Hydrogen: planning and consenting guidance

5. Control of Major Accidents & Hazards

5.1. Introduction

The COMAH Regime^[13] is the application of the Control of Major Accident Hazards Regulations 2015, which seek to prevent major accidents involving dangerous substances and limit the consequences to people and the environment of any accidents which do occur. COMAH brings specific reporting and site management requirements, as well as a general duty for operators of sites to take “all measures necessary (AMN) to prevent major accidents and limit their consequences for human health and the environment”. This is achieved through a combination of control measures for prevention of accidents (plant design and construction, and process operation, control, and maintenance systems and procedures), and mitigation measures (plant design and process control systems and procedures, and emergency procedures).

COMAH is regulated and enforced by the ‘Competent Authority’ (CA), which in Scotland comprises the Health & Safety Executive (HSE), or the Office for Nuclear Regulation (ONR) for nuclear establishments, acting jointly with the Scottish Environment Protection Agency (SEPA).

It is important to note that the COMAH Regulations require compliance through the regulation 5 general duty to take AMN, and are not a permissioning or licensed regime. They require continuous monitoring, review and updating of relevant equipment, systems, and procedures, as well as reporting to the CA to ensure safe operation of facilities that involve dangerous substances such as hydrogen.

It is therefore important that major accident hazards (MAH) are considered throughout the lifecycle of a hydrogen project – including planning & design, construction, commissioning, operation, maintenance, and decommissioning. At planning and consenting stage for hydrogen, it is particularly important to demonstrate that design principles around safety and major accident hazards are embedded into the project.

5.2. Tiers & Thresholds: Applying the COMAH Regulations

The COMAH regime is applied to two tiers of establishment – lower tier (LT) and upper tier (UT) depending on the level of dangerous substances such as hydrogen at a site^[14]. Additional duties and more detailed reporting requirements apply to upper tier establishments.

Hydrogen is a ‘Named dangerous substance’ in Schedule 1 Part 2 of the COMAH Regulations and therefore all proposals for the production or storage of hydrogen should review COMAH implications and carefully consider what requirements may apply. This should be undertaken early in the design and planning process, in order that appropriate notifications and reporting can be prepared and submitted to the CA.

Amount of hydrogen present on site

<5 tonnes

• Applicable COMAH Tier: May not be a COMAH Establishment subject to the quantity of other dangerous substances present.

>5 tonnes

• Applicable COMAH Tier: COMAH Lower Tier Establishment – the presence of other dangerous substances may make the establishment Upper Tier.

>50 tonnes

• Applicable COMAH Tier: COMAH Upper Tier Establishment

In addition to the presence of hydrogen, operators must fully consider the presence of other dangerous substances (DS) potentially involved in green and blue hydrogen production either as inputs, carriers, or waste and by-products. This could include methane, methanol, ammonia, oxygen and a range of other dangerous substances each of which have their own defined lower and upper tier thresholds. Generic categories of DS are specified in Part 1 of Schedule 1, and named DS are listed in Part 2 of Schedule 1 to the Regulations. The generic categories in Part 1 are based on those defined in the Classification, Labelling and Packaging Regulation 2008 (CLP) which is referred to in the Seveso III Directive.

A full inventory of dangerous substances must be developed. Inventories should consider the maximum quantity of dangerous substances that is or could be present (actual or anticipated presence of a dangerous substance in an establishment) during normal activities, or which it is reasonable to foresee may be generated during loss of control of the processes, including storage activities, in any installation within the establishment, and consider variations which may occur due to seasonal demand or other fluctuations in business activity.

Where more than one dangerous substance is present at the establishment operators must also carefully consider aggregation of substances using the aggregation rules in Part 3 to Schedule 1 and potential for this to result in lower tier or upper tier classification even if no single substance is above its defined threshold, and thus to determine the relevant thresholds and therefore regulatory requirements. Full detail and examples of the application of aggregation rules is contained within Schedule 1 of the COMAH Guidance on Regulations and Appendix 2.

