Nuclear Reactors

A nuclear reactor produces and controls the release of energy from splitting the atoms of certain elements. In a nuclear power reactor, the energy released is used as heat to make steam to generate electricity.

To turn nuclear fission into electrical energy, the first step for nuclear power plant operators is to be able to control the energy given off by the enriched uranium and allow it to heat water into steam.

Enriched uranium is typically formed into inch-long (2.5-cm-long) pellets, each with approximately the same diameter as a dime. Next the pellets are arranged into long rods, and the rods are collected together into bundles.

The bundles are submerged in water inside a pressure vessel. The water acts as a coolant.

To prevent overheating, control rods made of a material that absorbs neutrons are inserted into the uranium bundle using a mechanism that can raise or lower the control rods.

Raising and lowering the control rods allow operators to control the rate of the nuclear reaction.

When an operator wants the uranium core to produce more heat, the control rods are raised out of the uranium bundle (thus absorbing fewer neutrons). To create less heat, they are lowered into the uranium bundle.

The rods can also be lowered completely into the uranium bundle to shut the reactor down in the case of an accident or to change the fuel.

The uranium bundle acts as an extremely high-energy source of heat. It heats the water and turns it to steam. The steam drives a turbine, which spins a generator to produce power.

There are several components common to most types of reactors:› Fuel. Usually pellets of uranium oxide

(UO2) arranged in tubes to form fuel rods. The rods are arranged into fuel assemblies in the reactor core.

› Moderator. This is material in the core which slows down the neutrons released from fission so that they cause more fission. It is usually water, but may be heavy water or graphite.

› Control rods. These are made with neutron-absorbing material such as cadmium, hafnium or boron, and are inserted or withdrawn from the core to control the rate of reaction, or to halt it

› Coolant. A liquid or gas circulating through the core so as to transfer the heat from it. In light water reactors the water moderator functions also as primary coolant.

› Pressure vessel or pressure tubes. Usually a robust steel vessel containing the reactor core and moderator/coolant, but it may be a series of tubes holding the fuel and conveying the coolant through the moderator.

› Steam generator. Part of the cooling system where the primary coolant bringing heat from the reactor is used to make steam for the turbine.  Reactors may have up to four "loops", each with a steam generator.

› Containment. The structure around the reactor core which is designed to protect it from outside intrusion and to protect those outside from the effects of radiation in case of any malfunction inside. It is typically a metre-thick concrete and steel structure.

The CANDU reactor (Canadian deuterium uranium) uses uranium, bundled in the form of uranium oxide fuel pellets, to produce electricity.

Heavy water is used as the moderator in a CANDU reactor. Heavy water contains deuterium, an isotope of hydrogen having one neutron in the nucleus. Heavy water also transfers heat from the fuel into a heat exchanger which heats ordinary water to produce steam.

Saskatchewan has abundant deposits of uranium ore which is refined for use in nuclear reactors.

The refined uranium oxide fuel pellets are stacked into cylindrical rods. The rods are arranged into a fuel bundle which is then ready to be placed in special pressure tubes inside the reactor.

The reactor vessel is called the calandria.

Refuelling can be done by removing fuel bundles from the pressure tubes and replacing them with new bundles. In a CANDU reactor this can be done without having to shut the reactor down.

CANDU reactors need to be built near a large body of water. Fresh water is circulated through the condensers.

Nuclear reactors can not explode like a nuclear bomb. Even under a worst-case scenario, with a core meltdown, a critical mass of fuel would not be present and the fuel would burn into the ground. (This, of course, would lead to very serious consequences, including possible loss of life and environmental damage.)

Used nuclear fuel is both hot and radioactive. It is stored under water in large cooling pools for up to two years after use, until it cools. Some of the used fuel will still remain radioactive for up to several thousand years. This concerns many people.

The storage of used fuel is a contentious issue for those concerned about the protection of the environment. No ideal solution has yet been developed to dispose the waste. Current proposals for waste management merely offer temporary storage solutions until better methods become available.

One of the waste materials from a nuclear reactor is plutonium. It is known to cause cancer in extremely small quantities. It is also used to make nuclear weapons.

Positive Aspect The decay of a single U-235 atom

releases approximately 200 MeV (million electron volts). That may not seem like much, but there are a lot of uranium atoms in a pound (0.45 kg) of uranium. So many, in fact, that a pound of highly enriched uranium as us ed to power a nuclear submarine is equal to about a million gallons of gasoline.

Nuclear power's biggest advantages are tied to the simple fact that it doesn't depend on fossil fuels. Coal and natural gas power plants emit carbon dioxide into the atmosphere, contributing to climate change. With nuclear power plants, CO2 emissions are minimal.

The use of nuclear energy is controversial. Individuals need to have a solid knowledge base before taking a stand on the issue. One needs to weigh the risks against the benefits involved.