By Nina Chestney and Susanna Twidale
LONDON (Reuters) - Cracks in the core of a Scottish nuclear reactor could signal that most of Britain's ageing plants will not be able to supply the country with much needed power for as long as predicted.
Nuclear reactors generate just over 20 percent of Britain's electricity and even before EDF Energy said last week it would need to shut down one of two reactors at the Hunterston B plant, almost half of that capacity was scheduled to go offline by 2025.
"These reactors are over 40 years old. This is a generic defect which cannot be fixed so it would not surprise me if the older plants would all need to close within the next few years," said John Large, an independent nuclear engineering consultant.
Britain's electricity generation is under scrutiny due to a plan to close coal-fired power plants by 2025 and weak economic conditions for investment in new gas plants.
There are also doubts about the timetable for EDF's Hinkley Point C nuclear plant which is not expected to come online until the end of 2025, and the proposed Sizewell C plant, which is not even being built yet.
French utility EDF said the Hunterston B shutdown was due to new cracks developing faster than expected in graphite bricks in one reactor's core. These bricks are used in all 14 advanced gas-cooled nuclear reactors (AGRs) in Britain which drive seven out of eight of the country's plants.
"We believe that most of the AGRs will have their life limited by the progression of cracking," Britain's Office for Nuclear Regulation (ONR) says on its website, adding that this presents "unique challenges".
The bricks cannot be repaired or replaced and EDF says it will work with the ONR to show there would be no danger posed even in the event of an earthquake or other disruption.
"The longer-term safety case will build on work already completed and EDF Energy expects that this will demonstrate that there are large safety margins both now and for the projected reactor lifetime," EDF Energy said.
EDF has not changed its lifetime forecasts for Hunterston B, which is due to be decommissioned in 2023, although it predicts the reactor will remain shut until mid-November.
But experts such as Large, whose consultancy Large and Associates has worked for Britain's Atomic Energy Authority and environmental group Greenpeace, says this is over optimistic.
The graphite bricks form channels which contain nuclear fuel and the reactor control rods, while allowing carbon dioxide coolant to remove heat from the reactor fuel and core. They crack with age, but cannot be replaced due to their location.
Britain's ONR says the properties of the bricks change over time due to interaction with radiation and the reactor coolant.
There are around 3,000 graphite bricks in each reactor core which are bound by steel and contained in a concrete pressure vessel which is over three metres thick. Control rods are inserted through channels in the core to control the reaction and also used to shut down the reactor.
Keyway root cracking, as found at Hunterston B, is caused by tension in the graphite at the outer surface of the bricks due to changes in their internal stress. This can then progressively crack many bricks across the core.
John Loughhead, chief scientific adviser to the UK government's Department for Business, Energy and Industrial Strategy, said the micro-cracks have grown a little more quickly than projected and EDF Energy will now be working to understand how to make prediction of future growth more accurate.
"These reactors are in the later stages of their life but it is too early to be getting worried that this is the beginning of the end," he added.
Hunterston B and Hinkley Point B are Britain's oldest nuclear plants - each with two reactors - commissioned in 1976.
One of Hinkley Point B's two reactors is also offline - for a scheduled outage during which routine inspections of the graphite will also take place.
Reactors at two of EDF Energy's other plants in Britain - Heysham 1 and Hartlepool - were taken offline in 2014 after cracks on a boiler spine were found at one of them, caused by high operating temperatures.
In France, EDF's nuclear plants provide up to 75 percent of the country's power needs but the fleet has been dogged by repeated shutdowns and inspections over the integrity of some components.
Elsewhere in Europe, Belgium's regulator ordered production to be stopped at two nuclear reactors - not operated by EDF - in 2012 after finding indications of tiny cracks in core tanks.
EDF Energy said the shutdown at the Hunterston B reactor would result in a forecast reduction of 3 terrawatt hours in its total nuclear output for this year.
Based on current UK baseload power prices, that could equate to a loss of around 120 million pounds ($162 million).
The firm also says it has spent more than 100 million pounds in the last five years on graphite research.
"The thing which will close (these reactors) down in the end will be the cost of ensuring safety. It is possible to make a safety case for a significant amount of cracked bricks but it takes time and costs money," said Barry Marsden, professor of nuclear graphite technology at the University of Manchester.
($1 = 0.7399 pounds)
(Editing by Veronica Brown and Alexander Smith)
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Updated Date: May 10, 2018 00:07 AM