CANDU Reactors

A CANDU (CANada Deuterium Uranium) reactor is a Canadian designed pressurized heavy water reactor (PHWR). This reactor design uses natural uranium fuel and heavy water coolant and moderator. A high-level schematic is shown below:

  1. Fuel Bundles
  2. Calandria Vessel
  3. Adjuster Rods
  4. Pressurizer
  5. Steam Generator
  6. Light Water Condenser Pump
  7. Heavy Water Heat Transport Pump
  8. Fuelling Machine
  9. Heavy Water Moderator
  10. Pressure/Calandria Tube
  11. Steam to Turbine
  12. Condenser to Steam Generator
  13. Concrete Containment

Fission reactions (from thermal neutrons) occur in the fuel creating heat. The heat is removed by heavy water coolant (Heat Transport System) from the Primary Side and brought to the steam generator. Light water from the Secondary Side of the steam generator heats up to produce steam. The steam turns a turbine and produces electricity.

CANDU fuel is arranged in fuel bundles. Fuel bundles are made up of fuel elements (28 or 37) containing fuel pellets. Fuel pellets contain \(UO_2\) ceramic pellets with natural uranium \((0.71 \% \; ^{235}U)\).

Fuel bundles are contained in horizonal fuel channels. Fuel channels have pressure and calandria tubes with an annual gas inbetween them. a unqiue feature of CANDU reactors is online fuelling - fuelling the reactor with new fuel while it is operating. Refulling is required to maintain long term reactivity. As the fuel burns, it loses fissile material and becomes less reactivity. Furthermore, fission products build up and absorb valuable neturons.

Heavy water coolant circulates through the fuel channels (Primary Heat Transport System) and cools the hot fuel. Then it enters a heat exchanger in the steam generator where it creates steam from light water (Secondary Side). The steam is used to turn turbines and create electricity. Then, it condenses in the condenser and is recirculated to the steam generators.

Fuel channels are sourrounded by heavy water moderator. The purpose of the moderator is to thermalize (slow down) fast neutrons produced from fission to undergo subsequent thermal fissions and sustain a chain reaction. Heavy water has a lower neutron absorption propoerty, making it suitable for CANDU reactors with natural fuel. Other reactor designs may use light water moderators with enriched fuel. Neutrons scatter with the hyrogen in the moderator causing them to lose energy and increase the chances of fissioning.

Reactivity is controlled by refuelling and various neutron absorbing mechanisms. Adjuster Rods (21) are normally in-core and flatten the neutron flux distribution (naturally peaked in the center) to get more overall power without exceed individual channel and bundle power limits. Liquid Zone Controllers contain light water and are used to control reactivity in 14 zones of the reactor. Light water can be added to reduce zone reactivity or removed to increase zone reactivity. Mechanical Control Absorbers (4) are used to reduce overall core reactivity. Shutoff Rods (~28) are used to quickly shutdown the reactor (Shutdown System 1 or SDS1). Moderator Poison such as boron or gadolinium nitrate can also be used to quickly shutdown the reactor (Shutdown System 2 or SDS1).

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