Office for Nuclear Regulation

Atomic Weapons Establishment (AWE) Aldermaston – Inspection ID: 50384

Executive summary

Date(s) of inspection:

  • November 2022

Aim of inspection

In accordance with the Office for Nuclear Regulation’s (ONR’s) Atomic Weapons Establishment (AWE) Strategy, each year ONR performs a series of planned targeted compliance inspections associated with ONR’s safety and security regulatory purposes. This conventional safety compliance inspection was undertaken to assess if the high voltage (HV) site electrical distribution system equipment at AWE’s Aldermaston site is being suitably and sufficiently maintained to meet the requirements of the relevant statutory provisions of the Health and Safety at Work etc Act 1974.

Subject(s) of inspection

  • Health & Safety at Work Act – Rating: Amber

Key findings, inspector’s opinions and reasons for judgement made

This was a planned conventional safety compliance inspection conducted on 2 November 2022 at AWE’s Aldermaston site. The inspection assessed if the processes in place to maintain HV electrical distribution system equipment are in line with the practices contained in relevant regulatory guidance which reflect the requirements of the relevant statutory provisions of the Health and Safety at Work etc Act 1974.

The key elements of the inspection were as follows:

  1. AWE explained and evidenced how the HV electrical distribution systems equipment maintenance strategy is derived and verified that it is being effectively implemented.
  2. We witnessed the annual maintenance activities being undertaken on an HV transformer and sampled substation maintenance records.
  3. AWE explained and evidenced its processes for HV equipment condition assessment, investment planning and for ensuring maintainer competency.
  4. AWE explained and evidenced progress with the improvement activities resulting from its investigation report associated with Aldermaston Substation 5 failure event (INF-1663).

The inspection was carried out in line with ONR’s inspection guidance ONR-INSP-GD-064.


Whilst recognising that AWE is endeavouring to make relevant safety improvements, based on the evidence sampled at the time of the inspection, I judge that AWE was unable to demonstrate suitable and sufficient arrangements for compliance with legal requirements to prevent danger arising from its population of HV electrical distribution transformers. Noting the ONR guidance on inspection ratings, we judged that an inspection rating of Amber (seek improvement) was appropriate. ONR will hold AWE to account to identify, plan and implement the improvements by the addition of a new action in the existing Level 3 regulatory issue RI-10941 (Action 3). This will require AWE undertake optioneering and develop and present its case that it has suitable and sufficient arrangements for preventing, so far as is reasonably practicable, potentially dangerous failures in its population of HV transformers due to deterioration in the performance of the insulating oil, in accordance with the Electricity at Work Regulations 1989.

During the inspection AWE presented its argument for continuing to operate its population of HV transformers. Given these arguments and given the HV transformer play an important role keeping facilities on the Aldermaston site within their normal safety operating conditions I judge it would be disproportionate to require their de-energision whilst it improves its case regarding their safety management.