted, most recently, in new scheduled closure dates for four of its UK advanced gas-cooled reactors (AGRs). In February, the company announced that the Heysham 1 and Hartlepool plants could now close five years later than originally expected, in 2024, while Heysham 2 and Torness would close seven years later, in 2030. The four plants together supply electricity to more than one-quarter of UK homes.

EDF Energy’s other operating UK nuclear power plants include the Hunterston B and Hinkley Point B AGRs – which both started up in 1976 and are scheduled to close in 2023 – and the Dungeness B AGR which was commissioned in 1983 and set to shut in 2028. It also operates Sizewell B – the UK’s only pressurised water reactor (PWR) – which began operating in 1995 and is currently scheduled to close in 2035.

“Half the fleet would have been closed by today and the rest would have been closed by 2023,” Haller noted. Today, the company does not expect any of the AGRs to close before 2023 and subsequently to still be running in 2030, he said. The challenges of the AGR design fall into three categories – technical safety, economic viability and stakeholder support, he said. The nuclear island of an AGR is “largely irreplaceable” and this “creates a situation where EDF Energy is taking a long-term view on investment in components which is in the interests of safety”, he added.

From a regulatory safety perspective there is no change, he said. “We still require start-up permission after every three-yearly statutory outage, we have ten-year PSRs [periodic safety reviews] and of course it must be in our safety case at all times. So the strategic approach we’re taking to plan for what we think about the potential of the assets is entirely complementary to the regulatory position and really no detriment to that.”

EDF Energy has created a series of “through-life management strategies for every key plant system”, he said. These are “holistic ageing plans for all the key systems that underpin our lifetime planning case”, he said. These plans consider where investments need to be made, what additional inspections are required, what R&D work would be beneficial, and how the supply chain should be involved.

As an example, Haller said the lifetime strategy for AGRs “is generally not to replace our data processing systems”. Instead, the company is focused on working with the original equipment manufacturer (OEM) supply chain under long-term lifetime contracts. “This creates a level of certainty and confidence for the supply chain to invest in its people, facilities and R&D work,” he said.

AGRs feature a graphite moderator and are cooled using carbon dioxide. The graphite blocks cannot be replaced or repaired during the operating life of the reactors. However, radiation damage changes the shape and size of the crystallites that comprise graphite, a process known as dimensional change, which in turn degrades the mechanical properties of the graphite. For continued operation, it is therefore necessary to demonstrate that the graphite can still perform its intended role irrespective of the degradation.

In support of the “ageing management” of AGRs in the UK, EDF Energy – together with Atkins, Frazer Nash and NRG – launched a graphite irradiation research program ten years ago. This project aims to simulate accelerated ageing of reactor graphite and involves neutron irradiation at the right temperature combined with simultaneous radiolytic oxidation.

Greater investment in R&D and an increased number of inspections create a “win-win situation for safety”, Haller said.

On the “external” factors concerning EDF Energy’s strategy to push back the closure dates of its UK AGRs, the engagement of its stakeholders is very important, he said. Part of this approach involves “linking up with the key infrastructural partners for our fleet”, such as Sellafield, the nuclear fuel reprocessing and nuclear decommissioning site in Cumbria, and Springfields, the nuclear fuel production installation in Lancashire.

Canadian experience

Hawthorne contrasted the challenges of AGRs and Candu reactors, noting that there is no component in a Candu reactor that cannot be replaced.

“Where graphite is a limiter on an AGR, for Candu reactors it would be fuel channel integrity, either because of hydrogen uptake or because of elongation,” he said. “We can go in there and scrape those channels, take samples, we can do hydrogen uptake calculations and on the basis of that we can go from what the original design assumption was, of 175,000 full-power operating hours, and to what we are talking about now, which is 300,000.”

Hawthorne said this “massive difference” to the operating potential of Candu reactors is achieved by “much more in-depth examination”.

“We recover some of the safety margin by adding additional operating life. The original design of the Candu reactor expected an operating life for the fuel channels of 25 years, but that was based on 175,000 full-power hours. We will run these plants for 40 years before we change the fuel channels,” he said. “I’ve just signed a deal with the government of Ontario to allow us to refurbish our Candu fleet that will see them operating through to 2060. That’s almost 50 years more. These reactors will go for almost 80 years in operation. Of course, in order to do that we’ll replace the fuel channels and the steam generators, but the important thing is good inspection techniques, much better capability to analyse the group data and statistically to evaluate it. A much larger population of operating experience shared by the industry gives us the ability to recover safety margin.”

