Some Reactors CANDU®: What Others Cannot

CANDU® reactors are designed to operate with fuel that is composed of natural uranium dioxide formed into cylindrical pellets and inserted into zirconium alloy tubes. No enrichment is necessary.

An understanding of some of the features of the CANDU® reactor design makes it obvious that many of the negative perceptions about nuclear power are, in fact, based on characteristics of a single type of reactor.

Simple Fuel Manufacture

One common misconception about nuclear energy is that fuel manufacture is a complex endeavor that requires an enormous investment in enrichment and fabrication facilities. Part of the assumed cost is the fact that isotope enrichment is an energy intensive process. Occasionally, there are even statements made that the energy invested in enrichment is more than the energy that is obtained by fissioning the fuel. (That is not true by any measure, however that discussion will have to wait for another time.)

CANDU reactors are designed to operate with fuel that is composed of natural uranium dioxide formed into cylindrical pellets and inserted into zirconium alloy tubes. No enrichment is necessary.

Pellet production is a simple process amenable to high capacity automated machinery, but it can also be done with the same kind of machines that are often used to form plastic toys. Inserting the pellets into the cladding tubes is often done by hand, as is the bundling of the tubes into elements. A CANDU fuel element weighs approximately 24 kilograms, is half a meter long and contains 19 kilograms of uranium metal.

In contrast, commercial light water reactor fuel is normally produced in bundles that are four meters long and weigh 500 kilograms. The machinery required to produce, transport, handle, and store these large elements requires a much larger initial investment than that required for CANDU fuel bundles.

Flexible Fuel Cycle

Since CANDU reactors are designed to have sufficient neutron efficiency to operate on natural uranium, they can also operate quite effectively on many different fuel materials. As long as the ratio of fissile to fertile material is similar to that of natural uranium, which is 0.7 percent fissile and 99.3 percent fertile, the reactor will operate safely and effectively.

Potential candidate fuel cycles include mixed oxide (MOX) with some plutonium, uranium-thorium or even spent light water reactor fuel.

Though widely considered to be an expensive waste problem, spent light water fuel can serve quite adequately in a CANDU. In fact, the fuel that has been removed from an LWR has a higher fissile atom concentration than natural uranium and requires only minor physical reconfiguring. No chemical separation step is needed in this recyling scheme.

The South Koreans seem to have noticed that there is a potential synergy between the fuel requirements of CANDUs and those of PWRs. This far sighted country has active plans for using the by-product of their existing light water reactors as the initial fuel for their new CANDUs. Since advanced light water reactor fuel cycles can obtain about 30,000 MW days/tonne of heavy metal and CANDUs can obtain approximately 10,000 MWdays/tonne of heavy metal, series use of the fuel can produce 33 percent more electricity from a given quantity of uranium than a cycle that throws away the fuel after use in a light water reactor.

Though this cycle is under study in Canada, there has not been much incentive for agressive development. Canada has very low cost sources of natural uranium and does not own any light water reactor fuel. At one time there was a move to license CANDU reactors in the U.S. but the costs of the licensing action and the uncertain market caused AECL to abandon the program in 1994.

India, with its large reserves of thorium, is actively pursuing the possibility of using it as feed in their heavy water reactors derived from CANDU designs.

On-Line Refueling

The unique pressure tube design of the CANDU allows each tube to be individually refueled while the reactor is producing electricity.

This design characteristic gives the CANDU several advantages over its competitors. One major advantage is operational. Instead of being forced to shutdown based on a long planned schedule, a CANDU can respond to short lead time power needs caused by unexpected weather or unplanned failures in other power generators.

The system also helps to reduce complications that face fuel designers and operators for reactors that can only be refueled periodically. There is no need to add burnable poisons to the fuel or the reactor coolant to allow for the reactivity reduction caused by fuel consumption. Instead, the fuel management program is capable of shaping the neutron flux and making up for the uranium that has been fissioned.

Partially as a result of this on-line fueling capability, CANDU reactors consistently appear at or near the top of the list for lifetime performance ratings. This measure of reactor performance is obtained by dividing the number of kilowatt hours that the plant has produced divided by the total number of kilowatt hours that the plant would have produced if run at rated power since the time it was first syncronized to the grid.

The average lifetime performance rating for all CANDU reactors is about 72 percent compared to 69 percent for PWRs and 66 percent for BWRs. The most modern class of CANDUs, the CANDU 6 currently reports an average lifetime performance rating in excess of 81 percent.

Market Appeal

In many ways, it is obvious that CANDU design origniated in a country with limited capital resources. The same characteristics that made this system attractive in Canada in the 1960s make it attractive to countries that are trying to develop a nuclear industrial capability in the 1990s.

Though the overall cost of a CANDU is roughly equivalent to that of light water reactor, most of the components can be made locally with technology transfer agreements rather than requiring major components to be imported.

In most of her agreements with developing nations, however, Canada retains the ability to control the use of her nuclear technology by placing conditions on the use of heavy water, since she is one of the very few commercial sources of this rare and expensive material.