Reports of two radioactive spills at the nuclear power plant in Point Lepreau, N.B., late in 2011 have raised concerns with the head of Canada's Nuclear Safety Commission.

Michael Binder, the president of the commission, called the news “unsettling.”

In light of the meltdowns at Japan's Fukushima Daichi plant resulting from the tsunami in March 2011, the CNSC has made numerous assurances regarding the safety of plants in this country. It stressed that none of Canada's nuclear facilities are on or near fault lines capable of causing a major earthquake.

Point Lepreau is one of five nuclear generating facilities in operation in Canada. Three other plants are in Ontario and another is in Quebec. There are also a number of smaller institutional reactors, like the one in Chalk River, Ont., which are used for creating medical isotopes and research purposes.

All of the power-generating plants in Canada use the CANDU reactor. Short for CANada Deuterium Uranium, this pressurized heavy water reactor is a Canadian innovation.

Where are Canada’s nuclear plants?

Bruce Nuclear Generating Stations A and B

Opened in 1977 and located in Kincardine, Ont., about 150 kilometres northwest of London, the Bruce plant contains two generating stations, which each hold four reactors. Six of the units are currently operational, and produce over 4,700 megawatts (MW) of power.

Pickering stations A and B

This facility, which is about 30 kilometres east of Toronto, was built in 1971. Equipped with six working reactors, Pickering is one of the largest nuclear facilities in the world, and generates about 3,100 MW.

Darlington

Located in Bowmanville, Ont., about 70 kilometres east of Toronto, this four-unit station produces 3,512 MW — about 20 percent of Ontario’s needs. Plans are in the works to add up to four new reactors to the site, which, once built, will give Darlington a total generating capacity of 4,800 MW.

Gentilly-2

This facility near Bécancour, Que., was built in stages between 1966 and 1983 and originally featured two plants. Gentilly-1 is now closed and in the decommissioning process, while Gentilly-2 remains in operation and outputs about 675 MW. The site also contains a 381-MW gas turbine generation plant.

Point Lepreau

Built between 1975 and 1982 on the north shore of the Bay of Fundy, this station has a capacity of 635 MW.

Other uses of nuclear reactors

Nuclear reactors are primarily used for power generation, but they are also employed for uranium processing, research and processing for industrial and medical purposes. They can be found in the following Canadian locales:

Uranium processing and fuel fabrication facilities

• Blind River uranium refining facility (Blind River, Ont.)
• Port Hope uranium conversion facility (Port Hope, Ont.)
• Port Hope nuclear fuel facility
• Toronto nuclear fuel fabrication facility
• Peterborough nuclear fuel facility

Nuclear research and isotope production

• McMaster University (Hamilton, Ont.)
• École Polytechnique (Montreal)
• Saskatchewan Research Council (Saskatoon)
• University of Alberta (Edmonton)
• Royal Military College of Canada (Kingston, Ont.)
• Atomic Energy of Canada Ltd. in Chalk River, Ont. (four reactors, two of which are in extended shutdown state)

(Note: Dalhousie University's Life Sciences Centre in Halifax used a Slowpoke-2 reactor for research until 2008, and it was successfully decomissioned in 2011.)

What does a nuclear reactor do?

Nuclear energy is produced through the splitting, or fission, of the uranium 235 (U-235) isotope. When a neutron hits a U-235 atom, it creates an unstable uranium isotope that divides and releases two other neutrons, as well as heat and various radioactive particles. The newly released neutrons then go on to bombard other U-235 atoms, setting off a chain reaction that continues until the uranium fuel is used up.

If the neutrons move too fast, they pass through the U-235 atoms without affecting them, so they must be slowed down with the help of a so-called moderator, such as water.

A reactor needs uranium, a moderator to slow fast-moving neutrons, a coolant to absorb heat released during the reaction, and a system for shielding radiation.

What sets a Candu reactor apart?

Only 0.7 per cent of naturally occurring uranium consists of the U-235 isotope — not enough to sustain a chain reaction. Reactors, then, either need to enrich the uranium to increase the proportion of the isotope or use a more effective moderator. Most reactors use enriched uranium, a more expensive process.

Candu reactors use heavy water (deuterium oxide) to improve the likelihood of a chain reaction. The hydrogen atom, as present in ordinary water, is almost exactly the same size as the fast-moving neutrons created by nuclear fission. When a neutron collides with hydrogen, it will lose almost all of its energy and slow down enough to facilitate the fission reaction.

However, a regular hydrogen atom can also absorb the neutron, decreasing the likelihood of fission, which is why Candu reactors use the hydrogen isotope deuterium, known as heavy water. Deuterium will not absorb the neutron, improving the chances of a chain reaction.

Heavy water is more expensive than ordinary water, but it allows the use of natural uranium as an energy source.

Candu reactors have been sold to nuclear plants in Romania, China and South Korea. The reactors in Japan are not Candu reactors.

How does a Candu reactor work?

The reactor is best thought of as a giant tank filled with heavy water and a series of half-metre-long fuel rods bundled into what are called fuel assemblies. The fuel rods are filled with pellets of uranium in the form of uranium dioxide. The heat generated by the fission process is transferred to the heavy water and used to produce steam that powers a turbine connected to an electrical generator that feeds the energy grid.

In order to further control the fission process, solid cadmium rods that absorb unwanted neutrons are inserted into the reactor tank, perpendicular to the fuel assemblies. There are more of these control rods than necessary as a safety precaution.

The Candu reactor is surrounded by a thick wall designed to absorb dangerous radiation generated during the fission reaction. It consists of several nested layers made up of materials such as boron and cadmium, which act like neutron shields, as well as lead and concrete, which block gamma radiation.

Some notable nuclear accidents in Canada

Chalk River, 1952 and 1958

A power surge and partial loss of coolant led to significant damage to the NRX reactor core in 1952. It was the world's first major nuclear reactor disaster, and it resulted in 4.5 tonnes of radioactive water collecting in the cellar of the building. In 1958, a fuel rupture in the reactor led to a fire and complete contamination of the NRU building. The military was called in both times to aid in the cleanup.

Pickering, 1974 and 1983

The most serious nuclear accidents in Canada happened at the Pickering facility east of Toronto, in 1974 and in 1983. In each case, pressure tubes — which hold fuel rods — ruptured. Some coolant escaped, but was recovered before it left the plant, and there was no release of radioactive material from the containment building.

Darlington, 2009

In 2009, more than 200,000 litres of water containing trace amounts of tritium, the radioactive isotope of hydrogen, spilled into Lake Ontario after workers accidentally filled the wrong tank with a mixture of tritium and water. The level of the isotope in the lake was not considered enough to pose harm to residents.