Procedures to prevent power stations from becoming defunct, derelict or dangerous13 January 2016
Nuclear power plants have only been a part of the modern landscape for the past 50 years or so. As they were originally built with relatively short-life designs in mind, the energy industry continually faces issues relating to concrete-structure aging and the logistics of life extension. Sophie Peacock speaks to John Moore, a nuclear engineer, about the procedures in place that can prevent power stations from becoming defunct, derelict or dangerous.
Designed according to defensive concepts that put local barriers between radiation sources and the public, nuclear power plants (NPPs) can jeopardise these levels of protection if left to age untreated.
"What we try to do with aging management programmes is to demonstrate to our regulatory bodies and to the general public that aging issues of concern have been addressed for the life of the plant and any proposed extensions," explains John Moore, a nuclear engineer at the International Atomic Energy Agency.
The World Nuclear Industry Status Report 2014 suggested that without rapid measures, nuclear power in Europe could soon come to an end. The report warned that unless policymakers make swift plans to replace aging plants by the 2050s, nuclear power across the globe could become obsolete.
But in many ways, the nuclear industry is still fairly young, with the first major build of nuclear plants beginning in the early 1970s. The nuclear industry is investigating extending the life of these plants to 60-80 years, or possibly beyond, but effective management of aging components in NPPs is key to their safe and reliable operation. While new methods of maintenance and retrofitting are being researched, a distinct benefit for the nuclear business is the construction industry's long-term experience with concrete.
"The hydraulic industry has been creating large concrete structures for over a century and many of the people who made the original designs for NPPs have come from this sector," says Moore. "They are experienced with concrete constructions and have developed plants with large design margins built in."
If a condition in a concrete structure arises that hasn't been explicitly considered in the original design, the power plant operators carry out an assessment of the potential negative safety impacts. If the plant is outside of its design basis it will typically be shut down until it is safe to operate.
Sometimes these assessments offer time-limited solutions; for example, that a plant is safe to operate for another five years, and after that period more analysis will need to be carried out to perhaps determine a further extension. "Originally, if we have a plant life of 30-40 years, as research gets better, we can justify another ten years," says Moore. "And now they're looking at even longer than that."
The challenge for technicians is being able to adequately yet conservatively predict the behaviour of materials and components without the need for real life demonstration.
That being said, Moore points out that there are many established techniques in the industry, such as concrete blasting, to perform accelerated aging tests on materials. The difficulty lies in any scientific uncertainty as to how a material will respond to aging; this requires assumptions to be made that materials will age faster than perhaps, in reality, they would. "These conservative estimations tend to hurt the economics of the life extension because you may say you need to replace something after 20 years, when in reality it may last for 30 or 40 [years] or even longer," he says. The industry has to determine the right amount of money to invest in research and development to gain a full understanding of aging mechanisms and therefore support the maintenance economics.
It is therefore necessary to start inspections as soon as possible for components where aging is a concern; this way, conclusions about damage and corrections can more easily be made. "An example might be that if you see a crack in your container structure, it's important to know whether the crack has been there for 40 years and hasn't changed or whether it started last year and is rapidly growing," Moore says.
Having little or no inspection history means technicians must assume the worst, ultimately impacting on the amount of work that is carried out and the financial implications of the life extension. "We do actually recommend now for new plants that these inspections start as part of the original construction so you have good pictures and data of your concrete structure while it's brand new," he adds.
One-size does not fit all
The economics of power plant maintenance are quite different across jurisdictions. What may be financially viable in Eastern Europe for one-size plant, the work required on it would be quite different in North America or Asia. The electricity produced by nuclear plants can be in the broader energy market and so the economics of life extensions typically depend on competing sources of generation in the region. Government policy on nuclear power is also a factor in extension decisions.
One would think that an industry so young would still be continually developing its standards and finding methods of improvement. So why does it remain so reliant on mundane administrative tasks such as data keeping and maintenance reviews? In Moore's view, the relative infancy of the nuclear industry means it can advance at a faster rate as long as this groundwork is not abandoned: "We started to look at aging as a topic in the late 1980s and research has been going on since then. Things like reactor pressure loss or concrete containment were the first topics to come up, and we've been expanding our knowledge in these and other areas as time goes on."
