Atomic reactor

A nuclear reactor is a device in which nuclear chain reactions are initiated, controlled, and sustained at a steady rate (as opposed to a nuclear explosion, where the chain reaction occurs in a split second). Nuclear reactors are used for many purposes, but the most significant current uses are for the generation of electrical power and, in rare cases, for the production of plutonium for use in nuclear weapons. Currently all commercial nuclear reactors are based on nuclear fission, and are considered problematic by some for their safety and health risks. Conversely, some consider nuclear power to be a safe and pollution-free method of generating electricity. Fusion power is an experimental technology based on nuclear fusion instead of fission. There are other devices in which nuclear reactions occur in a controlled fashion, including radioisotope thermoelectric generators, which generate heat and power by passive radioactive decay, and Farnsworth-Hirsch fusors, in which controlled nuclear fusion is used to produce neutron radiation. neutron radiation

Applications

- Nuclear power:
- Heat for electricity generation
- Heat for domestic and industrial heating
- Desalination
- Nuclear propulsion:
- Nuclear marine propulsion
- Proposed nuclear thermal rockets
- Transmutation of elements:
- Production of plutonium, often for use in nuclear weapons
- Creating various radioactive isotopes, such as americium for use in smoke detectors
- Research applications including:
- Providing a source of neutron and positron radiation
- Development of nuclear technology

History

Enrico Fermi and Leó Szilárd, while both were at the University of Chicago, were the first to build a nuclear pile and demonstrate a controlled chain reaction on December 2, 1942. In 1955 they shared US patent number [http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=1&u=/netahtml/search-bool.html&r=1&f=G&l=50&co1=AND&d=pall&s1=2,708,656.WKU.&OS=PN/2,708,656&RS=PN/2,708,656 2,708,656] for the nuclear reactor. The first nuclear reactors were used to generate plutonium for nuclear weapons. Additional reactors were used in the navy (see United States Naval reactor) to propel submarines and aircraft carriers. In the mid-1950s, both the Soviet Union and western countries were expanding their nuclear research to include non-military uses of the atom. However, as with the military program, much of the non-military work was done in secret. On December 20, 1951, electric power from a nuclear powered generator was produced for the first time at Experimental Breeder Reactor-I (EBR-1) located near Arco, Idaho. On June 27, 1954, the world's first nuclear power plant generated electricity but no headlines--at least, not in the West. According to the Uranium Institute (London, England), the first reactor to generate electricity for commercial use was at Obninsk, Kaluga Oblast, Russia. The Shippingport Reactor (in Pennsylvania) was the first commercial nuclear generator to become operational in the United States. The Shippingport reactor was ordered in 1953 and began commercial operation in 1957. Even before the 1979 Three Mile Island accident, new orders for nuclear plants in the U.S. had ceased for economic reasons primarily related to greatly extended construction times. As of 2004, no new nuclear plants have been ordered in the USA since 1978 [http://www.pbs.org/wgbh/pages/frontline/shows/reaction/maps/chart2.html], although that may change by 2010 (see Future Of The Industry below). Surprisingly, and unlike the Three Mile Island accident, the 1986 Chernobyl accident did not increase regulations affecting Western reactors. This was because the Chernobyl reactors were known to be an unsafe design (RBMKs) without containment buildings and operated unsafely, and the West had nothing to learn from them [http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/fschernobyl.html]. There was however political fallout: Italy held a referendum the next year (1987), and it was decided to shut down the country's four nuclear power plants [http://breakingnews.iol.ie/news/story.asp?j=131104200&p=y3yyx49x6]. In 1992 the Turkey Point Nuclear Generating Station was hit directly by Hurricane Andrew. Over $90 million of damage was done, largely to a water tank and to a smokestack of one of the fossil-fueled units on-site, but the containment buildings were undamaged [http://www.nrc.gov/reading-rm/doc-collections/gen-comm/info-notices/1993/in93053.html] [http://www.nrc.gov/reading-rm/doc-collections/gen-comm/info-notices/1993/in93053s1.html]. The first organization to develop utilitarian nuclear power, the U.S. Navy, is the only organization worldwide with a totally clean record. This is perhaps because of the stringent demands of Admiral Hyman G. Rickover, who was the driving force behind nuclear marine propulsion. The U.S. Navy has operated more nuclear reactors than any other entity, other than the Soviet Navy, with no publicly known major incidents. Two U.S. nuclear submarines, USS Scorpion and Thresher, have been lost at sea, though for reasons not related to their reactors, and their wrecks are situated such that the risk of nuclear pollution is considered low.

The future of the industry

Some experts predict that electricity shortages, fossil fuel price increases and concern over Greenhouse gas emissions will renew the demand for nuclear power plants. Watts Bar 1, which came on-line in 1997, was the last U.S. commercial nuclear reactor to go on-line. As of 2004, the immediate future of the industry in many countries still appeared uncertain, the most notable exceptions being Japan, China and India, all actively developing both fast and thermal technology, South Korea and the United States, developing thermal technology only, and South Africa and China, developing versions of the Pebble Bed Modular Reactor (PBMR). Finland and France actively pursue nuclear programs; Finland has a new AREVA plant under construction. Japan has an active nuclear construction program with new units brought on-line in 2005. In the U.S., three consortia responded in 2004 to the U.S. Department of Energy's solicitation under the Nuclear Power 2010 Program and were awarded matching funds - the Energy Policy Act of 2005 authorized subsidies for up to six new reactors, and authorized the Department of Energy to build a reactor to produce both electricity and hydrogen. As of the early 21st century, nuclear power is of particular interest to both China and India to serve their rapidly growing economies - both are developing fast breeder reactors. See also future energy development. On September 22, 2005 it was announced that two sites in the U.S. had been selected to receive new power reactors (exclusive of the new power reactor scheduled for INL) - see Nuclear Power 2010 Program. It is possible that the first new nuclear power plant to be built in the United States since the 1970s may be installed in the remote town of Galena, Alaska. The town's City Council approved the idea, and Toshiba proposed to install its model 4S "nuclear battery" in Galena free of charge as a test. See also nuclear power phase-out, nuclear energy policy.

Method of operation

Essentially all existing nuclear reactors are critical fission reactors that produce heat and neutrons through a nuclear fission chain reaction in which nuclei of nuclear fuel are impacted by free neutrons, causing them to break apart (fission). In turn, these fission events yield more free neutrons which carry on to induce more fission events. The resulting nuclear fragments (fission products) are released with large amounts of kinetic energy that converts rapidly into heat. For details, see the articles on nuclear fission and nuclear reactor physics. Critical fission reactors usually consist of several parts: a reactor core that houses the reaction itself; a working fluid (usually water) to remove heat from the reactor core; some sort of shielding to prevent the reactor from emitting ionizing radiation and neutrons; and a containment vessel to prevent leakage of radioactive isotopes. Power reactors also include a heat exchanger to remove heat from the working fluid of the reactor itself and drive an attached electrical generator, while research reactors include facilities for inserting samples into the core (and later removing them), or for extracting neutron beams from the core. The reactor core contains nuclear fuel, a neutron moderator that aids the formation of a chain reaction, a working fluid that absorbs the heat of the nuclear reaction, a mechanism (such as control rods) to regulate and control the chain reaction by absorbing free neutrons, and possibly a neutron reflector that aids the reaction by keeping some neutrons from escaping the core. Most reactors are controlled by means of control rods that are made of a strongly neutron-absorbent material such as boron or cadmium. Inserting a control rod into the reactor core has the effect of absorbing all neutrons that would otherwise have passed through the space occupied by the rod; when the reactor is shut down, the control rods make the reaction subcritical, so that the chain reaction is not self-sustaining. Pulling a control rod partway out of the core reduces the neutron absorbent effect by permitting neutrons to survive (and impact fuel) where otherwise they would have been absorbed. When the rod is pulled sufficiently far from the core, the core becomes a critical assembly and the nuclear reaction is governed by the physics of the chain reaction rather than the usual laws of radioactive decay. Small research reactors or compact military power reactors may have only a small number of control rods, while large fixed power installations typically have dozens to hundreds. Relative placement of the control rods determines the neutron flux distribution in the core, which in turn can be used to adjust and even-out the rate of nuclear fuel consumption or to eliminate hot spots in the operating core. All nuclear reactors are built with at least one rapid-shutdown control, traditionally called a SCRAM, that inserts all or a large number of control rods into the core simultaneously. In many reactors, the rods are mounted vertically and suspended by electromagnets, so that interrupting the power to the electromagnets causes the reactor to fail safely by dropping all of the rods into the core. Because most reactors use water as a working fluid, many facilities can, as a last resort, insert borax, a readily soluble boron compound that absorbs neutrons, into the water in the core.

