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178-587: Chicago Pile-1 ( CP-1 ) was the world's first artificial nuclear reactor . On 2 December 1942, the first human-made self-sustaining nuclear chain reaction was initiated in CP-1 during an experiment led by Enrico Fermi . The secret development of the reactor was the first major technical achievement for the Manhattan Project , the Allied effort to create nuclear weapons during World War II . Developed by

356-464: A neutron moderator . The reactor contained 45,000 ultra-pure graphite blocks weighing 360 short tons (330 tonnes ) and was fueled by 5.4 short tons (4.9 tonnes) of uranium metal and 45 short tons (41 tonnes) of uranium oxide . Unlike most subsequent nuclear reactors, it had no radiation shielding or cooling system as it operated at very low power – about one-half watt. The pursuit of a reactor had been touched off by concern that Nazi Germany had

534-427: A neutron reflector surrounding the fissile material. Once the mass of fuel is prompt supercritical, the power increases exponentially. However, the exponential power increase cannot continue for long since k decreases when the amount of fission material that is left decreases (i.e. it is consumed by fissions). Also, the geometry and density are expected to change during detonation since the remaining fission material

712-479: A nuclear proliferation risk as they can be configured to produce plutonium , as well as tritium gas used in boosted fission weapons . Reactor spent fuel can be reprocessed to yield up to 25% more nuclear fuel, which can be used in reactors again. Reprocessing can also significantly reduce the volume of nuclear waste, and has been practiced in Europe, Russia, India and Japan. Due to concerns of proliferation risks,

890-461: A nuclear reactor , and with Robert Serber about how that plutonium might be separated from uranium. His report, submitted in November, stated that a bomb was feasible. The final draft of Compton's November 1941 report made no mention of plutonium, but after discussing the latest research with Ernest Lawrence , Compton became convinced that a plutonium bomb was also feasible. In December, Compton

1068-483: A rackets court. Stagg Field had been largely unused since the University of Chicago had given up playing American football in 1939, but the rackets courts under West Stands were still used for playing squash and handball . Leona Woods and Anthony L. Turkevich played squash there in 1940. Since it was intended for strenuous exercise, the area was unheated, and very cold in the winter. The nearby North Stands had

1246-486: A racquets court below the bleachers of Stagg Field at the University of Chicago . Fermi's experiments at the University of Chicago were part of Arthur H. Compton 's Metallurgical Laboratory of the Manhattan Project ; the lab was renamed Argonne National Laboratory and tasked with conducting research in harnessing fission for nuclear energy. In 1956, Paul Kuroda of the University of Arkansas postulated that

1424-478: A radiation shielding , with overhead protection from 6 inches (15 cm) of lead and 50 inches (130 cm) of wood. More uranium was used, so it contained 52 short tons (47 t) of uranium and 472 short tons (428 t) of graphite. No cooling system was provided as it only ran at a few kilowatts. CP-2 became operational in March 1943, with a k of 1.055. During the war, Walter Zinn allowed CP-2 to be run around

1602-558: A " neutron howitzer ") produced a barium residue, which they reasoned was created by fission of the uranium nuclei. In their second publication on nuclear fission in February 1939, Hahn and Strassmann predicted the existence and liberation of additional neutrons during the fission process, opening the possibility of a nuclear chain reaction . Subsequent studies in early 1939 (one of them by Szilárd and Fermi), revealed that several neutrons were indeed released during fission, making available

1780-494: A "pile". Emilio Segrè later recalled that: I thought for a while that this term was used to refer to a source of nuclear energy in analogy with Volta 's use of the Italian term pila to denote his own great invention of a source of electrical energy. I was disillusioned by Fermi himself, who told me that he simply used the common English word pile as synonymous with heap . To my surprise, Fermi never seemed to have thought of

1958-416: A bucket of concentrated cadmium nitrate , which he was to throw over the pile in the event of an emergency. The startup began at 09:54. Walter Zinn removed the zip, the emergency control rod, and secured it. Norman Hilberry stood ready with an axe to cut the scram line, which would allow the zip to fall under the influence of gravity. While Leona Woods called out the count from the boron trifluoride detector in

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2136-441: A crucial role in generating large amounts of electricity with low carbon emissions, contributing significantly to the global energy mix. Just as conventional thermal power stations generate electricity by harnessing the thermal energy released from burning fossil fuels , nuclear reactors convert the energy released by controlled nuclear fission into thermal energy for further conversion to mechanical or electrical forms. When

