The Three Mile Island Nuclear Generating Station is a commercial nuclear power plant. It is located on Three Mile Island in the Susquehanna River. The island is three miles downriver from the town of Middletown, Pennsylvania which is near the city of Harrisburg, Pennsylvania. There are two nuclear reactors referred to as Three Mile Island Unit 1(TMI-1) and Three Mile Island Unit 2 (TMI-2) at the station.
The boiling water reactor (BWR) design is used in about one third of the commercial power generation reactors in use in the world today. It is the second most popular reactor design behind pressurized water reactors. The BWR design was the result of collaboration between the Idaho National Laboratory and General Electric in the mid-1950s. Today, GE Hitachi Nuclear Energy is the main manufacturer of the BWRs.
Most of the hydrogen in the universe consists of a single proton orbited by a single electron and is also known as H-1 or protium. A small percentage of hydrogen atoms contain a neutron in the nucleus as well as the proton. This form of hydrogen is referred to as H-2, heavy hydrogen or deuterium. It is a stable atom like H-1. In the Earth's crust, for every six thousand four hundred and twenty H-1 atoms, there is a single H-2 atom.
Most of the hydrogen in the universe consists of a single proton orbited by a single electron and is also known as H-1 or protium. A small percentage of hydrogen atoms contain a neutron in the nucleus as well as the proton. This form of hydrogen is referred to as H-2, heavy hydrogen or deuterium. It is a stable atom like H-1. In the Earth's crust, for every six thousand four hundred and twenty H-1 atoms, there is a single H-2 atom.
Most of the water molecules in the universe contain two of these ordinary hydrogen atoms combined with an oxygen atom. A few of the water molecules will have H-2 atoms combined with oxygen. In all the water on Earth, in ten thousand water molecules there will be about 2 with H-2 atoms instead of H-1 atoms. Water with H-2 atoms is referred to as heavy water.
Just about all the deuterium in the universe is thought to have been created in the big bang. Heat can be used to separate the isotopes of hydrogen. The ratio of H1 to H-2 in gas giants and comets varies because of the effects of internal heat and solar heating. The fact that the ratio of H1 to H2 found in comets is close to that found in the oceans on Earth has been used to argue that the oceans were created by cometary impacts on the young Earth.
Deuterium was identified in the early 1930 soon after the discovery of the neutron. Harold Urey won a Nobel Prize in 1934 for discovering and naming deuterium. Since the discovery of deuterium, water containing deuterium in its molecules has been extracted from ordinary water through a steam distillation process. Canada used to be the leading world supplier of deuterium until its last heavy water production plant was closed in 1997.
The chemical and physical properties of compounds containing deuterium are similar to the behavior of the same compound without deuterium. However, there are still differences that are greater that those caused by any other change of particular isotopes in compounds. Heavy water is more viscous than ordinary water and ice created from heavy water will sink in ordinary water in contrast to ordinary ice which floats.
Heavy water is slightly toxic to multi-cellular creatures and single cell life forms whose cells contain a nucleus. More primitive single cell life that has no nucleus appear to not be harmed by it. A average person could consume five quarts of heavy water without serious injury but it half the water in the body was replace with heavy water, death would result.
Deuterium is used in experimental fusion reactors. When fusing hydrogen to helium, neutrons must be part of the mix because even though most hydrogen does not include neutrons, all helium nuclei do include neutrons. Heavy water is used as a moderator to slow neutrons in some nuclear reactor designs because it does not absorb neutrons like ordinary water. The Canada reactor design CANDU uses heavy water as a moderator. Deuterium is a useful tracer for chemistry and biochemistry because it is a non-radioactive and easily identified.
About two thirds of the heat generated by nuclear reactors is dumped into the cooling system. Ordinary water is a popular coolant for reactors. The water for cooling is drawn from either a large river or the ocean. While this makes it convenient to locate reactors near a river or the ocean, it also makes them more vulnerable to floods and tidal waves such as the recent disaster at Fukushima.
So far we have focused on uranium and plutonium in our discussion of nuclear fuel and reactors because they are the fuels for most of the world's reactors. There are other nuclear fuels used in existing reactors and atomic batteries or suggested for use in new designs. One of these alternative nuclear fuels that holds great promise is thorium.
When spent fuel rods are removed from a nuclear reactor, they are giving off heat and emitting radiation, primarily from fission products. They are stored in special pools of water or boric acid to allow the heat and radiation to diminish. The cooling fluid absorbs the radiation and is circulated through heat exchangers to get rid of the heat. It can take several years for the heat and radiation to drop to a safer level.
After manufacture, nuclear fuel is transported from the production facility to a nuclear power plant for use in a reactor. Specialized transport companies transport nuclear fuel assemblies which release little radioactivity and do not require special shielding.
In a typical nuclear reactor, sets of fuel rods called cells surround a control rod which can be inserted or withdrawn to control the neutron flux and thus, the rate of the chain reaction.
U-235 atoms are bombarded by neutrons and fission which produces heat and more neutrons. Some of the U-235 transmutes into plutonium which also undergoes fission producing about one third of the heat in the reactor core. The heat from the core is used to produce steam which drives the turbines that produce the electricity.
As the nuclear fuel in the rods fissions, the heat generated causes thermal expansion which can cause cracking. The nuclear fuel reacts with cladding materials such zirconium alloy which forms the shell of the fuel rod. The chemical composition of the fuel near the edge of the pellet changes as does its thermal conductivity. The purer uranium oxide in the center of the pellet will reach higher temperatures than the fuel near the outer edge of the pellet.
One ton of natural uranium can generate about fourty four million kilowatt hours. It would require over twenty thousand tons of coal or eight million cubic meters of natural gas to generate the same amount of electricity.
The rate at which the fuel is consumed is measured in gigawatt-days per ton of fuel and it is proportional to the level of concentration of U-235 in the nuclear fuel contained in the rods. The level of heat generation that can be safely handled by the current reactors limits the enrichment to about four percent which will yield a burn up rate of fourty gigawatt-days per ton. With improvements in materials and design, enrichment as high as five percent can be utilized, ultimately producing seventy gigawatt-days per ton.
Only a third of the heat produced by the core is captured in steam production. The other two thirds of the heat is passed to the water of the cooling system and either released in into a body of water such as a large river or the ocean. Alternatively, the water may be sent into cooling towers for evaporative cooling. Normally, a small amount of radioactivity is released into the cooling water.
The progression of nuclear fuel through a series of stages of production, use and disposal is referred to as the nuclear fuel cycle. When the spent fuel is used once and then disposed of, the cycle is called open or once-through. When the spent fuel is reprocessed and used again, the cycle is called closed.
