Executive summary
Date(s) of inspection:
- September 2025
Aim of inspection
Hunterston B, a lower-tier COMAH establishment, informed ONR of their intention to notify the Competent Authority (ONR & Scottish Environmental Protection Agency) that their holdings of dangerous substances will decrease below the COMAH lower tier threshold. A COMAH denotification was subsequently received on 15 July 2025.
The aims of this inspection are:
- to verify the removal of COMAH relevant dangerous substances, such as hydrazine, ammonia, hydrogen, methane, oxygen and fire resistant fluid (FRF) and the reduction to below COMAH qualifying quantities of sodium hypochlorite and diesel, and verify the condition and status of the residual plant and equipment.
- to provide reassurance on the approach taken to removing dangerous substances e.g. purging and clean down of pipework.
- to provide a judgement on the control of any residual risk, e.g. storage of remaining bulk chemicals, DSEAR arrangements, emergency arrangements
This inspection is being conducted jointly by ONR and SEPA.
Subject(s) of inspection
- COMAH - Control of Major Accident Hazards Regulations 2015 - Rating: Green
Key findings, inspector's opinions and reasons for judgement made
I conducted an inspection at Hunterston B power station (HNB) jointly with the Scottish Environment Protection Agency (SEPA), as the Competent Authority (CA). The inspection sought to verify the holdings of Control of Major Accident Hazards Regulations 2015 (COMAH) relevant dangerous substances on site to confirm the information contained in the COMAH denotification received in July 2025. The station has been defueled and is preparing for decommissioning. Significant quantities of dangerous substances have been removed which has brought the site to below the lower tier threshold for COMAH.
The site intervention centred on verifying the removal or reduction of key dangerous substances, that are either named substances or have the appropriate classification and labeling, to be considered ‘COMAH substances’.
The areas of site sampled were:
- Hydrazine and Ammonia tanks
- Hydrogen storage area
- Diesel tanks
- Oxygen storage area
- Methane tank
- Sodium hypochlorite tank
Regulatory judgment was based on determining compliance with COMAH including the associated guidance The Control of Major Accident Hazards Regulations Guidance on the Regulations (L111).
Conclusion
Based on the information provided in advance of the inspection and the site intervention, I judge the arrangements seen for the removal and reduction of the dangerous substance inventory, pursuant of the COMAH regulations as adequate. The operator has formally submitted its denotification from COMAH and as such is no longer considered to be COMAH operator by the CA.
The licensee was able to demonstrate evidence of the actions and arrangements that have been undertaken to exit COMAH, and those that are planned as decommissioning progresses.
A couple of observations were noted verbally on the day and are captured under Observations/Advice.
Based on my sample, an inspection rating of Green has been assigned to the inspection.