2314-437: A cubic lattice structure. A radium-beryllium neutron source was positioned near the bottom. The uranium oxide was heated to remove moisture, and packed into the cans while still hot on a shaking table. The cans were then soldered shut. For a workforce, Pegram secured the services of Columbia's football team. It was the custom at the time for football players to perform odd jobs around the university. They were able to manipulate

2492-512: A day shift under Zinn and a night shift under Anderson. For a work force they hired thirty high school dropouts who were eager to earn a bit of money before being drafted into the military. They machined 45,000 graphite blocks enclosing 19,000 pieces of uranium metal and uranium oxide. The graphite arrived from the manufacturers in 4.25-by-4.25-inch (10.8 by 10.8 cm) bars of various lengths. They were cut into standard lengths of 16.5 inches (42 cm), each weighing 19 pounds (8.6 kg). A lathe

2670-426: A factor k , the second generation of fission events will produce k , the third k and so on. In order for a self-sustaining nuclear chain reaction to occur, k must be at least 3 or 4 percent greater than 1. In other words, k must be greater than 1 without crossing the prompt critical threshold that would result in a rapid, exponential increase in the number of fission events. Fermi christened his apparatus

2848-427: A fissile atom undergoes nuclear fission, it breaks into two or more fission fragments. Also, several free neutrons, gamma rays , and neutrinos are emitted, and a large amount of energy is released. The sum of the rest masses of the fission fragments and ejected neutrons is less than the sum of the rest masses of the original atom and incident neutron (of course the fission fragments are not at rest). The mass difference

3026-486: A fissioning uranium nucleus produced 1.73 neutrons on average. It was enough, but a careful design was called for to minimize losses. (Today the average number of neutrons emitted per fissioning uranium-235 nucleus is known to be about 2.4). Szilard estimated he would need about 50 short tons (45 t) of graphite and 5 short tons (4.5 t) of uranium. In December 1940, Fermi and Szilard met with Herbert G. MacPherson and Victor C. Hamister at National Carbon to discuss

3204-445: A gas or a liquid metal (like liquid sodium or lead) or molten salt – is circulated past the reactor core to absorb the heat that it generates. The heat is carried away from the reactor and is then used to generate steam. Most reactor systems employ a cooling system that is physically separated from the water that will be boiled to produce pressurized steam for the turbines , like the pressurized water reactor . However, in some reactors

3382-442: A large fissile atomic nucleus such as uranium-235 , uranium-233 , or plutonium-239 absorbs a neutron, it may undergo nuclear fission. The heavy nucleus splits into two or more lighter nuclei, (the fission products ), releasing kinetic energy , gamma radiation , and free neutrons . A portion of these neutrons may be absorbed by other fissile atoms and trigger further fission events, which release more neutrons, and so on. This

3560-603: A larger share of uranium on Earth in the geological past because of the different half-lives of the isotopes U and U , the former decaying almost an order of magnitude faster than the latter. Kuroda's prediction was verified with the discovery of evidence of natural self-sustaining nuclear chain reactions in the past at Oklo in Gabon in September 1972. To sustain a nuclear fission chain reaction at present isotope ratios in natural uranium on Earth would require

3738-424: A less effective moderator. In other reactors, the coolant acts as a poison by absorbing neutrons in the same way that the control rods do. In these reactors, power output can be increased by heating the coolant, which makes it a less dense poison. Nuclear reactors generally have automatic and manual systems to scram the reactor in an emergency shut down. These systems insert large amounts of poison (often boron in

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3916-431: A loud voice, George Weil , the only one on the floor, withdrew all but one of the control rods. At 10:37 Fermi ordered Weil to remove all but 13 feet (4.0 m) of the last control rod. Weil withdrew it 6 inches (15 cm) at a time, with measurements being taken at each step. The process was abruptly halted by the automatic control rod reinserting itself, due to its trip level being set too low. At 11:25, Fermi ordered

4094-429: A mass of fissile fuel that is prompt supercritical. For a given mass of fissile material the value of k can be increased by increasing the density. Since the probability per distance travelled for a neutron to collide with a nucleus is proportional to the material density, increasing the density of a fissile material can increase k . This concept is utilized in the implosion method for nuclear weapons. In these devices,

4272-402: A moderator, while Leo Szilard and Enrico Fermi had asked suppliers about the most common contaminations of graphite after a first failed test. They consequently ensured that the next test would be run with graphite entirely devoid of them. As it turned out, both boron and cadmium were strong neutron poisons . In 1943, CP-1 was moved to Site A , a wartime research facility near Chicago, where it

