CANDU Inspection - CNSC Online

CANDU Inspection

Module icon

CANDU Inspection

Table of contents

Module icon

CANDU Inspection

CANDU Inspection Briefing

Illustration of an inspector.

The CNSC exists to ensure that all nuclear activities in Canada are safely operated. The most important consideration is safety - Public safety, employee safety, and the safety of the environment.

Nuclear power generating stations are designed with multiple overlapping safety features to prevent the release of radioactive materials.

Module icon

CANDU Inspection

CANDU Inspection Briefing

Illustration of an inspector.

Did you know that CNSC inspectors are on-site at every nuclear power plant operating in Canada? The inspectors ensure that plant operators adhere to all safety standards by monitoring 14 Safety and Control Areas. During your virtual inspection of a Canadian nuclear power plant, you will inspect an example of each of these areas.

Canada builds defense in depth into every piece of safety equipment and every security activity related to a nuclear power plant. Defence in depth means that if a system fails there is a backup system ready to take over. If the back-up fails, there is another back-up system in place to mitigate the failure. This ensures overlapping layers of protection.

Module icon

CANDU Inspection

Station Home

A multi-unit power generating station with 7 clickable hotspots.

The Virtual Station

It's your turn to inspect the virtual station and explore how its safety systems work. You will inspect a simplified version of a Canadian nuclear power plant and a key example of each safety system.

Click the marker to begin your inspection

Turbine Hall

Heat created in the reactor is used to boil water in a boiler or steam generator. The clean steam is then driven over a set of turbines. The spin of the turbines drives an electrical generator, which produces the electricity. The turbines and the generators are located in the Turbine Hall.

Primary Loop

The primary loop, which is filled with pressurized heavy water, transports the heat generated by the fuel bundles in the reactor’s pressure tubes to the steam generators located above the reactor. This is a closed circuit that does not mix with any other water loop in the plant.

Reactor

The reactor is a large, heavily shielded vessel called a calandria that holds the fuel channels loaded with fuel bundles. When a reaction is started inside the reactor, tiny particles called neutrons smash apart the uranium atoms of the pellets. As the atoms break, they release large amounts of heat energy and more neutrons, which break apart more atoms.

Cooling Loops

Three separate cooling loops constantly draw heat away from the reactor core, which is at its hottest during fission. Even when it is shut down, a reactor stays very hot for a long time and must continually be cooled.

Containment

Nuclear power plants are built to contain radioactivity. The reactor vault, sealed reactor building, and vacuum building, and constant monitoring help keep it safely contained at all times.

Spent Fuel

Over time, the amount of usable, or 'fissionable', material in uranium fuel pellets decreases until it can no longer sustain a strong enough reaction to generate electricity. The material that remains is referred to as 'spent fuel'. Spent fuel is removed (and fresh fuel is reloaded) by specialized robotic machines. Used fuel bundles are removed from the reactor and placed in water-filled bays called Spent Fuel Pools on the power plant site until they are cool enough to be put into dry storage.

Control Room

The nerve centre of the power plant is the control room. All automated systems are controlled and monitored from here. Drills, emergency scenarios and simulations are used to prepare the control room's highly trained personnel for nearly every eventuality. Should an event make this room inaccessible, the entire system can be transferred to a secondary control room further from the reactor site.

Module icon

CANDU Inspection

The Turbine Hall

A turbine hall with 10 clickable hotspots.

Turbine Hall

Power plants, whether they are coal, gas, oil, or nuclear all work in pretty much the same way. Heat is required to turn water into steam. The steam spins large turbines, which drive generators that produce electricity. In a nuclear power station, the reactor performs the same work as a boiler in a coal, gas or oil-fired station; it uses fuel to boil water.

Click the marker to begin your inspection

Boiler/Steam Generator

These huge tanks are full of fresh water. The water in the boilers is heated by loops of pipe carrying super-heated heavy water from the reactor. The fresh water boils into steam, which is carried by the steam pipe to the turbines. Boilers are very big. Some are as tall as 70 feet and hold 800 tons of water.

