Nuclear Fission Power
Nuclear power uses controlled nuclear reactions to release energy for work including propulsion, heat, and the generation of electricity. Nuclear energy is produced by a controlled nuclear chain reaction and creates heat, which is used to boil water, produce steam, and drive a steam turbine. The turbine can be used for mechanical work or to generate electricity. The United States produces the most nuclear energy, with nuclear power providing 20% of the electricity it consumes, while France produces the highest percentage of its electrical energy from nuclear reactors—80% as of 2006. In the European Union as a whole, nuclear energy provides 30% of the electricity. Many military and some civilian ships use nuclear marine propulsion, a form of nuclear propulsion. Wiki n.p.
One of the first organizations to develop useful nuclear power was the U.S. Navy, for the purpose of propelling submarines and aircraft carriers. It has a good record in nuclear safety, perhaps because of the stringent demands of Admiral Hyman G. Rickover, who was the driving force behind nuclear marine propulsion as well as the Shippingport Reactor. The U.S. Navy has operated more nuclear reactors than any other entity, including the Soviet Navy, with no publicly known major incidents. The first nuclear-powered submarine, USS Nautilus (SSN-571), put to sea in 1955. Wiki n.p.
The 1973 oil crisis had a significant effect on countries, such as France and Japan, which had relied more heavily on oil for electric generation (39% and 73% respectively) to invest in nuclear power. Today, nuclear power supplies about 80% and 30% of the electricity in those countries, respectively. A general movement against nuclear power arose during the last third of the 20th century, based on the fear of a possible nuclear accident, fears of radiation, nuclear proliferation, and on the opposition to nuclear waste production, transport and final storage. Perceived risks on the citizens' health and safety, the 1979 accident at Three Mile Island and the 1986 Chernobyl disaster played a part in stopping new plant construction in many countries, although the Brookings Institution suggests that new nuclear units have not been ordered in the US primarily for economic reasons rather than fears of accidents. Wiki n.p. In 2007, the U.S. had 104 nuclear reactors, 103 of which were operating. Approval of new reactors is expected at a rate of about 11 per year.
Conventional thermal power plants all have a fuel source to provide heat. Examples are gas, coal, or oil. For a nuclear power plant, this heat is provided by nuclear fission of a uranium, plutonium, or thorium fuel inside the nuclear reactor. When a relatively large fissile atomic nucleus is struck by a neutron it forms two or more smaller nuclei as fission products, releasing energy and neutrons in a process called nuclear fission. The neutrons then trigger further fission, and so on. When this nuclear chain reaction is controlled, the energy released can be used to heat water, produce steam, and drive a turbine that generates electricity. While a nuclear power plant uses the same fuel, uranium-235 or plutonium-239, a nuclear explosive involves an uncontrolled chain reaction, and the rate of fission in a reactor is not capable of reaching sufficient levels to trigger a nuclear explosion because commercial reactor grade nuclear fuel is not enriched to a high enough level. Naturally found uranium is less than 1% U-235, the rest being U-238. Most reactor fuel is enriched to only 3-4%, but some designs use natural uranium or highly enriched uranium. Reactors for nuclear submarines and large naval surface ships, such as aircraft carriers, commonly use highly enriched uranium. Although highly enriched uranium is more expensive, it reduces the frequency of refueling, which is very useful for military vessels. Naval reactors are able to use unenriched uranium because the heavy water they use as a moderator and coolant does not absorb neutrons like light water does. The chain reaction is controlled through the use of materials that absorb and moderate neutrons. In uranium-fueled reactors, neutrons must be moderated (slowed down) because slow neutrons are more likely to cause fission when colliding with a uranium-235 nucleus. Light water reactors use ordinary water to moderate and cool the reactors. When at operating temperatures if the temperature of the water increases, its density drops, and fewer neutrons passing through it are slowed enough to trigger further reactions. That negative feedback stabilizes the reaction rate. Wiki n.p.
The Nuclear Fuel Cycle begins when uranium is mined, enriched, and manufactured into nuclear fuel, which is delivered to a nuclear power plant. After usage in the power plant, the spent fuel is delivered to a reprocessing plant or to a final repository for geological disposition. In reprocessing 95% of spent fuel can be recycled to be returned to usage in a power plant. A nuclear reactor is only part of the life-cycle for nuclear power. The process starts with mining. Generally, uranium mines are either open-pit strip mines, or in-situ leach mines. In either case, the uranium ore is extracted, usually converted into a stable and compact form such as yellowcake, and then transported to a processing facility where the yellowcake is converted to uranium hexafluoride, which is then enriched using various techniques. At this point, the enriched uranium, containing more than the natural 0.7% U-235, is used to make rods of the proper composition and geometry for the particular reactor that the fuel is destined for. The fuel rods will spend about 3 years inside the reactor, generally until about 3% of their uranium has been fissioned, then they will be moved to a spent fuel pool where the short lived isotopes generated by fission can decay away. After about 5 years in a cooling pond, the spent fuel is radioactively cool enough to handle, and it can be moved to dry storage casks or reprocessed. Wiki n.p.
