02 May, 2007

CANDU Reactor

The CANDU reactor is a Pressurized Heavy Water Reactor developed initially in the late 1950s and 1960s by a partnership between Atomic Energy of Canada Limited (AECL), the Hydro-Electric Power Commission of Ontario (now known as Ontario Power Generation), Canadian General Electric (now known as GE Canada), as well as several private industry participants. The acronym "CANDU", a registered trademark of Atomic Energy of Canada Limited, stands for "CANada Deuterium Uranium". This is a reference to its deuterium-oxide (heavy water) moderator and its use of natural uranium fuel. This type of reactor is meant for those countries which do not prodce enriched uranium.Enrichment of uranium is costly and this reactor uses natural uranium as fuel and heavy water as moderator.

In heavy water reactors both the modeartor and coolant are heavy water (D2O). A great disadvantage of this type comes from this fact: heavy water is one of the most expensive liquids. However, it is worth its price: this is the best moderator. Therefore, the fuel of HWRs can be slightly (1% to 2%) enriched or even natural uranium. Heavy water is not allowed to boil, so in the primary circuit very high pressure, similar to that of PWRs, exists.

CANDU fuel is made from uranium that is naturally radioactive. Small amounts of uranium can generate large amounts of energy in the form of heat. The uranium is mined, refined and made into solid ceramic pellets (two pellets are the size of an AA battery). The pellets are put in metal tubes, which are welded together to form a fuel bundle that weighs around 23 kg.The bundle is about the size of a fireplace log and can provide enough energy for an average home for 100 years. The figure below shows the CANDU reactor and its main parts.

In CANDU reactors, the moderator and coolant are spatially separated: the moderator is in a large tank (calandria), in which there are pressure tubes surrounding the fuel assemblies. The coolant flows in these tubes only.

The advantage of this construction is that the whole tank need not be kept under high pressure, it is sufficient to pressurize the coolant flowing in the tubes. This arrangement is called pressurized tube reactor. Warming up of the moderator is much less than that of the coolant; its is simply lost for heat generation or steam production. The high temperature and high pressure coolant, similarly to PWRs, goes to the steam generator where it boils the secondary side light water. Another advantage of this type is that fuel can be replaced during operation and thus there is no need for outages.
Fission reactions in the reactor core heat a fluid, in this case heavy water (see below), which is kept under high pressure to raise its boiling point and avoid significant steam formation in the core. The hot heavy water generated in this primary cooling loop is passed into a heat exchanger heating light (ordinary) water in the less-pressurized secondary cooling loop. This water turns to steam and powers a conventional turbine with a generator attached to it. Any excess heat energy in the steam after flowing through the turbine is rejected into the environment in a variety of ways, most typically into a large body of cool water (lake, river, or ocean). More recently-built CANDU plants (such as the Darlington station near Toronto, Ontario) use a discharge-diffuser system that limits the thermal effects in the environment to within natural variations.
CANDU reactors employ two independent, fast-acting safety shutdown systems. Control rods penetrate the calandria vertically and lower into the core in the case of a safety-system trip.A second shutdown system is via gadolinium nitrate liquid "neutron poison" injection directly in to the low pressure moderator. Both systems operate via separate and independent trip logic.
CANDU-specific features and advantagesUse of natural uranium as a fuel
  • CANDU is the most efficient of all reactors in using uranium: it uses about 15% less uranium than a pressurized water reactor for each megawatt of electricity produced.
  • Use of natural uranium widens the source of supply and makes fuel fabrication easier. Most countries can manufacture the relatively inexpensive fuel .
  • There is no need for uranium enrichment facility.
  • Fuel reprocessing is not needed, so costs, facilities and waste disposal associated with reprocessing are avoided.
  • CANDU reactors can be fuelled with a number of other low-fissile content fuels, including spent fuel from light water reactors. This reduces dependency on uranium in the event of future supply shortages and price increases .
Use of heavy water as a moderator
  • Heavy water (deuterium oxide) is highly efficient because of its low neutron absorption and affords the highest neutron economy of all commercial reactor systems. As a result chain reaction in the reactor is possible with natural uranium fuel.
  • Heavy water used in CANDU reactors is readily available. It can be produced locally, using proven technology. Heavy water lasts beyond the life of the plant and can be re-used .
CANDU reactor core design
  • Reactor core comprising small diameter fuel channels rather that one large pressure vessel
  • Allows on-power refueling - extremely high capability factors are possible .
  • The moveable fuel bundles in the pressure tubes allow maximum burn-up of all the fuel in the reactor core.
  • Extends life expectancy of the reactor because major core components like fuel channels are accessible for repairs when needed.
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