Office for Nuclear Regulation

System Based Intervention (SBI) of the Essential Services and Supporting SSC's

Executive summary

Purpose of Intervention

The Office for Nuclear Regulation (ONR) undertakes all regulatory interaction with the Aldermaston site licensee (AWE plc) against a strategy defined by the ONR Weapons sub-Division. In accordance with that strategy, a planned System Based Inspection (SBI) was carried out on the Main Production Facility in March 2021.  The purpose of this inspection was for the ONR to determine the adequacy of the implementation of the extant safety case for a sample of the Essential Services in the context of the six Licence Conditions given below.

Interventions Carried Out by ONR

The intervention was performed in line with ONR's guidance requirements (as described in our technical inspection guides) in the areas inspected.

The ONR PTC Facility inspector and an ONR specialist mechanical engineering inspector conducted a SBI of a sample of the Main Production Facility Essential Services (Emergency electrical supplies and inert gas supplies) and their supporting Systems, Structures or Components (SSC’s). The adequacy of the implementation of AWE’s extant safety case for the Essential Services was sampled via compliance inspections against the Licence Conditions (LCs) below.

Explanation of Judgement if Safety System Not Judged to be Adequate

The safety system for the Main Production Facility Essential Services and its associated SSC’s is judged to be adequate.

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

The inspection consisted of a presentation from AWE of the extant safety case documentation for the Essential Systems.  A facility walk down was also carried out during a one and one-half day inspection, where documented evidence was presented and a number of facility operations staff were questioned to determine their level of understanding of the plant, the process, its hazards and the safety mechanisms and devices in place to prevent and/or mitigate against potential fault scenarios.

Discussions with plant operators and maintenance staff demonstrated an adequate understanding of the limits and conditions of operation required to maintain nuclear safety. Training records were examined for a sample of  operational staff and were seen to be completed and in date.  

A sample of Safety Mechanisms, Devices and Circuits were observed on the plant and their maintenance and calibration records examined. All SMDC’s were seen to have a periodic maintenance regime entry on the Asset Management System (AMS).

From the evidence sampled and presented during the inspection we consider AWE to have adequately implemented the claims made within the extant safety case that relate to the Essential Services and its supporting Systems, Structures or Components (SSC’s). There were some areas of concern with the provision of maintenance records from the Asset Management System (AMS) under LC28, as well as with the apparent tolerance of a long-standing defect on a safety related system under LC28.

However, AWE demonstrated adequate compliance with all other LC’s examined during the inspection. A potential concern with the resilience of the facility to an Extended Loss of Grid (ELoG) event was identified during this SBI and will be followed up by a Level 4 Regulatory Issue. This concern does not impact on the rating of this inspection, as strictly it is outside the immediate remit of an SBI. However, it still requires regulatory follow-up to ensure that risks are reduced ALARP in an ELoG event.

Conclusion of Intervention

Based on the evidence sampled during the inspection the overall ratings for the targeted assessment areas are LC 10 – Green, LC 23 – Green, LC 24 – Green, LC 27 – Green, LC 28 – Amber, and LC 34 - Green. Two level 4 Regulatory Issues were raised on concerns identified during this inspection. However, we are still of the opinion that ‘YES’ the system adequately fulfils the requirements of the extant safety case, and therefore the overall SBI rating is ‘Green’ – Adequate.