Reactor design

See also Nuclear power plant and nuclear reactor physics Most nuclear reactors are designed to carry out nuclear fission reactions on a large scale. This produces heat, fission products, and intense neutron radiation. In a nuclear power plant, that heat is used to do useful work. Some reactors, whether experimental or military, are designed with no concern for making use of the generated heat, as their goal is to make use of the neutron radiation to transmute elements. In either case, for all current nuclear reactors, it is essential that a nuclear chain reaction be continually sustained. In a sustained nuclear chain reaction, the fission of a single fuel nucleus releases a few neutrons. These neutrons initially carry a great deal of energy (and are therefore called fast neutrons). These neutrons may be captured immediately by another fuel nucleus, or they may interact with a neutron moderator or a neutron absorber. Most nuclear fuels are more likely to undergo induced fission when struck by a thermal neutron moving at low speed than when struck by a fast neutron, so it is necessary to slow down the neutrons by allowing the neutrons to scatter off nuclei of a neutron moderator. After a few such scattering events, the neutron radiation has a thermal energy spectrum (that is, they are moving with the same average energy as a gas at the same temperature as the reactor core) and is much more easily captured by a fuel nucleus. thermal neutron A nuclear reactor that uses a moderator is called a slow or thermal reactor, and it is normally categorized according to the type of moderator. Common moderators are heavy water and ordinary light water. Some reactors also use graphite, although it has a number of problems (see, for example, Wigner energy, the Windscale fire and the Chernobyl accident). A reactor that is not moderated is called a fast' reactor because fast neutrons are used to stimulate fission for the chain reaction, whereas a reactor that requires neutron moderation is called a slow reactor or thermal reactor. fast neutron When a neutron is captured by a fuel nucleus, the nucleus may undergo fission immediately, it may remain in an unstable state for a short while before undergoing fission, or it may fail to undergo fission at all. Fission events that occur immediately are called "prompt" fission events, and if there are enough prompt events for the reaction to be self-sustaining without the delayed fission events, then the reactor is said to be prompt critical. In such a situation, the amount of fission in the reactor will grow exponentially and very quickly; the result would be a large explosion (although not one comparable to a nuclear weapon). Thus a stable nuclear reactor must be maintained in a critical but not prompt critical state. Controls are also essential to ensure that the temperature does not rise so high that the reactor is damaged or destroyed. A nuclear reactor is controlled by adjusting the configuration of neutron absorbers in and around the central core, the configuration of the neutron moderator (if any), and sometimes the configuration of the fuel itself. The most common arrangement is to include neutron-absorbing control rods which can be partially inserted into the reactor in order to damp its reaction. Such control rods normally require sophisticated monitoring equipment, so a number of advanced reactor designs (such as the pebble-bed reactor) have tried to build in passive safety systems which require no action by electronic, mechanical, or human agents to prevent plant overheating (see Passively safe). In any nuclear reactor, some sort of cooling is necessary. In a nuclear power plant, the cooling system is designed to can make use of the heat by transferring it to a heat engine such as a steam turbine. Most nuclear reactors use water as a coolant, either in a pressurized liquid form or by boiling into steam. Since water acts as a moderator, fast reactors cannot be cooled with water. Molten sodium or sodium salts are in current use. Reactors designed for transmutation (breeder reactors and research reactors) generally do not use the heat but simply release it to the environment.
Safety
By regulation, as part of the design of any nuclear reactor provisions must be made for operator errors or failure of critical equipment. For this reason the "Defense in Depth" concept is employed to ensure operability of all systems when required for safety. All systems in nuclear plants have three main safety objectives:
- Control of Reactivity (ability to control the amount of neutron flux in the fuel either mechanically or chemically),
- Maintenance of Core Cooling (maintaining an adequate supply and backup supply of coolant to the core region) and
- Maintenance of Barriers to Release of Radiation (fuel cladding, primary barrier, containment and attenuation devices). Where Systems, Structures and Components (SSC) are required to perform any duties supporting the three safety functions, they are provided with frequent inspection, operational or functional tests, and increased design, purchase and repair scrutiny as part of a Quality Assurance (QA) plan. Part of the design of these SSC includes redundancy (having multiple backup components), provision of independent systems (such as a requirement to have two or more separate systems performing the same function in parallel) "voting" on an interpretation of a signal, fail-safe design (knowing how a SSC will fail and what effect it will have on companion SSC) monitoring instrumentation and protection against "Common Mode Failure". Common Mode Failure prevents a single failure from affecting both "trains" or systems of independent, redundant equipment. Engineering performance is tested on a frequent basis (surveillance) to provide assurance (QA) of readiness to perform its designed function. It should be noted that many of these same design features are mandated on commercial airliners. On detection of process (pressure, temperature, radiation, flow, etc) indications outside of a normal range an alarm will sound and be "acknowledged" in the control room, where an operator makes adjustments. If the alarming parameters exceed set points further, the reactor, turbine or generator may provide a fault signal which automatically places the system in a safer (lower energy) mode and may terminate operations without operator control. In the case of a generator or turbine fault, steam will be limited or shut off and the turbine will slow. If the problem is not corrected quickly, a SCRAM will occur automatically inserting the control rods into the reactor core and slowing the neutron flux by over 99% in seconds. The plant can be restarted, but only after an investigation is completed. Each facility operates to a set of license conditions (Final Safety Analysis Report, or FSAR) specific to the units' design, location and environment. The license conditions, condensed in a set of Technical Specifications, describes the limits of power, certain process parameters, staff, training and qualifications, minimum available equipment and other physical and administrative requirements which must be in place in order to operate the reactor. Violation of the license conditions may result in fines and inability to operate the facility - also, since the license conditions constitute part of a federal license, plant personnel may face criminal charges.
Types of reactors
SCRAM with 4% enriched, pin-type fuel consisting of UO₂ pellets in zircaloy cladding.]]zircaloy A number of reactor technologies have been developed. Fission reactors can be divided roughly into two classes, depending on the energy of the neutrons that are used to sustain the fission chain reaction.
- Thermal (slow) reactors use slow or thermal neutrons. These are characterised by having moderating materials which are intended to slow the neutrons until they approach the average kinetic energy of the surrounding particles, that is, until they are thermalised. Thermal neutrons have a far higher probability of fissioning U-235, and a lower probability of capture by U-238 than the faster neutrons that result from fission do. As well as the moderator, thermal reactors have fuel (fissionable material), containments, pressure vessels, shielding, and instrumentation to monitor and control the reactor's systems. Most power reactors are of this type, and the first plutonium production reactors were thermal reactors using graphite as the moderator. Some thermal power reactors are more thermalised than others; Graphite (ex. Russian RBMK reactors) and heavy water moderated plants (ex. Canadian CANDU reactors) tend to be more thoroughly thermalised than PWRs and BWRs, which use light water (normal water) as the moderator (due to the extra thermalization, these types can use natural uranium/unenriched fuel).
- Fast reactors use fast neutrons to sustain the fission chain reaction, and are characterised by the lack of moderating material. They require highly enriched fuel (sometimes weapons-grade), or plutonium in order to reduce the amount of U-238 that would otherwise capture fast neutrons. Some are capable of producing more fuel than they consume, usually by converting U-238 to Pu-239. Some early power stations were fast reactors, as are some Russian naval propulsion units, and construction of prototypes is continuing, see fast breeder, but overall the class has not achieved the success of thermal reactors in any application. An example of this type of reactor is the Fast Breeder Reactor (FBR). Thermal power reactors can again be divided into three types, depending on whether they use pressurised fuel channels, a large pressure vessel, or gas cooling.
- Pressure vessels holding steam heated by the reactor are used by most commercial and naval reactors. This serves as a layer of shielding and containment.
- Pressurised channels are used by the RBMK and CANDU reactors. Channel-type reactors can be refuelled under load, which has advantages and disadvantages discussed under CANDU reactor.
- Gas-cooled reactors are cooled by a circulating inert gas, usually helium, but nitrogen and carbon dioxide have also been used. Utilisation of the heat varies, depending on the reactor. Some reactors run hot enough that the gas can directly power a gas turbine. Older designs usually run the gas through a heat exchanger to make steam for a steam turbine. The pebble bed reactor uses a gas-cooled design. Since water serves as a moderator, it cannot be used as a coolant in a fast reactor. Most designs for fast power reactors have been cooled by liquid metal, usually molten sodium. They have also been of two types, called pool and loop reactors.
Current families of reactors

