|MadSci Network: Physics|
927824631.Ph The multiplication factor is simply the ratio of what neutrons are introduced to create a fission reaction to those resulting or produced from the fission reaction which then they are available to create the next fission. When the multiplication factor is 1.0, the ratio of neutrons going into a fission reaction is the same as those produced or emitted from the fission and are "available" to create the next fission reaction. There are always losses of neutrons in the overall reactor system in which neutrons either introduced or produced are being absorbed by the fuel cladding, moderator, etc. We take the ratio of one neutron creating a fission reaction to one produced neutron that will create the next fission reaction to be the multiplication factor of one (1.0). This ratio of 1 allows for better control and precision in operating a nuclear reactor. The rate of operation for the reactor is then at a steady, or constant, state of operation. This also means the nuclear or fission chain reaction is self sustaining. No other sources for neutrons are needed to make another fission occur. This state is termed the critical state for a reactor. When the multiplication factor is less than 1.0, this means less neutrons are produced from fission reactions and available for ...than are put into them. This is termed subcritical. When the multiplication number is less than 1.0, the nuclear chain reaction can not sustain itself and will eventually, as we say, die out to a great degree. Additional neutrons would then be needed in order to get reactions going again. In a reactor when conditions are subcritical, the reactor in a sense will not start. Somewhat similar to trying to start a car with too little fuel. It may try to start, but never really get going and get you where you need to go. More neutrons are needed to get the reactor running and generating enough heat to turn into electricity. Even though at subcritical states, fission reactions are going on but not enough to make the reactor system self sustaining. When the multiplication factor is greater than 1.0, this means more neutrons are being produced from the fission reactions than introduced. This is termed supercritical. This just simply means that there are more than enough neutrons to sustain a chain reaction for the reactor. When in a supercritical state more energy is released through the increased number of fissions. In a reactor when conditions are supercritical, more fuel is being consumed than desired. The temperatures rise with the increased energy output from the fissions. However, it is not necessarily an unsafe condition with many reactors. In water cooled reactors, there is a built in safety feature that if the water gets too hot, the reactor shuts itself down. The neutrons in some sense are moving too fast to be absorbed by the fuel and produce more fissions. This in a sense slows the reactions or reactor down again to a safe level. The only time that an unsafe condition might occur is if there was no water or other moderator to cool and moderate the reactor. A moderator is a substance or medium used to slow neutrons down slow enough to be absorbed by the fuel for a fission reaction. Just because there exists subcritical and supercritical states, does not mean a reactor can not be used or is not operational. Several experimental reactors often operate at subcritical states for a variety of experiments and other operations which require a lower temperature than operating at a critical state. Some reactors operate at supercritical states in order to raise temperatures at a faster rate or burn off certain impurities in the fuel. If there are any additional questions, please let me know and I'll see what I can do to help answer them.
Try the links in the MadSci Library for more information on Physics.