How a nuclear reactor works
In general, a nuclear reactor performs the same function as a furnace in a gas or oil-fired generating station. Rather than using gas or oil as a fuel to boil water to create high-pressure steam, the reactor uses the fissioning or splitting of uranium atoms to generate heat to make steam.
In all generating plants, the steam is then directed at the blades of a turbine, which causes the wheel and shaft to spin at high speed. The shaft is connected to the rotor of the generator, where mechanical energy is converted into electrical energy.
At Bruce Power, each reactor has its own turbine set, which includes one high-pressure cylinder and three low-pressure cylinders. The turbine rotor spins at 1,800 rpm.
Inside the reactor
In 1939, two scientists discovered that when uranium atoms are split by bombarding them with neutrons, three things happen: heat is generated and more neutrons are released and the process repeats itself. This is called fission.
To control the rate of fission and to produce only the specific amount of heat needed to generate the required electricity, a “moderator” – heavy water – is used to slow down neutrons enough so that they strike other nuclei and sustain a controlled chain reaction.
The power of uranium as a fuel
Uranium is the heaviest natural occurring atom within the 110 chemical elements we have on earth. It also contains a significant amount of energy. Fissioning one U-235 uranium atom releases 400,000 times more energy than burning one atom of carbon.
Uranium fuel for our reactors is first formed into ceramic pellets then sealed in metal tubes. Each seven-gram pellet – roughly the weight of two nickels – contains as much energy as three barrels of oil.
A fuel bundle is an assembly of 37 fuel elements – which are thin, pencil-like tubes about two feet long – containing 29 uranium dioxide pellets in a metal sheath that is welded together at both ends.
The pencils are assembled into roughly 22-kg fuel bundles. Each reactor has 480 fuel channels that each hold 12 or 13 fuel bundles, placed end to end.
A single fuel bundle, about the size of a fire log, can supply 100 homes with electricity for a year. The same electricity from a fossil station would require 400 tonnes of coal, or 270,000 litres of oil, or 300 million litres of natural gas.
What is a CANDU reactor?
CANDU stands for Canada Deuterium Uranium. Deuterium is another name for heavy water, which occurs naturally in all bodies of water. In Lake Huron, it occurs one part in every 7,000. Once extracted, heavy water is 10 per cent heavier than ordinary water due to an extra neutron in its nucleus giving it added weight.
Developed in Canada, the first CANDU reactor came on line in 1962. There are now 22 CANDU reactors in Canada and 17 abroad.