As opposed to current light water reactors which use uranium-235 (0.7% of all natural uranium), fast breeder reactors use uranium-238 (99.3% of all natural uranium). It has been estimated that there are up to five billion years’ (also the estimated remaining life of the Sun) worth of uranium-238 for use in these power plants. Breeder technology has been used in several reactors, but requires higher uranium prices before becoming justified economically. As of December 2005, the only breeder reactor producing power is BN-600 in Beloyarsk, Russia. Another alternative would be to use uranium-233 bred from thorium as fission fuel in the thorium fuel cycle. Thorium is three times more abundant in the Earth's crust than uranium, and theoretically all of it can be used for breeding, making the potential thorium resource orders of magnitude larger than the uranium fuel cycle operated without breeding. Unlike the breeding of U-238 into plutonium, fast breeder reactors are not necessary; it can be performed in more conventional plants. Wiki n.p.
Uranium enrichment produces many tons of depleted uranium (DU) which consists of U-238 with most of the easily fissile U-235 isotope removed. U-238 is a tough metal with several commercial uses, e.g., aircraft production, radiation shielding, and making bullets and armor, because it has a higher density than lead. There are concerns that U-238 may lead to health problems in groups exposed to this material excessively, like tank crews and civilians living in areas where large quantities of DU ammunition have been used. Wiki n.p.
The safe storage and disposal of nuclear waste is a significant challenge. The most important waste stream from nuclear power plants is spent fuel. A large nuclear reactor produces 3 cubic metres (25-30 tons) of spent fuel each year. It is primarily composed of unconverted uranium as well as significant quantities of transuranic actinides (plutonium and curium, mostly). In addition, about 3% of it is made of fission products. The actinides (uranium, plutonium, and curium) are responsible for the bulk of the long term radioactivity, whereas the fission products are responsible for the bulk of the short term radioactivity. Spent fuel is highly radioactive and needs to be handled with great care. However, spent nuclear fuel becomes less radioactive over time. After 40 years, the radiation flux is 99.9% lower than it was the moment the spent fuel was removed, although still dangerously radioactive. Spent fuel rods are stored in shielded basins of water (spent fuel pools), usually located on-site. The water provides both cooling for the still-decaying uranium, and shielding from the continuing radioactivity. After a few decades some on-site storage involves moving the now cooler, less radioactive fuel to a dry-storage facility or dry cask storage, where the fuel is stored in steel and concrete containers until its radioactivity decreases naturally ("decays") to levels safe enough for other processing. This interim stage spans years or decades, depending on the type of fuel. Most U.S. waste is currently stored in temporary storage sites requiring oversight, while suitable permanent disposal methods are discussed. As of 2003, the United States had accumulated about 49,000 metric tons of spent nuclear fuel from nuclear reactors. Underground storage at Yucca Mountain in U.S. has been proposed as permanent storage.
After 10,000 years of radioactive decay, according to United States Environmental Protection Agency standards, the spent nuclear fuel will no longer pose a threat to public health and safety. Wiki n.p.
The amount of waste can be reduced in several ways, particularly reprocessing. Even so, the remaining waste will be substantially radioactive for at least 300 years even if the actinides are removed, and for up to thousands of years if the actinides are left in. Even with separation of all actinides, and using fast breeder reactors to destroy by transmutation some of the longer-lived non-actinides as well, the waste must be segregated from the environment for one to a few hundred years, and therefore this is properly categorized as a long-term problem. Subcritical reactors or fusion reactors could also reduce the time the waste has to be stored. It has been argued that the best solution for the nuclear waste is above ground temporary storage since technology is rapidly changing. The current waste may well become a valuable resource in the future. Wiki n.p.
The nuclear industry also produces a volume of low-level radioactive waste in the form of contaminated items like clothing, hand tools, water purifier resins, and (upon decommissioning) the materials of which the reactor itself is built. In the United States, the Nuclear Regulatory Commission has repeatedly attempted to allow low-level materials to be handled as normal waste: landfilled, recycled into consumer items, etc. Most low-level waste releases very low levels of radioactivity and is only considered radioactive waste because of its history. For example, according to the standards of the NRC, the radiation released by coffee is enough to treat it as low level waste. In countries with nuclear power, radioactive wastes comprise less than 1% of total industrial toxic wastes, which remain hazardous indefinitely unless they decompose or are treated so that they are less toxic or, ideally, completely non-toxic. Overall, nuclear power produces far less waste material than fossil-fuel based power plants. Coal-burning plants are particularly noted for producing large amounts of toxic and mildly radioactive ash due to concentrating naturally occurring metals and radioactive material from the coal. Contrary to popular belief, coal power actually results in more radioactive waste being released into the environment than nuclear power. Wiki n.p.