Atomic Energy of Canada Limited, in cooperation with Canadian industry, began developing the first Candu (Canada deuterium uranium) reactor in the late 1950s. Candu reactors use heavy water (deuterium oxide) as a moderator and coolant, and are fueled using natural uranium (as opposed to enriched uranium). The advantages of the Candu reactor are savings in fuel cost, because the uranium does not have to go through the enrichment process, and reduced reactor downtime from refueling and maintenance. The first commercial Candu reactors began operations in Pickering, Ontario, in 1971. Sixteen of Canada’s 18 commercial reactors are located in Ontario (the others are in Quebec and New Brunswick).

The technology and design of Candu reactors have evolved through several generations, with the newest reactors the Enhanced Candu 6 (EC6, based on Qinshan in China). The next-generation Advanced Candu Reactor (ACR-1000) was not fully developed. Today, there are 31 Candu power reactors in seven countries, as well as 13 ‘Candu derivative’ reactors in India, with more being built. Export sales of 12 Candu units have been made to South Korea, Romania, India, Pakistan, Argentina and China, along with the engineering expertise to build and operate them. Three of the Canadian units are undergoing major refurbishment. As well as their use for electricity, Candu power reactors produce almost all the world’s supply of the cobalt-60 radioisotope for medical and sterilization use.

French strategy

Age need not impact operating performance and reliability, explained Olivier Lamarre, deputy senior vice president of nuclear generation at EDF SA. The company manages all of France’s 58 reactors, of which 22 will have been in operation for 40 years by the end of 2022. The French nuclear fleet represents 80% of the country’s power generation.

EDF is “expanding the operational lifespan” of the French nuclear fleet beyond 40 years, which was the initial assumption at the design stage of PWRs, Lamarre said. “Since 2010, the basic position of the French safety authority has been that the 40-year stage is a new domain for nuclear safety. This position is informed by the existence of Generation III reactors like the European pressurised reactor,” he said.

“EDF considers that there are four industrial positions for extending the operational lifespan well beyond 40 years. First, a high-performing fleet with error-free operation both day to day and in the long-term. Second, to set and meet nuclear safety and environmental protection targets that are acceptable to the general public for operational plants over the long-term against Generation III reactor construction. Third, to maintain and renew internal engineering and operating skills. Fourth, to maintain and renew industrial equipment,” he said.

The evolution of nuclear power plant design has reduced the risk of core events by a factor of 2, he said, while the radiological consequences of design-basis accidents have to be reduced by a factor of 5 to 10. To achieve this, EDF will implement design modifications, he added. The company has established three industrial processes to ensure the long-term “fitness for service” of equipment and facilities, he said. These are an ongoing in-service inspection and maintenance process; a management process for ageing systems structure and components; and a management process for equipment obsolescence.

EDF is focused on R&D, he said, and is involved in “large-scale international programs that are in line with international practice”. This commitment was confirmed by the International Atomic Energy Agency’s (IAEA) corporate operational safety review team (OSART) report on EDF in 2014, he added.

“EDF has embarked on a deliberate and challenging industrial program. For the non-replaceable equipment we demonstrate that it is able to perform as required for more than 50 years. And for replaceable equipment we justify its continued use in service or its replacement when its expected lifespan is less than 50 years,” he said.

“The continued operation of EDF’s reactors beyond 40 years is an ambitious and challenging but attainable step forward for nuclear safety for the nuclear fleet in operation in France. It will make it possible to reach a very high level of nuclear safety that is close to the worldwide requirements of new reactors in terms of the radiological consequences of an accident, meaning that the extension of the operational lifespan is indeed a possibility,” he said.

US difficulties

In the USA, operators are being forced to close nuclear power units, not because they lack potential for continued operation, but because of the “bad economics” of running them under deregulated electricity market conditions, said Bob Duncan, vice president of plant operations and supplier support at the Institute of Nuclear Power Operations.

Duncan told the conference that, since the Three Mile Island accident – a partial nuclear meltdown that occurred in March 1979 in unit 2 of Three Mile Island Nuclear Generating Station in Pennsylvania – operators have “delivered the promise” of safety, reliability, high capacity factors and higher measures of safety.

“What we haven’t delivered on is economics. So our biggest challenge in the US at this point is matching production costs of our nuclear power facilities against the natural gas prices that for the foreseeable future will be $4 per million BTUs for 50 years and production tax credits associated with solar and wind,” he said. Small single units have been “the first to go under the knife”, but “we also see bad economics against the large dual-unit plants in the Midwest”, he said.

