CANDU reactor

The CANDU (Canada Deuterium Uranium) is a Canadian pressurized heavy-water reactor design used to generate electric power. The acronym refers to its deuterium oxide (heavy water) moderator and its use of (originally, natural) uranium fuel. CANDU reactors were first developed in the late 1950s and 1960s by a partnership between Atomic Energy of Canada Limited (AECL), the Hydro-Electric Power Commission of Ontario, Canadian General Electric, and other companies. There have been two major types of CANDU reactors, the original design of around 500 MWe that was intended to be used in multi-reactor installations in large plants, and the rationalized CANDU 6 in the 600 MWe class that is designed to be used in single stand-alone units or in small multi-unit plants. CANDU 6 units were built in Quebec and New Brunswick, as well as Pakistan, Argentina, South Korea, Romania, and China. A single example of a non-CANDU 6 design was sold to India. The multi-unit design was used only in Ontario, Canada, and grew in size and power as more units were installed in the province, reaching ~880 MWe in the units installed at the Darlington Nuclear Generating Station. An effort to rationalize the larger units in a fashion similar to CANDU 6 led to the CANDU 9. By the early 2000s, sales prospects for the original CANDU designs were dwindling due to the introduction of newer designs from other companies. AECL responded by cancelling CANDU 9 development and moving to the Advanced CANDU reactor (ACR) design. ACR failed to find any buyers; its last potential sale was for an expansion at Darlington, but this was cancelled in 2009. In October 2011, the Canadian Federal Government licensed the CANDU design to Candu Energy (a wholly owned subsidiary of SNC-Lavalin), which also acquired the former reactor development and marketing division of AECL at that time. Candu Energy offers support services for existing sites and is completing formerly stalled installations in Romania and Argentina through a partnership with China National Nuclear Corporation. SNC Lavalin, the successor to AECL, is pursuing new Candu 6 reactor sales in Argentina (Atucha 3), as well as China and Britain. Sales effort for the ACR reactor has ended. The basic operation of the CANDU design is similar to other nuclear reactors. Fission reactions in the reactor core heat pressurized water in a primary cooling loop. A heat exchanger, also known as a steam generator, transfers the heat to a secondary cooling loop, which powers a steam turbine with an electric generator attached to it (for a typical Rankine thermodynamic cycle). The exhaust steam from the turbines is then cooled, condensed and returned as feedwater to the steam generator. The final cooling often uses cooling water from a nearby source, such as a lake, river, or ocean. Newer CANDU plants, such as the Darlington Nuclear Generating Station near Toronto, Ontario, use a diffuser to spread the warm outlet water over a larger volume and limit the effects on the environment. Although all CANDU plants to date have used open-cycle cooling, modern CANDU designs are capable of using cooling towers instead. Where the CANDU design differs is in the details of the fissile core and the primary cooling loop. Natural uranium consists of a mix of mostly uranium-238 with small amounts of uranium-235 and trace amounts of other isotopes. Fission in these elements releases high-energy neutrons, which can cause other 235U atoms in the fuel to undergo fission as well. This process is much more effective when the neutron energies are much lower than what the reactions release naturally. Most reactors use some form of neutron moderator to lower the energy of the neutrons, or 'thermalize' them, which makes the reaction more efficient. The energy lost by the neutrons heats the moderator and is extracted for power. Most commercial reactor designs use normal water as the moderator. Water absorbs some of the neutrons, enough that it is not possible to keep the reaction going in natural uranium. CANDU replaces this 'light' water with heavy water.Heavy water's extra neutron decreases its ability to absorb excess neutrons, resulting in a better neutron economy. This allows CANDU to run on unenriched natural uranium, or uranium mixed with a wide variety of other materials such as plutonium and thorium. This was a major goal of the CANDU design; by operating on natural uranium the cost of enrichment is removed. This also presents an advantage in nuclear proliferation terms, as there is no need for enrichment facilities, which might also be used for weapons. In conventional light-water reactor (LWR) designs, the entire fissile core is placed in a large pressure vessel. The amount of heat that can be removed by a unit of a coolant is a function of the temperature; by pressurizing the core, the water can be heated to much greater temperatures before boiling, thereby removing more heat and allowing the core to be smaller and more efficient. Building a pressure vessel of the required size is a significant challenge, and at the time of the CANDU's design, Canada's heavy industry lacked the requisite experience and capability to cast and machine reactor pressure vessels of the required size (which would also need to be much larger than the pressure vessel of an equivalent LWR). This issue was so major that even the relatively small pressure vessel originally intended for use in the NPD prior to its mid-construction redesign could not be fabricated domestically and had to be manufactured in Scotland instead, and domestic development of the technology required to produce pressure vessels of the size required for commercial-scale heavy water moderated power reactors was thought to be very unlikely.