Inside a CANDU power plant.
Follow the heat. Understand the parts. Remember the purpose.
CANDU stands for Canada Deuterium Uranium. To understand the plant, begin with the job it performs: release nuclear energy as heat, transfer that heat to steam, turn a shaft and generate electricity.
The memory chain
Make heat → Move heat → Make steam → Turn → Generate → Condense → Return.
For every component, ask: What enters? What changes? What leaves?
Before the plant: atoms, isotopes and energy.
An atom has a nucleus containing protons and neutrons, surrounded by electrons. The proton count identifies the element. Isotopes of one element have the same proton count but different neutron counts. The number after an isotope’s name counts protons plus neutrons.
For uranium-235, 92 protons + 143 neutrons = 235. For cobalt-60, 27 protons + 33 neutrons = 60. A neutron capture can change the isotope; a later beta decay can change the element.
Protons name it. Neutrons vary it. Protons + neutrons number it.
Reference: NRC — What is an atom? ↗
Fission splits. Fusion joins.
| Process | What changes? | Connection to CANDU |
|---|---|---|
| Fission | A heavy nucleus splits into lighter nuclei, releasing energy and neutrons. | The source of reactor heat. |
| Fusion | Light nuclei combine to form a heavier nucleus, releasing energy for suitable reactions. | CANDU is not a fusion reactor. |
| Neutron capture | A nucleus absorbs a neutron, often emitting a gamma ray. | Important in isotope production and neutron balance. |
| Radioactive decay | An unstable nucleus transforms and emits radiation. | Explains decay heat and radioisotope behaviour. |
In both energy-releasing fission and fusion, the products have less total rest mass than the starting particles. The difference appears as energy: E = Δmc². Fusion requires conditions that overcome the electrical repulsion between positively charged nuclei; heating ordinary water in a CANDU reactor does not cause fusion.
How one fission becomes useful heat.
- Capture: a uranium-235 nucleus absorbs a neutron and forms an excited uranium-236 nucleus.
- Split: the excited nucleus can break into two lighter fragments, releasing additional neutrons and radiation. Several product combinations are possible.
- Heat: the energetic fragments slow down within the fuel, depositing much of their energy as heat.
- Continue: some released neutrons cause further fissions. Others escape or are absorbed without causing fission.
- Control: moderation slows neutrons; neutron-absorbing devices regulate their availability. Coolant carries the heat to the steam generators.
²³⁵U + ¹n → ²³⁶U* → ¹⁴¹Ba + ⁹²Kr + 3¹n + energy
Check the bookkeeping: the mass numbers sum to 236 on both sides, and the product atomic numbers 56 + 36 equal uranium’s 92. The asterisk denotes an excited nucleus. This equation shows one possible split, not a unique pair of products.
The effective multiplication factor, keff, compares successive neutron generations: below one means a decreasing chain reaction; one means a self-sustaining chain; above one means growth. In reactor physics, critical means self-sustaining, not automatically dangerous. Delayed neutrons, emitted after certain fission products decay, strongly influence reactor-control behaviour.
After shutdown, radioactive products keep releasing energy. Stopping the chain reaction does not make the fuel instantly cold.
Capture → split → heat → repeat. Shutdown stops the chain; cooling removes the remaining heat.
Reference: CANTEACH — CANDU Fundamentals, fission and criticality ↗
Electricity and medical isotopes: two uses of reactor neutrons.
The medical isotope is lutetium-177 (Lu-177). Lithium is a different element. CANDU isotope production uses selected target materials exposed to neutrons; it is distinct from splitting uranium to generate heat.
Cobalt-60: capture a neutron.
Cobalt-59 targets incorporated into suitable adjuster assemblies absorb neutrons and become cobalt-60. The material is harvested during planned outages and sent for controlled processing into sources. Cobalt-60 provides gamma radiation used for medical-device sterilization and certain radiotherapy applications. Adjuster assemblies should not be confused with emergency shutoff rods.
Lutetium-177: capture, then transform.