5.3. All COMAH Establishments: Compliance Requirements

All Measures Necessary & General Duties

For all COMAH establishments the Regulations establish a general duty to take all measures necessary to prevent major accidents and limit their consequences to people and the environment (regulation 5). This principle of ‘all measures necessary’ is the basis of the major accident prevention policy (MAPP, Regulation 7) for the establishment, assessment of risk for human or environmental harm, and therefore the management of risks at an establishment and the safety management system (SMS) required by Schedule 2.^[15] ‘All measures necessary’ should inform the detail in the safety report (SR) and emergency plans (EP).

Prevention should be based on the principle of reducing risk to a level as low as is reasonably practicable (ALARP) for both human and environmental risks. However, wherever possible, the principle is to avoid a hazard and to reduce the risk. ‘All measures necessary’ includes control measures for both preventing major accidents and mitigating the effects of those major accidents. Guidance for application of “All Measures Necessary” in relation to the environment can be found at All measures necessary (sepa.org.uk). Please note that a review on this guidance is underway to be completed by March 2026.

Notifications

All COMAH establishments must notify the CA at key points prior to construction, the start of operations and certain changes to operations (regulation 6). This includes a Pre-Construction Notification before construction begins (suggested timescale 3-4 months before) and a Pre-Operation Notification before the start of operations^[16](suggested timescale 3-4 months before). Notification must also be provided if an establishment expands or alters its inventory of dangerous substances, becomes a COMAH establishment or changes tiers.

The HSE provides a template form for notifications that sets out the key information requirements and how these should be provided. Prior to completing the COMAH Notification Form operators (especially for UT establishments) should contact the CA to discuss the information required. The notification should be issued via email or post using the details below:

Comah.notifications@hse.gov.uk

COMAH Notifications, HSE

Chemicals, Explosives and Microbiological Hazards Division

CEMHD, 2.2 Redgrave Court, Merton Road

Bootle, Merseyside, L20 7HS

Further detail on the requirements for COMAH Notifications is provided in COMAH Guidance on Regulations (L111) for Regulation 6 and in Schedule 2.

Major Accident Prevention Policy & Safety Management Systems

All COMAH establishments are required to prepare and maintain a major accident prevention policy (MAPP, regulation 7). For LT establishments the MAPP is a standalone document, and for UT establishments it may be included (as a separate document) as part of the safety report (SR, further detailed below).

The MAPP should be proportionate in scope and structure to the major accident hazards and is usually a concise document outlining key aims and principles. Required information for a MAPP is typically provided in two parts:

• A policy setting out the aims and principles of action concerning the prevention of major accidents; and
• A summary description of the establishment’s ‘safety management system’ for achieving these aims – which may be through cross-reference and/or summary of other documentation such as health, safety and environmental policies or plant operating procedures.

Unlike notifications, there is no formal requirement to submit the MAPP to the CA, but it should be kept up to date and be available upon request from CA. As noted above, it should form part of the safety report for an UT establishment that is submitted to the CA.

For a new establishment, the MAPP must be prepared within a ‘reasonable period of time’ prior to the start of construction or operation. For most establishments this is normally between three and six months. Thereafter it must be reviewed in the event of changes that may have significant implications on major accident hazards, and at least every five years.

Further detailed guidance on the preparation of the MAPP is provided in COMAH Guidance on Regulations for regulation 7.

Safety Management Systems

The purpose of the safety management system (SMS) is to implement the MAPP through site procedures, management systems, and demonstrate through monitoring and auditing the effectiveness of the control measures, including showing continuous improvement. It can be produced in a format that reflects the hazards and complexity of the establishment, and assessment of the risks, provided that each of the specific requirements in Schedule 2 of the COMAH Regulations are addressed.

Schedule 2 to the COMAH Regulations do not explicitly requires preparation of an internal emergency plan for all COMAH establishments. Requirements for internal emergency plans are set out below.

Unlike notifications, there is no formal requirement to submit the SMS to the CA, but it should be kept up to date and be available upon request from CA. Most inspections at an establishment will focus on the adequacy of the SMS. As noted above, a summary of the principles of the SMS should form part of the safety report for an UT establishment that is submitted to the CA.

