Nuclear Reactors

How does a nuclear reactor work?
What types of reactors are there?

How does a nuclear reactor work?

Large electrical generating plants which provide most of our electricity all work on the same principle - they are giant steam engines. Power plants use heat supplied by a fuel to boil water and make steam, which drives a generator to make electricity. A generating plant's fuel, whether it is coal, gas, oil or uranium, heats water and turns it into steam. The pressure of the steam spins the blades of a giant rotating metal fan called a turbine. That turbine turns the shaft of a huge generator. Inside the generator, coils of wire and magnetic fields interact - and electricity is produced.

The reactor in a nuclear power plant does the same thing that a boiler does in a fossil fuel plant - it produces heat. The basic parts of a reactor are the core, a moderator, control rods, a coolant, and shielding. The core of a reactor contains the uranium fuel. For a light water reactor with an output of 1,000 megawatts, the core would contain about 75 tonnes of uranium enclosed in approximately 200 fuel assemblies.

The neutrons produced by fission are travelling at great speeds, and in most reactors, are deliberately slowed down by a material known as a moderator. Slow neutrons are much more likely, when they collide with the nuclei of U-235, to cause a fission and keep the reaction going. A moderator is composed of light atoms and the materials most commonly used are carbon in the form of graphite, and water.

For more precise control of the chain reaction, control rods are inserted into the core of the reactor. Pushed in, they absorb neutrons and slow down the reaction - pulled out they allow it to speed up again. In this way the chain reaction is controlled.

Fissions occurring in the reactor generate an enormous amount of heat. A liquid or gas coolant carries this heat away from the reactor to a boiler where steam is made.

Shielding, typically made of steel and concrete about two metres thick, is an outer casing that prevents radiation from escaping into the environment.

What types of reactors are there?

All nuclear reactors operate on the same basic principle, but various designs are in use throughout the world.

Reactor Types in Use Worldwide, January 2004

	Boiling Water Reactors (BWR) heat water in the core and allow it to boil into steam. The steam goes directly to the turbine outside the reactor.
	In a Pressurized Water Reactor (PWR) water is kept under pressure to keep it from boiling, even at 300 C. The pressurized water is pumped through a closed system of pipes called the primary circuit. Heat from the primary circuit warms up water in the secondary circuit. The water in the secondary circuit comes to a boil and its steam turns the turbine. The water in the primary circuit returns to the reactor core after giving up some of its heat.
	A Candu reactor is an example of a Pressurized Heavy Water Reactor (PHWR). Fuel assemblies are placed horizontally in a tank called a calandria. Heavy water coolant is pumped through tubes containing the fuel assemblies to pick up the heat generated from the nuclear reaction. The coolant then moves to the steam generators to produce steam from ordinary water and back to the reactor. Heavy water is a rare but natural form of water and is the most effective moderator used in nuclear reactors to maintain continuous fissioning. Ordinary water is a combination of one oxygen and two hydrogen atoms (H₂O). Heavy water is virtually identical, except each of the hydrogen atoms have an extra neutron. This hydrogen isotope is called deuterium (D). Since heavy water (D₂O) has almost all the extra neutrons it wants, it slows neutrons in the reactor without significantly absorbing them. The advantage of heavy water is that it permits the use of unenriched uranium as fuel. This means two less steps are required in the conversion process resulting in a more economical fuel source.
	The Fast Breeder Reactor (FBR) has a core of plutonium surrounded by rods of U-238. The U-238 nuclei absorb neutrons from the core and are transformed into plutonium (P-239). For every four atoms of plutonium that are used up in the core of the breeder, five new plutonium atoms are made from the U-238. Therefore, FBRs "breed" plutonium. Fast breeder reactors work at such a high temperature that they need a special coolant such as liquid sodium. In addition, they are not equipped with a moderator to slow down neutrons, and for this reason are called "fast" breeders.

High temperature gas cooled reactors (HTGR) offer an alternative to conventional light-water cooled and moderated reactors. They use graphite as the moderator and helium as the coolant. One design concept, called a pebble bed reactor, uses a fuel made of tennis-ball sized spheres known as "pebbles". Each pebble contains thousands of tiny "kernels" consisting of enriched uranium and graphite compressed together and coated externally with temperature resistant ceramic. The pebbles are stacked in the reactor and cooled by helium.

Nuclear Power Reactors: Typical Characteristics

Type of Reactor	Fuel Form	Coolant	Moderator
BWR	Enriched Uranium Dioxide	Water	Water
PWR	Enriched Uranium Dioxide	Water	Water
PHWR (Candu)	Natural Uranium Dioxide	Heavy Water	Heavy Water
GCR	Natural Uranium	Carbon Dioxide	Graphite
AGR	Enriched Uranium Dioxide	Carbon Dioxide	Graphite
LWGR	Enriched Uranium Dioxide	Water	Graphite
FBR	Plutonium Oxide and Uranium Dioxide	Liquid Sodium	None