Most of nuclear power plants introduce a ‘defence-in-depth‘ approach to achieve maximum safety, this approach is constituted of multiple safety systems supplementing the natural features of the reactor core. Level 3 and level 4 usually rely on various safety systems, structures and components. Engineered safety features and protection systems are provided to prevent evolution towards severe accidents and also to confine radioactive materials within the containment system. The measures taken at this level are aimed at preventing core damage in particular. Design and operating procedures are also aimed at maintaining the effectiveness of the barriers, especially the containment. For example, the emergency core cooling system (ECCS) is provided to mitigate the consequences of a loss-of-coolant accident (LOCA) even though the first level of defense makes such an occurrence highly unlikely.
In the regulatory arena, the term “safety-related” applies to systems, structures, components, procedures, and controls (of a facility or process) that are relied upon to remain functional during and following design-basis events. Safety-related systems, structures, and components have three characteristics. They ensure:
- the integrity of the reactor coolant pressure boundary (the reactor vessel and associated piping that circulates the reactor coolant);
- the capability to shut down the reactor and maintain it in a safe shutdown condition; or
- the capability to prevent or mitigate the consequences of accidents that could result in potential offsite exposures
A containment isolation valve is safety-related, for example, because by isolating the reactor coolant lines it confines radioactivity to the containment building and performs the functions defined above. It helps keep radioactivity away from the public.
An emergency diesel generator is safety-related because, by providing backup power to safety-related equipment, it ensures the capability to shut down the reactor and maintain it there safely.
The IEEE created its own term for “safety-related electric equipment,” which is “Class 1E.” In IEEE 308 it gives the definition of Class 1E as follows:
The safety classification of the electric equipment and systems that are essential to emergency reactor shutdown, containment isolation, reactor core cooling, and containment and reactor heat removal or that are otherwise essential in preventing significant release of radioactive material to the environment.
See also: IAEA Safety Standards, Safety Classification of Structures, Systems and Components in Nuclear Power Plants. Specific Safety Guide No. SSG-30, ISBN 978–92 –0–115413–2. Vienna, 2014.
Active and Passive Nuclear Safety
Nowadays, the most common nuclear reactors (PWRs and BWRs) rely mostly on active safety systems. Active in the sense that they involve electrical or mechanical operation on command systems (e.g., high-pressure water pumps). But the trend is to introduce more passive design features.
Passive nuclear safety is a design approach, which is more or less in use in nuclear power plants. Passive safety systems are designed to accomplish safety functions without any active intervention on the part of the operator or electrical/electronic feedback in order to bring the reactor to a safe shutdown state, in the event of a particular type of emergency (usually overheating resulting from a loss of coolant or loss of coolant flow). These systems take advantages of natural forces or phenomena such as gravity, pressure differences or natural heat convection.
The primary design objectives of the advanced passive technology is to provide greatly simplified nuclear plant designs that meet or exceed the latest regulatory requirements and safety goals, while being economically competitive with other systems.
Passive safety systems include: passive safety injection, passive residual heat removal, and passive containment cooling. All of these systems have been designed to meet the NRC single-failure criteria and other recent criteria
More recently, however, new reactor designs are making a more extensive use of passive safety features for a variety of purposes, for instance for core cooling during transients, design basis accident or even severe accidents or for containment cooling, with the claim that passive systems are highly reliable and reduce the cost associated with the installation and maintenance of systems requiring multiple trains of equipment requiring expensive pumps, motors and other equipment as well as redundant safety class power supplies.
Reactor Protection System
The Reactor Protection System, RPS, is one of safety systems and provides the following functions:
- Monitors the plant for abnormal conditions and alerts the operator to take appropriate action
- Automatically provides
- reactor trip (shutdown) signals
- engineered safeguards actuation signals when plant conditions, as monitored by nuclear instrumentation and process instrumentation, reach the plant safety limits
The RPS automatically initiates a rapid reactor shutdown (scram) by inserting control rods to preserve the integrity of the fuel cladding and reactor coolant pressure boundary. Also, the overall purpose of the reactor protection system is to prevent the release of radioactivity to the environment. The initiation of a reactor trip by the RPS prevents the core from operating in a condition that could cause damage to the core.