4450-471: A natural fission reactor may have once existed. Since nuclear chain reactions may only require natural materials (such as water and uranium, if the uranium has sufficient amounts of U ), it was possible to have these chain reactions occur in the distant past when uranium-235 concentrations were higher than today, and where there was the right combination of materials within the Earth's crust . Uranium-235 made up

4628-403: A nuclear chain reaction proceeds: When describing kinetics and dynamics of nuclear reactors, and also in the practice of reactor operation, the concept of reactivity is used, which characterizes the deflection of reactor from the critical state: ρ =  ⁠ k eff  − 1 / k eff ⁠ . InHour (from inverse of an hour , sometimes abbreviated ih or inhr) is a unit of reactivity of

4806-474: A nuclear reactor. In a nuclear reactor, k eff will actually oscillate from slightly less than 1 to slightly more than 1, due primarily to thermal effects (as more power is produced, the fuel rods warm and thus expand, lowering their capture ratio, and thus driving k eff lower). This leaves the average value of k eff at exactly 1 during a constant power run. Both delayed neutrons and the transient fission product " burnable poisons " play an important role in

4984-570: A number of ways: A kilogram of uranium-235 (U-235) converted via nuclear processes releases approximately three million times more energy than a kilogram of coal burned conventionally (7.2 × 10 joules per kilogram of uranium-235 versus 2.4 × 10 joules per kilogram of coal). The fission of one kilogram of uranium-235 releases about 19 billion kilocalories , so the energy released by 1 kg of uranium-235 corresponds to that released by burning 2.7 million kg of coal. A nuclear reactor coolant – usually water but sometimes

5162-478: A pair of ice skating rinks on the ground floor, which although they were unrefrigerated, seldom melted in winter. Allison used the rackets court area to construct a 7-foot (2.1 m) experimental pile before Fermi's group arrived in 1942. The United States Army Corps of Engineers assumed control of the nuclear weapons program in June 1942, and Compton's Metallurgical Laboratory became part of what came to be called

5340-465: A patent on reactors on 19 December 1944. Its issuance was delayed for 10 years because of wartime secrecy. "World's first nuclear power plant" is the claim made by signs at the site of the EBR-I , which is now a museum near Arco, Idaho . Originally called "Chicago Pile-4", it was carried out under the direction of Walter Zinn for Argonne National Laboratory . This experimental LMFBR operated by

5518-706: A pile (hence the name) of graphite blocks, embedded in which was natural uranium oxide 'pseudospheres' or 'briquettes'. Soon after the Chicago Pile, the Metallurgical Laboratory developed a number of nuclear reactors for the Manhattan Project starting in 1943. The primary purpose for the largest reactors (located at the Hanford Site in Washington ), was the mass production of plutonium for nuclear weapons. Fermi and Szilard applied for

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5696-407: A planned typical lifetime of 30–40 years, though many of those have received renovations and life extensions of 15–20 years. Some believe nuclear power plants can operate for as long as 80 years or longer with proper maintenance and management. While most components of a nuclear power plant, such as steam generators, are replaced when they reach the end of their useful lifetime, the overall lifetime of

5874-837: A plutonium semiworks, followed by larger water-cooled production reactors at the Hanford Site in Washington state . Enough plutonium was produced for an atomic bomb by July 1945, and for two more in August. Nuclear reactor Nuclear reactors have their origins in the World War II Allied Manhattan Project . The world's first artificial nuclear reactor, Chicago Pile-1, achieved criticality on 2 December 1942. Early reactor designs sought to produce weapons-grade plutonium for fission bombs , later incorporating grid electricity production in addition. In 1957, Shippingport Atomic Power Station became

6052-401: A preliminary chain reaction that destroys the fissile material before it is ready to produce a large explosion, which is known as predetonation . To keep the probability of predetonation low, the duration of the non-optimal assembly period is minimized, and fissile and other materials are used that have low spontaneous fission rates. In fact, the combination of materials has to be such that it

6230-441: A radium-beryllium source to bombard uranium with neutrons. They discovered significant neutron multiplication in natural uranium, proving that a chain reaction might be possible. Fermi and Szilard still believed that enormous quantities of uranium would be required for an atomic bomb , and therefore concentrated on producing a controlled chain reaction. Fermi urged Alfred O. C. Nier to separate uranium isotopes for determination of

6408-471: A reactor. One such process is delayed neutron emission by a number of neutron-rich fission isotopes. These delayed neutrons account for about 0.65% of the total neutrons produced in fission, with the remainder (termed " prompt neutrons ") released immediately upon fission. The fission products which produce delayed neutrons have half-lives for their decay by neutron emission that range from milliseconds to as long as several minutes, and so considerable time

