<! /u/ftp/jmc/progress/nuclear-faq.html > Unless one takes some rather complex facts on authority, which may be good enough depending on the authority, forming an opinion on nuclear energy requires just a little bit of physics. Let me assure the reader that nothing in what follows is controversial. Many readers will find nothing they don't already know. Here are some facts.
Power is measured in watts or kilowatts (1,000 watts) or megawatts (one million watts. An electric generator is rated in watts. A large nuclear power plant has a power of 1,000 megawatts (or one gigawatt). If a one kilowatt generator runs for an hour, it produces a kilowatt-hour of electric energy.
Nuclear reactions can change what element an atom is, occur on earth only under special conditions and involve something like ten million times the energy. Thus enormously more energy can be obtained from suitable nuclear reactions than from chemical reactions.
Here are some facts about nuclear power plants.
Present nuclear power plants consume uranium (specifically U-235) as fuel. When the power plant is loaded with fuel, it can run for 18 months or 2 years before it has to be refuelled, a process that takes a month or two. As the power plant operators have become more experienced, they have learned to operate longer between refuellings and take a shorter time for refuelling.
When an atom of U-235 absorbs a neutron it fissions, i.e. it breaks up in parts. These parts consist mainly of two atoms of smaller elements and some neutrons.
When a reactor is operating, fission of an atom of U-235 generates on the average a bit more than two neutrons.
If each of two neutrons produced by a fission was absorbed by an atom of U-235 the number of fissions would double in a fraction of second and then double again and again. If this were allowed to continue, in a few seconds the reactor would be generating enough power to melt.
When the reactor is turned on, the multiplication of fissions is allowed to continue until the reactor is generating power at the desired rate. Then control rods that absorb neutrons are inserted until exactly one neutron from each fission causes another fission.
Because some of the neutrons caused by a fission are emitted from the fission products only after a delay of a minute or so, it is not difficult to control the power level of the reactor. Nevertheless, there are safety systems that will shut down the reactor if the power level gets too high or if the cooling water stops flowing.
The power to produce electricity comes from the fact that the two atoms produced by the fission of a U-235 atom fly off at high speed, but they don't get even an inch before they hit something and are stopped. Stopping converts their energy of motion into heat, and the reactor heats up. If the heat weren't taken away, the reactor would melt.
The heat from fission is taken up by water or steam pumped through the reactor. The hot steam goes through turbines connected to electric generators.
About 2/3 of the heat energy is lost, and is emitted to the atmosphere or to a body of water, a river or the ocean. This loss is a consequence of the Second Law of Thermodynamics and applies to all power plants, nuclear or coal-burning.
If the highest temperature in the steam plant is T1 and the temperature at which heat is exhausted to the environment is T2, the fraction of the heat generated that can be turned into electricity is (T1 - T2)/T1. The fraction of the heat energy transformed into electric energy is called the efficiency of the power plant. For high efficiency T1 should be as high as possible and T2 as low as possible. How high T1 can be is determined by how high a temperature the materials of the reactor can be without softening. How low T2 can be is limited by the environment. Cold seawater gives a good T2.
After 18 months or two years, most of the U-235 in the fuel is used up, and the fuel rods consist mainly of the products of fission, which remain radioactive and continue to generate heat. The fuel rods are placed in large pools of water which takes the remaining heat. The fuel rods become less and less radioactive with time.
After the rods have cooled off for a while, they should be chemically reprocessed to extract left over uranium and some plutonium that has been produced. The left-over uranium and the plutonium can then be converted to more reactor fuel. The fission products can then be buried in stable rock formations.
France, Britain, Japan and most other countries have their used fuel rods reprocessed. For bad political reasons, the US stopped reprocessing and hasn't yet managed to agree on how to store the fission products. There is no actual hurry, because the fuel rods become less and less radioactive as time passes. 2002 Note: Congress has just passed a bill and the President has signed it that provides for storing the waste in tunnels dug into Yucca Mountain in Nevada. Probably this will happen, but first there will be lots of lawsuits from opponents of nuclear power.
Most environmental organizations mistakenly oppose nuclear energy. The consequence has been pollution from coal burning plants.
OK, the last two points are controversial.I don't know if this elementary exposition is useful to anyone. Up to: Sustainability FAQ
Send comments to firstname.lastname@example.org. I sometimes make changes suggested in them. - John McCarthyThe number of hits on this page since 1995 November 13.