CANDU reactor

CANDU reactor. The CANDU (CANada Deuterium Uranium) reactor is a pressurized heavy-water reactor design developed in Canada and one of the world's major commercial nuclear reactor types. It is distinguished by its use of heavy water as both moderator and coolant, enabling the use of unenriched, natural uranium fuel. This design philosophy, emphasizing independence from foreign uranium enrichment facilities, has led to its deployment in several countries and the development of advanced fuel cycle capabilities.

Design and operation

The fundamental design employs a horizontal calandria, a large cylindrical vessel containing the heavy water moderator at low pressure and temperature. Hundreds of individual pressure tubes, made from a zirconium alloy, pass horizontally through the calandria and contain the fuel bundles and high-pressure coolant. This pressure-tube design contrasts with the more common pressurized water reactor's single large pressure vessel. The primary coolant system, kept under high pressure to prevent boiling, transfers heat from the fuel to steam generators, producing steam to drive a turbine generator. The reactor's on-power refueling capability, managed by two fuelling machines, allows continuous operation without the need for lengthy shutdowns, contributing to high capacity factors. Key operational facilities include the Bruce Nuclear Generating Station and the Darlington Nuclear Generating Station in Ontario.

Fuel cycle and fuel handling

A defining characteristic is its ability to operate on natural uranium dioxide fuel, which avoids the economic and geopolitical complexities of uranium enrichment services dominated by entities like Urenco or Tenex. The standard fuel bundle is a simple cylindrical design containing sintered uranium dioxide pellets in zirconium alloy sheaths. The on-power refueling system allows precise management of neutron flux and fuel burnup. Furthermore, the design exhibits significant fuel cycle flexibility; it can utilize alternative fuels including slightly enriched uranium, recovered uranium from reprocessed light water reactor fuel, and thorium-based fuels. Research into the DUPIC (Direct Use of spent PWR fuel In CANDU) fuel cycle has been conducted in cooperation with Korea Atomic Energy Research Institute.

Safety features and performance

The design incorporates multiple, independent safety systems rooted in the principle of defence in depth. Two rapid shutdown systems are present: mechanical shutoff rods that drop into the core, and the injection of a neutron-absorbing gadolinium nitrate solution into the low-pressure moderator. The large heat capacity of the heavy water moderator and the separate low-pressure calandria vessel provide inherent safety advantages during loss-of-coolant accident scenarios. Containment structures, such as the reinforced concrete buildings at Point Lepreau Nuclear Generating Station, are designed to withstand internal pressures and external events. The performance record of operating units, particularly in Ontario Power Generation's fleet, has demonstrated high reliability and capacity factors over decades.

History and development

Development began in the 1950s under the auspices of Atomic Energy of Canada Limited (AECL), with key contributions from scientists like Harold Smith and engineers from Canadian General Electric. The first prototype, the Nuclear Power Demonstration reactor, began operation in 1962 at Rolphton, Ontario. This was followed by the larger Douglas Point station, which commenced operation in 1968. The success of these projects led to the design and construction of the first commercial-scale units at the Pickering Nuclear Generating Station in the early 1970s. The program was a cornerstone of Canada's industrial and energy policy during the Cold War, supported by federal agencies like the National Research Council Canada.

Variants and deployments

The original design evolved through several product lines, including the standardized 600 MWe-class CANDU 6, first deployed at the Gentilly-2 station in Quebec and internationally at the Cernavodă Nuclear Power Plant in Romania. The larger four-unit Bruce Nuclear Generating Station represents the multi-unit station concept. Major international deployments include units in Korea Hydro & Nuclear Power's Wolsong Nuclear Power Plant in South Korea and the Embalse Nuclear Power Station in Argentina. Advanced designs like the Enhanced CANDU 6 and the larger ACR-1000 were developed by AECL and its commercial successor, SNC-Lavalin, though these have not been constructed. India developed a derivative design, the IPHWR, at facilities like the Kakrapar Atomic Power Station following its early cooperation with Canada.