The Canadian deuterium uranium (CANDU) reactor was developed in Canada with the strategic aim of enabling nuclear power to be exploited without the need for imported enriched uranium. Uranium enrichment is an expensive and highly technical process. If it can be avoided, countries such as Canada with natural uranium reserves can more easily exploit their indigenous reserves to generate energy. This has made the CANDU reactor, which uses unenriched uranium, attractive outside Canada too.
The CANDU reactor uses, as its moderator and coolant, a type of water called heavy water. Heavy water is a form of water in which the two normal hydrogen atoms have been replaced with two of the isotopic form, deuterium. Each deuterium atom weighs twice as much as a normal hydrogen atom, hence the name heavy water. Heavy water occurs in small quantities in natural water.
Heavy water is much more expensive than light water but it has the advantage that it absorbs fewer neutrons. As a consequence, it is possible to sustain a nuclear reaction without the need to enrich the uranium fuel. The CANDU reactor has the additional advantage that it can be refuelled without the need to shut it down; in fact this is necessary with natural uranium fuel to keep the plant going. Avoiding lengthy refuelling shutdowns provides better operational performance.
The CANDU fuel is loaded in the form of uranium oxide pellets housed in zirconium alloy rods. Fuel replacement involves pushing a new rod into a pressure tube which passes through the vessel containing the heavy water (called a calandria) and forcing the old tube out of the other end.
The heavy water coolant in the CANDU reactor is maintained under pressure so that it cannot boil. Heat is transferred to a light water system in a steam generator and the secondary system drives a steam turbine in much the same way as a PWR. Efficiency is similar too.
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