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How is nuclear power generated? (continued)

Power reactors

CANDU reactor – Atomic Energy of Canada Ltd. developed the CANDU reactor to utilize Canada’s abundant supply of natural uranium as fuel without having to enrich the uranium, a process that could be supplied only by other countries. The CANDU reactor, as with other heavy water reactors, is fueled by natural uranium encased in fuel channels that run horizontally through a tank, or calandria, containing the heavy water moderator. The moderator slows the neutrons down thereby increasing the chances of fission. The coolant, a separate stream of pressurized heavy water, is pumped through the fuel channels to absorb the heat created by the uranium undergoing fission in the tubes. Once heated to about 300 degrees, the heavy water circulates through a boiler, transferring the heat to ordinary water in a separate circuit. The ordinary water is converted to a flow of high-pressure stream that turns the turbines and generates electricity.

The natural uranium fuel is in the form of uranium dioxide pellets sealed in zirconium alloy pressure tubes. The tubes are bundled into nuclear fuel bundles that are placed end-to-end in the fuel channels. Each fuel channel holds 12 to 13 bundles.

In a CANDU reactor, the fuel can be changed while the reactor is operating. New fuel bundles are inserted into one end of the fuel channel and spent fuel bundles are collected as they are pushed out of the other end.

Light-water reactors – Light-water reactors are power reactors that are cooled and moderated with ordinary water. There are two basic types: the pressurized-water reactor and the boiling-water reactor.

In pressurized water reactors, high-pressure, high-temperature water removes heat from the core and is then passed to a steam generator. Here the heat is transferred to a stream of water in the generator, causing the water to boil. The steam powers a steam turbine.

High pressure light water reactor - In a boiling-water reactor, water passing through the core boils, and the steam from the reactor is used directly in the power cycle.

The fuel used in light-water reactors is uranium dioxide pellets in zirconium alloy cladding as in heavy water reactors. Fuel for the boiling water reactor is slightly less enriched, but the pressurized water reactor’s fuel produces more energy before being discharged, so these two aspects balance each other out economically.

Boiling-water reactors are smaller than pressurized-water reactors, but are more complex. Size is reduced because boiling-water reactors eliminate the need for steam generators which are larger than the reactor itself. The complexity is greater because boiling-water reactors have internal recirculation pumps and complex steam separation and drying equipment within their vessels.

Other types of reactors

High-temperature gas-cooled reactors – These reactors use fuel consisting of a mixture of graphite and fuel-bearing microspheres. The coolant consists of pressurized helium. Experimental prototypes have been built, but no commercial plants are currently on order.

Liquid-metal reactors – Most liquid-metal reactors are fueled with uranium dioxide or mixed uranium–plutonium dioxides. Sodium is used to transfer the heat from the core to the steam generators.

Advanced gas-cooled reactor – Advanced gas-cooled reactors are fueled by two per cent enriched uranium dioxide loaded into zirconium-alloy fuel pins that are placed into zirconium-alloy channels that pierce a graphite moderator block. Carbon dioxide is used as a moderator to deliver heat to a steam generator, activating a steam-turbine cycle.

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