Steam going to steam turbine Cold water returning from turbine Containment building made of reinforced concrete 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. Natural uranium consists of a mix of mostly uranium with small amounts of uranium and trace amounts of other isotopes. Fission in these elements releases high-energy neutrons , which can cause other U 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. Calandria and fuel design[ edit ] 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.
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