Nuclear Fission
Nuclear Fission: is the splitting up of a large nucleus into two smaller nuclei of roughly the same size with the release of energy.
Discovered in 1939 by Hahn and Strassmann.
1. Fission is produced in a large nucleus by bombarding it with neutrons. The neutrons act as a trigger for the reaction.
2. During fi ssion very large amounts of energy are given out – about 200MeV per nucleus.
3. More neutrons are produced in the fission reaction; these can produce further fission.
Thus, a self-sustaining chain reaction can be initiated.
Note: uranium is a mixture of two naturally occurring isotopes:
1. 238 U which is relatively abundant (99:3% of all naturally occurring uranium). This isotope is not ideal for nuclear fission, however it can be processed (at some expense) into the next isotope.
2. 235 U which is relatively scarce (0:7% of all naturally occurring uranium). This isotope is ideal for nuclear fission because it is much more likely to undergo fission with slow neutrons.
Nuclear Fission Reaction:
Thermal/Slow Neutrons: are neutrons moving with kinetic energies equal to the average kinetic energy of the surrounding atoms – thermal energy.
Fission Fragments: are the products formed as a result of nuclear fission.
If at least one neutron from each atom that undergoes fission produces further fission then we have a self-sustaining chain reaction.
A substance is said to be fissile if it will undergo nuclear fission.
The Critical Size: is the minimum amount of a fissile substance which must be present for a self-sustaining chain reaction to occur.
Environmental Impact of Fission Reactors:
1. The mining of uranium ore.
2. Containment of radioactive materials within the reactor.
3. The removal and treatment of spent fuel rods.
4. Radioactive waste.
The Thermal Nuclear Reactor
These are used around the world to generate electric power in nuclear power stations.
They are based on a carefully controlled fission reaction.
For economic reasons the fuel used is natural uranium rather than plutonium or 235 U. A chain reaction can’t occur in natural uranium due to the great loss of neutrons by 238 U capturing them. This problem is overcome by using a moderator which quickly slows the fission neutrons to thermal energies, so that they are not captured by the 238 U. The number of neutrons in the reactor is thus conserved and a chain reaction can occur.
The uranium rods are inserted in the moderator and this plus the control rods make up the core of the reactor. The fission neutrons pass from the fuel rods to the moderator and are quickly slowed to their thermal value by colliding with the atoms of the moderator.
These neutrons then diff use through the moderator and re-enter the fuel rods where they produce further fi ssion in the 235 U atoms.
Substances of low atomic number should be used in the moderator as a neutron is slowed more e ectively by colliding with a particle of equal mass. Hydrogen would appear to be ideal for this purpose, but it is not used as it would capture slow neutrons, forming deuterium. The moderators most commonly used are heavy water (deuterium oxide) and carbon in the form of graphite.
If the reaction is to proceed under controlled conditions and not be explosive, some method of removing the unwanted neutrons must be used. This is done by inserting the control rods, which absorb neutrons, into the moderator. The control rods used are usually cadmium or boron. The reaction with boron is given by
The rate at which fission proceeds in the reactor core may be adjusted by varying the length of the control rods in the moderator. Increasing the length of the rods in the moderator increases the rate at which the neutrons are absorbed and slows down the reaction, while withdrawing the rods increases the number of neutrons available for fission and so speeds up the reaction. If an emergency develops the control rods drop fully into the moderator automatically and the reactor is said to be in shut down.
The energy released by fission in the reactor core appears as heat. This heat is removed by circulating a coolant such as CO2 through the system. The gas carries the heat to a heat exchange unit where it is used to boil water. The steam produced may then be used to drive turbines and produce electrical power.
The reactor core is enclosed in a reinforced concrete shield many feet thick to prevent radiation from escaping. Some of these outer shells are so strong that they can survive the impact of a Boeing-747 crashing into them!
Many nuclear accidents occur when a run-away fission reaction takes place. As a result (or for various other reasons) the temperature of the core increases drastically, causing the core to melt or otherwise collapse or become damaged; the core is then said to be in melt-down. Often, powerful explosions ensue which pierce the core’s protective shell, thus allowing radioactive material to escape. (Chernobyl, Three Mile Island, Fukushima.)