Pressure Release Valves

These valves release steam from the boiler when the pressure exceeds preset limits, avoiding accidental explosions. This passive system, which operates without any electrical or mechanical assistance, is an example of defence in depth design.

Closed Cooling Loops

The fresh water in the boiler runs in a closed loop from boiler to turbine and back again. Because the water in the boiler never mixes with the heavy water that runs through the reactor, it is not contaminated and does not carry radioactive material out of the containment building. Keeping loops of water apart to minimize risk is part of defence in depth design.

Steam Pipe

This heavy pipe carries the steam from the bouiler to the turbines.

Cold Water Return Pipe

This pipe carries cooled water from the condenser back to the boiler where it will be heated again. Recycling water in this way keeps the boiler from getting too hot.

Turbines

Made of thousands of metal vanes, the turbines spin when steam is forced over them. As they spin, the shaft they are attached to turns the electrical generator.

Electrical Generator

The electric generator converts the motion of the turbines into electricity by spinning a series of magnets over a coil. The electricity that the generator creates is sent out to the power grid.

Condenser

The condenser collects the cooled steam and returns it as water to the boiler where it can be heated again.

Radiation Protection - Safety and Control Area 7

CNSC inspectors check that personnel in the Turbine Hall are wearing their dosimeters and the proper clearance badges. They check gamma levels, verify that radiation and danger signs are posted in appropriate places, and ensure that interzonal whole body count monitors are functioning properly.

Security - Safety and Control Area 12

Monitored security cameras are in place throughout the plant. CNSC inspectors check to make sure the cameras are regularly inspected and that they are always in service.

Module icon

CANDU Inspection

Primary Loop

A representation of the primary loop with 10 clickable hotspots.

Primary Loop

Canadian reactors use fuel pellets made of non-enriched uranium. The pellets are inserted into rods made from a zirconium alloy and then assembled into bundles that are loaded into a fuel channel. Here, they generate heat through fission.

Deuterium, or 'heavy water', is pumped under high pressure through the fuel channels where it is heated by the nuclear reaction going on inside.

Click the marker to begin your inspection

Boiler/Steam Generator

Boilers, or steam generators, are huge tanks of fresh water. The water is boiled by loops of pipe carrying super-heated heavy water from the reactor core. The boiling water turns to steam which is carried away by the steam pipe. Steam generators are very big, some are as tall as 70 feet and they can hold up to 800 tons of water.

Uranium Fuel Pellet

A single fuel pellet (the size of the end of your pinkie finger) can power an average Canadian home for six weeks. Uranium used in CANDU reactors is mined in Canada and then processed into a powder called yellow cake, which is then baked into hard pellets the size of a thimble. These pellets are loaded into rods.

Ceramic Hardness

Nuclear fuel pellets have a ceramic-like hardness and can endure extreme temperatures without melting or distorting. The hardness and durability of the fuel pellets are the first of the five passive barriers built into a CANDU reactor.

Zircalloy Sheaths

The fuel pellets are inserted into tubes made from a zirconium alloy. This metal is a low neutron absorber and has a high resistance to corrosion.

Fuel sheaths made of high-integrity alloy are the second of the five passive barriers built into a CANDU reactor.

Fuel Bundles

The rods are inserted into a bundle containing hundreds of fuel pellets. A bundle is about the size of a fireplace log and weighs 24 kgs (52 pounds). A bundle spends 8 to 16 months in the reactor and will provide enough electricity to power one hundred homes during its lifetime. Each bundle is carefully inspected before it is used.

High Density Metal

The channels and pressurized tubes of the primary loop are made of a high-density metal. This specialized metal is the third of the five passive barriers built into a CANDU reactor.

Primary Heat Transport Loop

The primary loop is a sealed set of pipes that carries deuterium, or 'heavy water', through the reactor core where it gets super-heated. The deuterium is then pumped up to the boiler where it turns the fresh water into steam before going back down through the core again.

Fuel Channel

The fuel channel holds the bundles inside the reactor's core. The deuterium inside the channels is pressurized to keep it from boiling. It never mixes with the water inside the steam generator. A typical reactor has hundreds of fuel channels and thousands of fuel bundles.

