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Hinkley Point C: Fire Protection Design

Author : Frank Brooks, Fire Specialist, NNB GenCo

03 October 2014

EDF Energy is currently developing the detailed design of a twin-reactor nuclear power plant to be built at Hinkley Point near Bridgwater in Somerset. It is essential that all nuclear power plants are well protected from fire hazards and do not compromise nuclear safety if a fire was to occur. Designers must take into account modern design practices and standards, current conventional safety regulations and stringent nuclear safety requirements.
Artist's impression of Hinkley Point C - Image: EDF
Artist's impression of Hinkley Point C - Image: EDF

The design of the plant has considered protection from fire in great detail incorporating learning and features from other nuclear plants and the design has considered protection and prevention of fire through the application of the ETC-F code for fire protection. For the UK, this is the first time the fire safety design has been based on this specific code of practice that incorporates nuclear safety as well international standards and best practice.

EPR Design

Nuclear power reactor designs can be categorised by generation. Early prototype reactors are Generation I and most reactors in operation today are Generation II. Advanced reactors, the most modern reactors built about the turn of the century, are Generation III reactors. For Generation III+ reactors, the fundamental design is unchanged but the reactors are designed to be more efficient and incorporate enhanced safety features.

EDF Energy owns and operates eight nuclear power stations in the UK. Most of these are Advanced Gas-Cooled Reactors (AGRs) which use carbon dioxide as a coolant and graphite as a moderator. The exception is Sizewell B which is a Pressurised Water Reactor.  Sizewell B started generating in 1995 and was the last new nuclear power station to be built in the UK.

The EPR is a Pressurised Water Reactor and a Generation III+ reactor. It has an evolutionary design that has been developed to give enhanced safety using the operational experience and feedback from existing plants. The aim of the design is to incorporate the best features of the N4 reactors built in France and KONVOI reactors from Germany. An example is the design of the safeguard systems, which have four safety trains in common with the Konvoi reactor and incorporate containment spraying in common with the N4 reactor.

The enhanced safety features of the new design cover both the normal operation of the plant and also provide extra protection in the event of particular incidents. The plant design reduces the possibility of accidents occurring by reducing the frequency of initiating events, for example by improved design of the primary circuit to reduce the potential for loss of coolant accidents. The new design also aims to increase the availability of safety systems (in the unlikely event of severe accidents) to ensure that if accidents do occur, nuclear safety is not compromised. Supplementary design features have been included to prevent radiological releases outside of the reactor building containment in the event of a severe accident. For example, certain buildings on the site itself must be able to withstand and protect against external hazards. The reactor building and other safety classified buildings have been designed to be robust to extreme events such as earthquakes and aircraft crash scenarios.

Industrial accidents are sometimes caused from one catastrophic event, but may also occur when multiple lower-consequence events occur in succession. The use of Probabilistic Safety Analyses at the design basis phase has allowed the identification of multiple failure sequences that could lead to core damage accidents. Such multiple failure sequences were then considered from the outset in the design basis analysis to engineer safety features that can be used to prevent accidental damage to the core.

A new feature of the EPR design is the corium spreading area which provides a means of retaining fuel and introducing passive cooling in the event of a severe accident.  The idea is that in the event of a failure of the reactor pressure vessel, the corium (the molten mixture in the nuclear reactor core formed during a meltdown) is directed via a discharge channel into a specially designed pit which is lined with refractory bricks.

Other safety features include the containment heat removal system which maintains pressure below a set limit within the reactor building in the event of a severe accident, and the combustible gas control system which features passive hydrogen recombiners.

Safety systems must have back-ups that are available if one system were to fail. This can be achieved by duplicate pieces of equipment i.e. four way redundancy or functional diversity where different technology or equipment is used, and therefore provides protection if there was a common fault in the safety equipment.

EPR cutaway - Image: AREVA
EPR cutaway - Image: AREVA

In common with some other Pressurised Water Reactor designs such as Sizewell B, there are four redundant divisions (or “safety trains”) for the main safety systems and the design also incorporates functional diversity to protect against common mode failures.

Fire Protection

Fire can be an initiating event and has the potential to damage safety-related equipment essential for the operation of the site. The fire protection design must ensure that, in the event of a fire, the plant can reach and maintain a safe shutdown state.

The design for fire protection in the new EPR is based on the ETC-F code, developed by EDF SA and Areva and now available through AFCEN as an international EPR Technical Code for Fire Protection. Whilst there is the IAEA code[1] which sets established principles for fire protection in nuclear power plants, ETC-F is a detailed design code that gives the methodology to be applied to achieve those principles for the plant design.

For the detailed design phase of the UK EPR, a UK Companion Document which captures the UK context in terms of fire protection regulations, is being used in conjunction with the ETC-F.

The code details the safety requirements, the fire protection philosophy, the construction provisions, specific requirements relating to the provision of fire protection systems and the installation of electrical cabling with regard to fire risks.

