Office for Nuclear Regulation

AWE Aldermaston – Inspection ID: 51976

Executive summary

  • Inspection date(s): October 2022

Aim of inspection

To judge the adequacy of AWE’s implementation of HEPA filter replacement, confirming correct installation to support through design-life operations.

Subject(s) of inspection

  • LC28 – Examination, inspection, maintenance and testing – Rating – Green

Key findings

The purpose of this joint Environment Agency and ONR intervention was to obtain evidence towards closure of the outstanding Level 3 regulatory issue, on the management of aged HEPA filters by the Atomic Weapons Establishment Plc. This intervention was conducted against Licence Condition (LC) 28 (Examination, Inspection, Maintenance and Testing) and ONR’s Technical Assessment Guidance on ventilation.

LC28 relates to the licensee making and implementing adequate arrangements for the regular and systematic examination, inspection, maintenance and testing of all plant which may affect safety. ONR’s Technical Assessment Guidance on ventilation provides guidance on relevant good practice for ventilation systems including HEPA filters.

Licence conditions are inspected against ONR’s Technical Inspection Guides (TIG). ONR Technical Assessment Guides can be found at Office for Nuclear Regulation (ONR) Permissioning inspection – Technical assessment guides. ONR’s guidance on the rating of interventions can be found at ONR-INSP-GD-064 (Issue 5.3) – October 2025 – General Inspection Guide.

The inspection was conducted at a facility on the Aldermaston site that has had its HEPA filters replaced as a part of the organisation wide aged HEPA filter replacement project. This is the first of two interventions which will seek to collect evidence towards closing the Level 3 regulatory issue. This first intervention seeks evidence against the implementation of HEPA filter replacement, confirming correct installation to support through design-life operations. This intervention was undertaken via desktop discussions, semi-structured interviews with relevant AWE personnel and sample review of relevant documents and records.

We found that the organisational examination, inspection, maintenance, and testing arrangements at the facility were adequate and were being used effectively for the tasks that we sampled. We were satisfied that the arrangements for installation were adequate and that, with the exception of some shortfalls, relevant good practice was being followed, which supports design-life operations.

We made note of several findings against AWE’s management of HEPA filters. These related to lack of temperature-controlled stores for HEPA filters; AWE’s control of written arrangements derived from national forums and other licensees’ guidance; the premature degraded state of rubber gaskets on HEPA filter housings; and missing non-conformity justifications to support the validity of type testing certificates.


We judge that AWE has adequately demonstrated compliance against LC 28 – examination, inspection, maintenance and testing for the chosen sample of HEPA filters in the A** facility, meeting relevant good practices and providing supporting evidence. We further judge that AWE has adequately implemented HEPA filter replacement to support through design-life operations. Specifically, we note that where we sampled, our expectations (derived from the TIG for LC 28 – ONR Ref. No.’s NS-INS-INSP-GD-028 and the TAG for ventilation ONR Ref. No.’s NS-TAST-GD-022) were met, for example:

  • Maintenance has been carried out in accordance with a plant maintenance schedule and has been completed within the stated timescales.
  • Maintenance documents have been completed to the detail required by the arrangements.
  • Maintenance records are stored appropriately and are readily available.
  • HEPA filters have been type tested by manufacturers to ensure they meet GB nuclear standards.
  • The licensee has arrangements in place to perform appropriate, systematic pre-installation inspection of filters to confirm no damage or defect is present and the filter type is appropriate for the installation to minimise risks of unnecessary, contaminated, radioactive waste arising.
  • Filters are stored with clear labelling providing storage instructions, handling, and stacking requirements.
  • Evidence for the traceability of filters and filter housings are being maintained, and systems are in place to ensure that filters do not exceed their shelf/ design lives.

We therefore consider, noting the ONR guidance on inspection ratings, that an inspection rating of GREEN (no formal action required) is merited for the licence condition inspected.

The outcomes from this inspection will form part of the body of evidence which is taken to the programme sub-board with regards to the Level 3 regulatory issue.