For anyone who’s been glued to Chernobyl on TV recently, the safety of the world’s nuclear reactors might be more of a preoccupation than usual. But have you ever considered what would happen to a reactor’s graphite core in the event of an earthquake?
The University of Bristol’s Earthquake and Geotechnical Engineering Research Group, commissioned by EDF Energy, is doing exactly that. In order to test the response of a nuclear core to different magnitude earthquakes, it has built a high-precision model of an Advanced Gas-Cooled Reactor (AGR) graphite core and has been busy shaking it about with the university’s earthquake simulator.
AGRs are unique to the UK and were designed and built in the 1970s and 1980s (at a time when earthquake science was still in its infancy). Their core is made up of thousands of interlocking graphite bricks, some of which surround fuel rods and some of which surround control rods. Control rods are capable of absorbing neutrons which means they can literally ‘control’ the fission rate of uranium and/or plutonium and prevent it from accelerating too fast.
To replicate this system, and to test the way a core reacts to tremors, Bristol’s model contains over 43,000 components and 4,300 sensors hidden within the bricks. Talking through the research at the Royal Society’s Summer Science exhibition last week, Professor Colin Taylor, explained that the most cutting edge part of the model is the bespoke miniature sensor systems. ‘Our challenge was to work out how on Earth we could measure how all these different bricks interact together,’ he said. ‘Each smart brick has about 30 sensors on it so we couldn’t possibly use conventional instrumentation systems to collect all the data. We’ve had to build our own bespoke data acquisition systems which can eventually transfer data to the storage computer.’
To test the model, the team places it on the university’s ‘shaking table’ – a 3m x 3m platform, supported by eight hydraulic actuators. In doing so it can analyse how the graphite bricks respond. So far, the team has demonstrated that tremors with a magnitude of about 7.2 have no adverse impact on the structural integrity of the model reactor. This is comforting, particularly in the UK where the strongest earthquake ever recorded had a magnitude of 6.1 (it took place in 1931 near Dogger Bank). Though the British Geological Survey detects around 200 to 300 earthquakes in the UK each year, only around ten per cent can be felt by the public (Professor Taylor says humans start to feel tremors around magnitude two).
Nevertheless, this doesn’t stop the team preparing for the next big one. ‘In the nuclear sector we design for very low probability events – so a one in 10,000 year event,’ explained Taylor. ‘In the UK that’s about a magnitude 6.2 earthquake. It isn’t huge on a worldwide scale but is still significant enough to be taken seriously. We’re interested in seeing if and when the model falls apart – which hasn’t happened.’ In addition to the immediate reaction to the quake, the researchers are also working to detect any residual damage: ‘What we’re interested in finding out is if there’s any residual distortion in the core and particularly in the control columns which could stop the control rods going in.’
Once experiments have taken place, Bristol shares the data collected with EDF Energy who uses it to validate and improve computer models of the actual reactors used in plants across the UK. As well as testing the response of the reactors in their current state, the goal is also to predict the effect of an earthquake on the AGR cores as they age (older cores are likely to have fine cracks in the bricks).
The team is helped in this task by a computer vision system which looks down at the top of the model and tracks the movement of the bricks (which can be placed in different arrangements and with cracks in different locations). ‘We’re looking at doing experiments on core arrangements with many, many cracked bricks to once again help validate the EDF Energy computer models, but also to help us understand the physics of what’s going on,’ said Taylor.
Some arrangements of cracked bricks respond more violently to the incoming energy than others, but in general Taylor says that the model retains its structural integrity. ‘When you see the response for real it’s actually quite underwhelming, which is a kind of reassuring viewpoint. The model doesn't fall apart and we have no reason to believe the real thing would fall apart.’
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