The CANDU reactor is a pressurized-heavy-water, natural-uranium power reactor designed in the 1960s by a consortium of Canadian government and private industry. CANDU is a registered trademark and stands for "Canada Deuterium Uranium".
All power reactors in Canada are of the CANDU type, and Canada is also marketing the power-reactor product abroad.
CANDU reactors have some very unique design features that make them attractive:
CANDU are designed to react natural, unenriched uranium (0.7% U-235) oxide as fuel, hence needs a more efficient moderator, in this case heavy water (D2O). This means that they can operate without expensive fuel enrichment facilities. Most less-developed countries find this attractive because they cannot afford the enrichment facilities, and cannot be assured of access to enriched Uranium. The U.S. operates the Nuclear non-proliferation treaty in order to assure access to enriched Uranium, but has broken the terms of the treaty at least once.
The moderator is in a large tank called a calandria, penetrated by several hundred horizontal pressure tubes which form channels for the fuel, cooled by a flow of heavy water under high pressure in the primary cooling circuit, reaching 290°C. As in the pressurized water reactor, the primary coolant generates steam in a secondary circuit to drive the turbines. The pressure tube design means that the reactor can be refuelled progressively without shutting down, by isolating individual pressure tubes from the cooling circuit.
CANDU are designed to be constructed without large pressure vessels. The large pressure vessels commonly used in light-water reactors are extremely expensive, and require heavy industry that is lacking in many countries. In particular, Canada lacked such industries, and designed the reactor not to require them. Instead, the reactor pressurizes only small tubes that actually contain the fuel. These tubes are constructed of a zirconium alloy that is relatively transparent to neutrons. The core of the reactor is an unpressurized, low temperature tank of heavy water called a calandria.
A CANDU fuel assembly consists of a bundle of 37 half metre long fuel rods (ceramic fuel pellets in zircaloy tubes) plus a support structure, with 12 bundles lying end to end in a fuel channel. Control rods penetrate the calandria vertically, and a secondary shutdown system involves adding gadolinium to the moderator. The heavy water moderator circulating through the body of the calandria vessel also yields some heat.
Since the core of the reactor is maintained basically at room temperature and pressure, the equipment to monitor and act on the core is quite a bit less complex. It only has to cope with high radiation and high neutron flux. In particular, the control rods and emergency equipment are much less exotic.
The reactor has the least down-time of any known type. This is partly because so much of the reactor operates at low temperatures and pressures. It is also caused by the unique fuel-handling system. The pressure tubes containing the fuel rods can be individually opened, and the fuel rods changed without taking the reactor out of service.
Another advantage is that fuel use is the most efficient known. The in-operation refueling mechanism permits the fuel assemblies to be shuffled to the most efficient parts of the reactor core as their reactivity changes. Most other reactor designs insert degradeable poisons to lower reactivity.
Another advantage of the fuel management system is that the reactors can be operated as low temperature breeder reactors. CANDU can thereby operate in the most efficient known modes. They can breed fuel from natural Thorium, if Uranium is unavailable. CANDU can even be operated to "burn" nuclear waste to a less-reactive state.
Another less benign result is that CANDU can be operated to produce plutonium. This is popular with states that hope to produce Nuclear Weapons, as well as those that plan to operate CANDU as breeders.
There are two disadvantages of the CANDU reactor design. One is the cost of its heavy water. CANDU reactors require the purest grade of heavy water ever developed, "reactor grade," better than 99.975% pure. Tons of this expensive substance fill a CANDU's calandria. Such pure heavy water is expensive because heavy water is chemically indistinguishable from normal water, and mixes easily with it.
The second major disadvantage is that since the reactor can use unenriched uranium, the reactor could in principle be used to produce plutonium for nuclear weapons. Canada requires states to agree not to produce nuclear weapons in order to purchase CANDU designs, but the plutonium for the weapons programs of India and North Korea are believed to use reactors similar to CANDU.
Efficient CANDU installations use exotic means of controlling evaporation from the calandria, and actively separate tritium from the calandria to maintain the reactors' efficiencies. Some large CANDU installations use surplus power to operate their own small deuterium separation plants, to reduce costs.
The large thermal mass of the cool calandria acts as a substantial safety mechanism. If a fuel assembly were to overheat and melt, it would be cooled in the very process of changing the reactor geometry.
An interesting variant of the CANDU used a high-temperature organic coolant rather than pressurized water as the primary coolant of the fuel assemblies. This made the reactor even less exotic, because it removed high pressures from the fuel-tubes. The organic coolant was cooled in a steam-generator to generate power. The author believes that this design lost favor because the organic coolant was a toxic chlorinated hydrocarbon. The design might be revived if adapted to a biologically-inert fluorocarbon.