6586-504: A result of neutron capture , uranium-239 is produced, which undergoes two beta decays to become plutonium-239. Plutonium once occurred as a primordial element in Earth's crust, but only trace amounts remain so it is predominantly synthetic. Another proposed fuel for nuclear reactors, which however plays no commercial role as of 2021, is uranium-233 , which is "bred" by neutron capture and subsequent beta decays from natural thorium , which

6764-456: A schedule to achieve a controlled nuclear chain reaction by January 1943, and to have an atomic bomb by January 1945. In a nuclear reactor, criticality is achieved when the rate of neutron production is equal to the rate of neutron losses, including both neutron absorption and neutron leakage. When a uranium-235 atom undergoes fission, it releases an average of 2.4 neutrons. In the simplest case of an unreflected , homogeneous, spherical reactor,

6942-468: A series of attempts, the successful reactor was assembled in November 1942 by a team of about 30 that, in addition to Fermi, included scientists Leo Szilard (who had previously formulated an idea for non-fission chain reaction ), Leona Woods , Herbert L. Anderson , Walter Zinn , Martin D. Whitaker , and George Weil . The reactor used natural uranium. This required a very large amount of material in order to reach criticality, along with graphite used as

7120-529: A set of theoretical nuclear reactor designs. These are generally not expected to be available for commercial use before 2040–2050, although the World Nuclear Association suggested that some might enter commercial operation before 2030. Current reactors in operation around the world are generally considered second- or third-generation systems, with the first-generation systems having been retired some time ago. Research into these reactor types

7298-411: A slow enough time scale to permit intervention by additional effects (e.g., mechanical control rods or thermal expansion). Consequently, all nuclear power reactors (even fast-neutron reactors ) rely on delayed neutrons for their criticality. An operating nuclear power reactor fluctuates between being slightly subcritical and slightly delayed-supercritical, but must always remain below prompt-critical. It

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7476-410: A slow reaction like the one in a pile where the fission products build up, these neutrons account for about three percent of the total neutron flux . Fermi argued that by using the delayed neutrons , and by carefully controlling the reaction rates as the power is ramped up, a pile can reach criticality at fission rates slightly below that of a chain reaction relying solely on the prompt neutrons from

7654-475: A substantial scientific lead. The success of Chicago Pile-1 in producing the chain reaction provided the first vivid demonstration of the feasibility of the military use of nuclear energy by the Allies, as well as the reality of the danger that Nazi Germany could succeed in producing nuclear weapons. Previously, estimates of critical masses had been crude calculations, leading to order-of-magnitude uncertainties about

7832-670: A water-cooled production reactor. There remained concerns about the ability of a graphite-moderated reactor being able to produce plutonium on industrial scale, and for this reason the Manhattan Project continued the development of heavy water production facilities . An air-cooled reactor, the X-10 Graphite Reactor , was built at the Clinton Engineer Works in Oak Ridge, Tennessee, as part of

8010-400: Is a function of the incident neutron speed. Also, note that these equations exclude energy from neutrinos since these subatomic particles are extremely non-reactive and therefore rarely deposit their energy in the system. The prompt neutron lifetime , l {\displaystyle l} , is the average time between the emission of a neutron and either its absorption or escape from

8188-426: Is accounted for in the release of energy according to the equation E=Δmc : Due to the extremely large value of the speed of light , c , a small decrease in mass is associated with a tremendous release of active energy (for example, the kinetic energy of the fission fragments). This energy (in the form of radiation and heat) carries the missing mass when it leaves the reaction system (total mass, like total energy,

8366-553: Is almost 100% composed of the isotope thorium-232 . This is called the thorium fuel cycle . The fissile isotope uranium-235 in its natural concentration is unfit for the vast majority of nuclear reactors. In order to be prepared for use as fuel in energy production, it must be enriched. The enrichment process does not apply to plutonium. Reactor-grade plutonium is created as a byproduct of neutron interaction between two different isotopes of uranium. The first step to enriching uranium begins by converting uranium oxide (created through

8544-535: Is always conserved ). While typical chemical reactions release energies on the order of a few eVs (e.g. the binding energy of the electron to hydrogen is 13.6 eV), nuclear fission reactions typically release energies on the order of hundreds of millions of eVs. Two typical fission reactions are shown below with average values of energy released and number of neutrons ejected: Note that these equations are for fissions caused by slow-moving (thermal) neutrons. The average energy released and number of neutrons ejected

8722-420: Is considered as the first true nuclear-grade graphite . By November 1942, National Carbon had shipped 255 short tons (231 t) of AGOT graphite to the University of Chicago, where it became the primary source of graphite to be used in the construction of Chicago Pile-1. Szilard drafted a confidential letter to the U.S. President, Franklin D. Roosevelt , warning of a German nuclear weapon project , explaining

