1 Introduction
1.1 Purpose
1.1.1 This guide explains and refines the relevant clauses of HAF 102 Safety specifications for design of nuclear power plant (hereinafter referred to as Specifications), and provides guidance on the design of fire and explosion protection in nuclear power plant for the nuclear power plant designer and licence applicant.
1.1.2 The annexes to this guide are informative.
1.2 Scope
1.2.1 This guide is applicable to land-based stationary thermal-neutron reactor nuclear power plants. It may be referenced by other types of nuclear power plants in terms of their internal design of fire and explosion protection.
1.2.2 This guide only covers the internal design measures of fire and explosion protection adopted to protect items important to safety of nuclear power plants, excluding the general requirements for fire control, personnel safety protection and property protection in nuclear power plants.
1.2.3 The contents related to explosion protection in this guide focus on the protection of explosion caused by flammable liquids and gases released from systems and components of nuclear power plants, but not the protection of system and component explosion. The explosion protection problem of systems and components shall be solved through their own design.
2 General principles
2.1 General
2.1.1 The Specifications specifies basic requirements for the fire-fighting system of nuclear power plants. In the design and layout of structures, systems and components which are important to safety in nuclear power plants, attention shall be paid to reduce the possibility and result of internal fires and explosions caused by internal and external events. The ability to shutdown, remove waste heat, confine radioactive substances and monitor the status of nuclear power plants shall be held. The following objectives shall be achieved by using an appropriate combination of multiple components, various systems, physical isolation and fault safety:
(1) prevention of fire;
(2) rapid detection and extinguishing of the occurred fire, thus limiting the fire damage;
(3) prevention of the un-extinguished fire from spreading, thus minimizing its influence on the system that implements important safety functions.
2.1.2 The design of fire protection in nuclear power plants shall meet the following requirements:
(1) minimizing the probability of fire occurrence;
(2) realizing early detection and extinguishing of fire through combination of automatic and/or manual fire control;
(3) preventing fires from spreading by using fire barriers and physical members or by spatial isolation.
2.1.3 The design of explosion protection shall follow the following steps:
(1) prevent explosion;
(2) if the explosive environment is inevitable, minimize the risk of explosion;
(3) adopt design measures to limit the consequence of explosion.
Where steps (1) and (2) cannot be implemented, step (3) shall be adopted.
2.1.4 Multiple safety systems shall be included in the design of nuclear power plants to prevent postulated initiating events (e.g. fire or explosion) from disturbing safety systems to implement specified safety functions. Where the multiplicity and diversity of safety systems are reduced, protective measures against fire and explosion shall be strengthened for each safety system. In terms of fire, its protection is generally realized by passive protection, improvement of physical isolation and/or application of more automatic fire alarm systems and fire extinguishing systems.
2.1.5 The design of fire protection shall be carried out according to the following assumptions:
(1) fires may occur at any place where fixed or temporary combustible materials are present;
(2) the fire occurred at the same time is taken as a single event and subsequent fire spread shall be considered as its constituent part;
(3) fires may occur in nuclear power plants under any normal operating state.
In addition, consideration shall be given to fires and the combination of postulated initiating events that may be independent of fires (see 2.5).
2.1.6 Fire hazard analysis shall be conducted to prove that the design of nuclear power plants meets the safety objective specified in 2.1.1. The scope and guidance of fire hazard analysis are detailed in 3.5.
2.2 Fire prevention
2.2.1 The nuclear power plants shall keep at a reasonable and feasible minimum fire load. The material of nuclear power plants shall be non-combustible as much as possible, otherwise flame retardant.
2.2.2 The number of ignition sources shall be minimized.
2.2.3 For the systems of nuclear power plants, their design shall guarantee as much as possible that no fire will be caused by system failure.
2.2.4 For items important to safety whose function failure or fault may cause unacceptable release of radioactive substances, corresponding protective measures shall be taken to protect them from fire hazards caused by natural phenomena such as lightning.
2.2.5 Design measures shall be taken for the proper storage of temporary combustible materials in operation to keep them away from items important to safety or necessary protective measures shall be taken. For the guidance on fire protection during the operation of nuclear power plants, see relevant guides given in Fire safety for the operation of nuclear power plant.
2.3 Automatic fire alarm and fire extinguishing systems
2.3.1 There shall be automatic fire alarm and fire extinguishing systems, and other necessary systems determined by fire hazard analysis (see 3.5). In case of a fire, the automatic fire alarm and fire extinguishing systems shall give an alarm and/or extinguish the fire in time and minimize the adverse influence of fire on items important to safety and working personnel.
