SAFETY watchdogs have shut down a unit at the Aldermaston atom base after the discovery of corrosion in a steel structure.

The Office for Nuclear Regulation (ONR) declared it a safety risk and issued AWE managers with an improvement notice - automatically halting all non-essential work in the unit - while checks are carried out.

Anti-nuclear campaigners this week accused AWE of hushing up the discovery but spokeswoman Philippa Kent said: "During an inspection in May 2012 the company reported corrosion on structural steelwork in one of its older manufacturing facilities.

"Further inspections during July and August found incidence of degradation of some of the other structural components. At that time, AWE immediately suspended routine operations as a precaution."

Green Party MP Caroline Lucas is challenging defence secretary Philip Hammond to find out the extent of the corrosion and the repair costs.

She said: "I am glad that the Office for Nuclear Regulation has acted promptly to protect the public from any safety risks that could arise from the current problems at Aldermaston, but some serious questions remain over the adequacy of structural inspections conducted in the past."

Repairs are expected to take until the end of the year to complete and Ms Kent said: "Safety remains our top priority and AWE is working closely with the ONR to ensure that all inspections, safety measures and planning for remedial work are carried out with their full knowledge and consent."

But Peter Burt, of the Reading-based Nuclear Information Service, claims the building is the A45 facility, one of the largest nuclear production sites carrying out higher risk operations with radioactive uranium metal.

He said: "We should be concerned that AWE apparently tried to hush these safety problems up and that the matter only became known to the public when ONR insisted on reporting details in one of its regular reports on the site."

*AWE is facing prosecution by the Health and Safety Executive for safety breaches of safety following a fire in an explosives handling facility in August 2010.