8900-458: Is impossible for a nuclear power plant to undergo a nuclear chain reaction that results in an explosion of power comparable with a nuclear weapon, but even low-powered explosions from uncontrolled chain reactions (that would be considered "fizzles" in a bomb) may still cause considerable damage and meltdown in a reactor . For example, the Chernobyl disaster involved a runaway chain reaction, but

9078-413: Is inserted deeper into the reactor, it absorbs more neutrons than the material it displaces – often the moderator. This action results in fewer neutrons available to cause fission and reduces the reactor's power output. Conversely, extracting the control rod will result in an increase in the rate of fission events and an increase in power. The physics of radioactive decay also affects neutron populations in

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9256-427: Is known as delayed supercriticality (or delayed criticality ). It is in this region that all nuclear power reactors operate. The region of supercriticality for k > 1/(1 − β) is known as prompt supercriticality (or prompt criticality ), which is the region in which nuclear weapons operate. The change in k needed to go from critical to prompt critical is defined as a dollar . Nuclear fission weapons require

9434-428: Is known as a nuclear chain reaction . To control such a nuclear chain reaction, control rods containing neutron poisons and neutron moderators are able to change the portion of neutrons that will go on to cause more fission. Nuclear reactors generally have automatic and manual systems to shut the fission reaction down if monitoring or instrumentation detects unsafe conditions. The reactor core generates heat in

9612-405: Is mined, processed, enriched, used, possibly reprocessed and disposed of is known as the nuclear fuel cycle . Under 1% of the uranium found in nature is the easily fissionable U-235 isotope and as a result most reactor designs require enriched fuel. Enrichment involves increasing the percentage of U-235 and is usually done by means of gaseous diffusion or gas centrifuge . The enriched result

9790-478: Is now known as the Site ;A/Plot M Disposal Site . It is marked by a commemorative boulder. By the 1970s, there was increased public concern about the levels of radioactivity at the site, which was used for recreation by local residents. Surveys conducted in the 1980s found strontium-90 in the soil at Plot M, trace amounts of tritium in nearby wells, and plutonium, technetium, caesium, and uranium in

9968-401: Is produced. Fission also produces iodine-135 , which in turn decays (with a half-life of 6.57 hours) to new xenon-135. When the reactor is shut down, iodine-135 continues to decay to xenon-135, making restarting the reactor more difficult for a day or two, as the xenon-135 decays into cesium-135, which is not nearly as poisonous as xenon-135, with a half-life of 9.2 hours. This temporary state is

10146-448: Is reaching or crossing their design lifetimes of 30 or 40 years. In 2014, Greenpeace warned that the lifetime extension of ageing nuclear power plants amounts to entering a new era of risk. It estimated the current European nuclear liability coverage in average to be too low by a factor of between 100 and 1,000 to cover the likely costs, while at the same time, the likelihood of a serious accident happening in Europe continues to increase as

10324-416: Is required to determine exactly when a reactor reaches the critical point. Keeping the reactor in the zone of chain reactivity where delayed neutrons are necessary to achieve a critical mass state allows mechanical devices or human operators to control a chain reaction in "real time"; otherwise the time between achievement of criticality and nuclear meltdown as a result of an exponential power surge from

10502-513: Is separating the uranium hexafluoride from the depleted U-235 left over. This is typically done with centrifuges that spin fast enough to allow for the 1% mass difference in uranium isotopes to separate themselves. A laser is then used to enrich the hexafluoride compound. The final step involves reconverting the enriched compound back into uranium oxide, leaving the final product: enriched uranium oxide. This form of UO 2 can now be used in fission reactors inside power plants to produce energy. When

10680-424: Is the fissile isotope of uranium and it makes up approximately 0.7% of all naturally occurring uranium . Because of the small amount of U that exists, it is considered a non-renewable energy source despite being found in rock formations around the world. Uranium-235 cannot be used as fuel in its base form for energy production; it must undergo a process known as refinement to produce the compound UO 2 . The UO 2

10858-438: Is then converted into uranium dioxide powder, which is pressed and fired into 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. Nuclear chain reaction In nuclear physics , a nuclear chain reaction occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions, thus leading to

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11036-413: Is then pressed and formed into ceramic pellets, which can subsequently be placed into fuel rods. This is when UO 2 can be used for nuclear power production. The second most common isotope used in nuclear fission is plutonium-239 , because it is able to become fissile with slow neutron interaction. This isotope is formed inside nuclear reactors by exposing U to the neutrons released during fission. As

