THE CANDU REACTOR • Refer to pages 730-733 • The design and thought process in constructing the CANDU reactor • Safety Precautions
How Does the CANDU Reactor Work? • In the reactor, neutrons emitted in the fission reaction are slowed down by the heavy water, which acts as a coolant carrying the heat energy produced in the nuclear reaction from the uranium rods to the heat exchanger and then to the turbines to produce electric power. The products of fission are hot because the smaller atoms produced when a large atom breaks up, it has a great deal of kinetic energy.
Before the Reaction • The stored energy is brought into the generating system in the form of uranium rods. • The fuel is put into the section of the reator called the calandria. • In this process of nuclear fission some energy is stored in the uranium, and some transformed into heat.
After the Reaction • The heat heats up the uranium fuel bundles which in turn heats up the heavy water coolant and flows to the heat exchangers. • Here, some of the heat is transferred to a separate flow of ordinary water boiling and transforming into steam.
Before Generation • In the heat exchangers some of the energy released from the uranium fuel has been transformed into steam. • It flows from the heat exchangers to blades of the turbines. • This transfer has created the energy to transform into the format of motion.
After Generation • Not all the energy from the uranium is transformed into the electricity. Some of this energy stays in the steam passing the turbine blades, then is divided. • Some of it returns to the heat exchangers in the form of preheated steam. Here it reduces the reactors own energy consumption by helping to maintain the steam cycle. • The remainder is transferred one more time, it heats another flow of ordinary water which discharges into lakes or rivers beside the generating station.
Safety Precautions • The main safety precaution that is taken is to ensure that the core does not melt. It has three steps for shutting down the system. • The heavy water moderator can be dumped by gravity into a storage tank under the reactor vessel. This will stop the fission reaction because the neutrons won’t be slowed down. • Boron can be injected into the moderator absorbing the neutrons so the chain reaction is suppressed. • The Cadmium control rods are held above the reactor core by electromagnetic clutches. They automatically fall if the power fails, this stopping the chain reaction since cadmium absorbs the neutrons.