Ministry of Defence publishes list of submarine base nuclear safety incidents

A power supply failure to submarine berths, leakage from a tap left on in a radioactive waste tank, melting of an ice plug, and a loss of electricity caused when steam valves were mistakenly closed are among a series of unexpected events which have resulted in a string of nuclear incidents and near misses involving Britain’s nuclear submarine fleet over the last five years.

The catalogue of errors was revealed in response to a Parliamentary Question from Scottish National Party MP Angus Robertson, who asked the Ministry of Defence to provide a list of the most serious nuclear incidents that had happened at HM Naval Base Devonport and HM Naval Base Clyde – the two dockyards where the Royal Navy’s nuclear powered submarines are based.

The answer from Philip Dunne, Minister for Defence Equipment, Support, and Technology, revealed that a total of 11 ‘category B’ events had occurred at the two bases since 2008.  None of the most serious ‘category A’ events occurred over the period.

Five of the events took place at Devonport, where a category B incident is defined as having “a significant impact or potential for significant impact on safety or regulatory compliance. Investigation would be required to maintain compliance.  Includes Health and Safety Executive reportable events.”

The most recent event at Devonport occurred in 2012, when a loss of external power supply to submarine berths for a period of approximately 90 minutes left a submarine without a source of backup power.

The year before radioactively contaminated water overflowed from an Active Drain Tank in the Active Waste Facility when a tap was left open and a level alarm failed to operate, causing the tank to drain into a bunded area around it.  Another effluent leak occurred in 2008, when a discharge hose running between a submarine and an Active Waste tank split, discharging radioactive primary coolant water into the environment.

A more serious incident occurred in 2010, when two submarines set sail without protection from overpressure in their steam generators.  Hull test blanks that had been fitted when the submarines were undergoing maintenance had not been removed, leaving no route for venting high pressure steam in the event of an an accident.

Mr Dunne refused to release details of a fifth incident which happened at Devonport in 2011, claiming that “disclosure would be likely to prejudice the capability, effectiveness or security of the armed forces”.

Six events have occurred over the last five years at HM Naval Base Clyde, where a category B nuclear event is defined as “actual or high potential for a contained release within building or submarine or unplanned exposure to radiation.”

In 2012 maintenance workers entered an area adjacent to the reactor compartment without proper radiological controls being in place and received an unplanned exposure to a radiological dose.

Two events occurred in 2010: one when an ice plug, formed using liquid nitrogen, melted when the nitrogen supply failed during maintenance of a reactor system.  Melting of the plug resulted in leakage of radioactive reactor coolant into the bilge area of the submarine’s reactor compartment.  In the second incident a bag containing potentially contaminated clothing fell overboard from a submarine.  The bag was recovered and was later found not to be contaminated.

On two occasions in 2009 there were concerns that dockside cranes had exceeded the number of authorised lifts, and in 2008 valves which supplied steam to electrical generation equipment on board a submarine were shut in error, causing a loss of non-essential electrical power.

Control of the quality of work within the Ministry of Defence nuclear programme has regularly been highlighted as an area of concern by the Defence Nuclear Safety Regulator over recent years.  The regulator’s annual report for 2011 declared that “the number of incidents remains too high” while the 2012 report stated that “a number of ‘control of work’ issues have arisen during the reporting period and continued Duty Holder attention is required in this area.”

In July 2013 the Office for Nuclear Regulation served a formal Improvement Notice on Devonport Royal Dockyard Ltd (DRDL), the operators of HM Naval Base Devonport, “after being made aware of a number of incidents at the site in which operations were not carried out in accordance with DRDL’s own operating rules and instructions”.

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