Reprocessing can potentially recover up to 95% of the remaining uranium and plutonium in spent nuclear fuel, putting it into new mixed oxide fuel. This would produce a reduction in long term radioactivity within the remaining waste, since this is largely short-lived fission products, and reduces its volume by over 90%. Reprocessing of civilian fuel from power reactors is currently done on large scale in Britain, France and Russia, will be in China and perhaps India, and is being done on an expanding scale in Japan. The potential of reprocessing has not been achieved because it requires breeder reactors, which are not yet commercially available. France is generally cited as the most successful reprocessor, but it presently only recycles 28% (by weight) of the yearly fuel use, 7% within France and another 21% in Russia. Unlike other countries, the US has stopped civilian reprocessing as one part of US non-proliferation policy, since reprocessed material such as plutonium can be used in nuclear weapons. Spent fuel is all currently treated as waste. Wiki n.p.
Critics, including most major environmental groups, claim that nuclear power is an uneconomic and potentially dangerous energy source with a limited fuel supply, especially compared to renewable energy, and dispute whether the costs and risks can be reduced through new technology. They also point to the problem of storing radioactive waste, the potential for possibly severe radioactive contamination by accident or sabotage, and the possibility of nuclear proliferation. Proponents claim that these risks are small and can be further reduced by the technology in the new reactors. They further claim that the safety record is already good when compared to the other major kinds of power plants, that many renewables have not solved the problem with their intermittent power production, in effect limiting them to a minority share of power production, and that nuclear power is a sustainable energy source. Wiki n.p.
Most of human exposure to radiation comes from natural background radiation. Most of the remaining exposure comes from medical procedures. Several large studies in the US, Canada, and Europe have found no evidence of any increase in cancer mortality among people living near nuclear facilities. For example, in 1990, the National Cancer Institute (NCI) of the National Institutes of Health announced that a large-scale study, which evaluated mortality from 16 types of cancer, found no increased incidence of cancer mortality for people living near 62 nuclear installations in the United States. The study showed no increase in the incidence of childhood leukemia mortality in the study of surrounding counties after start-up of the nuclear facilities. The NCI study, the broadest of its kind ever conducted, surveyed 900,000 cancer deaths in counties near nuclear facilities. However, in Britain there are elevated childhood leukemia levels near some industrial facilities, particularly near Sellafield, where children living locally are ten times more likely to contract the cancer. The reasons for these increases, or clusters, are unclear, but one study of those near Sellafield has ruled out any contribution from nuclear sources. Apart from anything else, the levels of radiation at these sites are orders of magnitude too low to account for the excess incidences reported. Wiki n.p.
Nuclear generation does not directly produce sulfur dioxide, nitrogen oxides, mercury or other pollutants associated with the combustion of fossil fuels. It also does not directly produce carbon dioxide, which has led some environmentalists to advocate increased reliance on nuclear energy as a means to reduce greenhouse gas emissions, which contribute to global warming. Non-radioactive water vapor is the significant operating emission from nuclear power plants. According to a 2007 story broadcast on 60 Minutes, nuclear power gives France the cleanest air of any industrialized country, and the cheapest electricity in all of Europe. Wiki n.p.
Like any power source, including renewables like wind and solar energy, the facilities to produce and distribute the electricity require energy to build and subsequently decommission. Mineral ores must be collected and processed to produce nuclear fuel. These processes are either directly powered by diesel and gasoline engines, or draw electricity from the power grid, which may be generated from fossil fuels. Life cycle analyses assess the amount of energy consumed by these processes, given today's mix of energy resources, and calculate over the lifetime of a nuclear power plant the amount of carbon dioxide saved per amount of electricity produced by the plant vs. the amount of carbon dioxide used in construction and fuel acquisition. In a study conducted in 2006 by the UK's Parliamentary Office of Science and Technology (POST), nuclear power's lifecycle was evaluated to emit the least amount of carbon dioxide (very close to wind power's lifecycle emissions) when compared to the other alternatives (fossil oil, coal, and some renewable energy including biomass and PV solar panels). Wiki n.p.
The first successful experiment with nuclear fission was conducted in 1938 in Berlin by the German physicists Otto Hahn, Lise Meitner, and Fritz Strassmann.
The first reactor, Chicago Pile-1, achieved criticality on December 2, 1942 as part of the Manhattan Project.
On Dec. 20, 1951, the first nuclear-powered generator began to produce electricity at the U.S. Reactor Testing Station EBR-I near Arco, Idaho. It produced about 100 kW of power.
On June 27, 1954, the world's first nuclear power plant to generate electricity for a power grid started operations at Obninsk, USSR. The reactor produced 5 MW of electricity.
The world's first commercial nuclear power station, Calder Hall in Sellafield, Great Britain, was opened in 1956 with an initial capacity of 50 MW.
Enrico Fermi and Leó Szilárd in 1955 share U.S. Patent 2,708,656 for the nuclear reactor.
The first nuclear-powered submarine, USS Nautilus (SSN-571), put to sea in 1955.
The Shippingport Atomic Power Station in Shippingport, PA, was the first commercial reactor in the USA and was opened in 1957.