Primary Loop Pump

This huge pump keeps the pressurized deuterium circulating. It draws the deuterium through the reactor core and up through the boiler, and then forces the cooled deuterium back into the core to pick up more heat. Shedding heat to the boiler not only provides the heat necessary to spin the turbines, it keeps the reactor at the temperature required for optimum operation.

Primary Pump Back-ups

Because of its importance, the primary pump has numerous overlapping back-ups in case it, or the power running it, should fail. Built-in redundancy is an example of defence in depth design.

Module icon

CANDU Inspection

Reactor

A nuclear reactor with 12 clickable hotspots.

Reactor

The reactor is a large, heavily shielded vessel called a calandria that holds the fuel channels loaded with fuel bundles. When a reaction is started inside the reactor, tiny particles called neutrons smash apart the uranium atoms of the pellets. As the atoms break, they release large amounts of heat energy and more neutrons, which break apart more atoms. To stop the reaction, CANDU reactors have two independent shutdown systems: neutron absorbing rods, and gadolinium.

Click the marker to begin your inspection

Conventional Health and Safety - Safety and Control Area 8

CNSC Inspectors verify that scaffolding has been properly inspected and maintained. They also look for waste or loose items that could create a hazard, verify that hazardous materials are properly labelled, that work zones are delimited, that leaks or puddles are cleaned up, that rotating equipment is equipped with a guard, and that there is proper lighting for work to be done safely.

Physical Design

Equipment in a nuclear power plant is routinely checked, bench tested and taken apart to guarantee it is in perfect working order. CNSC inspectors verify that any changes are thoroughly reviewed by engineers, that all equipment is properly tagged, that valves are in their expected positions, that electrical grounds and cables are intact and undamaged,and that fire walls, floors and panels are sealed and in good repair.

Gadolinium Tanks - Shutdown System 2

Several tanks filled with gadolinium are ready to inject a "moderator poison" into the heavy water sealed inside the calandria. When large amounts of gadolinium are introduced to the moderator water fission stops and the reactor shuts down. The tanks, which are independent of the shut-off rods, are an example of overlapping protection and defence in depth redundancy.

Deuterium Moderator

Deuterium is called a moderator because it moderates the speed of neutrons. When uranium atoms split they release heat and many fast-moving neutrons. The deuterium slows them, which increases the number of unsplit atoms they can collide with-and therefore maximize the amount of fission going on. It is lower in temperature than the deuterium in the pressurized fuel channels and acts as an extra layer of safety by cooling fuel temporarily if other forms of cooling are interrupted. This redundancy is an example of Defence in Depth design.

Fuel Channels

The fuel channels hold the bundles inside the reactor's core. Deuterium, or 'heavy water', is pressurized to keep it from boiling inside the channels, and it flows up to heat the boiler. It never mixes with the water inside the boiler. This virtual reactor is highly simplified. A real reactor can have hundreds of fuel channels.

Control Rods

By remotely sliding these rods between the fuel channels, the reaction inside the calandria can be precisely controlled to keep the heat and power output steady. When more rods are inserted, their neutron-absorbing properties slow the reaction down. When rods are drawn out, the reaction speeds up. The rods are controlled remotely from the control room.

Calandria Vessel

The water-filled calandria body is made of high-density metal that can withstand extreme temperatures without melting or distorting. Along with the high-density metal of the Primary Loop pipes, it makes up the third of the five passive barriers built into a CANDU reactor.

Shut-off Rods - Shutdown System 1

These neutron-absorbing rods, called 'shut-off' rods, will automatically drop into the reactor in the event of a power failure, stoping the nuclear reaction. These rods are so sensitive they are sometimes triggered by routine testing. The rods are held out of the reactor by electro-magnets so that if the power fails, they drop automatically. The shut-off rods, are independent shut-off mechanisms that also overlap with the gadolinium tanks, are an example of defence in depth redundancy.

Security - Safety and Control Area 12

Like all parts of the plant, the reactor is closely monitored by security staff. CNSC inspectors check that the cameras are monitored and that staff are on hand and ready to act if needed.

