The earthquake and tsunami that hit Japan focused attention on the Fukushima nuclear power plant in the northern region of the island nation. Explosions at the plant and the failure of the systems used to cool its reactors heightened the risk of a meltdown.

The Canadian Nuclear Safety Commission (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.

CNSC has said that if an earthquake or similar disaster happened here, Canadian nuclear plants would be safe because the pressurized heavy-water used at Candu reactors is effective at cooling the reactor core.

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 apart a Candu reactor?

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.

How many reactors are in Fukushima?

There are six reactors at the Fukushima Daiichi power plant, all of which are boiled-water reactors. This means ordinary water is used to cool down the fuel rods, which contain enriched uranium. At the time of the earthquake on March 11, three reactors were operational.

What is a meltdown?

Nuclear fission creates immense amounts of heat and radiation. Under normal circumstances, the reactor core is cooled by a moderator or coolant, usually water or heavy water (Candu only). To avoid catastrophe, nuclear plants always have at least one secondary source of coolant. A meltdown occurs when the nuclear fuel overheats and essentially melts, leaking radioactive material.

To protect the surrounding environment in the event of a meltdown, all reactors have a series of walls surrounding the core. Significant damage to these walls could allow radioactivity to leak out.

What is the threat of meltdown at Fukushima?

When the 9-magnitude earthquake hit off the eastern coast of Japan, all three operational reactors immediately shut down and activated their emergency generators in order to remove residual heat. An hour later, tsunami-triggered flood waters damaged and deactivated the generators, causing heat from the reactor cores to evaporate the coolant.

At one of the reactors, efforts to use sea water and boric acid to keep fuel rods covered failed, leaving the rods exposed and increasing the risk of meltdown.

Radioactive material has been detected in the air and water surrounding the plant, and the Japanese government has issued evacuation orders for those living within 20 kilometres of the plant and told those within 30 kilometres to remain inside with doors and windows sealed. They have also distributed potassium-iodide tablets, which reduce or prevent the absorption of radioiodine, a byproduct of nuclear fission.

Japanese officials upgraded the international nuclear event scale rating, which gives an idea of the extent of radiation, for some parts of the plant from 4 to 5 on Friday, indicating an accident with wider consequences. Other parts of the plant were rated a 3, indicating a "serious incident."

What happened at Chornobyl?

A major accident is denoted with a ranking of 7 on the international nuclear event scale. The only incident to warrant this ranking was the 1986 disaster at the Chornobyl nuclear plant in Ukraine, which was assigned a ranking retroactively when the INES system was created.

An accidental power surge in a reactor at Chornobyl caused a severe meltdown. Chornobyl’s reactors were of a type known as RBMK (a Russian acronym) and used graphite as the reaction moderator.

The power surge, which occurred during a systems test, caused a series of explosions that damaged the reactor facility, leading to a leak of the graphite moderator. The graphite then ignited, resulting in a massive fire and subsequent plume of radioactive fallout, 60 per cent of which landed in nearby Belarus. The meltdown was directly responsible for killing 31 people while the fallout led the then Soviet government to relocate nearly 350,000 people and caused incidents of radiation-related illnesses in areas as far away as Berlin and Turkey.