Further detailed guidance on the preparation of the SMS is provided in COMAH Guidance on Regulations for Schedule 2.

Providing Information to the Public

Regulation 17 of the COMAH Regulations requires the CA to make information available to the public for all COMAH establishments. This is primarily fulfilled by providing key details about site operations, relevant dangerous substances, and information on public warning in the event of a major accident for inclusion in the CA’s web-accessible database. Additional duties for UT establishments are set out below.

Further detailed guidance on providing information to the public is provided in COMAH Guidance on Regulations for Regulation 17.

Domino Establishments

In its review of COMAH establishments the CA can define a ‘domino group’ where there is potential for an accident (whether major or not) at one establishment to trigger an incident at neighbouring establishments(s), that could be major accidents (regulation 24).

Within a ‘domino group’ risk and hazard management information for each of the respective establishments should be exchanged and documented by operators and all relevant operators must account for the overall combined hazard in emergency planning and risk management.

Further detailed guidance on domino establishments and requirement for coordination between operators provided in COMAH Guidance on Regulations for Regulation 18.

Hazardous Substances Consent

HSC is separate from compliance with COMAH Regulations but will be required for all hydrogen projects which are COMAH Establishments. It is important to note that the thresholds for HSC and COMAH may be different (this is particularly the case for hydrogen), so sites which do not meet COMAH thresholds may still require HSC. Preparation of HSC applications should be advanced as early as possible and in parallel to pre-construction and pre-operation requirements. Further detail on the HSC process is provided in Section 6 of this Guidance.

5.4. COMAH Upper Tier Sites: Additional Requirements

Safety Report

In addition to the MAPP and SMS, operators of UT establishments need to produce a safety report (regulations 8 to 10) and submit this to the CA as part of their demonstration that all measures necessary have been taken to prevent major accidents. In summary, the safety report is required to include the information specified in Schedule 3 paragraphs 2 to 6 to the COMAH Regulations.

New hydrogen projects are advised to liaise with the CA early in the preparation of safety reports to ensure clear understanding of requirements, timings and process based on the specific nature of their site. For new projects the safety report is typically produced in two stages.

A Pre-Construction Safety Report should be submitted a reasonable time before construction begins - typically 3-6 months before construction but variable depending on the scale and complexity of the development. At pre-construction stage the report should focus on conceptual design, selection process options and selection methods, the hazard and risk approach of the operating company, assessment of risk, what prevention and control measures are to be included, establishment layout and positioning of vessels, buildings, and descriptions of the surrounding environment including populations. The available information will vary between projects, but as much detail as possible should be included. It may be appropriate to provide a ‘rolling’ submission of pre-construction documents in liaison with the CA.

A Pre-Operation Safety Report is required to be submitted and assessed, before operations begin on site - typically 3-6 months before commencement of operations^[17]. The Pre-Operation Safety Report should update and add detail to the Pre-Construction Safety Report – ensuring outstanding issues have been resolved and including any elements not available at pre-construction stage to demonstrate that AMN have been taken.

All safety reports must be assessed by the CA and conclusions reached before the establishment begins to operate. Inspectors from the CA will provide advice to new operators on their proposed safety and environmental control arrangements and specific information required. The overall deadline for completion of a ‘full’ assessment will vary depending on the nature and complexity of the establishment, and the quality of information submitted.

Safety Report Review & Update

Where the operator proposes to modify the plant, the safety report must be reviewed and if any amendments are required, re-submitted to the CA (Regulation 10(2)(d)).

In addition, the safety report must be reviewed by the operator: following a major accident at the establishment; where a review is justified by new facts or by technological knowledge about safety matters, including knowledge arising from analysis of accidents or near misses; where a review is justified by developments in knowledge concerning the assessment of hazards; and at least every five years (Regulation 10(2)). For larger or more complex establishment the review for any of these purposes may be preceded by a pre-review discussion with the CA to discuss any changes and jointly review requirements.