Radiation Protection - Safety and Control Area 7

To enter or leave the reactor containment building an individual must pass through guarded security portals that can detect explosives and radioactive materials down to a single particle. CNSC inspectors verify that these monitors are working properly and that their calibrations are up to date.

Radiation Protection - Safety and Control Area 7

CNSC inspectors check that all personnel entering the containment building wear a dosimeter, radiation protection equipment, and a radiation exposure permit that does not exceed their allowable doses. CNSC verifies that the radiation protection equipment is properly calibrated, that whole-body monitors are functioning properly, that radiation protection supply stations are appropriately stocked, that emergency showers are working, and that radiation areas are properly marked.

Waste - Safety and Control Area 11

Low-level radioactive waste is collected inside the reactor containment building in special receptacles and then packaged for transport to a specialized facility in sealed containers. Inspectors check that waste is properly stored and frequently collected. They also verify that waste is minimized during jobs.

Module icon

CANDU Inspection

Cooling Systems

A nuclear reactor's three cooling loops with 8 clickable hotspots.

Cooling Systems

Three separate cooling loops constantly draw heat away from the reactor core, which is at its hottest during fission. Even when it is shut down, a reactor stays very hot for a long time and must continually be cooled. Engineers consider every possible loss-of-cooling scenario and build in overlapping safeguards into every CANDU design.

Click the marker to begin your inspection

Primary Heat Transport Loop

The Primary Loop is a sealed set of pipes that carries deuterium, also called heavy water, through the reactor core where it gets super-heated and then carried up into the steam generator. The hot primary loop pipes turn the fresh water in the boiler into steam.

Primary Loop Pump

This pump returns cooled, pressurized heavy water to the reactor where it can be heated again. This pump has multiple back-up systems to ensure the reactor is always receiving cooled heavy water.

Secondary Loop

The secondary loop brings the steam to the turbines and returns cooled water from the condenser back to the boiler.

Secondary Loop Pump

This pump keeps water circulating from the condenser to the boiler to ensure an even temperature is maintained in the boiler and the reactor core. Nuclear power plants keep several back-up generators on-site to keep the pumps circulating even if outside power is lost. If power fails, the generator (or one of its two to six back-ups) are ready to keep the system circulating safely. Multiple safety back-up systems is an example of defence In depth design.

Condenser

The condenser collects the steam as it cools back into water. It acts as a heat exchanger between the secondary loop and the tertiary loop. The cold tertiary loop pipe running through it is sealed and never mixes its water with the water from the secondary loop. Keeping the loops sealed and separate is part of the defense in depth design of the cooling system.

Tertiary Loop

The third, or tertiary, loop circulates fresh water from a nearby source-usually a large lake-through the condenser water to cool it further. This helps keep the reactor at a stable temperature. Like the primary and secondary loops, this cooling loop is a closed circuit. Water from this loop does not mix with anything else.

Tertiary Loop Pump

The third loop pump draws water from a large, exterior cooling source past the condenser, further cooling it.

Fitness For Service - Safety and Control Area 6

All emergency equipment in a nuclear power plant is regularly tested to ensure it is ready for service in case of an emergency. Here, a backup pump is fired up to ensure it can perform as needed. On-site, CNSC inspectors do thorough inspections of the plant and check even minute details for fitness.

Module icon

CANDU Inspection

Containment

A nuclear power station's containment barriers with 9 hotspots.

Containment

Nuclear power plants are built to contain radioactivity. The reactor vessel is contained in a vault made of steel and reinforced concrete. The vault is enclosed in a sealed reactor building with steel-reinforced concrete walls that are more than a meter thick. Monitoring stations and test wells sample the surrounding air, water and soil to check that no radiation is getting out. A well-monitored perimeter, strong security and a one-kilometer exclusion zone around the reactors keep the public safe.

Click the marker to begin your inspection

Vacuum Building

Power plants with more than one reactor are linked to a Vacuum Building. In the unlikely event of a large leak in the reactor cooling system, steam and water would be released into the containment building. The vacuum building's automated valves systems are an example of defence in depth.

Exclusion Zone

All nuclear power plants have a one-kilometre Exclusion Zone around them to ensure any releases to air and water would be diluted before reaching the plant's boundary. This is the fifth of the five passive barriers built into a CANDU reactor.