11214-412: Is torn apart from the explosion. Detonation of a nuclear weapon involves bringing fissile material into its optimal supercritical state very rapidly (about one microsecond , or one-millionth of a second). During part of this process, the assembly is supercritical, but not yet in an optimal state for a chain reaction. Free neutrons, in particular from spontaneous fissions , can cause the device to undergo

11392-423: Is unlikely that there is even a single spontaneous fission during the period of supercritical assembly. In particular, the gun method cannot be used with plutonium. Chain reactions naturally give rise to reaction rates that grow (or shrink) exponentially , whereas a nuclear power reactor needs to be able to hold the reaction rate reasonably constant. To maintain this control, the chain reaction criticality must have

11570-432: The Manhattan Project . Eventually, the first artificial nuclear reactor, Chicago Pile-1 , was constructed at the University of Chicago , by a team led by Italian physicist Enrico Fermi, in late 1942. By this time, the program had been pressured for a year by U.S. entry into the war. The Chicago Pile achieved criticality on 2 December 1942 at 3:25 PM. The reactor support structure was made of wood, which supported

11748-571: The Manhattan Project . Brigadier General Leslie R. Groves, Jr. became director of the Manhattan Project on 23 September 1942. He visited the Metallurgical Laboratory for the first time on 5 October. Between 15 September and 15 November 1942, groups under Herbert Anderson and Walter Zinn constructed 16 experimental piles under the Stagg Field stands. Fermi designed a new pile, which would be spherical to maximize k , which

11926-471: The Metallurgical Laboratory at the University of Chicago , CP-1 was built under the west viewing stands of the original Stagg Field . Although the project's civilian and military leaders had misgivings about the possibility of a disastrous runaway reaction, they trusted Fermi's safety calculations and decided they could carry out the experiment in a densely populated area. Fermi described the reactor as "a crude pile of black bricks and wooden timbers". After

12104-600: The National Defense Research Committee (NDRC) created a special project headed by Arthur Compton , a Nobel-Prize-winning physics professor at the University of Chicago , to report on the uranium program. Compton's report, submitted in May 1941, foresaw the prospects of developing radiological weapons , nuclear propulsion for ships, and nuclear weapons using uranium-235 or the recently discovered plutonium . In October, he wrote another report on

12282-517: The PWR , BWR and PHWR designs above, and some are more radical departures. The former include the advanced boiling water reactor (ABWR), two of which are now operating with others under construction, and the planned passively safe Economic Simplified Boiling Water Reactor (ESBWR) and AP1000 units (see Nuclear Power 2010 Program ). Rolls-Royce aims to sell nuclear reactors for the production of synfuel for aircraft. Generation IV reactors are

12460-524: The U.S. Atomic Energy Commission produced 0.8 kW in a test on 20 December 1951 and 100 kW (electrical) the following day, having a design output of 200 kW (electrical). Besides the military uses of nuclear reactors, there were political reasons to pursue civilian use of atomic energy. U.S. President Dwight Eisenhower made his famous Atoms for Peace speech to the UN General Assembly on 8 December 1953. This diplomacy led to

12638-477: The coolant also acts as a neutron moderator . A moderator increases the power of the reactor by causing the fast neutrons that are released from fission to lose energy and become thermal neutrons. Thermal neutrons are more likely than fast neutrons to cause fission. If the coolant is a moderator, then temperature changes can affect the density of the coolant/moderator and therefore change power output. A higher temperature coolant would be less dense, and therefore

12816-494: The four factor formula , which is the same as described above with P F N L {\displaystyle P_{\mathrm {FNL} }} and P T N L {\displaystyle P_{\mathrm {TNL} }} both equal to 1. Not all neutrons are emitted as a direct product of fission; some are instead due to the radioactive decay of some of the fission fragments. The neutrons that occur directly from fission are called "prompt neutrons", and

12994-413: The neutronic reactor no. 2,708,656. The Red Gate Woods later became the original site of Argonne National Laboratory , which replaced the Metallurgical Laboratory on 1 July 1946, with Zinn as its first director. CP-2 and CP-3 operated for ten years before they outlived their usefulness, and Zinn ordered them shut down on 15 May 1954. Their remaining usable fuel was transferred to Chicago Pile-5 at

13172-528: The reactor core ; the effective prompt neutron lifetime (referred to as the adjoint weighted over space, energy, and angle) refers to a neutron with average importance. The mean generation time , λ, is the average time from a neutron emission to a capture that results in fission. The mean generation time is different from the prompt neutron lifetime because the mean generation time only includes neutron absorptions that lead to fission reactions (not other absorption reactions). The two times are related by

