Important primary circuit parameters are continuously monitored and also serve as input signals for safety systems. If predefined safety limits are exceeded, the appropriate safety systems are automatically activated.

Under normal operating conditions, a nuclear power plant is controlled by active systems. These systems generally require electrical power and respond to commands from operators or automatic control systems. They also handle normal operational transients, such as changes in primary circuit pressure. In the event of a malfunction or accident, dedicated safety systems are activated to maintain the fundamental safety functions. Depending on the reactor design, these may include both active and passive systems.

Passive safety systems rely primarily on natural physical phenomena such as gravity, natural circulation, pressure differences or stored energy. They require little or no external power and minimal or no operator action to perform their safety function. The use of passive systems can therefore help maintain reactor safety even if electrical power or some active systems are unavailable.

Active safety systems require the operation of mechanical or electrical components, such as pumps, valves or emergency power supplies, to perform their safety function. They may be activated automatically or by the operator in response to abnormal conditions or an accident.

Safety systems of the VVER-440/V-213 reactor with a bubble condenser containment system.

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Safety systems of the VVER-440

Barriers

One of the fundamental principles of nuclear safety is to prevent the release of radioactive substances into the environment. Nuclear power plants therefore employ several successive physical barriers between the radioactive material and the environment.

Fuel matrix

Uranium dioxide is a ceramic material in which most fission products remain retained within the fuel during normal operation.

The containment forms the final physical barrier against the release of radioactive substances into the environment during an accident. It is a robust, highly leak-tight structure surrounding the reactor and other major components of the reactor coolant system.

Fuel cladding

Zircalloy cladding separates the fuel pellets from the coolant and retains fission products within the fuel rods.

Reactor coolant pressure boundary

The thick-walled steel reactor pressure vessel, together with the primary circuit piping and other pressure-retaining components, forms another barrier against the release of radioactive substances.

Containment

The containment forms the final physical barrier against the release of radioactive substances into the environment. It is designed to withstand the pressure and temperature that may arise during certain accidents, such as a loss-of-coolant accident, and to limit the release of radioactive material. There are many types of containment, but many consist of a thick reinforced or prestressed concrete structure with a steel liner surrounding the reactor and primary circuit. The containment is designed to be highly leak-tight and its walls are typically around one metre thick. Modern containments are also designed to withstand a range of internal and external hazards, which may include explosions and aircraft impact.

Safety barriers of a nuclear power plant.

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Safety barriers of a nuclear power plant

The integrity of these barriers is continuously monitored. Fuel cladding failures can be detected by monitoring the activity and composition of the reactor coolant for fission products. The integrity of the reactor coolant pressure boundary is monitored using pressure, coolant inventory, leakage detection and other measurements. Containment integrity is also regularly tested and monitored. If one barrier is impaired, the remaining barriers and safety systems provide additional levels of protection against the release of radioactive substances.

Inherent Safety Features

Inherent safety features arise from the physical characteristics of the reactor and its materials rather than from the operation of engineered safety systems. They can help stabilise the reactor or limit the progression of an abnormal event without requiring operator action or external power.

In older nuclear power plants, important operating parameters were recorded on paper by chart recorders and printers. Modern plants use digital systems to record, store and analyse operating data.

One of the most important inherent safety features of most modern power reactors is negative reactivity feedback. For example, a negative temperature coefficient of reactivity means that an increase in fuel or coolant temperature reduces reactor power, helping to counteract the initial temperature increase.

In light-water reactors, the formation of steam voids generally reduces the ability of the water to moderate neutrons and consequently reduces reactivity. This negative void coefficient provides an important inherent stabilising effect. However, the magnitude and even the sign of reactivity coefficients depend on the reactor design.

Natural circulation can also contribute to reactor cooling. When the coolant is heated in the core, its density decreases and it tends to rise, while cooler, denser coolant flows downward. Reactor coolant systems can be designed to make use of this effect, allowing some coolant circulation to continue even if the pumps are unavailable.

Some advanced reactor designs, such as lead-cooled fast reactors, are designed to remove decay heat by natural circulation of the coolant after shutdown. This can reduce their dependence on electrically powered pumps for residual heat removal.

Gravity is also used in some reactor designs for emergency shutdown. In many pressurised water reactors, control rods are held above the core during operation and can be released to fall into the core under gravity, rapidly shutting down the fission chain reaction.

Nuclear reactors incorporate a number of inherent safety features. Modern reactor designs make increasing use of both inherent safety features and passive safety systems. Together, they can enhance reactor safety and reduce reliance on active equipment, external power and operator intervention.

