LONDON - EDF Energy, French power utility, has shut down for an estimated eight weeks, four of its eight nuclear reactors in the UK for refueling and maintenance after routine ultrasonic inspection of a boiler spine at Heysham 1 nuclear power station revealed "unexpected cracking".

In an operational update concerning Heysham 1 and its sister station of Hartlepool, EDF Energy Monday said one of the two reactors at Heysham 1 nuclear power station had been shut down on 11 June for refueling and to allow investigations to take place on one of its eight boiler units.

This followed tests which took place during a period of planned maintenance and inspection in 2013. During the Heysham 1 Reactor 1 planned statutory outage in 2013, an unexpected result was found during routine ultrasonic inspection of a boiler spine.

No similar results were seen on the other seven boiler spines subjected to equivalent inspections on Heysham 1 Reactor 1, or during subsequent equivalent inspections of the boiler spines on Heysham 1 Reactor 2 and at Hartlepool as the design of the boilers of both the plants are similar.

Heysham 1 Reactor 1 was returned to service early in 2014 on reduced load with the affected boiler quadrant isolated pending further investigations to confirm the source of the unexpected inspection result.

In June this year, subsequent more detailed inspections of the affected boiler spine during an outage on Heysham 1 Reactor 1 confirmed a defect in the location indicated by the initial findings.

"Heysham 1 Reactor 1 remains shut down while work continues to characterise the nature of the defect," stated EDF Energy.

Although routine inspections of other boiler spines have not previously indicated any similar defects EDF Energy has taken the conservative decision to shut down Heysham 1 Reactor 2 and Hartlepool Reactors 1 and 2 that are of similar design over the next few days to carry out further inspections in order to satisfy itself and the Regulator that the reactors can be safely returned to service.

"Until the results of the further inspections are known it is not possible to advise exact return to service dates for these four reactors, however, an initial estimate is that these investigations will take around eight weeks. EDF Energy's other nuclear power stations are not affected by this issue as they are of a different design," the nuclear power company stated..

Heysham 1 Reactor 1 is likely to continue to operate on reduced load when it returns to service until a suitable repair strategy can be implemented, EDF Energy added.

"As a result of these further inspections at Heysham 1 and Hartlepool with an initial estimate of around eight weeks the revised likely maximum theoretical output before unplanned losses for the EDF Energy Nuclear Generation Fleet in respect of the period from the 1 January 2014 to 31 December 2014 inclusive will now be c61TWh,"the company stated.

Office of the Nuclear Regulation in a statement expressed satisfaction at the steps taken by the power utility. "The shut downs will allow the licensee to undertake accelerated inspections of all of the boilers in each of the reactors, which are a similar design, and to fit equipment that will enable monitoring of the condition of the boiler spines," ONR stated.

The nuclear regulator assured that the "unexpected cracking in a boiler spine of Heysham 1 Reactor 1" had not resulted in any "release of radioactive material and no persons have been injured".

As the UK nuclear regulator, ONR will require an acceptable safety case from EDF Energy before the planned return to service of these four reactors.

Heysham 1 and Hartlepool have a combined power generating capacity of more than 2.3 gigawatts - equivalent to about 4 percent of the UK's peak winter demand. However, National Grid said there was plenty of spare capacity to meet summer demand, which is much lower in the warm weather.

The shutdown highlights the fragility of Britain's ageing energy infrastructure at a time when spare power capacity is forecast to fall to less than 2 percent by winter 2015-16, The Telegraph reported.