* Heysham 1, Hartlepool nuclear plants offline for inspections
* Will return to service gradually, depending on findings
* Two inspections already completed (Adds detail, links to graphic, table)
By Nina Chestney
LONDON, Sept 4 (Reuters) - EDF Energy expects two of its nuclear power stations in Britain, currently offline for inspections, to be back in service between the end of October and end of December, it said on Thursday.
The company's Heysham 1 and Hartlepool nuclear plants were taken offline last month to conduct inspections of their boilers after a fault was found on a boiler spine at Heysham 1.
Each plant has two reactors surrounded by eight boiler units that create steam to drive the turbines. The boiler spine is a central metal tube that supports the weight of the boiler coiling tubes around it.
The company said it has completed the inspections at two of the reactors and no defects have been found. The remaining boiler inspections are under way.
"Depending on the progress of the programme and any necessary modifications, the company expects there to be a phased return to service between the end of October and the end of December 2014," the company said in a statement.
"Dates for returning the stations to service depend on the findings and completion of the inspections," it added.
The Heysham 1 and Hartlepool nuclear plants have combined capacity of about 2.4 gigawatts (GW). Another 460 megawatt reactor, Hunterston B-8, is also offline for a statutory outage, taking unavailable nuclear capacity to nearly 3GW, equivalent to a third of the UK's total nuclear capacity.
The prolonged outages have prompted fears about a supply gap this winter. Electricity network operator National Grid this week brought forward by a year a scheme to tap additional power capacity over the winter.
Britain is already facing a power capacity crunch as ageing and polluting stations are shutting down, while new plants are slow to start operating because of a government overhaul of its energy policy.
Utility Centrica, which has a 20 pct stake in EDF's nuclear fleet, said on Thursday that the shutdown would hit its full-year earnings.
EDF Energy said that its parent, EDF Group, has already identified actions to mitigate the financial impact of the lower nuclear output.
The boiler inspection programme is to check there are no further defects that could develop in to cracks, discover the cause of the crack on the boiler spine at Heysham 1, develop measures to mitigate the impact of any defects and implement those measures if necessary, EDF Energy said.
"During the coming weeks the programme team will focus on completing the inspections and work to build a robust case for the safe continuing operation of the boilers," the company said.
"This 'safety case' is subject to approval from the independent nuclear regulator, the ONR (Office for Nuclear Regulation)," it added.
EDF Energy operates 15 nuclear reactors in Britain. However, its other reactors have a different boiler design and the company said it does not see a risk of them suffering from the same fault.
Editing by David Goodman