- Pool type reactor
- Pressurized water reactor (PWR)
- Boiling water reactor (BWR)
- Fast breeder reactor (FBR)
- Pressurised Heavy Water Reactor (PHWR or CANDU)
- United States Naval reactor
Obsolescent types still in service

- Magnox reactor
- Advanced gas-cooled Reactor (AGR)
- Light water cooled graphite moderated reactor (RBMK)
- Fast neutron reactor
Other types of reactors

- Molten salt reactor
- Aqueous Homogeneous Reactor
Advanced reactors
More than a dozen advanced reactor designs are in various stages of development. Some are evolutionary from the PWR, BWR and CANDU designs above, some are more radical departures. The former include the Advanced Boiling Water Reactor (ABWR), two of which are now operating with others are under construction. The best-known radical new design is the Pebble Bed Modular Reactor (PBMR), a High Temperature Gas Cooled Reactor (HTGCR). Possible designs of subcritical reactors exist on the drawing board, notably the energy amplifier, awaiting political support and funding. Some, such as the Integral Fast Reactor (IFR), have been cancelled due to a political climate unfavorable to nuclear power.
Nuclear fuel cycle
Main article: nuclear fuel cycle Thermal reactors generally depend on refined and enriched uranium. Some nuclear reactors can operate with a mixture of plutonium and uranium (see MOX). The process by which uranium ore is mined, processed, enriched, used, possibly reprocessed and disposed of is known as the nuclear fuel cycle. Uranium is sampled and mined as other metals are, via open-pit mining or leach mining. Raw uranium ore found in the United States ranges from 0.05% to 0.3% uranium oxide. Uranium ore is not rare; the largest probable resources, extractable at a cost of US$80 per kilogram or cheaper, are located in Australia, Kazakhstan, Canada, South Africa, Brazil, Namibia, Russia, and the United States. The raw ore is then milled, where it is ground and chemically leached. The resulting powder of natural uranium oxide is called "yellowcake". The yellowcake powder is then converted to uranium hexafluoride to prepare for enrichment. Since under 1% of the uranium found in nature is the easily fissionable U-235 isotope, the uranium must be enriched to about 4% U-235, usually by means of gaseous diffusion or gas centrifuge. The enriched result is then converted into uranium dioxide powder, which is pressed and fired onto pellet form. These pellets are stacked into tubes which are then sealed and called fuel rods. Many of these fuel rods are used in each nuclear reactor.
Fueling of nuclear reactors
The amount of energy in the reservoir of nuclear fuel is frequently expressed in terms of "full-power days," which is the number of 24-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-235 (U-235) contained in the fuel assemblies at the beginning of the cycle. A higher percentage of U-235 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," and is discharged and replaced with new (fresh) fuel assemblies. Although in practice, it is the buildup of reacton poisons in nuclear fuel that determines the lifetime of nuclear fuel in a reactor; long before all possible fissions have taken place, the buildup of long-lived neutron absorbing fission products damps out the chain reaction. The fraction of the reactor's fuel core replaced during refueling is typically one-fourth for a boiling-water reactor and one-third for a pressurized-water reactor. Not all reactors need to be shut down for refueling; for example, pebble bed reactors, molten salt reactors and CANDU reactors allow fuel to be shifted through the reactor while it is running. In a CANDU reactor, this also allows individual fuel elements to be moved about within the reactor core to places that are best suited to the amount of U-235 in the fuel element. The amount of energy extracted from nuclear fuel is called its "burn up," 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.
Waste management
The final stage of the nuclear fuel cycle is the management of the still highly radioactive, "spent" fuel, which constitutes the most problematic component of the nuclear waste stream. After fifty years of nuclear power the question of how to deal with this material remains fraught with safety concerns and technical problems, and one of the most important lines of criticism of the industry is based on the long-term risks and costs associated with dealing with the waste. Management of the spent fuel can include various combinations of storage, reprocessing, and disposal. In practice storage has been the primary modality so far. Typically the spent fuel rods are stored in a pool of water which is usually located on-site. The water provides both cooling for the still-decaying uranium, and shielding from the continuing radioactivity. Another, more permanent method of disposal of high-level nuclear waste calls for the material to be buried deep underground in certain geological formations. The Canadian government, for example, is seriously considering this method of disposal, known as the Deep Geological Disposal concept. Under the current plan, a vault is to be dug 500 to 1000 meters below ground, under the Canadian Shield, one of the most stable landforms on the planet. The vaults are to be dug inside geological formations known as batholiths, formed about a billion years ago. The used fuel bundles will be encased in a corrosion-resistant container, and further surrounded by a layer of buffer material, possibly of a special kind of clay (bentonite clay). The case itself is designed to last for thousands of years, while the clay would further slow the corrosion rates of the container. The batholiths themselves are chosen for their low ground-water movement rates, geological stability, and low economic value. (See [http://www.nuclearfaq.ca/cnf_sectionE.htm#v The Canadian Nuclear FAQ, Waste Management section], by Dr. Jeremy Whitlock) Reprocessing is attractive in principle because (1) it can recycle nuclear fuel and (2) it can prepare the waste material for disposal. Considerable experience with reprocessing in France however, has indicated that a one way fuel cycle based on extracting and processing fresh supplies of uranium and storing the spent fuel is more economical than reprocessing.
Natural nuclear reactors
A natural nuclear fission reactor can occur under certain circumstances that mimic the conditions in a constructed reactor. The only known natural nuclear reactor formed 2 billion years ago in Oklo, Gabon, Africa. [http://www.ans.org/pi/np/oklo] Such reactors can no longer form on Earth: radioactive decay over this immense time span has reduced the proportion of U-235 in naturally occurring uranium to below the amount required to sustain a chain reaction. 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. 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.
Related articles

- Nuclear Reactor Operator Badge
- United States Naval reactor
- List of nuclear reactors
- Green Field status
See also

- Nuclear reactor physics
- Nuclear power
- Nuclear fission
- Nuclear fusion
- Nuclear power plant
- Nuclear meltdown
- Power plant
- Nuclear waste
- Electricity generation
- Nuclear physics
- Enrico Fermi
- Manhattan Project
- Nuclear marine propulsion
- Technology assessment
- List of nuclear accidents
- Energy amplifier
- Future energy development
- SCRAM
- SSTAR - LLNL design for a "world" reactor
References and links