The latest example was Entergy’s announcement earlier this month that its Pilgrim nuclear power plant, a single 680 MWe boiling water reactor, will be refuelled for the final time in 2017 and cease operations in 2019. The company cited poor market conditions, reduced revenues and increased operational costs behind its decision to close the only nuclear power station in the state of Massachusetts. The unit entered service in 1972 and is currently licensed to operate until 2032.

Duncan, who is also senior vice president for nuclear operations at Duke Energy Corporation, said the US nuclear industry responded to this state of affairs late last year when the Nuclear Energy Institute, the Institute of Nuclear Power Operations and the Electric Power Research Institute “put together a task force to work on economics”. This task force established a strategy it named Delivering the Nuclear Promise to maintain operational focus, increase value and improve efficiency.

“Those three strategies broke into four building blocks” that are very similar to the response to the Fukushima Daiichi nuclear power plant accident that occurred in Japan in March 2011, he said. “Our objective is within three years to create $3 billion worth of operating margin in our costs to run our nuclear power plants. That became 30% of our operating cost, which at that point some didn’t believe, but we had to create an ambitious goal so that we would be able to enliven the industry. This isn’t just another cost cutting measure; this is the life and death of nuclear power in the US,” he said.

The first three of the four building blocks are “economic analysis, economic viability and development of the teams necessary to make the economics and the efficiencies work”, he said. “The fourth centres around stakeholders because when you start to talk about a 30% reduction in O&M [operation and maintenance] costs, who cares about the job – the mechanic, the I&C [instrumentation and control] tech, the unions – many of the stakeholders that we may have taken for granted earlier, taken into the fold.”

International support

Peter Tarren, head of operational safety at the IAEA, described the Vienna-based organisation’s position on “safety over extended life”.

The IAEA “stole the concept” of the US Nuclear Regulatory Commission’s Generic Ageing Lessons Learned (GALL) report, he said, and used it as the starting point for its International Generic Ageing Lessons Learned (IGALL).

“This is a truly international effort spread over several years and has several different technological areas. These efforts are documented in databases and publications, in the form of downloadable safety guides and reports in this area,” he said.

The agency also offers peer reviews. An OSART includes assessment of the long-term operation of a plant, but an IAEA Member State can also request a Safe Long Term Operation (SALTO) review. This can be tailored to focus on Ageing Management Plans and/or on other programs related to long-term operation to support the given Member State in enhancing the safety of its nuclear power plants. The SALTO peer review service can also support regulators in establishing or improving regulatory and licensing strategies for the long-term operation of plants.

Tarren, who prior to joining the IAEA was the director of nuclear training at Ontario Power Generation, encouraged the “interplay between utilities and regulators”. Regulators have a “real desire”, he said, “to find out what long-term operation is all about”.

The IAEA also looks at the human resource side of long-term operation of a plant, Tarren said. “Just at the time the nuclear fleet is getting towards the end of its normal licence lifetime, sure enough the people working at the plants are also. So it’s all about replacing the people and skills.”

He encouraged delegates to engage with the IAEA, “We welcome your assistance, not just in drawing up standards, but also in helping us to do the reviews themselves,” he said. “The benefit that we hope that you will get and appreciate, especially as the global nuclear fleet becomes more mature, is an increased consistency approach among operators and regulators, sharing information and experience and reducing overall costs to safely extend the lives of these hugely valuable assets.”

Safety and security

Chairing the 14 April conference’s panel session, which was titled ‘Cycle Safety over Extended Life’, Stuart Crooks, managing director of EDF Energy Generation, said the subject “requires huge professionalism, engineering skill and business leadership”.

“This subject is in many ways at the forefront and the cutting edge of science and technology in a context of economic, political and societal change,” Crooks said. “Life extension of existing nuclear assets is likely to be a key part of many national strategies, continuing the provision of low-carbon baseload generation and bridging the gap with new nuclear assets. Our primary concern when making life extension decisions should be that of maintaining and enhancing nuclear safety and security. This ensures that the unique and powerful technology we have and the technology that we manage remains a significant and positive contributor to our society and to our environment.”

The environment in which each of the operators speaking on the panel work “may be very different”, Crooks said, “but their consideration of safety and security within life extension decisions is of a consistent and clear measure of importance”.

Source: World Nuclear News

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One comment

  1. Ramesh Chandra

    Agreed the safety margins in the earlier designs were kept high from mechanical design point of view,but the Station Blackout,Independent layout of pipelines/diverse routes for power and safety cables,use of fire fighting water for direct injection in Rxs core ,better monitoring of parameters and automatic controls were not adequately addressed.These provisions have been felt necessary after Fukushima,chernobyl,TMI incidents.So constant improvement in safety provisions have to be continued.

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