Nuclear power plant safety systems
Each nuclear power plant in Canada has multiple robust safety systems designed to prevent accidents or, should one occur, mitigate their consequences. These systems are maintained, inspected and upgraded as needed to ensure that plants meet or exceed the strict safety requirements of the Canadian Nuclear Safety Commission (CNSC). Together, the systems provide multiple layers of protection by performing 3 fundamental safety functions: controlling the reactor, cooling the fuel and containing radioactive material.
How a nuclear power plant works
All currently operating nuclear power plants in Canada use the CANDU design – a safe and reliable reactor technology.
The reactor
CANDU reactors produce electricity through a process known as fission. Fission is the process of splitting atoms of natural uranium inside the reactor, releasing energy in the form of radiation and heat.
The split atoms then produce a chain reaction: more atoms continue to be split, resulting in more radiation and heat.
The heat energy is used to produce steam, which powers the turbines and generators that produce electricity.
Used nuclear fuel pool
After the nuclear fuel (uranium) has been used in the reactor, it is removed and stored securely in a water-filled pool for a period of 6 to 10 years.
The water in the pool continues to cool the fuel and provides shielding against radiation.
In Canada, used fuel pools are located below ground level and are housed in robust structures designed to withstand external hazards such as earthquakes.
Controlling the reactor
Controlling the reactor involves increasing, decreasing or stopping the chain reaction happening inside the reactor.
Normal operation
When the reactor is operating, the chain reaction (or power level) is controlled by moving adjuster rods and varying the water level in vertical cylinders.
Sensitive detectors constantly monitor different aspects, like temperature, pressure and the reactor power level.
When necessary, CANDU reactors can safely and automatically shut down within seconds.
Shutdown systems
All nuclear power reactors in Canada have 2 independent, fast-acting and equally effective shutdown systems.
The first shutdown system is made up of rods that drop automatically and stop the chain reaction if something irregular is detected.
The second system injects a neutron-absorbing liquid, or poison, into the reactor to immediately stop the chain reaction.
Both shutdown systems are independent and fast-acting, designed to automatically shut down the reactor when specific conditions are detected. Operator intervention is not required to initiate the shutdown systems, although operators can also manually activate them.
These systems are regularly and safely tested.
Restarting the reactor
Once shut down, the reactor cannot restart without deliberate operator intervention and until required operating conditions are met. The reactor must be manually restarted. This is another important safety feature.
Cooling the fuel
Following shutdown, the amount of energy produced by the reactor decreases rapidly.
The nuclear fuel will, however, continue to produce some heat and must be cooled.
Decay heat
That heat, called decay heat, represents a small fraction of the heat produced during normal operation.
Main cooling systems
Fuel cooling involves 3 main systems:
- the heat transport system
- the steam system
- the condenser cooling system
The heat transport system brings the heat produced by the reactor to the steam generators.
This system is made up of robust piping filled with heavy water – a naturally occurring but rare form of water that allows nuclear fission to produce a chain reaction. Pipes and other components are maintained and inspected regularly and replaced if necessary.
Inspections include measuring pipe wear and tear and identifying any microscopic cracks or changes well before they become a problem.
Quick fact
Approximately 1 in 6,400 naturally occurring water molecules consists of this rare form of water. It is 10% heavier than regular water because it includes a heavy form of hydrogen called deuterium.
The second system, the steam system, uses normal water. The heat from the reactor turns this water into steam to run the turbines and generators.
That steam is then indirectly cooled and condensed using a third system that pumps in cold water from a body of water such as a lake or the sea. This is called the condenser cooling system.
Like other components, the steam and condenser cooling systems are regularly inspected.
These inspections take place throughout the life of the nuclear facilities to confirm that aging equipment is functioning as originally designed.
Shutdown cooling system
A simpler cooling system is used when the reactor is shut down for an extended period, for example during a planned outage.
It requires little power to function and is connected directly to the heat transport system. It allows the primary coolant system to be partly drained to perform inspection and maintenance work (e.g., inspection of the steam generator tubes or replacement of pump components).
Multiple power supplies
Cooling systems need electricity to operate. Under normal operation, the reactor’s turbine generator is able to provide electricity back to itself to power all its cooling systems.
These cooling systems may also get their electricity from the same power grid as the rest of us as an alternative source of backup power. Nuclear power plants in Canada are equipped with multiple sources of backup power in case they get disconnected from the grid.
These include:
- 2 or 3 standby power generators
- 2 or 3 emergency power generators
- emergency batteries
Some plants include even more equipment.
You can learn more by watching how Canadian nuclear power plants would respond in the very unlikely event of a total station blackout – the condition that contributed to the Fukushima accident after a large tsunami disabled available onsite power sources.
