Nuclear reactors, which produce heat by splitting uranium atoms, do the same job as conventional power producing equipment in the generation of electricity – they produce heat to convert waterinto steam, which spins a turbine or generator to make electricity. Instead of coal, oil or natural gas, Canadian nuclear reactors use natural uranium for fuel. But the uranium is not burned. Uranium atoms make heat by splitting – the technical term is fissioning.
atoms) strikes an atom of uranium, the uranium atom splits into two lighter atoms(which are called fission products) and releases heat at the same time. The fissioning process also releases from one to three more neutrons that can split other uranium atoms. This is the beginning of a "chain reaction" in which more and more uranium atoms are split, releasing more and more neutrons (and heat).
In a power reactor, the chain reaction is tightly controlled to produce only the amount of heat needed to generate a specific amount of electricity.
In a power reactor, the chain reaction is tightly controlled to produce only the amount of heat needed to generate a specific amount of electricity.
Heat makes SteamThe fission process generates a huge amount of heat. In order to be useful, the heat has to be moved to boilers to make steam. In a reactor, heavy water does this job. It is pumped constantly through the fuel channels in the reactor and takes the heat from the fuel bundles up to boilers above the reactor. In the boilers the heated heavy water heats up ordinary water to make steam. The steam is piped out of the boilers and over to the turbine
hall where it drives the huge turbines/generators that make the electricity we use.
Creating A Chain Reaction
Canadian reactors use fuel made of natural uranium. Like uranium in the ground, almost all of the uranium in fuel is U-238. This is the common form of the element. The ore also contains tiny amounts (0.7%) of U-235, an unstable isotope of uranium that fissions spontaneously – that’s why Geiger counters react to ore-carrying rock. The fact that U-235 atoms fission spontaspontaneously makes it possible to get a controlled chain reaction
going inside the mass of fuel in the reactor. But no chain reaction can take place in this fuel unless three conditions are all satisfied at the same time:
hall where it drives the huge turbines/generators that make the electricity we use.
Creating A Chain Reaction
Canadian reactors use fuel made of natural uranium. Like uranium in the ground, almost all of the uranium in fuel is U-238. This is the common form of the element. The ore also contains tiny amounts (0.7%) of U-235, an unstable isotope of uranium that fissions spontaneously – that’s why Geiger counters react to ore-carrying rock. The fact that U-235 atoms fission spontaspontaneously makes it possible to get a controlled chain reaction
going inside the mass of fuel in the reactor. But no chain reaction can take place in this fuel unless three conditions are all satisfied at the same time:
- several tons of fuel are present;
- the tubes containing the fuel are stacked in a special arrangement, neither too close together, nor too far apart; and,
- a material called a "moderator" surrounds the fuel. The moderator slows, or moderates, the speed of the neutrons resulting from the fission so they are more likely to collide with, and split, more uranium atoms. The moderator in Canadian reactors is heavy water which is
Reactor Fuel
The Calandria
The heart of an Power Generation reactor is a large cylindrical tank filled with the heavy water moderator. This tank, or calandria, is penetrated horizontally by several hundred fuel channels. Twelve or thirteen fuel bundles are placed end-to-end in each fuel channel as shown in the picture. Pressurized heavy water is pumped through the fuel channels where it is heated by the fuel to 300ºC. It then travels to a boiler to boil ordinary
water into high-pressure steam that drives the turbine/ generator to produce electricity. Upon cooling the heavy water is returned to the reactor to pick up more heat and the ordinary water is recirculated to the boiler to be reheated.
Safety and Reactor ControlThe reactor is automatically controlled to the required reactor power using liquid zone controllers and mechanical control absorbers. These specially designed tubes and control rods can be activated by the computer or manually controlled. During routine operation, operators can shut down a reactor by completely inserting the control rods. In emergency
situations, however, a separate set of neutron absorbing rods, called shut-off rods, will automatically drop into the reactor and shut it down. In all Ontario Power Generation reactors, the safety systems are independent of the process systems and independent of each other. They do not function during normal operation of the reactor. They activate only if the
process systems are unable to ensure the safe shut down or cooling of the unit.
Curtesy :Ontario Power Generation Labels: Power Plant