One fluid, the primary coolant, is water – albeit heavy water – which is kept under sufficient pressure to prevent boiling, even at the design temperature of 316 C (600 F). This high pressure water is pumped through the reactor and steam generators by several large, carefully sealed pumps.
The other fluid is also water, but it is allowed to boil to form saturated steam at approximately 260 C (500 F). The steam drives a turbine, which is essentially identical to those used in other nuclear or fossil stations. Some amount of leakage is normal from this part of the system, but the fluid is ordinary, non-radioactive water.
The term “heavy water” often brings to mind tales of intrigue related to dramatized stories of attempts by the Germans to obtain the material during World War II. The material is chemically indistinguishable from ordinary water. What makes it “heavy” is the fact that the hydrogen is an isotope called deuterium which has a neutron in the nucleus, giving it an atomic mass of two instead of one.
This is important in a nuclear reactor because deuterium is 600 times less likely than hydrogen to absorb an additional neutron. However, it is effective in moderating the energy level of neutrons to improve the likelihood of fission in uranium 235.
Because similarities in physical and chemical constrains exist between the CANDU and the PWR, several common hurdles had to be overcome in order to make the system work. However, since Canadian resources were different from American resources, the approach taken was somewhat different.
One problem that designers had to solve was the need for very high pressures. Water naturally becomes a vapor at temperatures above the normal boiling point of 100 C (212 F) and the only way to force it to remain a liquid is to keep it under pressure. To allow temperatures sufficient for adequate steam production, primary pressure in a CANDU or PWR is maintained at approximately 10 MPa (1740 psi). In contrast, the pressure in a home pressure cooker might be just a few kPa (tens of psi).
Sufficient pressure is not too difficult to obtain in a piping system, but it is very challenging to build a tank large enough to hold a critical arrangement of natural uranium in such a way that it is strong enough to withstand that pressure. The American solution to the problem was to obtain a smaller minimum core size by using enriched uranium and to develop specialized manufacturing machinery with the capability to form the vessels required.
The development of the needed manufacturing capability was not considered economic for the Canadians, who had a much smaller domestic market for their power systems. For a time, it appeared that the goal of an independent nuclear industry might not be possible and construction was begun on a reactor plant that used an imported pressure vessel.
One alternative with some promise was a system using a multitude of pressure tubes full of fuel and coolant that could be grouped together to form a critical assembly. This arrangement was dependent, however, on the availability of a high strength, high temperature, corrosion resistent material with a low neutron absorption cross section.
The low neutron absorption cross-section was important because neutrons cannot be wasted in a natural uranium reactor, while the other criteria were necessary because of the use of high temperature, high pressure water as the coolant.
Aluminum has a small affinity for neutrons and good corrosion resistance, but its performance suffers at elevated temperatures. It works well in low temperature research reactors but cannot function in a power reactor.
Stainless steel alloys resist corrosion, and meet the temperature and strength goals, but they absorb too many neutrons to allow reactor operation.
Just in time to make a major impact on the design of the CANDU system, a commercial supply of zirconium alloys became available. These low absorption alloys have sufficient strength and corrosion resistance to meet all requirements.
Though the overall system is similar to a PWR, the reactor proper that resulted from the meshing of all of the above constraints looks significantly different from the familiar pressurized water reactor. The main component is a large stainless steel cylindrical tank mounted horizontally with hundreds of pressure tubes running from one end to the other.
The tank is full of low temperature, low pressure heavy water, which fills in the space between the pressure tubes and acts as a moderator. The pressure tubes, which are surrounded by a gas filled space and a second tube, contain the fuel bundles along with high pressure, high temperature coolant. The gas space serves as an insulator to minimize the heat loss to the moderator.
Though the system does not require the same level of heavy industry capability as a light water reactor, there is a visible increase in the piping complexity caused by the need for individual piping to each of several hundred pressure tubes. Since each pipe is filled with high pressure, high temperature, very expensive heavy water, high quality piping and connections are essential. Skilled pipefitters and welders are even more important for a CANDU than for a light water reactor.
The heavy water in a CANDU requires a capital investment equal to approximately 20 percent of the cost of the plant. Overall, the initial capital cost of a CANDU is ten to twenty percent higher than a comparable light water reactor depending on local labor costs.
On a lifecycle basis, however, lower fuel costs tend to make the two systems roughly comparable on price, so decisions between the two are often made on the desire for independence, the availability of local labor, the availability of capital investment, the existing infrastructure of the customer, and the availability of vendor incentives.
It is important to understand, however, that nuclear power plants do not compete solely against other nuclear plants. They compete against hydro-electric plants and fossil fuel plants. According to recent data from Ontario Hydro, which, despite its name, obtains 60 percent of its electricity from CANDU reactor plants, nuclear power costs 5 cents (CDN) per kilowatt hour compared to 7 cents per kilowatt hour (CDN) for electricity from its fossil plants. That gives the CANDU a 35 percent cost advantage over one of its major competitors.