The CANDU Reactor: Inside a Nuclear Power Plant

Boiling water, the hard way

A CANDU reactor is one of the most advanced machines Canada has ever built. Engineers have to control a nuclear chain reaction, handle highly radioactive fuel, and keep hundreds of tonnes of hot, high-pressure water flowing safely.

And the reason for all of it is... to boil water. A nuclear generating station is a giant steam engine: the reactor heats water into steam, the steam spins a turbine, and the turbine drives a generator. In this lesson you will follow the energy from a uranium nucleus all the way to the outlet in your wall, and find out why two-thirds of it ends up warming a lake.

Inside a CANDU reactor

CANDU stands for CANada Deuterium Uranium. It was designed in Canada, and all of Ontario's operating nuclear stations (Pickering, Darlington and Bruce) use CANDU reactors.

• Fuel: pellets of natural uranium oxide (mostly U-238, with about 0.7% U-235), sealed in metal tubes and grouped into fuel bundles about half a metre long. CANDU reactors do not need enriched uranium.
• Moderator: fission neutrons are fast, but U-235 captures slow neutrons best. The fuel sits in a large tank of heavy water (D₂O), which slows (moderates) the neutrons by collisions while absorbing very few of them. That is why natural uranium is enough.
• Coolant: heavy water under high pressure is pumped past the fuel through pressure tubes, carrying heat away at about 300 °C. The pressure keeps it from boiling.
• Control: adjustable control rods and other devices containing neutron absorbers keep the chain reaction steady (each fission causing exactly one more on average). Two independent shutdown systems can stop the reaction within seconds: one drops shutoff rods in, the other injects a neutron-absorbing liquid into the moderator.
• On-power refuelling: machines remove used bundles and insert fresh ones while the reactor keeps running.

From the reactor to your outlet

The heat leaves the reactor and is turned into electricity in the rest of the station:

1. Steam generator: the hot heavy-water coolant flows through thousands of tubes inside a steam generator. Heat is conducted through the tube walls into ordinary water on the outside, which boils into steam. The two loops never mix, so the radioactive coolant stays inside the reactor building.
2. Turbine: high-pressure steam blasts against the blades of a turbine, making it spin. Thermal energy becomes kinetic energy.
3. Generator: the turbine shaft turns a generator, where spinning magnets and coils convert kinetic energy into electrical energy (you study how in the Electricity and Magnetism unit).
4. Condenser: the used, lower-pressure steam goes to a condenser, where cold water pumped from the lake flows through tubes. The steam releases its latent heat and condenses back into water, which is pumped back to the steam generator. The lake water, now a few degrees warmer, is returned to the lake.

The laws of thermodynamics at work

First law (conservation of energy): energy is never created or destroyed. In steady operation, all the thermal energy produced in the reactor each second must leave the station each second, either as electrical energy or as heat to the surroundings:

thermal energy in = electrical energy out + waste heat

Second law: heat flows naturally from hot to cold, and a heat engine can turn heat into work only while heat is flowing from a hot place (the steam) to a cold place (the lake). Some heat must always be released to the cold place. No heat engine, however well built, can turn all of its heat into work.

That is why the condenser and the lake are not optional extras: without a cold place to dump heat, the turbine could not keep running. In a CANDU station, roughly one-third of the reactor's thermal energy becomes electrical energy; about two-thirds is released as waste heat, mostly into the lake. (Coal and natural-gas steam plants also lose much of their energy as waste heat for the same reason. You compare the efficiencies of different energy sources in the sibling unit Work, Energy and Power.)

Safety and the environment

Defence in depth. Radioactive material is kept in by several barriers: the ceramic fuel pellet, the sealed metal fuel sheath, the pressure tubes and cooling system, and a thick concrete containment building. Multi-unit Ontario stations also have a large vacuum building that can pull in and hold steam if a pipe breaks.

Benefits. Nuclear stations produce large amounts of electricity steadily, day and night, with very low greenhouse gas emissions while operating.

Concerns. Used fuel is highly radioactive and must be managed for a very long time; uranium mining affects land and communities; accidents, though rare, can be serious; and the warm water returned to the lake can affect fish and other aquatic life near the outflow. Stations are designed and regulated to limit these effects, and they are monitored by the Canadian Nuclear Safety Commission. You will weigh these issues in Nuclear and Geothermal Technology and Society.