Pressure Relief Duct

The vacuum building is connected to each reactor containment building by a sealed channel. During an accident, any steam pressure that builds up in the reactor containment would be automatically sucked into the Vacuum Building through this channel by the opening of special, self-actuated Pressure Relief Valves. The use of passive Pressure Relief Valves that require no on-site power to function is an example of the Defence in Depth principle.

Containment Building

The reactor is sealed inside a containment building. This building has special sumps to collect any contaminated water that might need to be treated in the case of an extreme event. The walls of the containment building are made of steel-reinforced concrete and are a metre thick. Together with the vault, the containment building is are part of the passive barriers built into a CANDU reactor.

Administration Building and Turbine Hall

For simplicity during this inspection tour, we combined the administration building, control room and turbine hall. Different power plants have different configurations and the three zones have different security rules, radiation protection requirements and access restrictions.

Reactor Vault

The entire reactor and primary heat transport loop are contained in a steel and concrete vault. Its walls are over a metre thick. The vault is hollow and surrounds the calandria with an enormous tank of fresh water that acts as an extra layer of cooling should every one of the many other sources of cooling be somehow compromised. Having a series of fail-safes for the back-up systems is an example of the defense in depth principle.

Spent Fuel Building

Spent fuel is kept in closely watched cooling pools until it can be put into dry storage on site.

Environmental Protection - Safety and Control Area 9

The area around the plant is carefully monitored. Water sampling wells and air stack monitors are tested regularly for contamination. To keep the environment and the public safe, CNSC inspectors make sure the environmental monitoring equipment is in service and that emissions are well below the limits of the plant's licence. CNSC makes sure filtration equipment has been tested and functions and verify that there is no sign of leakage around liquid effluent lagoons.

Security - Safety and Control Area 12

Nuclear power plants are highly secure areas with heavily guarded, fenced perimeters. CNSC inspectors perform thorough inspections and verify the fence has no penetrations or obstructions. Highly trained personnel patrol the plant grounds against intrusion for the safety of workers and the public.

Module icon

CANDU Inspection

Spent Fuel

A nuclear power plant's spent fuel facilities with 10 hotspots.

Spent Fuel

Over time, the amount of usable, or 'fissionable', material in uranium fuel pellets decreases until it can no longer sustain a strong enough reaction to generate electricity. The material that remains is referred to as 'spent fuel'. Spent fuel is removed (and fresh fuel is reloaded) by specialized robotic machines. Used fuel bundles are removed from the reactor and placed in water-filled bays called Spent Fuel Pools on the power plant site until they are cool enough to be put into dry storage.

Click the marker to begin your inspection

Dry Storage Building

All nuclear power plants are responsible for safely storing the spent fuel they generate. Dry waste containers hold bundles that have cooled and can now be placed in temporary storage until permanent storage facilities can be developed.

Safeguards

All dry waste areas are monitored by plant personnel as well as by the International Atomic Energy Agency (IAEA) to ensure that all fissionable material is accounted for. CNSC inspectors verify that IAEA cameras are clearly marked and not tampered with and that the waste storage area doors are always secured.

Dry Storage Containers

Dry storage containers are filled remotely underwater with cooled bundles, then drained and sealed shut before being transported to the dry storage building. The containers undergo several tests before being moved and must be perfectly sound to be placed in dry storage. All containers are marked with unique codes for tracking and security.

Packaging and Transport - Safety Control Area 14

When spent fuel bundles have cooled down sufficiently (after 5 to 10 years), they are packaged into highly specialized, extremely resilient dry storage containers. These containers are sealed and labeled with a unique code for tracking. CNSC inspectors record and track all waste generated by power plants and ensure that Transportation of Dangerous Goods Regulations are followed.

Spent Fuel Cooling Pools

Water in the Spent Fuel Pools helps cool the fuel bundles and provides shielding from radiation while their radioactivity naturally decreases through radioactive decay. Spent fuel is transferred underwater when it is outside the containment building.

Operating Performance - Safety Control Area 3

The plant is operated with strict operating policies. On-site inspectors verify that safety equipment is poised and ready to perform its function, that gauges and indicators are functioning properly, and that warning alarms are addressed by the operator.

