Reactor type 3 of 10
Heavy water reactor (CANDU)
by Joe Miller
Canada's CANDU uses heavy water, water made with deuterium, a heavier form of hydrogen. Heavy water slows neutrons without absorbing many, so the reactor runs on natural uranium with no enrichment, and it refuels while running.
Plain-English explainer by Joe Miller, a nuclear engineer. Not investment advice. Figures are the latest public numbers we could find and change fast; sources are at the bottom of each page.
What it looks like
How the coolant circulates
- Heat transport pumps push pressurized heavy water through the pressure tubes.
- The coolant picks up heat from the fuel bundles (about 310°C).
- It flows to steam generators and boils light water on the secondary side.
- Separately, the moderator heavy water in the calandria is cooled by its own loop.
- Steam drives the turbine as in other plants.
The main components
| Component | What it does |
|---|---|
| Calandria | A large horizontal tank of cool, low-pressure heavy water that acts as the moderator. |
| Pressure tubes | About 380 to 480 horizontal zirconium tubes running through the calandria, each holding 12 or 13 fuel bundles in hot, pressurized heavy-water coolant. |
| Fuel bundles | Short bundles, about 20 inches long, of natural uranium oxide, about the size of a fireplace log. |
| Fueling machines | Robotic machines that lock onto both ends of a tube while the reactor runs, push new bundles in and take spent ones out. |
| Steam generators and heat transport pumps | Like a PWR: hot heavy water boils ordinary water in steam generators. |
| Shutdown systems | Two independent systems: spring-driven shutoff rods and liquid gadolinium injected into the moderator. |
Key details
| Item | Detail |
|---|---|
| Coolant | Pressurized heavy water |
| Moderator | Low-pressure heavy water, separate |
| Fuel | Natural uranium oxide (0.7% U-235) |
| Pressure | About 1,450 psi coolant |
| Efficiency | About 30 to 32% |
| Typical size | 500 to 900 MWe |
| Refueling | Online, every day |
How it is maintained
- Online refueling, so no refueling outages; planned outages are for maintenance only.
- Pressure tube inspections for creep, sag and hydrogen pickup; tubes are the life-limiting part.
- Major refurbishment after about 30 years: every pressure tube and feeder is replaced in a 3 to 4 year outage, extending life another 30 years or more (now under way at Darlington and Bruce).
- Heavy-water management: leaks are recovered, and tritium is removed and sold.
Where it runs
Canada (Bruce, Darlington, Pickering, Point Lepreau), India (its own PHWR designs), Argentina, Romania, South Korea and China.
Who's writing this? I'm Joe Miller, a nuclear engineer since 1974 and an ASME Fellow. I've run safety analyses for PWRs and BWRs, directed nuclear engineering at a working BWR, and reviewed new reactor designs for the NRC. This site explains every kind of reactor in plain English. My background and papers.