A typical fossil fuel or renewable plant will usually have a shorter design life than a nuclear power plant, meaning that they don't necessarily experience some of the same degradation issues that nuclear power plants do over a 60-80-year lifetime. One possible exception to that rule may be hydraulic plants or large dams, as they have lives of 80-100 years and do undergo refurbishments during that time.
But even plants with short lifetimes need to be maintained in order to maximise the return on investments in their ongoing functionality; there is economic interest in keeping the plants reliable and operating well. Because fossil fuel plants don't have nuclear safety issues, the amount of investment in maintenance and the scope of the work required are typically much less than for nuclear plants.
NPP owners have prime accountability for their plant's safety, ensuring that high-quality maintenance is being carried out and that all systems are reliable. The actual inspection and repairs to the plants are carried out by utility staff or supplemental staff to assist with specialised activities; the type of work involved can vary from routine corrective maintenance such as inspections, relay calibrations or topping up oil levels, to much more substantial activities that might require a longer station shutdown than average.
"A very good analogy would be the type of work you might get done on your car," Moore suggests. "You have small things such as getting an oil and filter change or putting your winter tyres on. Then you have larger activities such as getting the bumper repaired, wheel bearings fixed - other big tasks that you'd only do maybe once in the car's life."
If a condition in a concrete structure arises that hasn't been explicitly considered in the original design, the power plant operators carry out an assessment of the potential negative safety impacts.
A longer life
Plant owners contemplating life extensions need to show their regulators, via a periodic safety review - that the plant will be safe for the proposed time frame. The plant goes through a systematic review against modern safety codes and standards for all systems that are subject to aging, as well as their normal operations. "There's some discussion and agreements made with the national regulators as to what modifications or upgrades may be necessary to allow the life extension. The operator then considers the cost of that upgrade and the price they'll get for the power as to whether the extension is justified," Moore says.
Operators may also consider implementing upgrades that aren't safety related in order to increase plant efficiency or reduce reinstatement costs. They may make adjustments, such as rearranging the turbine system, in order to get more out of the plant, provide benefits for maintenance and maybe reduce outage time in the future.
In terms of retrofitting power plants, each reactor has varying requirements depending on its age, location and maintenance history. Moore explains that in a lot of the older plants, obsolescence is an issue: "An example I always like to give is if you were to try to buy parts for an old stereo system that was made in the 1970s, would you be able to buy them? It would be really difficult. A lot of the instrumentation that was designed for these plants in the '70s or '80s has changed over the years."
There are methods of reviewing equipment for retrofitting such as finding equivalencies or using digital software, and some designs have particular peculiarities to them that affect life extension decisions. "You sometimes see corrections such as piping system upgrades that are carried out to cut off material unsuited to corrosion in order to reduce maintenance costs or outage times going forward," Moore says.
Practice makes perfect
Different organisations around the world recommend effective practices for managing outages, the priority being to keep them as short as possible to minimise the amount of time that electricity is not being generated. Although there's no particular world standard of how this is carried out, the industry makes a concerted effort to learn from each other in terms of how to be more efficient.
According to Moore, it's never too late in a power plant's life cycle to intervene with vital maintenance. "Fortunately, most of the components that we're concerned about from an aging or life-extension perspective are those that you don't typically do a lot of routine maintenance on. In a concrete structure, you carry out inspections of the building but you may not need to carry out repairs on it if you're not retrofitting at all."
The numerous components that do need scheduled maintenance at a nuclear plant are generally in good shape by the time a life extension is required as they're being fixed and refurbished on a regular basis; a small amount of upkeep might be required once a year or a larger refurbishment every two to three years. "By the time you get to life extension, the station is usually in reasonable shape or, if you've had obsolescence issues you've got a big investment to correct what's been going wrong."
Will nuclear facilities and other energy plants be eventually replaced entirely or continue to have their lives extended - as has been the procedure for the past few decades? It is interesting to note that the original lifespan of NPPs was simply an arbitrary number from the original designers. "That's just what they thought the plant could reasonably achieve. All our research has been to put a lot more science and data around those numbers, and increase them to say '30's okay, 40's okay, 50's okay'," Moore explains. "So that's really what the current philosophy of the industry is: looking to restore NPPs for 60 to 80 years, and possibly beyond that - how to look at a concrete structure that's 60 years old and validate that it'll be here for some years to come."