- [http://www.insc.anl.gov/pwrmaps/ Worldwide maps of nuclear power stations]
- [http://www.uranium.info Uranium.Info] publishing uranium price since 1968.
- [http://eia.doe.gov Energy Information Administration] provides lots of statistics and information on the industry.
- [http://www.antenna.nl/wise/uranium/efac.html World Nuclear Fuel Facilities]
- [http://www.uic.com.au/ The Uranium Information Centre] provided some of the original material in this article.
- [http://www.nrc.gov/ The US Nuclear Regulatory Commission] supervises the US Nuclear industry
- The Idaho National Engineering and Environmental Laboratory developed nuclear reactor technology in the United States - [http://newsdesk.inel.gov/press_releases/2001/05-21EBR_I_summer_tours.htm INEL Newsdesk - Experimental Breeder Reactor-I opens for summer tours]
- The International Atomic Energy Agency (IAEA) works with its Member States and multiple partners worldwide to promote safe, secure and peaceful nuclear technologies.
- [http://www.iaea.org/ IAEA Website]
- [http://www.iaea.org/programmes/a2/ IAEA's Power Reactor Information System (PRIS)]
- [http://www.iaea.org/inis/aws/htgr/ IAEA's Knowledge Base] on Gas Cooled Reactors
- [http://www.iaea.org/inis/ws/ IAEA's Web directory of nuclear related resources on the Internet]
- [https://www.pbmr.co.za/ The Pebble Bed Modular Reactor] - [http://whyfiles.org/130nukes/3.html Whyfiles.org - On a bed of pebbles]
- [http://www.world-nuclear.org World Nuclear Association] - A pro nuclear site
- [http://www.greenpeace.org/~nuclear/ Greenpeace Nuclear Campaign] - An anti-nuclear site
- [http://www.democracynow.org/article.pl?sid=04/09/24/1359225 A Debate: Is Nuclear Power The Solution to Global Warming?]
- [http://www.ecolo.org Environmentalists for Nuclear Power, pro nuclear site]
- [http://www.sckcen.be SCK.CEN Belgian Nuclear Research Centre - pro nuclear site]
- [http://www.phyast.pitt.edu/~blc/book/BOOK.html The Nuclear Energy Option] by Bernard Cohen. Pro nuclear book which compares risks of nuclear power with other methods of energy generation.
- [http://www.ucsusa.org/clean_energy/nuclear_safety/page.cfm?pageID=1408 Union of Concerned Scientists, Concerns re: US nuclear reactor program]
- [http://www.nuclearfaq.ca The Canadian Nuclear FAQ] - a very information-rich resource about Canadian CANDU reactors.
- [http://www.eagletv.co.uk/home/nuclear.htm The Nuclear Boy Scout] - Eagle and Eagle TV production about David Hahn's nuclear reactor experiment Category:Energy conversion Category:Nuclear technology Category:Electric power ja:原子炉

Nuclear chain reaction
A nuclear chain reaction occurs when on average more than one nuclear reaction is caused by another nuclear reaction, thus leading to an exponential increase in the number of nuclear reactions. An uncontrolled chain reaction within a sufficiently large amount of fission fuel (critical mass) can lead to an explosive energy release and is the concept behind nuclear weapons. The chain reaction could also be adequately controlled and used as an energy source (nuclear reactor). The only known natural self-sustaining nuclear chain reaction was at Oklo. The first artificial self-sustaining nuclear chain reaction was initiated by the Metallurgical Laboratory, led by Enrico Fermi, in a racquets court below the bleachers of Stagg Field at the University of Chicago on December 2, 1942 during the Manhattan Project. Some fission equations, showing averages:
- U-235 + neutron = fission fragments + 2.52 neutrons + 180 MeV
- Pu-239 + neutron = fission fragments + 2.95 neutrons + 200 MeV. This excludes 10 MeV for unusable and hardly detectable neutrinos. When a heavy atom undergoes nuclear fission it breaks into two or more fission fragments. Each of these fission fragments is an atom of a more lightweight element on the periodic table of the elements. Thus a neutron can cause a nuclear fission reaction which releases ca. 2.5 or 3 neutrons. Crucial is how many of these cause another fission reaction. The effective neutron multiplication factor k is the average number of neutrons from these 2.5 or 3 cause another fission reaction, as opposed to neutrons produced by the fission which are being absorbed without causing a new fission, and those travelling out of the system. The value of k for a combination of two objects is always more than the larger of the two k values of each separately. It may or may not be more than the sum of these two: for two objects far apart it is little more than the larger value, for an object inserted in a hole in the other, as the assembled parts of a gun method weapon, it may well be more than the sum. The average generation time is the average time from neutron emission to fission capture. This time is very short: the distance is something like the diameter of a critical mass; the speed may be ca. 10 000 km/s and the distance 10 cm, so the time is of the order 10 ns = 1 shake. We can distinguish the following cases:
- k < 1 (sub-critical mass): starting with one fission, we have on average a total of 1/(1 − k) fissions. Any begin of a chain reaction dies out quickly.
- k = 1 (critical mass): Starting with one free neutron, the expected value of the number of free neutrons resulting from it is 1 at any time; in the course of time there is a decreasing additional probability that the beginning chain reaction has died out, which is compensated by the possibility of multiple neutrons still being present.
- k > 1 (super-critical mass): starting with one free neutron, there is a non-trivial probability that is does not cause a fission or that a beginning chain reaction dies out. However, once the number of free neutrons is more than a few, it is very likely that it will increase exponentially. Both the number of neutrons present in the assembly (and thus the instantaneous rate of the fission reaction), and the number of fissions that have occurred since the reaction began, is proportional to $e^$ , where g is the average generation time and t is the elapsed time. This cannot continue, of course: k decreases when the amount of fission material that is left decreases; also the geometry and density can change: the geometry radically changes when the remaining fission material is torn apart, but in other circumstances it can just melt and flow away, etc. When k is close to 1, this calculation somewhat over-estimates the 'doubling rate'. When a uranium nucleus absorbs a neutron it enters a very-short-lived excited state which then decays by several possible routes. Typically it decays into two fragments, fission products, typically isotopes of Iodine and Cesium, with expulsion of a number of neutrons. The fission products are themselves unstable, with a wide range of lifetimes, but typically several seconds, and decay producing further neutrons. It is usual to split the population of neutrons which are emitted into two sorts - 'prompt neutrons' and 'delayed neutrons' Typically, the 'delayed neutron fraction' is less than 1 percent of the whole. In a nuclear reactor the variable k is typically around 1 to have a steady process. When a value of k = 1 is achieved when all neutrons produced are considered the reaction is said to be 'critical'. This is the situation achieved in a nuclear reactor. The power changes are then slow, and controllable e.g. with control rods. When k = 1 is achieved counting only the 'prompt' neutrons, the reaction is said to be 'prompt critical' - much shorter doubling rates can then occur, depending on the excess criticality (k-1). The change in reactivity needed to go from critical to prompt critical (ie the delayed neutron fraction) is defined as a dollar. The value of k is increased by a neutron reflector surrounding the fissile material, and also by increasing the density of the fissile material: the probability for a neutron per cm travelled to hit a nucleus is proportional to the density, while the distance travelled before leaving the system is only reduced by the cube root of the density. In the implosion method for nuclear weapons, detonation takes place by increasing the density with a conventional explosive.
Numerical example for the probability of a chain reaction
Suppose a fission caused by a neutron hitting a nucleus produces 3 neutrons (i.e. 2 extra). Also suppose k > 1. The probability that a neutron causes a fission is k / 3. The probability that a free neutron does not cause a chain reaction is (1 - k / 3) (no fission at all) plus the probability of at least one fission, while none of the 3 neutrons produced causes a chain reaction. The latter has a probability of k / 3 times the cube of the first-mentioned probability that a free neutron does not cause a chain reaction. This equation can be solved easily, giving a probability of a chain reaction of : $1.5 - 0.5 \sqrt$ which ranges from 0 for k = 1 to 1 for k = 3. For values of k which are little above 1 we get approximately k - 1.
Predetonation
Detonation of a nuclear weapon involves bringing fissile material into its optimal supercritical state very rapidly. During part of this process the assembly is supercritical, but not yet in optimal state for a chain reaction. Free neutrons, in particular from spontaneous fissions, can cause predetonation. To keep the probability low, the duration of this period is minimized and fissile and other materials are used for which there are not too many spontaneous fissions. In fact, the combination has to be such that it is unlikely that there is even a single spontaneous fission during the period of assembly. In particular the gun method cannot be used with plutonium, see nuclear weapon design.
See also

- chain reaction
- critical mass
- criticality accident
- nuclear physics
- nuclear reaction
- nuclear weapon design
External links

- [http://www.atomicarchive.com/Docs/Begin/Einstein.shtml Einstein's Letter to President Roosevelt - 1939] Chain reaction, nuclear

