Office for Nuclear Regulation

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Agreement to implement the Category A modification/experiment on existing plant

License Instrument no 536

Permission requested

Devonport Royal Dockyard Ltd (DRDL), the site Licensee at Devonport Royal Dockyard, Plymouth, has requested the Executives agreement to implement a modification to the activities defined in the current Submarine Refit Complex (SRC) Plant Safety Case. Their submission justifies the use of the Welding Argon Purge Rig for welding on PWR1 Nuclear Steam Raising Plant (NSRP) whilst in the SRC in the light of new criteria on pressure and flow to ensure the critical safety function of control of reactivity is maintained. This permission will be limited to use solely on HMS Talent at the present time.


The SRC at Devonport Royal Dockyard consists of two docks, 14 and 15 Dock and supporting facilities. These support the maintenance, repair and refit of Swiftsure and Trafalgar Class hunter-killer submarines, which are fitted with PWR1-type NSRP.

The justification for activities in the facility is covered by a number of extant Facility Plant Safety Cases:

  1. PSC-240 14 Dock Facility Plant Safety Case
  2. PSC-250(1) 15 Dock Refuel Docking Safety Case
  3. PSC-250(2) 15 Dock LOP(R) Safety Case

In addition, there is an Interim Safety Justification for Fleet Time Docking (non-refuelling maintenance and upgrade periods) in the SRC which is a modification to the above safety cases.

During certain docking periods, it is necessary to undertake welding operations on the NSRP. A potential fault sequence has recently been identified that had not been fully considered within the SRC safety cases. As a result of identification of this fault sequence, a High Nuclear Safety Significance (Category A) modification to the extant safety cases has been raised. This modification considers the potential of argon gas pressure to cause a potential hazard to the NSRP, which could result in the fault leading to a radiological consequence or damage to the NSRP.

As required under their arrangements for Licence Condition 22(1), DRDL has requested the Executives agreement to implement the Category A Modification for the purpose of enabling the Welding Argon Gas Purge Rig to continue to be used to provide an inert gas purge during welding operations on the NSRP. The Category A modification submission has been through DRDL due process, including Independent Peer Review (IPR) of the safety arguments and the advice of its Management Safety Committee and Nuclear Safety Committee.

Under the HSE/MoD agreement, the regulation of activities at DRDL, including carrying out technical assessment and reaching regulatory judgements is shared with the Defence Nuclear Safety Regulator (DNSR) according to vires and relevant expertise. This Category A modification was assessed by DNSR in relation to activities on the 5 Basin Non-Tidal X-Berth during 2009 and permission granted for use of the Welding Argon Gas Purge Rig at locations within 5 Basin. The current submission seeks to extend the safety justification to cover activities within the SRC.

ONR has considered the Licensees submission and has sought the assurance of DNSR on the aspects of the case where it was not appropriate or necessary for us to reach an independent conclusion.

Assessment and inspection work carried out by ONR in consideration of this request

ONR's assessment of the safety case modification was targeted at consideration of any differences between the use of the Welding Argon Gas Purge Rig in 5 Basin, which has already been permissioned by DNSR, and the SRC. ONR's permissioning decision has been supported by DNSR who have led on the technical assessment of the safety case relating to the NSRP.

Matters that arose from ONR's work and outcome of ONR's consideration

Following ONR's review of the submission, specific assurance was requested from DNSR. ONR has received DNSR's assurance that their assessment of the safety case submission has not identified any issues that would give reason for ONR not to issue an agreement to the activities proposed. However, it has highlighted a caveat that will currently limit the application of this rig to decay heat levels below 6kW. As a result, DRDL's request has been amended such that the Licensee will now only implement the modification for HMS Talent (which has a decay heat level below this level) pending satisfactory resolution of DNSR's concerns. Once these concerns have been addressed, a further Licence Instrument will be required to allow the modification to be implemented on submarines with higher decay heat levels.


As a result of ONR's assessment and DNSR's assurance relating to its assessment, ONR has concluded that the justification provided in DRDL's submission forms an acceptable basis for the proposed modification, enabling the Welding Argon Gas Purge Rig to be used to provide an inert gas purge during welding operations on the NSRP solely on HMS Talent. In considering this request, we have not identified any other factors that would challenge nuclear safety for these activities.


It was recommended that ONR granted permission for DRDL to implement the modification to the justified activities and Licence Instrument 536 was issued on 9 May 2011.