As a rough guide, decay heat is about 6—7% of the reactor’s previous thermal power immediately after shutdown, around 1% after an hour and a fraction of one percent after a week.

Nuclear Reactor Safety

The nuclear reactor is the central component of a nuclear power plant. Accurate information about its current state is essential for operators both to control the reactor and to ensure its safe operation.

Nuclear reactor safety has three fundamental goals:

1. Control the fission reaction;
2. Ensure adequate cooling of the nuclear fuel;
3. Prevent the uncontrolled release of radioactive substances.

Control rods containing neutron-absorbing materials are used to control the fission reaction. Reactor power can be changed by inserting or withdrawing these rods from the reactor core. In an emergency, the reactor protection system rapidly inserts the control rods to shut down the chain reaction, an action commonly known as a scram. In many pressurised water reactors, the control rods are held above the core by electromagnets and, when released, fall into the core by gravity. This allows the reactor to be shut down even in the event of a loss of electrical power. Other reactor designs use different rapid shutdown mechanisms. The fission chain reaction is reduced to a very low level within seconds.

Even after the fission chain reaction has been stopped, radioactive decay continues to generate considerable heat in the fuel. This decay heat must be removed to prevent the fuel from overheating. Nuclear power plants therefore have redundant systems for residual heat removal and emergency core cooling. The ultimate heat sink, to which heat from the reactor is eventually transferred, is also essential. Depending on the plant design and location, heat may ultimately be discharged to the atmosphere through cooling towers or transferred to a large body of water, such as a river, lake or sea.

Access to the containment and other hermetically sealed areas is generally restricted while the reactor is operating. Personnel normally enter these areas during refuelling and maintenance outages, when access can be permitted under controlled conditions.

In the event of a loss-of-coolant accident (LOCA), emergency core cooling systems (ECCS) provide additional coolant to the reactor and ensure continued core cooling and heat removal. Different systems operate at different pressures and stages of an accident. Depending on the reactor design, coolant may be supplied by electrically driven pumps, pressurised accumulators or passive systems using gravity or pressure differences.

High-pressure injection systems can supply coolant while the reactor coolant system is still at relatively high pressure. As the system pressure decreases, low-pressure injection systems can provide larger quantities of coolant and contribute to long-term core cooling. Some reactor designs also employ passive injection systems, such as pressurised accumulators, which can deliver coolant without electrically powered pumps. Emergency electrical supplies, typically including diesel generators and batteries, provide power to safety systems if normal electrical supplies are lost.

The pressuriser automatically controls the pressure in the primary circuit. It is a vessel connected to the primary circuit and contains both coolant and steam. If the pressure becomes too low, electric heaters increase steam generation in the pressuriser, raising the pressure. If the pressure becomes too high, water is sprayed into the steam space, causing some of the steam to condense and reducing the pressure.

If the primary circuit pressure exceeds specified limits, safety and relief valves protect the system against overpressure. In some reactor designs, discharged steam is directed to a relief tank where it condenses in water. Some VVER-440/V-213 reactors also use a bubble condenser containment system. During a loss-of-coolant accident, steam released into the hermetic compartments is directed through pools of water in the bubble condenser tower, where much of it condenses, limiting the pressure rise. Containment spray systems can provide additional cooling and steam condensation.

If core cooling is lost, the fuel rods begin to overheat. At high temperatures, zirconium-alloy fuel cladding loses strength and reacts rapidly with steam, producing additional heat and hydrogen. As temperatures continue to rise, fuel assemblies and other core structures can become severely damaged and eventually melt. Uranium dioxide fuel melts at about 2,800 °C. The resulting mixture of molten fuel, cladding and structural materials is known as corium. Even after the fission chain reaction has stopped, radioactive decay continues to generate heat within this material.

Redundancy

Cylinders of a large diesel engine used to drive an emergency generator at a nuclear power plant. If normal electrical power is lost, emergency diesel generators automatically start and provide power to essential safety systems and other equipment required to maintain the plant in a safe condition.

Safety systems are designed with redundancy so that essential safety functions can be maintained even if individual components fail. Many safety systems consist of two or more independent trains or divisions.

A nuclear power plant requires reliable electrical power for many of its safety functions. During normal operation, power is available from the plant generator and/or the external grid. If these sources are lost, emergency power supplies are available, typically including diesel generators and batteries. Batteries provide uninterrupted DC power to essential instrumentation, control and protection systems until other power sources are restored or started.