Science of Conflict

Nuclear reactor
The nucleus of the atom contains a great deal of energy. When protons and neutrons come together to form a nucleus the nucleus weighs slightly less than the sum of the masses of the protons and neutrons that formed it, as shown on the right. This energy is used to bind the protons and neutrons tightly in the nucleus. These type of reactions are known as nuclear fusion reactions. Another way of releasing energy is to split the nucleus of an atom.
Atoms with large nuclei, such as uranium, tend to be unstable. Occasionally an atom will split into two smaller atoms and in the process give off a great deal of energy in the form of heat and light. This process is called nuclear fission. Unlike nuclear fusion, nuclear fission is commonly harnessed to provide energy.
Click for more information on nuclear fission.

Uranium is the fuel used to power nuclear reactors. It is a heavy metal which is mined in many countries including Australia. 99% of uranium is composed of the U238 isotope while the U235 isotope makes up 0.7% of the natural uranium found on earth. U235 is used for the production of nuclear weapons and as a fuel for nuclear power production. This particular isotope is one of the few substances that will undergo nuclear fission when bombarded by neutrons. When struck by a neutron, the U235 nucleus absorbs the neutron, becomes unstable and splits into two smaller nuclei with the evolution of a great deal of energy according to the equation E = mc2.
The animation on the right shows an enriched piece of uranium, with up to 3% U235, undergoing fission in a nuclear reactor. The control rods (shown in black) absorb neutrons.

 

For the production of nuclear weapons 90% enrichment is required. That is 90% of the uranium atoms in the fuel must be U235. Construction of a nuclear bomb is not difficult, the difficulty lies in the separation of U235 from the rest of the uranium. It took the Manhattan Project scientists 2 years to gather enought U235 to use in the bomb detonated over Hiroshima.
The 3% enriched uranium is packed into small circular pellets about 2 cm in diameter and 2 cm long. These are arranged into long rods that are collected together in bundles. These bundles are lowered into the reaction vessel and immersed in water, which acts as a coolant. If left on its own the fission reaction would proceed and eventually overheat and melt the reaction vessel.

 

 

To prevent this melt down from occurring, control rods are inserted into the reaction chamber. These rods are made of a material that absorbs neutrons and so can slow the rate of the fission reaction. For example, if an operator wishes to increase the temperature of the reactor they will raise the control rods out of the reaction chamber. If they wish to decrease the temperature they will lower the control rods further into the reaction chamber until the rate of the reaction ceases.


Look at the animation above. The more control rods present in the reaction chamber the slower the fission reaction and hence the slower the rate of energy production.

Why is it unlikely that a nuclear reactor can explode like a nuclear bomb?

What is the difference, in chemical behaviour, between the U235 isotope and the U238 isotope?

Why is the U235 isotope used to make a nuclear bomb and not the U238 isotope?

What is the role of control rods in a nuclear reactor?

Continue with nuclear bombs

Continue with thermonuclear weapons