Office for Nuclear Regulation

Hunterston B – Inspection ID: 51007

Executive summary

Date(s) of inspection:

November 2022

Aim of inspection

  1. CA to gain confidence in HNB’s approach to hazardous substance removal and decommissioning of associated plant systems.
  2. CA to understand HNB’s MOC strategy and its implementation where it relates to COMAH.
  3. CA to receive more detailed information on the plans to handover to Magnox, including potential to exit COMAH regime.

Subject(s) of inspection

  • COMAH – Control of Major Accident Hazards Regulations 2015 – Rating: GREEN

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

The purpose of this intervention was to provide regulatory confidence in the management of conventional health and safety hazards present at EDF Energy Nuclear Generation Limited (NGL) Hunterston B power station (HNB). Specifically, the hazard associated with substances defined in the Control of Major Accident Hazards Regulations 2015 (COMAH). The inspection forms part of the Operating Reactors Integrated Intervention Strategy and is part of the annual intervention programme agreed with the facility and follows on from an intervention 1 year ago.

The visit was led by the Office for Nuclear Regulation as part of the Competent Authority (CA), to assess the adequacy of arrangements for the control of COMAH hazardous substances. This inspection sampled planning of work, removal of bulk quantities of flammable gases, isolation and purging activities and the control of any residual hazard remaining until handover of the site.

The objective of the inspection was to verify the removal of flammable gasses and the associated reduction in major accident risk to inform our proportionate and targeted intervention process.

The key regulatory activities undertaken during the inspection, via sampling of arrangements, focused on determining stations compliance in the following areas:

  • Management and control of the removal of flammable gases (specifically substances defined by the COMAH Regulations)
  • Management of change in relation to reductions in resource for roles with responsibilities for COMAH compliance
  • Arrangements for gas sampling and management of chemistry labs

Regulatory advice and judgment were based on determining compliance with the COMAH Regulations 2015 and the associated guidance document, L111.

Overall HNB demonstrated application of the hierarchy of control in line with CA expectations. A structured approach to the removal of hazardous substances was applied through the EDF Engineering Change process. During the site walk down, minor observations made by the CA to the dutyholder were taken on board and commitment made to resolve them, which was accepted by the regulator.

Procedures and techniques for gas sampling and testing were inspected, this aspect of the inspection was unannounced. Good standards were observed, and this aspect of the visit provided confidence in the gas sampling results that underpin purging operations.

The planned reduction of staff in the Chemistry team is subject to Management of Change (MoC) control as part of wider changes to the Operations department. Demonstration of sufficient workload reduction and approval by the MoC steering committee are required before reductions in staff are enacted. CA was satisfied that the risks of reducing Chemistry staffing levels had been assessed and suitable hold points to review the process are in place.

In line with the hierarchy of control, flammable gases have been removed from site and a significant proportion of COMAH major accident scenarios (as defined in the HNB Major Accident Prevention Policy) are no longer credible.

CA judges HNB approach to the reduction of flammable gas inventory and major accident risk to be adequate. A summary of inspection findings, verbal advice and additional document requests was shared with DNB at the close of the intervention. An inspection rating of Green, no formal action has been assigned to this intervention.


An inspection rating of Green, no formal action has been assigned to this intervention.

No regulatory issues were raised.