Nuclear weapon

, 1945, rose some 18 km (11 mi) above the hypocenter.]]
- A nuclear weapon is a weapon which derives its destructive force from the nuclear reactions of nuclear fission and/or fusion. As a result, even a nuclear weapon with a small yield is significantly more powerful than the largest conventional explosives, and a single weapon can be capable of destroying or seriously disabling an entire city. In the history of warfare, nuclear weapons have been used on two occasions, both during the closing days of World War II. The first event occurred on the morning of 6 August 1945, when the United States dropped a uranium gun-type device code-named "Little Boy" on the Japanese city of Hiroshima. The second event occurred three days later when a plutonium implosion-type device code-named "Fat Man" was dropped on the city of Nagasaki. The use of the weapons, which resulted in the immediate deaths of at least 120,000 individuals (mostly civilians) and about twice that number over time, was and remains controversial — critics charged that they were unnecessary acts of mass killing, while others claimed that they ultimately reduced casualties on both sides by hastening the end of the war. (See Atomic bombings of Hiroshima and Nagasaki for a full discussion.) Since that time, nuclear weapons have been detonated on over two thousand occasions, mostly for testing purposes, chiefly by the following seven countries: the United States, Soviet Union, France, United Kingdom, People's Republic of China, India and Pakistan. These countries are the declared nuclear powers (with Russia inheriting the weapons of the Soviet Union after its collapse). Various other countries may hold nuclear weapons, but they have never publicly admitted possession, or their claims to possession have not been verified. For example, Israel has modern airbourne delivery systems and appears to have an extensive nuclear program (see Israel and weapons of mass destruction); North Korea has recently stated that it has nuclear capabilities (although it has now stated that it will abandon all of its nuclear weapons programs); Ukraine may possess an obsolete Soviet-era nuclear stockpile due to a post-Soviet administrative error; and Iran is believed to be attempting to develop nuclear capabilities (for more information see List of countries with nuclear weapons). Nuclear weapons in modern times have been used primarily as a method of creating a strategic threat. For example, the worry that North Korea will use nuclear weapons has dominated the relations between the United States and North Korea. Apart from their use as weapons, nuclear explosives have been proposed for various non-military uses.

Types of nuclear weapons

non-military uses The simplest nuclear weapons derive their energy from nuclear fission. A mass of fissile material is rapidly assembled into a critical mass, in which a chain reaction begins and grows exponentially, releasing tremendous amounts of energy. This is accomplished by rapidly creating supercriticality, either by shooting one piece of subcritical material into another, or compressing a subcritical mass. A major challenge in all nuclear weapon designs is ensuring that a significant fraction of the fuel is consumed before the weapon destroys itself. These are colloquially known as atomic bombs. More advanced nuclear weapons also contain a nuclear fission device, but the energy is used to trigger nuclear fusion, releasing even more energy. In such a weapon, the X-ray thermal radiation from a nuclear fission explosion is used to heat and compress a capsule of tritium, deuterium, or lithium, in which fusion occurs. These weapons, colloquially known as hydrogen bombs, can be many hundreds of times more powerful than fission weapons. The so-called "Teller-Ulam design" is thought to be applied for megaton range thermonuclear weapons. More exotic nuclear weapons also exist, designed for special purposes. The detonation of a nuclear weapon is accompanied by a blast of neutron radiation. Surrounding a nuclear weapon with suitable materials (such as cobalt or gold) creates a weapon known as a salted bomb. This device can produce exceptionally large quantities of radioactive contamination. A nuclear weapon may also be designed to permit as many neutrons as possible to escape; such a weapon is called a neutron bomb.

Effects of a nuclear explosion

neutron bomb The energy released from a nuclear weapon comes in four primary categories:
- Blast – 40-60% of total energy
- Thermal radiation – 30-50% of total energy
- Ionizing radiation – 5% of total energy
- Residual radiation (fallout) – 5-10% of total energy The amount of energy released in each form depends on the design of the weapon, and the environment in which it is detonated. The residual radiation of fallout is a delayed release of energy, while the other three forms of energy release occur immediately. The damage from each of the three initial forms of energy release differs with the size (or "yield", see below) of the weapon. Thermal radiation drops off the slowest with distance, so the larger the weapon the more significant the impact of this effect. Ionizing radiation is strongly absorbed by air, so it is only dangerous by itself for smaller weapons. Blast damage falls off more quickly than thermal radiation but more slowly than ionizing radiation. The energy released by a nuclear weapon is generally measured by the explosive power of an equivalent amount of trinitrotoluene, known as the weapon's yield. The yield of nuclear weapons may be rated as equivalent to several kilotons or megatons of TNT. The first fission weapons had yields measurable in the tens of kilotons, while the largest practical hydrogen bombs have yields around 20 megatons. In practice, nuclear weapon yields will vary significantly, from fractional kiloton weapons designed for tactical use on the battlefield (eg. the man-portable Davy Crockett warheads developed by the United States), to the record Tsar Bomba created by the Soviet Union which had a theoretical maximum design yield of around a hundred megatons. Although a nuclear weapon is capable of causing the same destruction as conventional explosives through the effects of blast and thermal radiation, it does so by releasing much larger amounts of energy in a much shorter period of time. Most of the damage caused by a nuclear weapon is not directly related to the nuclear process of energy release, and would be present for any explosion of the same magnitude. In human terms, nuclear weapons are enormously destructive. A weapon with a ten-megaton yield can destroy most of the buildings of a modern city, while a weapon with a hundred-megaton yield (although the deployment of such a weapon would be considered impractical) would set wooden structures and forests alight up to 60-100 miles (100-160 km) from ground zero. A nuclear weapon detonated in the upper atmosphere will also generate an electromagnetic pulse which can disrupt or disable electronic communications and instruments over a wide area, causing more difficulties for those who survive the effects of a detonation. Concerns over the health and environmental effects of nuclear testing led to the passing of the Partial Test Ban Treaty in 1963 which prohibited atmospheric (above-ground), underwater, or outer space nuclear tests (underground testing continued, however). Since most of the effects of nuclear weapons are blast, thermal, or fallout, well-known civil defense efforts could greatly reduce the total loss of life in a nuclear war.

Nuclear strategy

civil defenseed delivery system. Each missile can contain up to ten nuclear warheads (shown in red), each of which can be aimed at a different target. These were developed to make missile defense very difficult for an enemy country.]] Nuclear warfare strategy are ways for either fighting or avoiding a nuclear war. The policy of trying to ward off a potential attack by a nuclear weapon from another country by threatening nuclear retaliation is known as the strategy of nuclear deterrence. The goal in deterrence is to always maintain a second strike status — the ability to respond to a nuclear attack against your country with a nuclear attack of your own. During the Cold War, theorists used game theory to work out models of what sorts of policies could prevent one from ever being attacked by a nuclear weapon. However, many critics have noted that there could be many exceptions to this in practice, and if an attack ever was truly made then many hundreds of thousands if not millions of people would lose their lives as a result. Additionally, the presence of nuclear weapons by one country can spur nuclear proliferation in countries who feel threatened by them and look to deterrence (which requires a nuclear weapon in the first place) as the only solution. Sometimes this theory has been called Mutual Assured Destruction. Weapons which are designed to threaten large populations or to generally deter attacks are known as "strategic" weapons. Weapons which are designed to actually be used on a battlefield in military situations are known as "tactical" weapons. Different forms of nuclear weapons delivery (see below) allow for different types of nuclear strategy, primarily by making it difficult to defend against them and difficult to launch a pre-emptive strike against them. Sometimes this has meant keeping the weapon locations hidden, such as putting them on submarines or train cars whose locations are very hard for an enemy to track, and other times this means burying them in hardened bunkers. Other responses have included attempts to make it seem likely that the country could survive a nuclear attack, by using missile defense (to destroy the missiles before they land) or by means of civil defense (using early warning systems to evacuate citizens to a safe area before an attack).

Weapons delivery

civil defense" weapon dropped on Nagasaki, Japan. These weapons were very large and could only be delivered by larger bomber aircraft.]] Nuclear weapons delivery— the technology and systems used to bring a nuclear weapon to its target—is an important aspect of nuclear weapons relating both to nuclear weapon design and nuclear strategy. Historically the first method of delivery, and the method used in the two nuclear weapons actually used in warfare, is as a gravity bomb, dropped from bomber aircraft. This method is usually the first developed by countries as it does not place many restrictions on the size of the weapon, and weapon miniaturization is something which requires considerable weapons design knowledge. It does, however, limit the range of attack, response time to an impending attack, and number of weapons which can be fielded at any given time. More preferable from a strategic point of view are nuclear weapons mounted onto a missile, which can use a ballistic trajectory to deliver a warhead over the horizon. While even short range missiles allow for a faster and less vulnerable attack, the development of intercontinental ballistic missiles (ICBMs) and submarine-launched ballistic missiles (SLBMs) has allowed some nations to plausibly deliver missiles anywhere on the globe with a high likelihood of success. More advanced systems, such as multiple independently targetable reentry vehicles (MIRVs) allow multiple warheads to be launched at a number of targets from any one missile, reducing the chance of any successful missile defense. Today missiles are by far the most common among systems designed for delivery of nuclear weapons. To make a warhead small enough to fit onto a missile, though, can be a difficult task. "Tactical" weapons (see above) have involved the most variety of delivery types, including not only gravity bombs and missiles but also artillery shells, land mines, and nuclear depth charges and torpedoes for anti-submarine warfare. An atomic mortar was also tested at one time by the United States. Small, two-man portable tactical weapons (erroneously referred to as suitcase bombs), such as the Special Atomic Demolition Munition, have been developed, although the difficulty to combine sufficient yield with portability limits their military utility.

