Pressurised Heavy Water Reactor

A pressurised heavy water reactor (PHWR) produces heat by maintaining a controlled chain reaction in uranium fuel. It uses heavy water, water containing deuterium, as both a neutron moderator and a pressurised primary coolant, while the fuel is placed in pressure tubes.

Sustaining the chain reaction

Fission of a uranium nucleus releases energy and fast neutrons. The heavy-water moderator slows these neutrons without absorbing many of them, increasing their probability of causing further fissions in natural uranium fuel.

• A self-sustaining reaction is achieved when, on average, one neutron from each fission produces another fission.
• The favourable neutron economy of heavy water enables PHWRs to use uranium with little or no enrichment.
• Fuel bundles are contained in horizontal pressure tubes surrounded by heavy-water moderator.

Keeping fission controlled

The reactor is operated near the critical state, where neutron production balances neutron loss. Neutron-absorbing control devices regulate reactivity, while independent shutdown systems can rapidly terminate the chain reaction.

• Control devices absorb excess neutrons and prevent an uncontrolled rise in reactor power.
• Periodic or online replacement of spent fuel maintains the required fuel inventory and reactivity.

Converting fission heat into electricity

Pressurised heavy-water coolant flows through the fuel channels and removes fission heat. High pressure keeps the primary coolant liquid at high temperature; it transfers heat through steam generators to a separate water circuit.

• Steam from the secondary circuit drives a turbine-generator to produce electricity.
• The steam is condensed and recirculated, while the radioactive primary coolant remains isolated from the turbine circuit.

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