Reactors in many UK nuclear power stations are in danger of developing cracks in their graphite cores. This could force some plants to close down earlier than expected, dealing a blow to the idea that nuclear power can become a "green" option in the fight against global warming.
Documents obtained by New Scientist under the UK's Freedom of Information Act have revealed unsuspected problems with the country's ageing advanced gas-cooled reactors (AGRs). Government nuclear inspectors say they have uncovered weaknesses in the safety analyses carried out by British Energy, the company that runs the reactors.
The UK's 14 AGRs provide nearly a fifth of the country's electricity. The graphite bricks that form part of their core help sustain the nuclear reaction by slowing down fast-moving neutrons. They also play a vital part in maintaining the core's structural integrity.
While irradiation and thermal stress would eventually cause the graphite bricks to crack, new estimates suggest these cracks could develop up to two years earlier than thought, according to British Energy. In a letter in August 2004, the company warned the Nuclear Installations Inspectorate (NII) of "possible errors" in the computer models used to predict the onset of cracking.
In December, British Energy found cracks in the graphite bricks at one of the two AGRs in Hartlepool, County Durham. Cracking is now suspected in six other reactors at Heysham in Lancashire, Hinkley Point in Somerset and Hunterston in Ayrshire.
Widespread cracks in these bricks could cause the core to distort, overheat and leak radiation. To ensure this does not happen, the NII has asked British Energy to conduct more inspections of the bricks. This means that reactors may have to be closed down for maintenance more frequently and for longer, at a cost in lost income of £250,000 a day for each reactor that is shut down.
Two reactors at Hinkley Point and Hunterston are scheduled for maintenance shutdowns later this year. In letters sent in February, the NII told the stations' directors that their safety analyses of the graphite cores were "weak", and demanded answers to 47 technical questions about cracking before it will permit the reactors to restart.
British Energy has warned its shareholders that graphite cracking could kill its hopes of extending the lives of AGRs, and that there may have to be "early closures". The reactors, which were built in the 1970s and 1980s, were expected to operate for 35 years, and not start closing until after 2010.
British Energy stresses that, at present, no reactors have been shut down because of graphite cracking. "The implication is one of periodic monitoring and inspection within our normal programme of planned maintenance shutdowns," a spokeswoman says.
Critics of nuclear power argue that the safety and economic implications are more serious. The worst-case scenario is that cracking could cause part of the core to collapse and the reactor to be written off, says John Large, an independent nuclear engineering expert.
Pete Roche, a consultant to the anti-nuclear group Greenpeace, says graphite cracking highlights the unreliability of nuclear power. "Generic problems can shut down several large nuclear stations all at the same time," he says. "So they are not the best solution to combat climate change."