The scope of the code covers nuclear safety, life safety, environmental protection and asset protection. The fire protection philosophy is based on three layers of defence; fire prevention, fire containment, and fire control.

Fire Prevention

Fire prevention is achieved through four methods; fire load reduction, segregation, separation, and elimination or isolation of ignition sources. Fire load reduction includes consideration of combustible materials both in the building design, equipment use and other materials on site. Combustible materials are strictly controlled on nuclear sites so from an operational perspective fire loads should always be monitored. Segregation isolates the high hazards and allows special precautions to be taken for those hazards. Separation of fire loads can be either physically through a barrier or geographically, preventing combustible materials from being involved in a fire. Where possible, ignition sources are eliminated or isolated from combustible materials.

ETC-F specifies the construction materials which can be used, which are largely non-combustible, and gives some requirements for the mechanical and electrical installations which include, amongst others, the following aspects:

The EPR is designed to contain the core after a meltdown
The EPR is designed to contain the core after a meltdown

·  Oil containment.

·  Fire properties of cables.

·  Electrical installation.

·  Piping layout.

·  Tank and storage layout.

For buildings which are designed to withstand the effects of external hazards, such as earthquakes, the plant equipment must be designed so as not to release combustible materials or to create an ignition source in the event of seismic movement or other external hazards.

Fire Containment

The benefit of the ETC-F approach to fire containment is that there is a systematic way to identify fire compartmentation requirements and the purpose of any particular fire compartment is clear. By classifying compartments the code allows a structured and rigid assessment of ‘zones’ which then have specific requirements according to their function. The table below details the main types of fire compartments outlined in ETC-F.

The requirements also apply to doors, hatches, service penetrations, ducts and dampers – basically to any element of the compartment boundary. For structural elements the fire resistance requirement is for fire resistance of structure in addition to the integrity and insulation requirements.

In cases where it is not practicable to fully enclose an area to form a fire compartment then “fire cells” may be used, which are designed to give full fire protection without the area being fully enclosed. They are also defined in the ETC-F as: safety fire cells, storage cells and unavailability limitation fire cells. Fire cells are separated by distance (or using thermal shields) and an analysis must be undertaken to ensure that the fire cannot spread and cannot have any adverse effects beyond the fire cell notional boundaries.

The use of the Intervention Fire Compartment (SFI) detailed above is a very practical way of reducing overall fire risks. The approach is similar to that taken in conventional fire safety design. For example Approved Document B[2] which provides general guidance on the fire safety design of buildings, defines a “place of special fire hazard” which should be a separate fire compartment with 30 minutes fire resistance. Another example is BS 9999[3] which refers to high fire risk areas and provides a list of typical areas with recommendations for fire resistance which can be 30 minutes or 60 minutes depending on the hazard.

According to ETC-F an Intervention Fire Compartment (SFI) should be formed if the room is designated as one in which there is a Possibility of a Generalised Fire (PFG), The code gives the criteria for when a room is designated a PFG room. The criteria relates to the amount of cabling installed, the amount of combustible liquid or the amount of solid fuel. For combustible liquids the PFG criteria is different depending on whether the fuel is contained within operating machinery or whether it is simply being stored. Rooms not classed as PFG may be designated as a room in which there is the Possibility of a Localised Fire (PFL) or a room that is neither PFG nor PFL.

Fire containment compartment types
Fire containment compartment types

This methodology is well adapted to the types of fire hazard found on the nuclear power plants. It provides a consistent approach to identifying higher fire hazard areas, and where additional fire compartmentation can be provided to reduce overall fire risks. The designation of rooms in accordance with these criteria makes it possible to see at a glance where the higher fire risk areas are.

Fire Control

Fire detection and fixed fire fighting systems are used to bring a fire quickly under control and limit the consequences of the fire.

ETC-F is consistent with the principle of the hierarchy of risk control in that it states that passive fire protection measures should be adopted in preference to active fire protection systems. Therefore the safety case for Hinkley Point C is typically based upon ensuring adequate fire containment without being reliant upon the operation of active fire protection systems. Additionally the fire protection systems are engineered to be very robust and are essentially based on systems that are installed on conventional sites. This brings the benefits of being able to use tried and tested products which are easier to maintain.

The systems are designed for high reliability, for example, the water supplies for the fixed fire fighting systems on the Nuclear Island of the plant are designed with the same four-way redundancy as the nuclear safety related systems. There are four fire pumps (in this case electric pumps), each pump belonging to a particular safety train which is backed up by the diesel generator associated with that safety train.

The ‘single failure criterion’ is applied to all active items of the fire protection systems which perform a safety function so that if one component fails the system is still able to perform its safety function.

ETC-F also defines the criteria for when fixed fire suppression is required and also specifies the key parameters of the system such as the discharge density and area of operation for the water based fire suppression systems.

