Office for Nuclear Regulation

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Barrow - Criticality Systems SBI Inspection

Executive summary

Purpose of Intervention

This Intervention Record summarises a system based inspection (SBI) on the implementation of safety case documentation with regards to criticality control in the New Core Facility (NCF) at BAE Systems Marine Limited’s (BAESML) Barrow licensed site.

This was a planned SBI, in line with the ONR Integrated Intervention Strategy plan for 2019/20. The intervention was undertaken by ONR criticality and fault studies inspectors; the Defence Nuclear Safety Regulator (DNSR) also participated in the inspection. This intervention was undertaken in accordance with an intervention scope sent in advance of the inspection.

Interventions Carried Out by ONR

The scope of this SBI was to inspect structures, systems and components (SSCs) related to the control of criticality in the New Core Facility, specifically during receipt and build of submarine reactor core components and storage of the completed core prior to transfer to the Devonshire Dock Hall. These SSCs are intended to prevent an inadvertent criticality involving the core or core components. This is achieved by maintaining a safe geometry, by ensuring that neutron poisons remain in their designated place and by preventing accumulation of water which could act as a moderator.

The SBI was carried out in accordance with ONR’s Guidance for Intervention Planning and Reporting (ONR-INSP-GD-059). SBIs are intended to establish that the basic elements of the safety case (as implemented by the relevant systems SSCs) are fit for purpose and will fulfil their safety functional requirements as defined in the safety case. A SBI is not an inspection of the arrangements that the licensee has put in place for each of the relevant licence conditions; instead it is an inspection of the adequacy of their implementation in relation to a particular ‘system’.

The inspection considered compliance against a number of licence conditions to test the adequacy of the implementation of the site safety case. The standard six licence conditions inspected against were:

The inspection was informed by ONR’s Technical Inspection Guides. Each licence condition was rated using the intervention ratings in ONR’s General Inspection Guide (ONR-INSP-GD-064). ONR’s guidance for compliance inspection can be found at

Explanation of Judgement if Safety System Not Judged to be Adequate

Not applicable.

Key Findings, Inspector's Opinions and Reasons for Judgements Made

The following judgements were made:

LC10 (Training) – I examined training records for two ‘Duly Appointed Person’ role holders. This included the BAESML Nuclear Build Process Manager responsible for the facility and the Senior Manufacturing Engineer responsible for supervising the build activity (employed by Rolls Royce Submarine Limited (RRSL), but appointed by BAESML to this role). Records were found to be adequate and I therefore assign an inspection rating of Green for LC10 compliance.

LC23 (Operating Rules)/LC24 (Operating Instructions) - I sampled links between the LC23 Operating Rules (‘operating limits’ using BAESML terminology) identified in the safety case and the relevant LC24 operating instructions. In my opinion there is evidence that the safety case operating limits for core build have been adequately implemented in BAESML’s operating instructions. I found that the safety case requirement and verification records in BAESML’s Admin. Control Document did not provide a clear record of the sequence of acceptances and verifications that were carried out at the time to implement the safety case, however I am satisfied that in this instance the limits have been adequately implemented. To ensure this is addressed for future safety cases I will raise a Level 4 Regulatory Issue. Consistent with findings from previous inspections, I also found an example where BAESML is not keeping records of compliance – this is covered by an existing Regulatory Issue. Overall, I consider the implementation of BAESML’s arrangements for LC23 and LC24 to be adequate for the inspection sample and consider the inspection rating for both to be Green.

LC27 (Safety Mechanisms, Devices and Circuits) – The safety case identifies a single set of Safety Mechanisms, Devices and Circuits (SMDCs); these are the hold down caps used to secure control rods within fuel assemblies. The arrangements for securing the availability of the hold down caps, for example the physical constraints preventing removal of more than a single device in accordance with operating rules, were considered to be adequate. BAESML also identifies a number of crane trips which appear to provide fault protection; these could prevent dropping of core components or impacts onto the build stand. However, these are not formally identified as SMDCs and are not identified as safety measures in the safety case. I intend to follow this up through planned safety case interactions on an existing Regulatory Issue covering BAESML’s process for identification of SMDCs. Overall, compliance against LC27 is considered to be adequate with a Green rating.

LC28 (Examination, Inspection, Maintenance and Testing) –BAESML presented evidence that the Examination, Inspection, Maintenance and Testing requirements of the safety case are being implemented, for example through the sampled maintenance records for the facility drainage and calibration of the crane over-speed trip. I raised a minor observation regarding periodicity of inspection of the facility drainage. Overall, compliance against LC28 is considered to be adequate with a Green rating.

LC34 (Leakage and escape of radioactive material and radioactive waste) - I considered it to be inappropriate to provide a rating for this licence condition due to the limited potential for radioactive release or generation of waste for the facilities considered.

Conclusion of Intervention

Based upon the sample of evidence gathered during the SBI, I consider that the requirements of the safety case with regard to control of criticality during activities in the New Core Facility have been adequately implemented. There are no findings resulting from this SBI that have raised any immediate concerns with regard to nuclear safety.

A minor regulatory observation has been raised regarding periodicity of inspection of the facility drainage; this was shared with BAESML at the time of the inspection and I am content for this to be progressed by the licensee without further involvement from ONR.

In relation to LC23/24, I intend to raise a Level 4 action to ensure that BAESML can demonstrate an adequate process for implementing their safety cases:

Level 4 Regulatory Issue: ‘Review of arrangements for producing and managing the Admin. Control Document and associated implementation of safety case requirements’.