Nuclear Fission Reactors

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The thorium beta decays to protactinium and then to uranium , which in turn is used as fuel. Hence, like uranium , thorium is a fertile material.

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The amount of energy in the reservoir of nuclear fuel is frequently expressed in terms of "full-power days," which is the number of hour periods days a reactor is scheduled for operation at full power output for the generation of heat energy. The number of full-power days in a reactor's operating cycle between refueling outage times is related to the amount of fissile uranium U contained in the fuel assemblies at the beginning of the cycle. A higher percentage of U in the core at the beginning of a cycle will permit the reactor to be run for a greater number of full-power days.

At the end of the operating cycle, the fuel in some of the assemblies is "spent", having spent 4 to 6 years in the reactor producing power. This spent fuel is discharged and replaced with new fresh fuel assemblies. Plants typically operate on 18 month refueling cycles, or 24 month refueling cycles. This means that 1 refueling, replacing only one-third of the fuel, can keep a nuclear reactor at full power for nearly 2 years. This nuclear waste is highly radioactive and its toxicity presents a danger for thousands of years.

The spent fuel pool is a large pool of water that provides cooling and shielding of the spent nuclear fuel. After loading into dry shielded casks, the casks are stored on-site in a specially guarded facility in impervious concrete bunkers.

On-site fuel storage facilities are designed to withstand the impact of commercial airliners, with little to no damage to the spent fuel. An average on-site fuel storage facility can hold 30 years of spent fuel in a space smaller that a football field. In a CANDU reactor, this also allows individual fuel elements to be situated within the reactor core that are best suited to the amount of U in the fuel element. The amount of energy extracted from nuclear fuel is called its burnup , which is expressed in terms of the heat energy produced per initial unit of fuel weight.

Burn up is commonly expressed as megawatt days thermal per metric ton of initial heavy metal. Nuclear safety covers the actions taken to prevent nuclear and radiation accidents and incidents or to limit their consequences. The nuclear power industry has improved the safety and performance of reactors, and has proposed new safer but generally untested reactor designs but there is no guarantee that the reactors will be designed, built and operated correctly. Some serious nuclear and radiation accidents have occurred.

Nuclear power plant accidents include the SL-1 accident , the Three Mile Island accident , Chernobyl disaster , and the Fukushima Daiichi nuclear disaster Nuclear reactors have been launched into Earth orbit at least 34 times. A number of incidents connected with the unmanned nuclear-reactor-powered Soviet RORSAT radar satellite program resulted in spent nuclear fuel reentering the Earth's atmosphere from orbit. Almost two billion years ago a series of self-sustaining nuclear fission "reactors" self-assembled in the area now known as Oklo in Gabon , West Africa.

The conditions at that place and time allowed a natural nuclear fission to occur with circumstances that are similar to the conditions in a constructed nuclear reactor. First discovered in by French physicist Francis Perrin , they are collectively known as the Oklo Fossil Reactors. Self-sustaining nuclear fission reactions took place in these reactors approximately 1.

Such reactors can no longer form on Earth in its present geologic period. Radioactive decay of formerly more abundant uranium over the time span of hundreds of millions of years has reduced the proportion of this naturally occurring fissile isotope to below the amount required to sustain a chain reaction with only plain water as a moderator.

The natural nuclear reactors formed when a uranium-rich mineral deposit became inundated with groundwater that acted as a neutron moderator, and a strong chain reaction took place. The water moderator would boil away as the reaction increased, slowing it back down again and preventing a meltdown. The fission reaction was sustained for hundreds of thousands of years, cycling on the order of hours to a few days.

These natural reactors are extensively studied by scientists interested in geologic radioactive waste disposal. They offer a case study of how radioactive isotopes migrate through the Earth's crust. This is a significant area of controversy as opponents of geologic waste disposal fear that isotopes from stored waste could end up in water supplies or be carried into the environment. Nuclear reactors produce tritium as part of normal operations, which is eventually released into the environment in trace quantities. As an isotope of hydrogen , tritium T frequently binds to oxygen and forms T 2 O.

This molecule is chemically identical to H 2 O and so is both colorless and odorless, however the additional neutrons in the hydrogen nuclei cause the tritium to undergo beta decay with a half-life of Despite being measurable, the tritium released by nuclear power plants is minimal.

The United States NRC estimates that a person drinking water for one year out of a well contaminated by what they would consider to be a significant tritiated water spill would receive a radiation dose of 0. The amounts of strontium released from nuclear power plants under normal operations is so low as to be undetectable above natural background radiation.

Nuclear fission

Detectable strontium in ground water and the general environment can be traced to weapons testing and the Chernobyl accident that occurred during the mid- 20th century. From Wikipedia, the free encyclopedia. This article is about constructed nuclear fission reactors. For nuclear fusion reactors, see fusion power. For natural nuclear reactors, see natural nuclear fission reactor. Main article: Nuclear reactor physics. Main article: Nuclear fission. PWR: Number of reactors by type end [21]. Net power capacity GWe by type end [21].

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This section does not cite any sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. June Learn how and when to remove this template message. Main article: Fusion power.

Main article: Nuclear fuel cycle. Main article: Nuclear safety. See also: Lists of nuclear disasters and radioactive incidents. Main article: Natural nuclear fission reactor. US Department of Energy. Archived from the original PDF on 23 April Retrieved 24 September The Nuclear Tourist.

Retrieved 25 September Archived from the original on 27 September Retrieved 18 March Cambridge University Press. Retrieved 17 March Archived from the original PDF on 11 December Krivit, Steven ed. Hoboken, NJ: Wiley. American Nuclear Society Nuclear news. November Retrieved 12 January BBC News. Retrieved 9 November Camp Century, Greenland.

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Nuclear fission - Energy Education

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When more heat is desired, the control rods are raised so they catch fewer neutrons, and the chain reaction speeds up and more heat is generated. The control rods are operated in a fail-safe system so that power is necessary to hold them up; during a power failure, gravity will pull the control rods down into the shut off position. Fission reactors use a moderator surrounding the fuel rods to slow down the neutrons. Water is not only a good coolant but also a good moderator so a common type of fission reactor has the fuel core submerged in a huge pool of water.

You can follow the operation of an electricity-generating fission reactor in the figure. The reactor core is submerged in a pool of water. The heat from the fission reaction heats the water and the water is pumped into a heat exchanger container where the heated water boils the water in the heat exchanger. The steam from there is forced through a turbine which spins a generator and produces electricity.

After the water passes through the turbine, it is condensed back to liquid water and pumped back to the heat exchanger.

In the United States, heavy opposition to the use of nuclear energy was mounted in the late 's and early 's. Every environmentalist organization in the US opposed the use of nuclear energy and the constant pressure from environmentalist groups brought increased public fear and therefore, opposition. This is not true today; at least one environmental leader has published a paper in favor of nuclear powered electricity generation. In , a reactor core meltdown at Pennsylvania's Three Mile Island nuclear power plant reminded the entire country of the dangers of nuclear radiation.