Basic Design and Operation
A CANDU power plant generates power in the same fashion as a fossil-fuel power station; heat is generated by "burning" fuel, and that heat is used to drive a steam turbine, normally located in a separate "power hall". A typical coal-fired plant burns coal and air and produces mostly carbon dioxide and fly ash, the CANDU burns nuclear fuel in-situ; when the fuel is "burned up" it is removed from the reactor and stored.
The CANDU reactor is similar to most light water reactors in principle (however the design has considerable differences). Fission reactions in the reactor core heat pressurized heavy water in a primary cooling loop. A heat exchanger, also known as a steam generator, transfers the heat to a light-water secondary cooling loop, which powers a steam turbine with an electrical generator attached to it (for a typical Rankine thermodynamic cycle). The exhaust steam from the turbines is then condensed and returned as feedwater to the steam generator, often using 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 condensor outlet water over a larger volume and limit the effects on the environment. A cooling tower can be used, but it reduces efficiency and increases costs considerably. Some of the unique features of the CANDU design are listed below:
- Use of online refuelling- A CANDU plant uses robotic machines to fuel the reactor with natural uranium while it is in operation. Unlike BWRs and PWRs, CANDU reactors do not undergo batch refuelling, and 2 machines simply hook up to the reactor faces, open the end caps (located on the pressure tubes) and push in the new fuel, while spent fuel comes out at the other end.
- Use of natural uranium- Since CANDU uses heavy water moderator and heavy water coolant, it has the luxury of maintaining a very high neutron economy. This means that the subsequent neutrons resulting from fission are used more effectively and there are fewer losses (compared to PWRs and BWRs). This further allows the use of natural uranium as the fuel source and saves the cost of enrichment.
- Pressure-tube design: PWRs and BWRs are mostly pressure-vessel type reactors. However, CANDU uses pressure tubes. Each pressure tube is inside the calandria tubes and there are normally 380-480 such tubes assembled in a reactor. This design enables the use of online refuelling and many other unique features of CANDU.
In a light water reactor (LWR), the entire reactor core is a single large pressure vessel containing the light water, which acts as moderator and coolant, and the fuel arranged in a series of long bundles running the length of the core. At the time of its design, Canada lacked the heavy industry to cast and machine the pressure vessels. In CANDU the pressure (and the fuel bundles) is contained in much smaller (10 cm diameter), easier to fabricate tubes. Each bundle is a cylinder assembled from alloy tubes containing ceramic pellets of fuel. In older designs the assembly had 28 or 37 half-meter-long fuel tubes with 12 such assemblies lying end to end in a pressure tube. The newer CANFLEX bundle has 43 tubes, with two pellet sizes (so the power rating can be increased without melting the hottest pellets). It is about 10 centimetres (3.9 in) in diameter, 0.5 metres (20 in) long and weighs about 20 kilograms (44 lb) and replaces the 37-tube bundle. To allow the neutrons to flow freely between the bundles, the tubes and bundles are made of neutron-transparent zircaloy (zirconium + 2.5% wt niobium).
The zircaloy tubes are surrounded by a much larger low-pressure tank known as a calandria, which contains the majority of the moderator. To keep the hot coolant from boiling the moderator, a so-called calandria tube surrounds each pressure tube, with insulating carbon dioxide gas in between. Slowing down neutrons releases energy, so a cooling system dissipates the heat. The moderator is actually a large heat sink that acts as an additional safety feature. CANDU's fuel channels in a low-pressure calandria design makes it easier to refuel: a pressure-vessel reactor must be shut down, the pressure dropped, the lid removed, and a sizeable fraction of the fuel, e.g. one-third, replaced all at once. In CANDU, individual channels can be refuelled without taking the reactor offline, improving the capacity factor. One fueling machine inserts new fuel into one end of the channel while the other receives discharged fuel from the opposite end. One significant operational advantage of online refuelling is that a failed or leaking fuel bundle can be removed from the core once it has been located, thus reducing the radiation fields in the primary systems.
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