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UPDATE 1-EDF Energy extends outages at two UK nuclear plants

* Heysham 1-1 outage extended to Dec. 31 from Nov. 30

* Hartlepool 1 set to return earlier than estimates (Adds detail, comment from EDF Energy)

LONDON, Oct 17 (Reuters) - EDF Energy has extended the unplanned outages at three reactors at two of its nuclear plants in Britain, which were taken offline for safety inspections after the company found a fault.

In August, EDF Energy had to take three reactors at Hartlepool and Heysham 1 off the grid for inspection after a crack was discovered on a boiler spine of one of the Heysham 1 reactors. Heysham 1 had already been offline since June for refuelling.

Each plant has two reactors.

The British energy supplier, whose parent company is France’s EDF, said last month that it expected to bring the two plants back online by the end of December, depending on the findings and completion of the inspections.

On its website on Friday, EDF Energy specified that its 610 MW Heysham 1-1 reactor was expected to return to service on Dec. 31, extended from Nov. 30.

Many UK power traders are monitoring the exact dates for hedging purposes.

Its 585 MW Heysham 1-2 reactor is expected to return on Nov. 9, revised from Oct. 31, and the outage at its 620 MW Hartlepool 2 is also expected to end on Nov. 9, extended from Oct. 31.

However, the 620 MW Hartlepool 1 is expected to return on Nov. 22, eight days earlier than the previously estimated return date of Nov. 30.

EDF Energy said in a statement that good progress was being made with the inspections and no further defects have been found.

“EDF Energy will issue a full update around the end of October....Return service dates will be subject to approval from the independent nuclear regulator, the ONR (Office for Nuclear Regulation),” the company said.

It added that as boiler spine defects only develop into cracks at very high temperatures, the Hartlepool reactors and one at Heysham 1 could be returned to service at 75-80 percent power to reduce the temperature to which the spines are exposed.

“Specific modifications would then take place during planned maintenance periods in 2015 and 2016 to allow a return to full power for these three reactors,” EDF Energy said.

The other Heysham 1 reactor would return to service only when the affected are boilers isolated.

EDF Energy had said on Thursday the outage at its 460 MW Hunterston B-8 nuclear unit would be extended by six days to Nov. 3. This reactor went offline on Oct. 5 after cracks were found in two graphite bricks in the core of the reactor.

The company said at the time the cracks did not have any safety implications.

The unplanned outages, combined with planned, statutory outages, mean that nine of EDF Energy’s 15 British nuclear reactors, with a total capacity of around 5,200 megawatts (MW), will be offline by the end of Friday.

Britain’s nuclear power stations are ageing. Hunterston B was built in the 1970s and its two operating reactors are expected to be retired by 2023.

EDF Energy’s nuclear plants, and Magnox’s Wylfa plant, usually provide around a fifth of Britain’s energy, and the outages have prompted fears of a power supply squeeze this winter. (Reporting by Nina Chestney; Editing by David Holmes and Keiron Henderson)

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