Heavy-water reactor

Heavy-water reactor. A heavy-water reactor is a class of nuclear reactor that uses deuterium oxide, known as heavy water, as its neutron moderator and often as its coolant. This key design feature allows the reactor to sustain a fission chain reaction using natural uranium as fuel, bypassing the need for uranium enrichment facilities. The most commercially successful design is the CANDU reactor, developed by Atomic Energy of Canada Limited. These reactors have been deployed for electricity generation and the production of radioisotopes for medical and industrial use.

Overview

The fundamental principle of the heavy-water reactor stems from the superior neutron economy provided by heavy water compared to light water. The reduced neutron absorption cross-section of deuterium allows more neutrons to remain available to cause fission in fissile material like uranium-235. This enables the use of low-grade fuels, a significant advantage for nations without access to large-scale enrichment technology. Major projects utilizing this design include the Bruce Nuclear Generating Station in Canada and the Rajasthan Atomic Power Station in India. The International Atomic Energy Agency monitors the operation of such reactors under safeguards agreements.

Design and operation

A typical pressurized heavy-water reactor, like the CANDU, features a horizontal calandria vessel containing the moderator. Hundreds of pressure tubes, made from an alloy like Zircaloy, pass through the calandria, each containing fuel bundles and high-pressure coolant. This pressure tube design contrasts with the single large reactor pressure vessel used in light-water reactors. The primary coolant system transfers heat to a secondary light water circuit in a steam generator, driving a turbine connected to an electrical generator. On-line refueling is a hallmark operational feature, allowing continuous operation without lengthy shutdowns for refueling outages.

Types of heavy-water reactors

The primary classification is based on the coolant used in the primary circuit. The Pressurized Heavy Water Reactor, exemplified by the CANDU and Indian IPHWR designs, uses pressurized heavy water coolant. The SGHWR in the United Kingdom used light water as coolant within pressure tubes. An early experimental design was the Organic Cooled Deuterium Moderated Reactor, which used an organic compound like terphenyl as coolant. Research reactors, such as the Jules Horowitz Reactor under construction at the Cadarache centre in France, also sometimes employ heavy water moderation for their high neutron flux.

Fuel cycle and fuel handling

The classic fuel cycle relies on natural uranium, typically as uranium dioxide pellets in zirconium alloy cladding. The ability to use alternative fuels is a significant flexibility; many designs can operate on slightly enriched uranium, reprocessed uranium, MOX fuel blending plutonium with uranium, or fuels derived from the thorium fuel cycle. The on-line refueling system uses dedicated fuelling machines that connect to the reactor channels, allowing individual fuel bundles to be replaced while the reactor operates at full power. Spent fuel is initially stored in water-filled spent fuel pools located at the reactor site.

Safety and proliferation considerations

Safety systems are designed to handle loss-of-coolant accident scenarios and include emergency core cooling systems and containment structures like the CANDU 6's double containment. A specific safety topic is the positive void coefficient of reactivity in some large-core designs, which is managed through automated control systems. The use of natural uranium fuel produces plutonium-239 with a lower fraction of plutonium-240, creating so-called "weapons-grade" material in the spent fuel. This has raised nuclear proliferation concerns, leading to strict IAEA monitoring under the Treaty on the Non-Proliferation of Nuclear Weapons. The Rokkasho Reprocessing Plant in Japan handles material from such reactors.

Historical development and deployment

Early theoretical work was conducted by Hans von Halban and Lew Kowarski following the discovery of nuclear fission. The first operational heavy-water reactor was the CP-3 at the Argonne National Laboratory in the United States in 1944. The commercial development was pioneered by Canada, with the first CANDU prototype, the Nuclear Power Demonstration, becoming operational in 1962. India developed its indigenous program after the Tarapur Atomic Power Station agreement, leading to the IPHWR series. Other countries with operational units include Pakistan with the Karachi Nuclear Power Plant, Argentina with the Embalse Nuclear Power Station, Romania with the Cernavodă Nuclear Power Plant, and South Korea with the Wolsong Nuclear Power Plant. The Advanced CANDU reactor represents ongoing Generation III+ design evolution. Category:Nuclear reactors