TWO Devonport-based nuclear submarines have been operating with a safety defect which put the vessels and crew at 'serious' potential risk, a leaked memo has revealed.

Valves in the hulls of HMS Turbulent and HMS Tireless, pictured, were blocked off during testing of their hulls, preventing the boats venting excess steam pressure.

These blocking devices should have been removed before the submarines re-entered service after their maintenance periods — but were instead left in place.

HMS Turbulent was operating for more than two years before the fault was discovered, while HMS Tireless operated with blocked valves for more than a year.

A top independent consultant and nuclear engineer today said as a result radioactive water could have burst into manned compartments of each submarine, possibly exposing the crew to radiation.

The leaked document described the issue as a 'serious incident', adding: "Procedures were weak and ambiguous."

A Ministry of Defence spokesman said a detailed investigation had been launched.

City council leader Vivien Pengelly said the authority would be asking for talks with the MoD, Royal Navy and dockyard operator Babcock over the issue.

"I demand an answer to this," Cllr Pengelly said.

"I want to know exactly what has happened and how they are going to improve things in the future.

"We will be very interested in the results of the investigation, and will be asking further questions."

The revelation comes as the MoD prepares to announce a consultation exercise over possible plans to base a new Submarine Dismantling Project (SDP) in Plymouth.

The MoD is considering 12 sites for the storage part of the work but only two, including Plymouth, for the more controversial dismantling work.

Plymouth already refits, defuels and refuels submarines at Devonport Naval Base.

Ian Avent, of the Plymouth-based Campaign Against Nuclear Storage and Radiation, said the consequences of the mistake involving Tireless and Turbulent 'could have been horrendous.

"It just doesn't bear thinking about. It's shocking," he said.

"It's yet another example of how they're relying on luck as a management tool."

Anti-nuclear campaigner Tony Staunton, who is also secretary of the Plymouth Trades Union Council, said: "This is one of a series of serious problems that the privatised dockyard owners have had with the nuclear fleet and at some point, there will be serious consequences.

"This memo proves that even the MoD has a concern that they're missing something as serious as this, because the maintenance procedures are very complex.

"There's the clear potential that they're not picking things up they should."

He added: "Basically, they [the submarines] could have overheated and 'blown' at any time."

Mr Staunton said he had grave concerns, especially given the prospect of Devonport playing host to a submarine dismantling project and a multi-million-pound waste incinerator in a working naval base.

"There is a madness at the heart of those who lead and run our city that makes them blind to what's in front of their faces," Mr Staunton added.

The situation surrounding HMS Turbulent and HMS Tireless involves 'test blanks' which were fitted to the submarines during on-land maintenance to test for leaks.

These were meant to be removed from the submarines following the work, but were left blocked off, states the leaked memo which was written last month.

This left the submarines, both nuclear-powered hunter-killers, without over-pressure protection on the steam generators.

The memo states: "Despite there being several potential safety nets, none succeeded in identifying and arresting the events before the submarines undertook PSA (Platform Sea Active) operations."

Independent consultant John Large said the oversight was 'an accident waiting to happen.

"This was a serious incident," he said. "It illustrates the breakdown of the rigid procedures required to ensure submarines are fit for service."

An MoD spokesman said: "We can confirm that, as part of routine maintenance checks, an issue was identified on HMS Turbulent and HMS Tireless which has now been resolved.

"We take safety extremely seriously and as soon as we were aware of this potential issue we took action to address the problem," he said.

"Detailed investigations to assess the cause and any possible safety implications are ongoing.

"It is too early to speculate on the outcome of those investigations," he added.

HMS Turbulent, which has a crew of 130, is the second of seven Trafalgar-class submarines and was launched in 1982.

HMS Tireless, which also has a complement of 130, is the third of her class and was launched in March 1984.

In 2007 two sailors were killed after an explosion in the oxygen-generating equipment on board HMS Tireless during an exercise under the Arctic ice.

The submarine was also forced to dock at Gibraltar for a year in 2000 to undergo repairs when a fracture was found in a pipe close to its nuclear reactor.