History

Special Atomic Demolition Munition The first nuclear weapons were created by the United States, with assistance from the United Kingdom and Canada, during World War II as part of the top-secret Manhattan Project. While the first weapons were developed primarily out of fear that Nazi Germany would first develop them, they were eventually used against the Japanese cities of Hiroshima and Nagasaki in August 1945. The Soviet Union developed and tested their first nuclear weapon in 1949, based partially on information obtained from Soviet espionage in the United States. Both the USA and USSR would go on to develop weapons powered by nuclear fusion (hydrogen bombs) by the mid-1950s. With the invention of reliable rocketry during the 1960s, it became possible for nuclear weapons to be delivered anywhere in the world on a very short notice, and the two Cold War superpowers adopted a strategy of deterrence to maintain a shaky peace. Nuclear weapons were symbols of military and national power, and nuclear testing was often used both to test new designs as well as to send political messages. Other nations also developed nuclear weapons during this time, including the United Kingdom, France, and China. These five members of the "nuclear club" agreed to attempt to limit the spread of nuclear proliferation to other nations, though at least three other countries (India, South Africa, Pakistan, and most likely Israel) developed nuclear arms during this time. At the end of the Cold War in the early 1990s, the Russian Federation inherited the weapons of the former USSR, and along with the USA pledged to reduce their stockpile for increased international safety. Nuclear proliferation has continued, though, with Pakistan testing their first weapons in 1998, and the state of North Korea claiming to have developed nuclear weapons in 2004. Nuclear weapons have been at the heart of many national and international political disputes, and have played a major part in popular culture since their dramatic public debut in the 1940s, and have usually symbolized the ultimate ability of mankind to utilize the strength of nature for destruction. There have been (at least) four major false alarms, the most recent in 1995, that almost resulted in the US or USSR/Russia launching its weapons in retaliation for a supposed attack.[http://www.pbs.org/wgbh/nova/missileers/falsealarms.html] Additionally, during the Cold War the US and USSR came close to nuclear warfare a number of times, most notably during the Cuban Missile Crisis. As of 2005, there are estimated to be at least 29,000 nuclear weapons held by at least seven countries, though 96% of these are in the possession of just two (the United States and the Russian Federation)

Media

References

- p. 54. Bethe, Hans Albrecht. The Road from Los Alamos. Simon and Schuster, New York. (1991 ISBN 0-671-74012-1)
- Glasstone, Samuel and Dolan, Philip J., [http://www.cddc.vt.edu/host/atomic/nukeffct/ The Effects of Nuclear Weapons (third edition)], U.S. Government Printing Office, 1977. [http://www.princeton.edu/~globsec/publications/effects/effects.shtml PDF Version]
- [http://www.fas.org/nuke/guide/usa/doctrine/dod/fm8-9/1toc.htm NATO Handbook on the Medical Aspects of NBC Defensive Operations (Part I - Nuclear)], Departments of the Army, Navy, and Air Force, Washington, D.C., 1996.
- Hansen, Chuck. U.S. Nuclear Weapons: The Secret History, Arlington, TX: Aerofax, 1988.
- Hansen, Chuck. The Swords of Armageddon: U.S. nuclear weapons development since 1945, Sunnyvale, CA: Chukelea Publications, 1995 [http://www.uscoldwar.com/].
- Smyth, Henry DeWolf. [http://nuclearweaponarchive.org/Smyth/ Atomic Energy for Military Purposes], Princeton University Press, 1945. (The first declassified report by the US government on nuclear weapons) (Smyth Report)
- [http://www.fas.org/nuke/intro/nuke/7906/index.html The Effects of Nuclear War], Office of Technology Assessment (May 1979).
- Rhodes, Richard. Dark Sun: The Making of the Hydrogen Bomb. Simon and Schuster, New York, (1995 ISBN 0684824140)
- Rhodes, Richard. The Making of the Atomic Bomb. Simon and Schuster, New York, (1986 ISBN 0684813785)
- Weart, Spencer R. Nuclear Fear: A History of Images. Cambridge, Mass.: Harvard University Press, 1988.

External links

- [http://intergate.cccoe.k12.ca.us/abomb/ "The Race to Build the Atomic Bomb"] educational resource
- [http://nuclearweaponarchive.org Nuclear Weapon Archive from Carey Sublette] is a reliable source of information and has links to other sources and an informative [http://nuclearweaponarchive.org/Nwfaq/Nfaq0.html FAQ].
- [http://www.fas.org/main/content.jsp?formAction=297&contentId=367 Nuclear weapon simulator for several major cities]
- [http://www.fas.org/main/content.jsp?formAction=297&contentId=409 Fallout Calculator for various regions]
- [http://www.neis.org/literature/Brochures/weapcon.htm "Nuclear Power and Nuclear Weapons: Making the Connections"] – an article about the connections between nuclear power and nuclear weapons development by an anti-nuclear group
- The [http://fas.org Federation of American Scientists] provide solid information on weapons of mass destruction, including [http://fas.org/nuke/ nuclear weapons] and their [http://www.fas.org/nuke/intro/nuke/effects.htm effects]
- [http://www.oism.org/nwss/ Nuclear War Survival Skills] is a public domain text about civil defense.
- [http://www.atomicarchive.com/Example/Example1.shtml Step by step scenario of a 150 kiloton bomb exploding in Manhattan] - click on the Next >> button at the bottom of each slide.
- [http://www.ippnw.org IPPNW: International Physicians for the Prevention of Nuclear War] Nobel Peace Prize-winning organization with information about the medical consequences of nuclear weapons, war and militarization.
- [http://www.thebulletin.org Bulletin of the Atomic Scientists] - Magazine founded in 1945 by Manhattan Project scientists. Covers nuclear weapons proliferation and many other global security issues. See [http://www.thebulletin.org/nuclear_weapons_data this page] for comprehensive data on nuclear weapons worldwide.
- [http://alsos.wlu.edu/ Alsos Digital Library for Nuclear Issues] – contains many resources related to nuclear weapons, including a historical and technical overview and searchable bibliography of web and print resources. ko:핵무기 ms:Senjata nuklear ja:核兵器 simple:Nuclear weapon th:อาวุธนิวเคลียร์

Nuclear fission

Nuclear fission (in nuclear physics, simply fission) is a process in which the nucleus of an atom splits into two or more smaller nuclei (fission products) and usually some by-product particles. Hence, fission is a form of elemental transmutation. The by-products include free neutrons, photons (usually gamma rays), and other nuclear fragments such as beta particles and alpha particles. Fission of heavy elements can release substantial amounts of useful energy both as gamma rays and as kinetic energy of the fragments. Nuclear fission is used to produce energy for nuclear power and to drive explosion of nuclear weapons. Fission is useful as a power source because some materials, called nuclear fuels, both generate neutrons as part of the fission process and also undergo triggered fission when impacted by a free neutron. Nuclear fuels can be part of a self-sustaining chain reaction that releases energy at a controlled rate (in a nuclear reactor) or a very rapid uncontrolled rate (in a nuclear weapon). The amount of free energy contained in nuclear fuel is millions of times the amount of energy contained in a similar mass of chemical fuel such as gasoline, making nuclear fission a very tempting source of energy; however, the waste products of nuclear fission are highly radioactive and remain so for millennia, giving rise to a nuclear waste problem. Concerns over nuclear waste accumulation and over the immense destructive potential of nuclear weapons counterbalance the desirable qualities of fission as an energy source, and give rise to intense ongoing political debate over nuclear power.