2.3.2 The fire extinguishing system shall be able to start automatically when necessary. The design and layout of fire extinguishing system shall guarantee that its operation, breakage or misoperation will not (1) affect the function of structures, systems and components important to safety, (2) damage the protective measures of critical accidents and (3) affect multiple safety systems at the same time, so as to ensure the effectiveness of measures adopted for meeting single fault criterion.
2.3.3 Consideration shall be given to the possibility of fire extinguishing system fault and the influence of system effluent from places adjacent to fire compartments or adjacent fire cells.
2.3.4 Appropriate emergency lighting and communication equipment shall be set up to ensure the smooth implementation of manual fire extinguishing action.
2.4 Fire confinement and mitigation of fire consequences
2.4.1 Multiple components of safety systems shall be fully isolated to ensure that the fire will only affect a certain series of the safety systems and will not prevent another redundant series from performing safety functions. Each redundant series of safety systems may be placed in an independent fire compartment, or at least in an independent fire cell, to achieve the above objectives (see 3.3-3.4). The number of penetrating components between fire compartments shall be minimized.
2.4.2 The consequence of hypothetical fires shall be analyzed for all areas provided with safety systems and other parts posing fire hazards to safety systems. In the analysis, it should be assumed that the functions of all safety systems in the fire compartment or fire cell where hypothetical fires are located are all failed, except that the safety systems are protected by qualified fire barriers or can take the consequences of the fire. For the exceptional case, the rationality of the analysis shall be proved.
2.4.3 The automatic fire alarm system, fire extinguishing system and their supporting systems (such as ventilation and drainage systems) in each fire compartment shall be as independent as possible from their corresponding parts in other fire compartments to maintain the operability of these systems in adjacent fire compartments.
2.5 Event combination
2.5.1 If the probabilistic safety analysis can prove that the frequency of random combination of certain extremely unlikely events is so low as to be negligible, such event combination may not be considered as a postulated accident.
2.5.2 In the design of fire-fighting system and equipment, consideration shall be given to the combination of fire and other postulated initiating events that may be independent of fire and appropriate countermeasures shall be taken. For example, for the combination of loss of coolant accident and independent fire events, consideration shall be given to the independent fires in the long-term stage after the accident, but not to the combined independent fires in short-term stages such as accident occurrence and mitigation system startup.
2.5.3 A postulated initiating event shall not lead to a fire that endangers any safety system. The possible causes of the fire shall be determined in the fire hazard analysis, such as serious earthquakes or the disintegration of steam turbine generators, and specific countermeasures (e.g. adopting cable wrapping, automatic fire alarm system, fire extinguishing system, etc.) shall be taken if necessary. In the fire hazard analysis, special attention shall be paid to the possibility of failure of high-temperature equipment and pipelines conveying flammable liquids and gases.
2.5.4 Fire-fighting systems and equipment that still need to maintain their functions (e.g. integrity, and/or functionality, and/or operability) under various effects of the postulated initiating events shall be identified, and they shall be properly designed and qualified so that they have the appropriate capability to resist the impact of postulated initiating events.
2.5.5 For the fire-fighting systems and equipment that do not need to maintain their functions after a postulated initiating event, their design and qualification shall ensure that their failure modes will not endanger nuclear safety related items.
2.6 Protection against explosion hazards
2.6.1 The explosion hazards of nuclear power plants shall be eliminated as much as possible through design. Measures to prevent or limit the formation of explosive environment shall be given priority in the design.
2.6.2 Flammable gases, combustible liquids and combustible materials that may produce or contribute to the production of explosive mixed gases shall be excluded from fire compartments, fire cells, areas adjacent to fire compartments and fire cells, and areas connected through ventilation systems as far as possible. If this is impossible, the quantity of these materials shall be strictly limited, sufficient storage facilities shall be provided, and the active substances, oxidizing agents and combustible materials shall be isolated from each other. Compressed flammable gas cylinders shall be properly stored in a special enclosure far away from the main plant building and shall be properly protected according to the local environmental conditions. The automatic fire alarm system, automatic flammable gas detection system and automatic fire extinguishing system shall be set up to prevent fire-induced explosion from affecting the items important to safety in other plant buildings.
2.6.3 The explosion hazards shall be identified for fire compartments, fire cells and other areas that are obviously affected by explosions. In identifying explosion hazards, consideration shall be given to the physical explosion (such as rapid air expansion caused by high-energy electric arc), chemical explosion (such as gas mixture explosion and oil-filled transformer explosion) and explosion caused by fire, as well as the effect of postulated initiating events (such as rupture of flammable gas transmission pipeline).
2.6.4 Appropriate electrical components (such as circuit breakers) shall be selected, and the probability, size and duration of electric arc may be limited by design to minimize the harm of physical explosion.