The protection system normally uses 2/3 or 2/4 logic. A 2/3 logic means that a trip occurs when at least 2 signals out of 3 indicate a trip condition.
Reactor trip signals provided by the system are usually as follows:
- Manual trip signal – Redundant control board pushbuttons are provided.
- Neutron flux trip signals – Source, intermediate and power range signals are provided for protection during startup, full power, and shutdown operations.
- Primary coolant trip signals – DNBR, kw/ft, high-power-to-flow, and reactor coolant pump under-speed trip signals provide trips. The main purpose is to prevent core coolant conditions which could lead to excessive fuel or cladding temperatures or excessive bulk boiling of the coolant.
- Pressurizer trip signals – High pressure, low pressure, and high-level trip signals initiate a reactor trip
- Steam generator trip signals – Low feedwater flow and low water level trip signals protect against loss of steam generator water inventory. High water level trip signals protect against excessive water carryover into the turbine.
- Turbine-generator trip – Trip of the turbine-generator above a certain power level initiates a reactor trip
- Safety injection signal – Actuation of the Safety Injection System simultaneously initiates a reactor trip to prevent excessive reactor coolant temperature and/or pressure.
Due to its importance to safety, the RPS is designed, constructed, and tested to the highest standards. These include requirements for the ability to withstand single failures and still provide full protection, for the independence of separate trains, and for testability to insure continued reliability.
Engineered Safety Features
The main purpose of the engineered safety features is to prevent or limit the escape of radioactivity to the environment in cases of a highly unlikely transient or accident that is too severe to be managed by the reactor protection system alone. A protective action is generated when sufficient number and combinations of these set-points have been met or exceeded.
The ESF functions include:
- Safety injection actuation. The safety injection actuation shuts down the reactor (if it is still operating), maintains the reactor in a shutdown state (via injection of borated water), ensures sufficient core cooling to limit possible fuel damage.
- Containment spray actuation. The containment spray system is automatically actuated by a very high containment pressure signal.
- Containment isolation. Redundant isolation valves and dampers, both inside and outside the containment building, are shut in all non-essential lines which penetrate the containment building to ensure no leakage from containment to the environment.
- Steam line isolation. This isolation ensures that a steam break accident downstream of the valves is isolated, and if upstream (in containment) only one steam generator is discharged.
- Feedwater isolation. A high steam generator water level and SI actuation also trip the main feed pumps and the main turbine.
- Emergency feedwater actuation. The primary function of the Emergency Feedwater System is to supply feedwater to the steam generators following accident or transient conditions when the main feedwater system is not available. The system thereby maintains the capability of the steam generators to remove plant stored heat and core decay heat by converting the emergency feedwater to steam which is then discharged to the condenser or to the atmosphere.
Emergency Core Cooling System – ECCS
The purpose of the Emergency Core Cooling Systems (ECCS) is to provide core cooling under loss-of-coolant accident (LOCA) conditions to limit fuel cladding damage. The ECCS limits the fuel cladding temperature below the limit so that the core will remain intact and in place, with its essential heat transfer geometry preserved. The Code of Federal Regulations, CFR, requires the ECCS to be designed so that after any LOCA the reactor core remains in a geometrical configuration amenable to cooling. The basic criteria are to limit fuel cladding temperature and oxidation to minimize clad fragmentation, and to minimize the hydrogen generation from clad oxidation to protect the containment.
The ECCS usually consists of redundant high-pressure systems (e.g.3×100%) and redundant low-pressure systems (e.g.3×100%).
- HPCI. The high-pressure systems are the High Pressure Coolant Injection system (HPCI) and the Automatic Depressurization system (ADS). The HPCI system maintains adequate reactor vessel water inventory for core cooling on smallbreak LOCAs, depressurizes the reactor vessel to allow the low pressure ECCS to inject on intermediate-break LOCAs.
