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Playing it safe with reactors

With new nuclear reactors on the horizon, Mike Yule explains why helping to keep the UK's existing plants running safely is a great job for a physicist

After a period of decline, there is now a real sense of excitement in the UK's nuclear industry. The previous UK government's commitment to allow new reactors to be built on 10 sites has proved invigorating for the entire sector. Even firms that had traditionally focused on keeping the current power stations operational are now making "new build" a growing part of their business.

My own employer, AMEC, is one such firm. As contractors, we do work that other companies cannot do, either because they do not have enough people or because they lack the right skills. Following the company's acquisition of the nuclear consulting firm NNC in 2005, the nuclear part of AMEC's UK business has expanded considerably. There are now offices around the country, including one in London near the headquarters of EDF Energy (one of the main companies planning to build new reactors in the UK), which lends project-management support and other expertise.

I joined AMEC two years ago after completing an MSci in maths and physics at Durham University. I had planned to continue my studies with a PhD, but after doing a final-year project in theoretical physics, I decided that this was enough theory for me. At a careers fair, I came across AMEC, which was looking for physicists to work in its nuclear sector. I applied, was accepted onto its graduate scheme and I now work as an analyst at AMEC's Booths Park site in Cheshire.

AMEC is split into three divisions: Natural Resources, which deals with the oil and gas industries; Power and Process, which works in the nuclear sector; and Earth and Environmental, which provides geoscience services. The Booths Park site is part of the Power and Process division, and has been home to nuclear scientists since the 1950s. Back then, the offices were in a mansion house, and the workers were helping to design the Magnox reactors – the UK's first commercial nuclear plants. Today, the offices are more modern, but the same view of a small local lake, Booths Mere, is available – along with cows making the daily parade for milking! There is also a nice link to the original work, as AMEC has recently been involved in decommissioning the Magnox reactors.

Safety first
In my team of about 30 analysts, project managers and planners, most of our consulting work comes from British Energy, although we also work with Rolls Royce and BAE Systems. British Energy (now part of EDF Energy) owns the majority of the UK's commercial reactors, which are mainly advanced gas-cooled reactors (AGRs). Some of these reactors were constructed 30 years ago and they are now coming to the end of their lives. But with power stations able to generate £1m worth of electricity a day, keeping them online for even a few extra years is big business. A lot of AMEC's work therefore deals with life extension – determining whether or not the reactors are safe to operate for a little longer.

Deciding when a reactor needs to be decommissioned can be a challenge. Everything gets weaker as it ages – even in coal power stations, parts must be replaced continuously. But once you throw irradiation into the mix, dealing with ageing power plants gets more complicated. For example, in an AGR, the reactor core is constructed from graphite blocks that fit together to form what is known as the diagrid. Over time, the graphite is irradiated, and also gets oxidized by any small quantities of air present, making it weaker and lighter. Eventually, such "core ageing" processes will render the reactor unsafe for continued operation.

Safety is paramount in this industry, so if the Nuclear Installations Inspectorate – the regulatory body that issues licences for running a reactor in the UK – is not happy, then a reactor will simply be shut down. Getting the evidence we need to prove that a reactor is still safe has required some novel approaches to testing. One of these is AMEC's "test rig", which is essentially a steel cage with a quarter of the footprint of a real reactor core, containing quarter-size "graphite" blocks (actually made from aluminium) in various states of wear and tear. Tilting this rig allows cameras to see whether the ageing blocks would cause problems during normal operation, or in more extreme conditions, such as a major earthquake.

Measuring the damage
One of the main projects I have been involved in is proving that it is safe to connect inspection equipment to empty reactor channels, to allow video equipment to be slid into the channels and video footage to be obtained from inside the core. The idea is to see how many cracks are present: even though it will not be possible to fix any that are found, given the extreme environment, it is still useful to know how much the core has degraded.

Within this project, my specific job is to evaluate the consequences of what are known as "dropped fuel faults". In a reactor, the uranium fuel is contained in bullet-like pellets within stainless-steel tubes, called fuel rods or pins. An arrangement of 36 fuel pins in three concentric rings, held inside a graphite "sleeve" using steel braces, is known as a fuel element. These fuel elements are lifted into and out of the reactor and various other components during refuelling or discharging operations. If a element drops at any point, then its potential energy goes into processes such as crumbling the sleeves and buckling the pins.

Thanks to specially conducted drop tests of fuel elements, we have a good idea of how they can be damaged, and how much energy can be absorbed in the various processes. By incorporating energy-absorption mechanisms into spreadsheets, we can calculate how much damage would occur in a particular situation, and can show how damage depends on drop height, channel diameter, ductility and many other factors. Another problem is that because the pins contain fuel, they generate heat even when the reactor is shut down (so-called decay heat). If the pins buckle due to mechanical loading in a dropped fuel fault, the fuel becomes more concentrated and less easy to cool. We need to model this too, so some of the work I do uses software (written in good old Fortran) to predict how faults will affect fuel temperatures, which have to stay within limits to ensure safety.

I have found this work interesting because it has built on knowledge gained over the course of my degree, particularly that of spreadsheets and programming languages. My degree has also helped by giving me a better understanding of physical situations, such as where heat is being transported by flows and radiation. The AMEC graduate scheme is a great introduction to the nuclear industry, and a good way of meeting the other graduates who joined the firm at around the same time. I have greatly benefited from the supervision of specialists, who are always ready to share their expertise. I enjoy the work I do and it has given me confidence in the safety of our reactors. In a growing industry, that is no bad thing.

About the author
Mike Yule (mike.yule@amec.com) is an analyst in AMEC's Power and Process division


This article originally appeared in the June 2010 issue of Physics World.

last edited: November 05, 2014



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