Physical overview

Nuclear fission differs from other forms of radioactive decay in that it can be harnessed and controlled via a chain reaction: free neutrons released by each fission event can trigger yet more events, which in turn release more neutrons and cause more fissions. Chemical isotopes that can sustain a fission chain reaction are called nuclear fuels, and are said to be fissile. The most common nuclear fuels are ²³⁵U (the isotope of uranium with an atomic mass of 235) and ²³⁹Pu (the isotope of plutonium with an atomic mass of 239). These fuels break apart into a range of chemical elements with atomic masses near 100 (fission products). Most nuclear fuels undergo spontaneous fission very slowly, gradually disintegrating over periods of eons. In a nuclear reactor or nuclear weapon, most fission events are induced by bombardment with another particle such as a neutron. Typical fission events release several hundred MeV of energy for each fuel atom that undergoes fission, which is why nuclear fission is used as an energy source. By contrast, most chemical oxidation reactions (such as burning coal or TNT) release at most a few tens of eV per event, so nuclear fuel contains at least ten million times more usable energy than does chemical fuel. The energy of nuclear fission is released as kinetic energy of the fission products and fragments, and as electromagnetic radiation (gamma rays); in a nuclear reactor, the energy is converted to heat as the particles and gamma rays collide with the atoms that make up the reactor and its working fluid (usually water). Nuclear fission produces energy because the binding energy of intermediate-mass nuclei (with atomic numbers and atomic masses close to ⁵⁶Fe) is greater than the binding energy of very heavy nuclei, so that energy is released when heavy nuclei are broken apart. The total mass of the fission products from a single reaction is less than the mass of the original fuel nucleus, and the excess is released as energy via Einstein's relation E=mc². The variation in binding energy is due to the interplay of the two fundamental forces acting on the component nucleons (protons and neutrons) that make up the nucleus. Nuclei are bound by an attractive strong nuclear force between nucleons, which overcomes the intense electrostatic repulsion between protons. However, the strong nuclear force acts only over extremely short ranges (it follows a yukawa potential), so that large nuclei are less tightly bound than small nuclei, and breaking a large nucleus into two or more intermediate-sized nuclei releases energy. yukawa potential Because of the short range of the strong binding force, large nuclei contain proportionally more neutrons than do light elements, which are most stable with a 1-1 ratio of protons and neutrons. Fission products have, on average, the same ratio of neutrons and protons as their parent nucleus, and are therefore usually very unstable because they have too many neutrons compared to stable isotopes of similar mass. This is the fundamental cause of the problem of radioactive high level waste from nuclear reactors. Fission products tend to be beta emitters, emitting fast-moving electrons to conserve electric charge as neutrons convert to protons inside the nucleus. The most common nuclear fuels, ²³⁵U and ²³⁹Pu, are not major radiologic hazards by themselves: ²³⁵U has a half-life measured in billions of years, and although ²³⁹Pu has a half-life of only about 25,000 years it is a pure alpha particle emitter and hence not particularly dangerous unless ingested. Once a fuel element has been used, the remaining fuel material is intimately mixed with highly radioactive fission products that emit energetic beta particles and gamma rays. Some fission products have half-lives as short as seconds; others have half-lives of tens of thousands of years, requiring long-term storage in facilities such as Yucca mountain until the fission products decay into non-radioactive stable isotopes.

Spontaneous and induced fission; chain reactions

Many heavy elements, such as uranium, thorium, and plutonium, undergo both spontaneous fission (a form of radioactive decay) and induced fission (a form of nuclear reaction). Elemental isotopes that undergo induced fission when struck by a free neutron are called fissionable; isotopes that undergo fission when struck by a thermal (slow moving) neutron are also called fissile. A few particularly fissile and readily obtainable isotopes (notably ²³⁵U and ²³⁹Pu) are called nuclear fuels because they can sustain a chain reaction and can be obtained in large enough quantities to be useful. All fissionable and fissile isotopes undergo a small amount of spontaneous fission, which releases a few free neutrons into any sample of nuclear fuel. The neutrons typically escape rapidly from the fuel and either decay into protons (with a half-life of about 15 minutes) or impact and are absorbed by other nuclei in the vicinity. However, some neutrons will impact fuel nuclei and induce further fissions, releasing yet more neutrons. If enough nuclear fuel is assembled into one place, and/or if the escaping neutrons are sufficiently contained, then these freshly generated neutrons outnumber the neutrons that escape from the assembly, and a sustained chain reaction will take place. An assembly that supports a sustained chain reaction is called a critical assembly or, if it the assembly is almost entirely made of a nuclear fuel, a critical mass. The word "critical" refers to a cusp in the behavior of the differential equation that governs the number of free neutrons present in the fuel: if less than a critical mass is present, then the amount of neutrons is determined by radioactive decay, but if a critical mass or more is present, then the amount of neutrons is controlled instead by the physics of the chain reaction. The actual mass of a critical mass of nuclear fuel depends strongly on the geometry and surrounding materials. Not all fissionable isotopes can sustain a chain reaction. For example, ²³⁸U, the most abundant form of uranium, is fissionable but not fissile: it undergoes induced fission when impacted by an energetic neutron with over 1 MeV of kinetic energy. But the neutrons produced by ²³⁸U fission are not, themselves, energetic enough to induce further fissions in ²³⁸U, so no chain reaction is possible with that isotope. Instead, bombarding ²³⁸U with slow neutrons causes it to absorb them (becoming ²³⁹U) and decay by beta emission to ²³⁹Pu.

Fission reactors

Critical fission reactors are the most common type of nuclear reactor. In a critical fission reactor, neutrons produced by fission of fuel atoms are used to induce yet more fissions, to sustain a controllable amount of energy release. Devices that produce engineered but non-self-sustaining fission reactions are subcritical fission reactors. Such devices use radioactive decay or particle accelerators to trigger fissions. The home-built fission pile built by David Hahn is an example. Critical fission reactors are built for three primary purposes, which typicallly involve different engineering trade-offs to take advantage of either the heat or the neutrons produced by the fission chain reaction:
- power reactors are intended to produce heat for nuclear power, either as part of a generating station or a local power system such as a nuclear submarine.
- research reactors are intended to produce neutrons and/or activate radioactive sources for scientific, medical, engineering, or other research purposes.
- breeder reactors are intended to produce nuclear fuels in bulk from more abundant isotopes. The most common type makes ²³⁹Pu (a nuclear fuel) from the naturally very abundant ²³⁸U (not a nuclear fuel). While, in principle, all fission reactors can act in all three capacities, in practice the tasks lead to conflicting engineering goals and most reactors have been built with only one of the above tasks in mind. (There are several early counter-examples, such as the Hanford N reactor, now decommissioned). Power reactors generally convert the kinetic energy of fission products into heat, which is used to heat a working fluid and drive a heat engine that generates mechanical or electrical power. The working fluid is usually water with a steam turbine, but some designs use other materials such as gaseous helium. Research reactors produce neutrons that are used in various ways, with the heat of fission being treated as an unavoidable waste product. Breeder reactors are a specialized form of research reactor, with the caveat that the sample being irradiated is usually the fuel itself, a mixture of ²³⁸U and ²³⁵U. For a more detailed description of the physics and operating principles of critical fission reactors, see nuclear reactor physics. For a description of their social, political, and environmental aspects, see nuclear reactor.

Fission bombs

One class of nuclear weapon, a fission bomb, otherwise known as an atomic bomb, is a fission reactor designed to liberate as much energy as possible as rapidly as possible, before the released energy causes the reactor to explode (and the chain reaction to stop). Development of nuclear weapons was the motivation behind early research into nuclear fission: the Manhattan Project of the U.S. military during World War Two carried out most of the early scientific work on fission chain reactions, culminating in the Little Boy and Fat Man bombs that were exploded over Japan in August of 1945. Fission bombs are thousands of times more explosive than a comparable mass of chemical explosive. For example, Little Boy weighed a total of about four tons (of which 60 kg was nuclear fuel) and yielded an explosion equivalent to 10,000 tons of TNT, destroying a large part of the city of Hiroshima. Modern nuclear weapons are literally thousands of times more energetic, so that a single bomb could destroy an entire city. The strategic importance of nuclear weapons is a major reason why the technology of nuclear fission is politically sensitive. Viable fission bomb designs are within the capabilities of bright undergraduates (see John Aristotle Phillips), but nuclear fuel to realize the designs is thought to be difficult to obtain (see uranium enrichment and nuclear fuel cycle).