13350-402: The "iodine pit." If the reactor has sufficient extra reactivity capacity, it can be restarted. As the extra xenon-135 is transmuted to xenon-136, which is much less a neutron poison, within a few hours the reactor experiences a "xenon burnoff (power) transient". Control rods must be further inserted to replace the neutron absorption of the lost xenon-135. Failure to properly follow such a procedure

13528-580: The 1986 Chernobyl disaster and 2011 Fukushima disaster . As of 2022 , the International Atomic Energy Agency reported there are 422 nuclear power reactors and 223 nuclear research reactors in operation around the world. The US Department of Energy classes reactors into generations, with the majority of the global fleet being Generation II reactors constructed from the 1960s to 1990s, and Generation IV reactors currently in development. Reactors can also be grouped by

13706-403: The 60-inch (150 cm) cyclotron at the University of California, Berkeley and found that it had 1.7 times the thermal neutron capture cross section of uranium-235. At the time only such minute quantities of plutonium-239 had been produced in cyclotrons, and it was not possible to produce a sufficiently large quantity that way. Compton discussed with Wigner how plutonium might be produced in

13884-665: The Argonne National Laboratory's new site in DuPage County , and the CP-2 and CP-3 reactors were dismantled in 1955 and 1956. Some of the graphite blocks from CP-1/CP-2 were reused in the reflector of the TREAT reactor. High-level nuclear waste such as fuel and heavy water were shipped to Oak Ridge, Tennessee , for disposal. The rest was encased in concrete and buried in a 40-foot-deep (12 m) trench in what

14062-599: The German chemist Max Bodenstein for a situation in which two molecules react to form not just the final reaction products, but also some unstable molecules that can further react with the original substances to cause more to react. The concept of a nuclear chain reaction was first hypothesized by the Hungarian scientist Leo Szilard on 12 September 1933. Szilard realized that if a nuclear reaction produced neutrons or dineutrons , which then caused further nuclear reactions,

14240-408: The U.S. government. An Advisory Committee on Uranium was formed under Lyman J. Briggs , a scientist and the director of the U.S. National Bureau of Standards . Its first meeting on 21 October 1939 was attended by Szilard, Teller, and Wigner. The scientists persuaded the U.S. Army and Navy to provide $ 6,000 for Szilard to purchase supplies for experiments—in particular, more graphite. In April 1941,

14418-736: The U.S. military sought other uses for nuclear reactor technology. Research by the Army led to the power stations for Camp Century, Greenland and McMurdo Station, Antarctica Army Nuclear Power Program . The Air Force Nuclear Bomber project resulted in the Molten-Salt Reactor Experiment . The U.S. Navy succeeded when they steamed the USS Nautilus (SSN-571) on nuclear power 17 January 1955. The first commercial nuclear power station, Calder Hall in Sellafield , England

14596-535: The United States does not engage in or encourage reprocessing. Reactors are also used in nuclear propulsion of vehicles. Nuclear marine propulsion of ships and submarines is largely restricted to naval use. Reactors have also been tested for nuclear aircraft propulsion and spacecraft propulsion . Reactor safety is maintained through various systems that control the rate of fission. The insertion of control rods, which absorb neutrons, can rapidly decrease

14774-516: The United States entered World War II, Compton decided on his own location, the University of Chicago, where he knew he had the unstinting support of university administration. Chicago also had a central location, and scientists, technicians and facilities were more readily available in the Midwest , where war work had not yet taken them away. In contrast, Columbia University was engaged in uranium enrichment efforts under Harold Urey and John Dunning, and

14952-448: The United States require a negative void coefficient of reactivity (this means that if coolant is removed from the reactor core, the nuclear reaction will tend to shut down, not increase). This eliminates the possibility of the type of accident that occurred at Chernobyl (which was caused by a positive void coefficient). However, nuclear reactors are still capable of causing smaller chemical explosions even after complete shutdown, such as

15130-571: The United States' major urban areas in radioactive fission products. But the physics of the system suggested that the pile could be safely shut down even in the event of a runaway reaction . When a fuel atom undergoes fission, it releases neutrons that strike other fuel atoms in a chain reaction. The time between absorbing the neutron and undergoing fission is measured in nanoseconds. Szilard had noted that this reaction leaves behind fission products that may also release neutrons, but do so over much longer periods, from microseconds to as long as minutes. In

