MORE than 350 cracks have been discovered in an Ayrshire nuclear reactor which has been shut since March, taking the total over government safety limits.

Owners EDF Energy closed Reactor 3 of Hunterston B in March this year for more detailed investigation.

A smaller number of cracks had been known about but the figure has risen through projections and following recent inspections.

Office for Nuclear Regulation (ONR) details about Hunterston outages said said that while cracks of the graphite bricks were expected due to the reactor's age - the number of cracks led to EDF carrying out further inspections.

HeraldScotland: FUTURE IMPERFECT? Hunterston B is due for decommissioning in 2011 but, contrary to what outsiders may think, many locals say they don't have a problem with the plant. Picture: Angela Catlin

Hunterston B's Reactor 4 was taken offline in October for an inspection.

ONR said safety cases were being prepared for both reactors and will be assessed by officials to determine whether they are safe to return to service.

EDF said Reactor 3 was safe to operate, and it hoped to bring it back into service.

A spokeswoman insisted: "Nuclear safety is our overriding priority."

The ONR said the number of cracks was over the "operational limit" and that EDF were working ona revised safety case to justify Reactor 3's return to service.

An ONR spokesperson said: "A conservative assessment of the inspection results shows that the number of cracks in Reactor 3 exceeded the operational limit of 350 cracks in the existing safety case. However, it should also be noted that the safety case demonstrates a significant margin beyond this limit and safe operation was ensured.

"Once we have received the safety case from EDF we will fully assess it and permission will only be granted for the reactor to return to service if we are satisfied that it is safe to do so. This assessment will include consideration of the timing of further inspections."

An EDF Energy spokeswoman said: "The most recent results support the work we are doing on the long-term safety case and underline our confidence that the normal operations at the station are unaffected and that there would be safe shutdown in the event of a 1 in 10,000 year earthquake.

"We are preparing to present a safety case for return to service of reactor three to the regulator, the ONR, for their assessment.

"We have also carried out similar inspections on reactor four and the case for return to service for that unit is currently with the ONR for review."