An incident which left an electrical ring main at the base without power for over 90 minutes had “potential nuclear implications” according to an official report, leading to a risk that reactors on board nuclear powered submarines berthed in the dockyard could have overheated.
The failure, which took place on 29 July 2012 in HM Naval Base Devonport in Plymouth, followed warnings in previous years that just such a situation might arise. The Devonport base is operated by Devonport Royal Dockyard Ltd (DRDL) on behalf of parent company Babcock Marine, and contains refit facilities for the UK's nuclear powered submarines and is the home port for Trafalgar class attack submarines.
According to a Ministry of Defence site safety report for Devonport, a nuclear ring at the base “experienced a complete loss of supplies for over 90 minutes” when the loss of an electrical power supply was followed by “unsuccessful connection of diesel generators” which were meant to provide back-up. Electrical power is essential at the dockyard to ensure that coolant is supplied to a submarine's reactor to prevent it from overheating after the submarine has berthed
The electrical failure was the result of “a combination of factors and additional unidentified defects”, according to the report. Subsequent investigations revealed that the power blackout was caused by a defect on a central nuclear switchboard, with concerns being expressed about “the potential for hidden defects in the switchboard gear.”.
The faults were revealed in a copy of the Devonport Site Event Report Committee's annual report for 2012 which was provided to the Independent on Sunday under the Freedom of Information Act. The report revealed that 50 events which affected or had the potential to affect nuclear or radiological safety had taken place at Devonport during the year, of which the electrical failure was the “most significant”, rated as a category B event with category A being the most severe.
The number of events which took place during 2012 had gone up by 10 compared with 2011, but was "broadly similar" to the numbers recorded in 2008 and 2009. The site committee considered that it was “reasonable” to expect an increase in the number of events as there had been an increase in the total number of days when submarines were berthed at the base.
Two previous electrical failures, both of which had been formally investigated, had taken place at Devonport before the July 2012 incident: a loss of primary and alternative shore electrical supply to HMS Talent in 2009 and a loss of AC shore supply to HMS Trafalgar in 2011. The report described DRDL's "inability to learn from previous incidents and to implement the recommendations from previous event reports" as “areas of concern”, and stated that the Devonport Base Nuclear Safety Organisation had questioned the "effectiveness of the maintenance methodology and its management" at the dockyard, highlighting a need for DRDL to "address the shortfalls in their current maintenance regime".
The Ministry of Defence confirmed that there was a “brief loss of electrical shore supply in July 2012, which temporarily removed one method of supplying electrical power to a submarine”. A spokesperson for the Ministry said that: "Our submarines have a number of safety measures to provide cooling to reactors which do not rely on electrical shore supplies. At no time was there any threat to the safety of any submarine, people working at the Devonport site or members of the public."
Following recent problems unrelated to the July 2012 power failure, the Office for Nuclear Regulation has served DRDL with a formal Improvement Notice on July 16 2013 “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”.
Inspections by ONR revealed that DRDL had breached the conditions of the Nuclear Site Licence which permits the company to operate the Devonport site and had not fully complied with the Management of Health and Safety at Work Regulations. The regulator decided that an Improvement Notice was necessary to ensure that DRDL make improvements to their management arrangements, although they accepted that the company had already acted to remedy some of the problems and that the issue “did not have an immediate impact on safety”. DRDL has until 31 March 2014 to comply with the terms of the Notice.
This article was modified on 21 October 2013 in the light of information received from the Office for Nuclear Regulation about the improvement notice issued to HM Naval Base Devonport (see comment below).