History

The results of the bombardment of uranium by neutrons had proved interesting and puzzling. First studied by Enrico Fermi and his colleagues in 1934, they were not properly interpreted until several years later. On January 16 1939, Niels Bohr of Copenhagen, Denmark, arrived in the United States to spend several months in Princeton, N. J., and was particularly anxious to discuss some abstract problems with Albert Einstein. (Four years later Bohr was to escape to Sweden from Nazi-occupied Denmark in a small boat, along with thousands of other Danish Jews, in large scale operation.) Just before Bohr left Denmark, two of his colleagues, Otto Robert Frisch and Lise Meitner (both refugees from Germany), had told him their guess that the absorption of a neutron by a uranium nucleus sometimes caused that nucleus to split into approximately equal parts with the release of enormous quantities of energy, a process that they dubbed nuclear "fission." The occasion for this hypothesis was the important discovery of Otto Hahn and Fritz Strassmann in Germany (published in Naturwissenschaften in early January 1939) which proved that an isotope of barium was produced by neutron bombardment of uranium. Bohr had promised to keep the Meitner/Frisch interpretation secret until their paper was published to preserve priority, but on the boat he discussed it with Léon Rosenfeld, but forgot to tell him to keep it secret. Rosenfeld immediately upon arrival told everyone at Princeton University, and from them the news spread by word of mouth to neighboring physicists including Enrico Fermi at Columbia University. As a result of conversations among Fermi, John R. Dunning, and G. B. Pegram, a search was undertaken at Columbia for the heavy pulses of ionization that would be expected from the flying fragments of the uranium nucleus. On January 26, 1939, there was a conference on theoretical physics at Washington, D. C., sponsored jointly by the George Washington University and the Carnegie Institution of Washington. Fermi left New York to attend this meeting before the Columbia fission experiments had been tried. At the meeting Bohr and Fermi discussed the problem of fission, and in particular Fermi mentioned the possibility that neutrons might be emitted during the process. Although this was only a guess, its implication of the possibility of a chain reaction was obvious. A number of sensational articles were published in the press on this subject. Before the meeting in Washington was over, several other experiments to confirm fission had been initiated, and positive experimental confirmation was reported from four laboratories (Columbia University, Carnegie Institution of Washington, Johns Hopkins University, University of California) in the February 15 1939, issue of the Physical Review. By this time Bohr had heard that similar experiments had been made in his laboratory in Copenhagen about January 15. (Letter by Frisch to Nature dated January 16 1939, and appearing in the February 18 issue.) Frédéric Joliot in Paris had also published his first results in the Comptes Rendus of January 30 1939. From this time on there was a steady flow of papers on the subject of fission, so that by the time (December 6 1939) L. A. Turner of Princeton wrote a review article on the subject in the Reviews of Modern Physics nearly one hundred papers had appeared. Complete analysis and discussion of these papers have appeared in Turner's article and elsewhere. A major focus of early fission research was on producing a controllable chain reaction, which would mark the first harnessing of nuclear power. This led to the Manhattan project to develop a nuclear weapon and the development of Chicago Pile-1, the world's first man-made critical nuclear reactor (which used uranium, the only natural nuclear fuel available in macroscopic quantities). But producing a fission chain reaction in uranium fuel is far from trivial. It requires separation of the rare ²³⁵U isotope from the far more common ²³⁸U isotope, and inclusion of extremely chemically pure neutron moderator materials such as deuterium, beryllium, and graphite (The high purity is required because many chemical impurities such as boron are very strong neutron absorbers and poison the chain reaction). Up to 1940 the total amount of uranium metal produced in the USA was not more than a few grams and even this was of doubtful purity, of metallic beryllium not more than a few kilograms, concentrated deuterium not more than a few kilograms, and carbon had never been produced in quantity with anything like the purity required of a moderator, so the problem of producing and purifying materials was a major one. The problem of producing large amounts of high purity uranium was solved by Frank Spedding using the thermite process. Ames Laboratory was established in 1942 to produce the large amounts of uranium that would be necessary for the research to come. For more detail on the early development of nuclear reactors and nuclear weapons, see Manhattan Project.

Links

- [http://alsos.wlu.edu/qsearch.aspx?browse=science/Fission Annotated references on nuclear fission from the Alsos Digital Library]
- [http://www.aip.org/history/mod/ The Discovery of Nuclear Fission] historical account complete with audio and teacher's guides from the American Institute of Physics History Center Fission, nuclear Category:Nuclear chemistry Category:Radioactivity ko:핵분열 ja:核分裂反応 th:ปฏิกิริยานิวเคลียร์ฟิซชัน

Fusion power

Fusion power is useful energy generated by nuclear fusion reactions. In this kind of reaction two light atomic nuclei fuse together to form a heavier nucleus and release energy. There are ongoing experimental attempts to build fusion power generators, but so far none continuously generates more energy than it uses. In June 2005, it was announced that the first experimental reactor intended to achieve this goal, ITER, will be built at Cadarache in Southern France.

Fuel cycle

In order for two nuclei to fuse, they must collide with enough energy to overcome the repulsive electrostatic force between them. Most fusion generation experiments therefore raise their fuel to very high temperatures. If two light nuclei come close enough to each other, they may fuse to form a single nucleus with a slightly smaller mass than the sum of their original masses. The difference in mass is released as energy according to Einstein's relationship E = mc². (If the input nuclei are sufficiently massive, the resulting fusion product will be heavier than the reactants, and the reaction requires the addition of energy to convert into the additional mass; in this case the reverse process of nuclear fission will release energy which can be used, for example, in nuclear reactors or bombs.) Hydrogen, the most abundant element in the universe, also has the smallest nuclear charge and therefore reacts at the lowest temperature. Helium has an extremely low mass per nucleon and therefore is energetically favored as a fusion product. As a consequence, most fusion reactions combine isotopes of hydrogen ("protium", deuterium, or tritium) to form isotopes of helium (³He or ⁴He). Perhaps the three most widely considered fuel cycles are based on the D-T, D-D, and p-¹¹B reactions. Other fuel cycles (D-³He and ³He-³He) would require a supply of ³He, either from other nuclear reactions or from extra-terrestrial sources, such as the surface of the moon or the atmospheres of the gas giant planets. The details of the calculations comparing these reactions can be found here.

The D-T fuel cycle

here The easiest reaction to utilize for fusion power is :D + T → ⁴He + n Deuterium is a naturally occurring isotope of hydrogen and as such is universally available. The large mass ratio of the hydrogen isotopes makes the separation rather easy compared to the difficult uranium enrichment process. Tritium is also an isotope of hydrogen, but it occurs naturally in only negligible amounts due to its radioactive half-life of 12 years. Consequently, the deuterium-tritium fuel cycle requires the breeding of tritium from lithium using one of the following reactions: :n + ⁶Li → T + ⁴He :n + ⁷Li → T + ⁴He + n The reactant neutron is supplied by the D-T fusion reaction shown above, the one which also produces the useful energy. The reaction with ⁶Li is exothermic, providing a small energy gain for the reactor. The reaction with ⁷Li is endothermic but does not consume the neutron. At least some ⁷Li reactions are required to replace the neutrons lost by reactions with other elements. Most reactor designs use the naturally occurring mix of lithium isotopes. The supply of lithium is more limited than that of deuterium, but still large enough to supply the world's energy for hundreds of years. Several drawbacks are commonly attributed to D-T fusion power. # It produces substantial amounts of neutrons that result in induced radioactivity within the reactor structure, and it requires the handling of the radioisotope tritium. # Only about 20% of the fusion energy yield appears in the form of charged particles (the rest neutrons), which limits the extent to which direct energy conversion techniques might be applied. # The use of D-T fusion power depends on lithium resources, which are less abundant than deuterium resources. The neutron flux expected in a commercial D-T fusion reactor is about 100 times that of current fission power reactors, posing problems for material design. Design of suitable materials is underway but their actual use in a reactor is not proposed until the generation after ITER. After a single series of D-T tests at JET, the largest fusion reactor yet to use this fuel, the vacuum vessel was sufficiently radioactive that remote handling needed to be used for the year following the tests. On the other hand, the volumetric deposition of neutron power can also be seen as an advantage. If all the power of a fusion reactor had to be transported by