Waste Management - Safety Control Area 11

The bundles of spent fuel in the cooling pools are carefully monitored as they slowly cool. CNSC on-site inspectors verify that the licensee is storing waste in approved locations, that the waste is labelled correctly, separated appropriately, and that surveys are routinely done. The inspectors also regularly check the integrity of the waste storage installations, checking for leaks or faults in the concrete.

Spent Fuel Cooldown

Besides being a coolant for the hot fuel, water is also an excellent method of shielding. Four metres of water over the bundles makes the area over the spent fuel safe to walk around with no protective clothing. Storing spent fuel in pools outside the reactor containment building there is no chance of the spent fuel creating a problem with the reactor. This is an example of Defence in Depth design.

Spent Fuel Transfer Channel

Robotic conveyors move spent, irradiated fuel to the cooling pools by way of a sealed transfer channel under the containment building. This minimizes the risks of exposure and keeps the containment building sealed when spent fuel is being removed. This is an example of defence in depth design.

Refueling Robots

A CANDU reactor is refuelled remotely. This reduces the risk of exposure to workers and allows defective fuel to be detected and safely removed while the reactor is on power. Remote fuelling is an example of defence in depth.

Module icon

CANDU Inspection

Control Room

A nuclear power plant's control room with 7 hotspots.

Control Room

The nerve centre of the power plant is the control room. All automated systems are controlled and monitored from here. Drills, emergency scenarios and simulations are used to prepare the control room's highly trained personnel for nearly every eventuality. Should an event make this room inaccessible, the entire system can be transferred to a secondary control room further from the reactor site.

Click the marker to begin your inspection

Simulation Room

All control room staff are trained in a control room simulator. It is identical to the real control room. All control room personnel, including the Operator and Supervisor, train in simulations on a weekly basis and are evaluated on every performance. Simulator room tests are part of the certification process. Tests are recorded and reviewed by CNSC inspectors.

Emergency Management and Fire Prevention - Safety Control Area 10

CNSC inspectors ensure the plant is up to date with its fire drills and that fire- management equipment is available and in good repair. They verify that emergency response teams are adequately staffed, that personal protection equipment is available and fully stocked, that fire exits are clear, that PA speakers are functioning, and that fire doors are closed and well sealed. They also observe and participate in regular emergency preparedness exercises.

Conventional Health and Safety - Safety Control Area 8

To prevent workplace hazards and to protect personnel and equipment, safety equipment must be worn in designated areas. CNSC inspectors verify that personal protection equipment is worn at all times, that pre-job briefs are conducted and documented when required, and that emergency stations like eyewash stations and emergency showers are inspected regularly.

Human Performance - Safety Control Area 2

Power plant personnel undergo regular simulator exercises to train for various scenarios. CNSC inspectors review their performance during these simulations and during daily activities. Inspectors ensure staff use proper communication habits designed to reduce miscommunications and always use independent verification when making decisions. The inspectors also verify that no one is drowsy or unfit for duty and check that each shift has a full complement of people.

Safety Analysis - Safety Control Area 4

Nuclear design engineers consider multiple potential scenarios to put preventative measures in place before they are needed. CNSC inspectors ensure that even the smallest details are addressed by verifying safety equipment and mitigations related to these scenarios. For example, to mitigate the effects of a possible earthquake, inspectors routinely check that back-up equipment is available for service, that seismic corridors are free of obstructions, that equipment is always safely tied down, and that solid structures have been evaluated against damage by seismic activity.

Control Room Operator and Supervisor

The control room Operator and Supervisor are highly trained, certified professionals. Operator certification requires an engineering degree followed by eight years of training. An additional four years of training are needed for Supervisor certification. The Operator and Supervisor oversee the plant's daily functioning and are extensively trained to handle extreme events such as natural disasters.

Management Systems - Safety Control Area 1

To gauge the overall performance of the plant, CNSC inspectors review management policies and procedures, and the performance of personnel. They attend daily operation meetings and meet with managers about employee and plant reviews. CNSC inspectors also witness performance and coaching activities.