A pressurized heavy-water reactor (PHWR) is a class of nuclear power reactor that uses heavy water (deuterium oxide, D2O) as both the neutron moderator and—often—its primary coolant. PHWR designs are distinguished by their use of heavy water to slow neutrons more efficiently than ordinary water, producing a strong neutron economy that permits the use of low-enrichment or natural uranium fuel. For general reactor context see reactor overview, and for the country that first developed the class consult Canadian development.

Design features and components

Key elements of many PHWRs include pressure tubes or a reactor vessel that keep the heavy water under pressure so it can reach elevated temperatures without boiling. The pressurized coolant transfers heat to steam generators or directly to turbines in some layouts. Heavy water serves as the moderator to slow neutrons to energies that are more likely to induce fission in natural uranium; see uranium fuel and heavy water details. The moderator and coolant roles may be separated or combined depending on the variant. Pressure control and boiling prevention are central to operation (pressurization, higher operating temperature, and avoidance of boiling).

Characteristics and operational advantages

  • Neutron economy: Heavy water absorbs fewer neutrons than light water, enabling sustained chain reactions with less enriched fuel.
  • Fuel flexibility: PHWRs can run on natural or lightly enriched uranium and can be adapted to alternative fuel cycles such as thorium-based fuels.
  • Online refuelling: Many PHWRs support refuelling without shutting down, improving capacity factors.
  • Cost trade-offs: Heavy water is more expensive to produce and maintain, a cost often offset by not requiring large enrichment facilities (economics).

The Canadian CANDU family is the best known PHWR lineage; its development emphasized modular pressure-tube architecture rather than a single large pressure vessel. That configuration has particular maintenance and safety implications and has been exported and adapted in several countries.

History, uses and notable distinctions

PHWR technology originated and matured in Canada, later adopted and modified elsewhere to match national fuel strategies and resources. Countries with limited enrichment infrastructure have favored PHWRs for electricity production and strategic fuel-cycle flexibility. The design contrasts with pressurized light-water reactors (PWRs) and boiling-water reactors (BWRs) by its moderator choice and, in many implementations, by online refuelling and pressure-tube layout. For information about moderation and neutron behaviour see moderator concepts. Additional resources and technical summaries are available through specialized nuclear energy references: heavy water properties, coolant behaviour, and thermal design.

PHWRs remain important where fuel-resource optimization, operational flexibility, and specific national fuel strategies—such as thorium utilization—are priorities. Their distinctive balance of engineering trade-offs continues to influence reactor choices and fuel-cycle planning in several nuclear programs worldwide.