- LPCI. The low pressure systems are the Low Pressure Coolant Injection (LPCI) made of the Residual Heat Removal (RHR) system and the Core Spray (CS) system. The LPCI is an emergency system which consists of a pump that injects a coolant into the reactor vessel once it has been depressurized. The CS system (typical for BWRs) provides spray cooling to the reactor core to help mitigate the consequences of the large-break LOCAs when reactor pressure is low enough for the system to inject water into the reactor vessel. For low pressures, the accumulator injection system is also available. The accumulators are independent tanks containing borated coolant stored under nitrogen gas at a given pressure.
See also: Decay Heat Removal
The containment building is primarily designed to prevent or mitigate the uncontrolled release of radioactive material to the environment in operational states and in accident conditions. Therefore it is considered to be the fourth and final barrier in the Defence in depth strategy.
While the containment plays a crucial role in Design Basis Accidents or in Design Extension conditions, it is “only” designed to condense steam from primary coolant and to contain it inside the building.
In case of Design Basis Accidents such as the Large Break Loss of Coolant Accident (LBLOCA) the pressure increase is usually significant and active containment systems (pressure-suppression systems) must be available in order to maintain the integrity (to keep the pressure and temperature under certain limits) of the containment building.
Pressure-suppression systems are critical to safety and greatly affect the size of containment. Suppression refers to condensing the steam after a major break has released it from the cooling system. There are many designs of suppression systems around the world.
Most of Pressurized Water Reactors (PWRs) containments use two stage pressure-suppression systems:
- Fan Cooler System. This system circulates air through heat exchangers and filters to provide the cooling of containment atmosphere. Since this system is not sufficient for suppression during severe loss of coolant accidents, the containment spray system must be available as the secondary active pressure-suppression system.
- Containment Spray System. This system consist usually from three elements:
- Spray System Pump
- Spray System Tank
- Spray System Rings and Nozzles
When pressure increase inside the containment is indicated, the containment spray system is automatically started and the pumps (usually with 3×100% redundancy) take a suction from the tank (refueling water storage tank can be also used) and pump the water into spray nozzles located in the upper part of the containment. The water droplets, being cooler than the steam, will remove heat from the steam, which will cause the steam to condense. This will cause a reduction in the pressure of the building and will also reduce the temperature of the containment atmosphere. The spray system usually contains extra chemical additives dissolved in the tank to enhance the removal of particular radionuclides from the containment atmosphere. Especially radioiodine, which is of particular importance, can be effectively bonded to potassium hydroxide or sodium hydroxide.
Most of Boiling Water Reactors (BWR) containments use pressure-suppression pools in order to maintain the integrity of the containment building. The major containment designs are the Mark I, Mark II and the Mark III. The Mark I and Mark II containments consist of two main parts:
- A drywell houses the reactor coolant system.
- A wetwell is a suppression chamber, which stores a large body of water and therefore it is commonly called as the suppression pool.
Water spray systems are usually installed in both the drywell and the wetwell. The Mark III design consists of a primary containment and a drywell.
Containment buildings and containment pressure-suppression systems vary widely depending on certain reactor design. In some cases really unique technologies can be installed. For example, the containment building of Loviisa NPP uses two ice condensers as the pressure-suppression system.
Hydrogen Mitigation in Water Cooled Power Reactors
Hydrogen mitigation in water cooled power reactors is an important area of study in the field of safety of nuclear reactors. Hydrogen and oxygen can be generated during normal operation of a power reactor primarily as a consequence of the radiolysis of the water in the core.
During accidents hydrogen and oxygen can be generated also as a consequence of:
- Metal–water reactions in the core
- Radiolysis of the water in the sump or the suppression pool
- Degassing of hydrogen dissolved in the primary coolant
- Chemical reactions with materials in the containment ( interactions of molten core debris with concrete)
- Thermolysis of water – in extreme cases
During DBAs (Design Basis Accidents) such as the large break loss of coolant accident, the production of hydrogen (metal–water reactions in the core) is limited at low values by the operation of the emergency core cooling systems. Also the radiolysis of the water in the core is relatively slow process. Therefore, from the DBAs point of view the hydrogen hazards can be eliminated by maintaining the local hydrogen concentration below its flammability limit (4% of volume). This requirement can be ensured by mixing devices or thermal hydrogen recombiners.