2.6.5 If the formation of explosive environment is inevitable, appropriate design or necessary operation procedures shall be adopted to minimize risks. Relevant measures include: limiting the volume of explosive gas, eliminating ignition sources, sufficient ventilation, selecting electrical equipment suitable for explosive environment, inerting, explosion venting (such as explosive plates or other pressure-relief devices) and isolation from items important to safety. Equipment that need to maintain their functions after a postulated initiating event shall be identified, and appropriately designed and qualified.
2.6.6 The risk of fire-induced explosion (e.g. explosion caused by boiling liquid expansion vapor explosion) shall be minimized by isolating potential fire from potential explosive liquids and gases, or by active measures (e.g. fixed water-based fire extinguishing system capable of providing cooling and vapor diffusion). Consideration shall be given to the positive pressure of shock wave and projectiles generated by the boiling liquid expansion vapor explosion, as well as the possibility of gas cloud explosion caused by ignition of flammable gas far away from the release point.
2.6.7 Explosion hazards that cannot be eliminated shall be identified and design measures shall be taken to limit explosion consequences (e.g. overpressure, generation of projectiles or fires). The influence of hypothetical explosion on safety systems shall be evaluated according to the requirements of 2.1.1. In addition, the evacuation and rescue routes for personnel in main control room and auxiliary control room shall also be evaluated. If necessary, specific design measures shall be adopted.
3 Plant building design
3.1 General
3.1.1 In order to ensure the reflection of fire safety objectives mentioned in Clause 2 in the design of nuclear power plants, this clause describes the necessary design activities.
3.2 Layout and construction
3.2.1 In the early stage of design, the plant building shall be divided into fire compartments which isolate items important to safety from high fire loads and isolate multiple safety systems from each other. By isolation, the risk of fire spread is reduced, the secondary effect of fire is reduced, and common mode faults are prevented.
3.2.2 Plant building structures shall have appropriate fire resistance. The fire stability grade (bearing capacity) of plant building structural components that are placed in or constitute the boundary of the fire compartment shall not be inferior to the fire- resistance requirements of the fire compartment itself.
3.2.3 Non-combustible or flame-retardant and heat-resistant materials shall be used as far as possible in the whole nuclear power plant (especially in reactor containment and control room).
3.2.4 A list of combustible materials and their locations in the plant building shall be established at the initial stage of design. This list is an important input for fire hazard analysis and shall be continuously updated throughout the service life of the nuclear power plant.
3.2.5 Combustible materials shall be avoided as far as possible near the items important to safety.
3.2.6 Adequate evacuation and rescue routes shall be arranged (see Annex II).
3.3 Application of fire compartments: fire blocking method
3.3.1 In order to embody the isolation principle described in Clause 2 and isolate items important to safety from high fire loads and other fire hazards, priority shall be given to arranging multiple items important to safety in mutually isolated fire compartments. This method is called fire blocking method.
3.3.2 A fire compartment is a plant building or area completely surrounded by fire barriers. The fire resistance of the fire barrier in fire compartment shall be high enough to keep the fire barrier undamaged even when the fire load therein is completely burnt.
3.3.3 The fire shall be confined in the fire compartment to prevent the fire and its effects (such as smoke and heat) from spreading between fire compartments, so as to avoid the simultaneous failure of multiple items important to safety.
3.3.4 The isolation provided by fire barriers shall be reliable and shall not be weakened by the temperature or pressure effect of fire on common plant building components (such as building equipment system or ventilation system).
3.3.5 Since any penetrating component will reduce the reliability and overall effect of fire barriers, the number of penetrating components shall be minimized. For passage blocking devices (such as fire doors, fire dampers, containment gates and fire blocks) and fire compartment boundaries that form part of the fire barrier, their fire resistance shall be at least the same as that required by the fire barrier itself.
3.3.6 For the fire compartment adopting fire blocking method, a fire extinguishing system shall be set up in the area with high fire load determined by fire hazard analysis to control the fire as soon as possible.
3.3.7 The fire resistance of the fire barrier constituting the boundary of fire compartment shall be determined in the fire hazard analysis, and it shall be at least 60min. Annex III provides relevant information on fire barriers and penetrating components.
1 Introduction
2 General principles
3 Plant building design
4 Fire preventive measures and explosion hazard control
5 Automatic fire alarm and fire extinguishing systems
6 Mitigation of the secondary effect of fires
7 Safety classification and quality assurance
Terms and definitions
Annex I Application of fire blocking method and fire extinguishing method
Annex II Evacuation and rescue routes
Annex III Fire barriers
Annex IV Cable fire protection
Annex V Fire detectors
Annex VI Automatic water sprinkler and water mist spray systems
Annex VII Gas fire extinguishing system