Natural circulation
One of the inherent and proven safety features of CANDU reactors is their ability to cool the reactor through natural circulation.
In CANDU reactors, natural circulation takes over when the pumps that normally push the coolant through the heat transport system stop functioning.
For natural circulation to continue over time, steam generators need to be filled with cool water.
How natural circulation works
This cooling feature of CANDU reactors works because of the difference in temperature and elevation between the steam generators (cooler and physically higher than the reactor core) and the reactor core (hotter and lower than the steam generators).
Emergency injection systems
In the unlikely event of a loss-of-coolant accident, such as one caused by a pipe break, the emergency injection system is designed to provide large amounts of cooling water to the reactor fuel to rapidly cool it.
The system uses pressurized nitrogen tanks and pumps to deliver cooling water to the reactor.
A collection basin located in the basement of the reactor building would recover the water and pump it back into the reactor to maintain fuel cooling.
Emergency mitigation equipment
As mandated by the CNSC following the Fukushima accident, nuclear power plant operators in Canada have acquired emergency mitigation equipment, such as portable power generators and pumps, which can be used to support safe reactor shutdown and accident mitigation during a severe accident.
The equipment, located onsite and offsite, is easily transported and could be used in several ways.
For instance, it could be used to provide a long-term supply of cooling water to the reactor, supply power to the control room, or add water to the used nuclear fuel pools to keep the used nuclear fuel cooled.
Containing radioactive material
Nuclear reactors are designed with multiple physical barriers to contain radioactive material and prevent its release to the environment.
Containment layers
At the heart of all CANDU reactors are hardened ceramic pellets made of natural uranium.
These pellets help contain radioactive material and form the first barrier to release.
The pellets are enclosed in rods, which form the second barrier to release. CANDU fuel rods are made of zircaloy, a metal alloy extremely resistant to heat and corrosion.
The rods are then loaded into pressure tubes, which are part of the heat transport system. This is the third barrier to release.
The pressure tubes are contained inside a metal tank called the calandria, which itself is contained inside a thick vault made of reinforced concrete.
The fourth barrier to release is the containment building that houses and protects the reactor.
The walls of the reactor building are made of at least 1 metre of reinforced concrete.
The reactor building is surrounded by an exclusion (buffer) zone. This exclusion zone is the fifth barrier to release so that very small releases that may occur do not impact the health and safety of the public or the environment.
Minimizing radiation releases
During normal operation, nuclear power plants release very small amounts of radioactive materials into the air and water.
These releases originate from reactor systems and waste management activities.
In order to reduce airborne releases, highly efficient filters and radiation monitors are installed as part of the ventilation systems.
High-efficiency filtration systems remove over 99% of radioactive particulates and contaminants from exhaust air before it is released to the environment.
Similar systems are also installed to remove radioactivity from waterborne releases.
These releases usually come from wash water used to clean surfaces, floors and laundry, as well as from water draining from showers and sinks.
All radiation releases from nuclear facilities in Canada are very small. They are monitored and controlled by the plant operator and are reported to the CNSC.
The release levels are well below regulatory limits and do not pose any risk to the health and safety of persons or to the environment.
Filtering systems are regularly inspected, and power plant operators must, by law, report all radioactive releases into the environment.
Protecting containment in case of an accident
Safety systems are in place to protect containment structures from increases in pressure caused by steam releases during an accident.
In a single-unit station, internal pressure would be lowered by spraying water from a dousing tank.
In a multi-unit station, pressure would be lowered by releasing steam and hot gases from the reactor building to the vacuum building.
The vacuum building is a structure specifically designed to quickly and safely lower pressure inside the reactor building. This building also has a dousing system to control pressure.
The vacuum and dousing systems work without power and must be periodically tested by the plant operator.
Text version.
There are 3 main parts to the plant: the vacuum building, the reactor building and the turbine hall. The image identifies key components and where they are located in the plant. The vacuum building contains the dousing tank and pressure relief valves. The reactor building contains the steam generators and the reactor. There is also a collection basin in the basement of the reactor building. The turbine hall is connected to the reactor building and contains the turbines, the generator, and the condenser for cooling water from a lake or sea. Some parts outside the plant are also identified, namely the air filter system beside the vacuum building, emergency and standby power generators, and a transmission tower. Large image version
Hydrogen management
Hydrogen gas can be produced during severe accidents. Hydrogen gas is flammable and, if allowed to accumulate, could lead to an explosion that could damage containment systems and other plant structures.
To deal with the potential hazards of hydrogen gas, most CANDU plants are equipped with hydrogen igniters or burners.
Recently, nuclear power plant operators have installed passive autocatalytic recombiners.
These devices passively (without the need for external power) remove hydrogen from the containment and effectively reduce the risk of an explosion or fire.
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