Nuclear Safety

The ETC-F code is a nuclear safety code and is supplemented by a whole raft of other related nuclear safety codes and fire safety standards which form the basis of the UK EPR design.

The general UK approach to nuclear safety classification which is outlined in the Safety Assessment Principles for Nuclear Facilities[4] has been adopted in the design of the UK EPR. This is a means of classifying structures, systems and components according to their significance with regard to nuclear safety. Once classified, the required reliability and integrity of the structures, systems and components can be determined. This is applicable to fire protection elements such as fire barrier elements and also to fire protection systems.

EPR Reactor and fuel buildings - Image: AREVA
EPR Reactor and fuel buildings - Image: AREVA

The fire protection measures are determined by applying the code at the design stage of the plant. Part of the design process is to perform a vulnerability analyses. A vulnerability analysis is performed to search for potential common mode failures and to carry out a functional analysis of the consequences of these failures by taking into account the fire hazards. The analysis is then used to identify any further protection measures required for specific scenarios, even if they are extremely unlikely to occur.

Whilst the three layers of defence; prevention, containment and control, provide protection from fire, particular attention is paid to fire containment. Each fire zone has a defined fire load and for the operational site the fire load must be controlled. The operator must ensure that the designated allowable amount of combustible material is not exceeded and that new types of combustible materials are not introduced.

The code has a specific methodology to ensure the adequacy of the fire barrier elements and this is an important verification for the fire barriers which have nuclear safety significance. This is another example of where the code provides consistent, detailed methodologies that consider specifically the nuclear industry.

The fire barrier elements are typically tested in accordance with the relevant European standard. This means that the heating regime to which the element is exposed is based on a standard fire curve which may not be representative of a real fire. One example is where there is a high level of compartmentation and a fire is ventilation controlled. In this case the duration of the burn may be for several hours and the time-temperature profile for such a fire differs greatly from the standard test fire.

Using data taken from the fire testing of a particular product, the response of the equipment is modelled for a number of different heating regimes and a performance diagram developed to show the behaviour of the equipment when subject to the specific heat regime. For the specific fire compartment being studied, fire modelling is performed to develop a reference fire curve which corresponds to a real fire. The adequacy of the fire barrier element can be verified by comparing the reference fire curve to the performance diagram for the fire barrier element.

This methodology makes use of advanced fire engineering analysis tools such as computer modelling of compartment fires and finite element analysis techniques to model the response of the fire barrier elements. It is a new approach to verifying the adequacy of fire compartmentation which goes beyond that which has been done in the past.

One further point to add is that when the fire barrier has nuclear safety significance, the single failure criterion is applied to the active fire barrier elements. Therefore two fire dampers are installed in series in the Safety Fire Compartment boundaries. Fire barrier elements in these boundaries are also seismically qualified.

Conclusions

The EPR has a robust fire protection design which is based on the ETC-F code. For the UK EPR, UK regulatory requirements with regard to fire safety have been detailed in a UK Companion Document. Both the ETC-F and the UK Companion Document make up the fire protection code which is used in the detailed design phase of the UK EPR. This code is a nuclear code and is relevant to the potential fire hazards typically found at a nuclear power plant. 

Photo: Alan Franck
Photo: Alan Franck

The use of the code brings a clear, systematic approach to the design and provides the designers with specific methodologies to develop and assess the design with regard to fire safety. Some specific methodologies are unique to the nuclear industry and provide a very robust means of verifying the design.

The ETC-F code is consistent with the principles given in the International Atomic Energy Agency safety guide Protection against Internal Fires and Explosions in the Design of Nuclear Power Plants[1] and, in fact, the assessment of the UK EPR design against this guidance formed a part of the Generic Design Assessment.

As the first commercial nuclear power plant to be built in the UK since Sizewell B was completed in the 1990s, the construction of Hinkley Point C will be a challenge. Conventional safety requirements have changed significantly since the existing nuclear power stations were built and using the ETC-F code, designers now have a systematic way to approach the design which takes into account modern design practices and standards, current conventional safety regulations and the stringent nuclear safety requirements.

References

[1] IAEA (2004), Protection against Internal fires and Explosions in the design of Nuclear Power Plants, NS-G-1.7, International Atomic Energy Agency.

[2] HM Government (2013), Approved Document B. Volume 2 – Buildings other than dwellinghouses.

[3] BS 9999 (2008), Code of practice for fire safety in the design, management and use of buildings. British Standards Institute. 

[4] Health and Safety Executive (2006), Assessment Principles for Nuclear Facilities.

About the author:

Frank Brooks is the fire specialist working for the Design Authority within NNB GenCo and is responsible for the technical ownership of the fire protection and assessment for the new reactor design. Previously, he worked for a London-based fire safety consultancy as a Principal Fire Engineer and prior to that specialised in working on fire protection systems. Brooks has experience of working in a range of industries including the power generation, industrial, commercial and transport sectors.


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