NUCLEAR FISSION REACTORS

A nuclear fission reactor is a machine where controlled nuclear fission reactions take place. Nuclear fission reactors use uranium as fuel source, typically uranium which contains a greater amount of an isotope of uranium known as U-235, because they are easily split apart or fissile by neutron bombardment. The other isotopes of uranium and their percentage composition in nature are U-238; with an abundance of 99.3% and with a half-life of 4.5 billion years. Uranium 238 are fissionable but are not fissile that means they cannot sustain a chain reaction. Uranium 235 isotope has an abundance of 0.7% of natural uranium and they have a half-life of 703.8 million years. Uranium 235 is fissile that means they can sustain a chain reaction, making it suitable for nuclear reactor fuel. Uranium 234 the last known isotope has an abundance of 0.005% of natural uranium and they have a half-life of 245500years. Uranium 234 is radioactive but not fissile.

The components of a nuclear fission reactor are; fuel, moderator, control rods, coolant, reactor vessel and the heat exchanger.

The fuel source is typically enriched uranium 235 or any other fissile material.

The moderator is typically light water, heavy water or graphite. The function of the moderator is to slow down neutrons to increase the probability of fission.

The control rods are made up of boron, cadmium or hafnium. Their function is to absorb neutrons to regulate the reaction rate and maintain a stable power output.

The coolant material is water, gas (e.g. carbon dioxide or helium) or liquid metal such as sodium. The function of the coolant is to remove heat from the reactor core and transfer it to a heat exchanger.

The reactor vessel material is thick walled steel or concrete. The main function of the reactor vessel is to house the reactor core and the coolant.

The heat exchanger is made up of steel or other heat conductive materials. The function of the heat exchanger is primarily to transfer heat from the coolant to a secondary circuit producing steam. This steam is used to drive a steam turbine in a nuclear power plant to generate electricity.

There are several types of nuclear reactors and they are; pressurized water reactors (PWR), boiling water reactors (BWR), gas cooled reactors (GCR), heavy water cooled reactors (HWR), liquid metal fast breeder reactors (LMFBR} and advanced reactors.

Pressurized water reactors (PWR) use enriched uranium as fuel, water as coolant and moderator and a steel pressure vessel to contain the reactor core.

Boiling water reactor (BWR) also use enriched uranium as a fuel source and water as coolant amd moderator but produce steam directly thereby eliminating the need for a separate steam generator.

Gas cooled reactors (GCR) use gas as coolant such as carbon dioxide or helium and graphite as moderator.

Heavy water reactors (HWR) use heavy water (deuterium oxide) as coolant and moderator allowing for more efficient use of natural uranium.

Liquid metal fast breeder reactors (LMFBR) uses liquid metal usually sodium as coolant and operate on a fast neutron cycle, breeding more fuel than consumed.

Advanced nuclear reactors are more compact modular designs for smaller scale power generation. They cost less to construct, operate and less time to deploy.

The applications of nuclear reactors are diverse and are as follows; they are used for large scale electricity generation, industrial process heat, medical applications such as radioisotopes production and scientific research.

The future of nuclear fission reactors is based on the following; the development of advanced reactors with improved safety, efficiency and flexibility, reduced waste in its operation, safe disposal and storage of raw and spent nuclear fission materials.

 

SOURCES:

• Nuclear reactor analysis by J.J. Duderstadt and L.J Hamilton.
• Introduction to nuclear engineering by J.R Lamarsh and A.J Baratta.
• Nuclear systems by Neil E. Todreas.
• Nuclear reactor theory by George Bell and Samuel Glasstone.
• Handbook of Nuclear engineering by Dan Gabriel Cacuci.