¹⁷⁷Yb → ¹⁷⁷Lu + β⁻ + electron antineutrino
Bruce Power’s isotope-production approach uses ytterbium-176 targets. Neutron capture forms ytterbium-177; beta-minus decay then produces lutetium-177. In beta-minus decay, a neutron becomes a proton, so the atomic number increases while the mass number remains 177.
The isotope subsequently needs chemical separation, purification, quality control and pharmaceutical preparation. Lu-177 can be linked to a targeting molecule for specific cancer therapies. A reactor produces the radionuclide; it does not directly produce a ready-to-administer medicine.
Co-59 catches one → Co-60. Yb-176 catches one, then changes element → Lu-177.
Reference: Bruce Power–Saugeen Ojibway Nation partnership — Isotopes ↗
Reference: Kinectrics — Isotope Production System engineering ↗
Three circuits. Two heat-transfer boundaries.
Trace one circuit at a time. Primary coolant returns to the reactor; condensed secondary water returns to the steam generators; condenser cooling water carries rejected heat to the heat sink. Following a pipe path is different from following energy: heat can cross a tube wall while the fluids remain separate.
The parts, explained by their jobs.
These explanations describe the CANDU system family, using CANDU 6 for the two shutdown-system examples. Equipment arrangements vary between stations.
Fuel bundles
Heat starts here.
Ceramic uranium-dioxide pellets sit inside sealed metal elements assembled into bundles. Fission releases energy within the fuel; that energy passes through the pellet and sheath to the surrounding coolant.
Fuel = the heat source.
Pressure tubes
Carry fuel and hot coolant.
Horizontal pressure tubes contain the fuel bundles and the high-pressure coolant flowing over them. They provide a pressure boundary at each fuel channel rather than enclosing the whole core in one large pressure vessel.
Pressure tube = pressurized pathway.
Calandria and moderator
Give neutrons the right pace.
The calandria holds heavy-water moderator around the fuel channels. Collisions slow fast neutrons, improving their ability to cause further fission in uranium-235. Heavy water absorbs relatively few neutrons, supporting the use of natural-uranium fuel.
Moderator slows neutrons; coolant carries heat.
Calandria tubes and annulus
Separate hot channels from cooler moderator.
A calandria tube surrounds each pressure tube. The gas-filled annular gap limits heat transfer between the hot pressure tube and the much cooler moderator. These concentric tubes perform different jobs.
Inner tube: pressure. Outer tube: separation.
Headers, feeders and heat-transport pumps
Collect, distribute and circulate.
Headers connect many channels. Feeders carry coolant between headers and individual channels. Heat-transport pumps maintain circulation through the reactor and steam generators, moving energy out of the fuel.
Headers gather; feeders connect; pumps circulate.
Steam generators
Pass heat across a boundary.
Primary coolant flows through tubes while separate secondary water receives heat and boils. The tube wall transfers energy while keeping the two fluids apart during normal operation.
Heat crosses. Water stays in its circuit.
Pressure and inventory control
Keep the coolant system within its working range.
Pressure control manages coolant conditions; inventory control manages the amount of coolant. CANDU arrangements differ by station, including the use of pressurizers and associated feed-and-bleed systems.
Pressure is a condition; inventory is an amount.
Steam turbine
Turn flowing steam into rotation.
Steam expands through stationary and rotating blade rows. Its energy produces shaft torque. Moisture separation and reheating between turbine sections help manage wetness and improve cycle performance.
Steam pushes; the shaft turns.
Generator and transformer
Turn rotation into usable electrical power.
A rotating magnetic field induces electrical voltage in the generator’s stator windings. A step-up transformer raises voltage for transmission, reducing the current needed to deliver a given power.
Generator makes electricity; transformer changes voltage.
Condenser
Turn exhaust steam back into water.
Steam leaving the turbine condenses on cool tube surfaces. Removing latent heat maintains low turbine-exhaust pressure and recovers water for reuse. Separate cooling water flows through the condenser tubes.
Condenser closes the steam cycle.
Condensate and feedwater systems
Return the working fluid.
Pumps move condensed water through the return train. Feedwater heaters recover useful heat; the deaerator removes dissolved gases. Feedwater pumps raise pressure so water can enter the steam generators.
Recover, condition, pressurize, return.