Further detailed guidance on the preparation of safety reports is contained within the COMAH Guidance on Regulations (2015) for Regulations 8, 9 and 10 and in Schedule 3. The following links and references are also relevant to the preparation of safety reports:

• Safety Report Assessment Guidance (Technical Aspects)
• COMAH 2015 Safety Report Assessment Manual (SRAM)
• Preparing safety reports: Control of Major Accident Hazards Regulations 1999 (COMAH)

Emergency Plans (EP)

UT COMAH establishment operators are also required to prepare an internal EP, with the local authority required to prepare an external EP based on information provided by the operator (regulations 11 to 14), to ensure that the consequences of a major accident are minimised if it does occur.

Internal Emergency Plan

The internal EP should cover the full range of possible major accidents, including the operator’s response to reasonably foreseeable low probability, high consequence events. Consideration must be given to potential on-site consequences to people, environment, and property, and how to initiate the internal EP and escalate to initiation of the external EP, assist and link with off-site mitigation action.

It should include how the operator would communicate to the public, emergency services and other authorities in the area so that they may determine what actions they need to take. Other authorities will be defined based on the nature, scale and location of the site, but will typically include the Emergency Services, Local Authority, SEPA, Local Health Board, and Scottish Water. In preparing the internal EP, the Operator must consult these authorities and agencies to ensure coordination and understanding of arrangements in the event of a major accident.

For new COMAH establishments or sites moving from LT to UT, the internal EP should be prepared alongside the SR. It must be prepared before the start of operation, or when any inventory changes or modifications are made. It should be reviewed at least every three years, considering changes at the establishment or with the relevant emergency services, and any relevant new technical knowledge or best practice. Testing of the plan and relevant communication arrangements should be undertaken to inform the review process.

Further detailed guidance on the preparation, review and testing of the internal EP is provided in COMAH Guidance on Regulations for Regulations 11 and 12 and in Schedule 4.

As noted above, the COMAH Regulations require preparation of an internal EP for LT establishments as part of the SMS.

External Emergency Plans

To complement the internal EP (prepared by the operator), the Local Authority in whose administrative area an UT establishment is located must prepare, review and test an external EP. This will specify the measures to be taken by organisations and authorities other than the operator, either on the establishment, or outside the establishment in the event of a major accident.

As early as possible before the commencement of operations, the operator should consult with the Local Authority and initiate sharing of information necessary to inform preparation of the external EP and ensure this is fully coordinated with their internal EP.

The external EP will detail the roles to be carried out by emergency services, local authorities and other external agencies in the event of a major accident. The Local Authority will consult with these agencies in preparing the plan and must also consult the public. This could include consultation with elected councillors and/or with established groups representing nearby residents.

Further detailed guidance on the preparation, review and testing of the external EP is provided in COMAH Guidance on Regulations for regulations 13 and 14 and in Schedule 4.

Providing Information to the Public (Upper Tier Establishments)

For UT establishments the CA will define a Public Information Zone (PIZ) around the establishment for which there is an additional duty for the operator to provide specific information to the public.

This must be provided pro-actively by the operator to all premises (including schools, hospitals, workplaces, etc) and residences within the PIZ, using straightforward and simple language (regulation 18). It should include details of the dangerous substances, possible major accidents and their consequences, how members of the public will be alerted to an incident occurring at the establishment, and what to do in the event of an accident. Specific details of information provided, and its distribution should be reviewed with the CA as well as the relevant local authority.

No time period is specified in the Regulations, however the provision of the information is one of the mitigation measures under AMN, and therefore the information must be supplied prior to the commencement of operations.

Further detailed guidance on providing information to the public for UT establishments is provided in COMAH Guidance on Regulations for Regulation 18.

Domino Establishments (UT)

Where they are UT establishments, operators in the same ‘domino group’ must coordinate with others in the group to inform the public and neighbouring properties (regulation 24(5)). This can be addressed as part of duties under Regulation 18 for UT establishments to notify the public within a defined PIZ. UT establishments must also ensure that they give the Local Authority the information needed to take account of ‘domino’ effects in their external EPs.

Further detailed guidance on domino establishments and requirement for coordination between operators provided in COMAH Guidance on Regulations for Regulation 18.