CANDU reactor

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The CANDU reactor is a pressurized-heavy water, natural-uranium power reactor designed in the 1960s by a partnership between Atomic Energy of Canada Limited and the Hydro-Electric Power Commission of Ontario as well as several private industry participants. CANDU is a registered trademark and stands for "CANada Deuterium Uranium". All current reactors in Canada are of the CANDU type. Canada markets the power-reactor product abroad.


Design advantages

CANDU reactors have some unique design features that may provide advantages over other reactor designs:

  • CANDU reacts natural, unenriched uranium (0.7% U-235) oxide as fuel, hence it needs a more efficient moderator, in this case heavy water (D2O, deuterium oxide). This means that they can be operated 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 Nuclear Non-Proliferation Treaty (NPT), which implements a safeguard regime under the auspices of the International Atomic Energy Agency, regulates access to nuclear materials such as enriched uranium.
  • 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 continuously, without shutting down, as the fuel channels can be accessed individually.
  • 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. At the time, 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 special zirconium alloy that is relatively transparent to neutrons.
  • 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 injecting gadolinium nitrate solution to the moderator.1 The heavy water moderator circulating through the body of the calandria vessel also yields some waste heat.
  • Since the bulk moderator of the reactor is maintained at relatively low 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 simpler and more reliable than in other reactor types.
  • 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. This is due to the use of heavy water as the moderator. The efficiency is also increased because of the in-operation refuelling 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 need to insert degradable poisons in order to lower the high reactivity of their initial fresh fuel load. This is not necessary in a CANDU.
  • Another advantage of the fuel management system is that the reactors can potentially be operated as low temperature breeder reactors. CANDU can operate very efficiently because their neutron economy is so good. They can breed fuel from natural thorium, if uranium is unavailable. CANDU can even be operated to "burn" former nuclear weapons material to a less-reactive state effectively rendering it useless for warheads.

Economic and political concerns

One (economic) disadvantage of the CANDU reactor design is the initial, one-time cost of its heavy water, although this high capital-cost penalty is generally offset by the CANDU reactor's lower fuelling cost compared to other designs, since it does not require enriched uranium. CANDU reactors require the purest grade of heavy water (better than 99.75% pure2). Tonnes of this expensive material are required to fill a CANDU's calandria and heat transport system. Such high-purity heavy water is expensive because heavy water is almost indistinguishable, chemically, from normal water, and occurs in such low natural concentrations (roughly one part in 7000) in normal water. The next generation reactor (the Advanced CANDU reactor, also called the "ACR") overcomes this disadvantage by having a smaller moderator size and by not using heavy water in the heat transport system (it uses light water as a coolant).

A political issue with the CANDU reactor is the contention that its ability to refuel without shutting down also makes it easier to produce plutonium. All commercial reactor designs produce plutonium as a natural byproduct of uranium fission, and since plutonium similarly undergoes fission it actually generates a portion of a reactor's energy output. In a CANDU reactor this portion is roughly one-half. Canada is a signatory to the Nuclear Non-Proliferation Treaty, which requires states to agree not to produce nuclear weapons in order to purchase CANDU designs (which are in use or being built in China, South Korea, Argentina, India, Pakistan, and Romania). All CANDU reactors operate under IAEA safeguards that ensure their compliance with that UN agency's global non-proliferation standard. There is a common misconception that the plutonium for India's Operation Smiling Buddha nuclear test was produced in a CANDU-like design; in fact the plutonium was produced in a reactor based on the NRX design. India has some unsafeguarded reactors based on the CANDU design, used for power generation. While these reactors could in principle be used for plutonium production, India has a locally-designed and built reactor (Dhruva) which is designed for plutonium production. It is this reactor which is thought to have produced the plutonium for India's more recent Operation Shakti nuclear tests.

Measures that address concerns

Efficient CANDU installations are careful to control heavy water losses from the calandria, and also actively separate tritium from the moderator to sell in the secondary medical market. Some large CANDU installations use surplus power to operate their own small deuterium separation plants, to upgrade the heavy water inventory and 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. Furthermore, due to the use of natural uranium as the fuel, the reactor cannot sustain a chain reaction if its original fuel channel geometry is altered in any significant manner.

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 simplified the design even further by removing 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 favour because the organic coolant was a toxic chlorinated hydrocarbon. The design might be revived if adapted to use a biologically-inert fluorocarbon.

As mentioned above, by burning it as fuel, CANDU could actually render existing stocks of weapons-derived plutonium less reactive and less toxic. A proposal to do this submitted by Atomic Energy of Canada to the United States Department of Energy is currently being debated by government agencies and non-governmental organizations 3.

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