Cooling-water system
Carry rejected heat to the surroundings.
Cooling water absorbs heat in the condenser and transfers it to the ultimate heat sink. This circuit is separate from the steam/feedwater circuit and primary heat transport.
The third circuit removes the heat not converted to electricity.
Fuelling machines
Replace fuel through the channel ends.
Specialized machines connect to opposite ends of a channel to transfer fresh and used bundles while preserving the channel’s pressure boundary. On-power refuelling is a defining CANDU capability.
Fuel moves through channels, not an opened reactor vessel.
Used-fuel pool
Cool and shield discharged fuel.
Fuel continues producing decay heat after removal. Pool water removes heat and provides radiation shielding while the fuel is stored under controlled conditions.
Used fuel still needs cooling.
Reactor regulating system
Keep power and its distribution controlled.
Neutron detectors and process instruments supply measurements. Control logic uses regulating devices, including liquid-zone controllers and rods, to manage reactor power and its spatial distribution.
Measure → compare → adjust.
Shutdown systems
Stop the chain reaction.
Independent systems provide different ways to add strong neutron absorption. In CANDU 6, one inserts shutoff rods; the other injects neutron-absorbing liquid into the moderator.
Two independent brakes on fission.
Shutdown cooling and emergency core cooling
Remove heat in different conditions.
Shutdown cooling removes heat during appropriate shutdown conditions. Emergency core cooling supplies coolant following a loss of coolant. Stopping fission does not eliminate decay heat.
Stop the reaction; keep removing heat.
Containment and shielding
Limit release and reduce exposure.
Containment limits escape of radioactive material; shielding attenuates radiation. Pressure-control features differ: single-unit designs can use dousing, while Ontario multi-unit stations use vacuum-building arrangements.
Containment holds material; shielding reduces radiation.
Electrical supplies and batteries
Support essential plant functions.
Normal and backup supplies support plant loads. Batteries provide uninterrupted DC for selected functions; inverters supply selected AC loads. Standby generation supports loads that can tolerate a transfer interval.
Different loads need different continuity.
Service systems
Keep the main equipment available.
Cooling services, instrument air, ventilation, lubrication and water chemistry support reliable plant operation. A major machine depends on these less-visible systems as much as on its main energy supply.
Support systems make the headline systems work.
Four distinctions that make the plant easier to understand.
| Often confused | Remember the different jobs |
|---|---|
| Moderator / coolant | Neutron speed / heat removal. Heavy water can serve both functions in separate systems. |
| Steam generator / electrical generator | Transfers heat to boil water / converts shaft work into electricity. |
| Shutdown / cooling | Stops the chain reaction / removes continuing heat. |
| Shielding / containment | Reduces radiation exposure / limits escape of radioactive material. |
The engineering behind the flow.
Heat-transfer rate follows the energy balance Q̇ = ṁ(hout − hin). Mass flow tells us how much fluid passes each second; enthalpy change tells us how much energy each kilogram gains. A turbine extracts work as steam enthalpy falls. A condenser removes the heat needed to turn that exhaust back into liquid.
Remember why the condenser is essential: a continuous heat engine needs both a heat source and a heat sink. The turbine cannot turn all incoming heat into electrical work. Pumps, heat exchangers, valves, bearings and instrumentation each support that energy balance.
Close the diagram. Explain it back.
Answer aloud, then reveal the explanation.
Does reactor coolant normally flow through the turbine?
No. It transfers heat through steam-generator tubes to the separate secondary circuit.
What does the moderator slow down?
Neutrons. Its main purpose is different from carrying heat away from the fuel.
Why cool a reactor after shutdown?
Radioactive decay continues to release heat after the chain reaction stops.
Which component turns steam into shaft work?
The turbine. The generator then converts shaft work into electrical power.
Where does exhaust steam go?
To the condenser, then through the condensate and feedwater systems back to the steam generators.
Why is one diagram not enough to understand a station?
An energy-flow map omits support systems, control relationships and station-specific arrangements. Study those connections separately.
Continue with the technical references.
Source material: CNSC and The Essential CANDU, published by UNENE. The explanations and memory cues above are written for this learning guide.
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