PHWRs

PHWR stands for Pressurized Heavy-Water Reactor. It is a type of nuclear reactor that uses heavy water (deuterium oxide, D2O) as both coolant and moderator, and typically runs on natural uranium fuel. The core consists of many horizontal fuel channels placed inside a calandria, a large vessel filled with heavy water. Each fuel channel contains a bundled fuel element and is enclosed in a pressure tube, through which the primary coolant circulates at high pressure to transfer heat to a secondary steam system. The heavy-water moderator remains around the fuel channels to sustain a favorable neutron economy, enabling efficient use of natural uranium.

In a PHWR, heat is carried away by the primary heavy-water coolant and transferred through heat exchangers or steam generators to a low-pressure secondary water loop that produces steam to drive turbines. Because each fuel channel can be individually refueled, PHWRs commonly support on-load refueling, allowing extended operation between outages and a high capacity factor.

Fuel and neutron economy are notable advantages. Natural uranium requires no enrichment, and the reactor’s design supports use of thorium-based fuels in some fuel cycles, offering potential fuel-division flexibility. The heavy-water moderator also contributes to a strong negative void coefficient, aiding passive safety features in certain designs.

Disadvantages include the cost and complexity of heavy-water production and containment, as well as the engineering challenges and material stress associated with long-term pressure-tube operation. The approximately large, complex calandria and zirconium alloy tubes require careful manufacturing and maintenance.

PHWRs originated in Canada with the CANDU design and have been deployed in several countries, including India and others, contributing to diversified nuclear power capacity in those regions.