15308-569: The area was contaminated, like Fukushima, Three Mile Island, Sellafield, and Chernobyl. The British branch of the French concern EDF Energy , for example, extended the operating lives of its Advanced Gas-cooled Reactors (AGR) with only between 3 and 10 years. All seven AGR plants were expected to be shut down in 2022 and in decommissioning by 2028. Hinkley Point B was extended from 40 to 46 years, and closed. The same happened with Hunterston B , also after 46 years. An increasing number of reactors

15486-565: The area. In 1994, the United States Department of Energy and the Argonne National Laboratory yielded to public pressure and earmarked $ 24.7 million and $ 3.4 million respectively to rehabilitate the site. As part of the cleanup, 500 cubic yards (380 m) of radioactive waste was removed and sent to the Hanford Site for disposal. By 2002, the Illinois Department of Public Health had determined that

15664-795: The beginning of his quest to produce the Einstein-Szilárd letter to alert the U.S. government. Shortly after, Nazi Germany invaded Poland in 1939, starting World War II in Europe. The U.S. was not yet officially at war, but in October, when the Einstein-Szilárd letter was delivered to him, Roosevelt commented that the purpose of doing the research was to make sure "the Nazis don't blow us up." The U.S. nuclear project followed, although with some delay as there remained skepticism (some of it from Enrico Fermi ) and also little action from

15842-458: The choices of coolant and moderator. Almost 90% of global nuclear energy comes from pressurized water reactors and boiling water reactors , which use water as a coolant and moderator. Other designs include heavy water reactors , gas-cooled reactors , and fast breeder reactors , variously optimizing efficiency, safety, and fuel type , enrichment , and burnup . Small modular reactors are also an area of current development. These reactors play

16020-482: The clock, and its design was suitable for conducting experiments. CP-2 was joined by Chicago Pile-3 , the first heavy water reactor, which went critical on 15 May 1944. The reactors were used to undertake research related to weapons, such as investigations of the properties of tritium . Wartime experiments included measuring the neutron absorption cross-section of elements and compounds. Albert Wattenberg recalled that about 10 elements were studied each month, and 75 over

16198-467: The complexities of handling actinides , but significant scientific and technical obstacles remain. Despite research having started in the 1950s, no commercial fusion reactor is expected before 2050. The ITER project is currently leading the effort to harness fusion power. 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

16376-401: The control circuits, but after 28 minutes, the alarm bells went off to notify everyone that the neutron flux had passed the preset safety level, and he ordered Zinn to release the zip. The reaction rapidly halted. The pile had run for about 4.5 minutes at about 0.5 watts. Wigner opened a bottle of Chianti , which they drank from paper cups. Compton notified Conant by telephone. The conversation

16554-439: The control rods reinserted. He then announced that it was lunch time. The experiment resumed at 14:00. Weil worked the final control rod while Fermi carefully monitored the neutron activity. Fermi announced that the pile had gone critical (reached a self-sustaining reaction) at 15:25. Fermi switched the scale on the recorder to accommodate the rapidly increasing electric current from the boron trifluoride detector. He wanted to test

16732-475: The course of a year. An accident involving radium and beryllium powder caused a dangerous drop in his white blood cell count that lasted for three years. As the dangers of things such as inhaling uranium oxide became more apparent, experiments were conducted on the effects of radioactive substances on laboratory test animals. Though the design was held secret for a decade, Szilard and Fermi jointly patented it, with an initial filing date of 19 December 1944 as

16910-440: The critical radius was calculated to be approximately: R c r i t ≈ π M k − 1 {\displaystyle R_{crit}\approx {\frac {\pi M}{\sqrt {k-1}}}} , where M is the average distance that a neutron travels before it is absorbed, and k is the average neutron multiplication factor . The neutrons in succeeding reactions will be amplified by

17088-418: The critical size and geometry ( critical mass ) necessary in order to obtain an explosive chain reaction. The fuel for energy purposes, such as in a nuclear fission reactor, is very different, usually consisting of a low-enriched oxide material (e.g. uranium dioxide , UO 2 ). There are two primary isotopes used for fission reactions inside of nuclear reactors. The first and most common is uranium-235 . This

17266-688: The dissemination of reactor technology to U.S. institutions and worldwide. The first nuclear power plant built for civil purposes was the AM-1 Obninsk Nuclear Power Plant , launched on 27 June 1954 in the Soviet Union . It produced around 5 MW (electrical). It was built after the F-1 (nuclear reactor) which was the first reactor to go critical in Europe, and was also built by the Soviet Union. After World War II,

17444-494: The energy of the neutrons that sustain the fission chain reaction : In principle, fusion power could be produced by nuclear fusion of elements such as the deuterium isotope of hydrogen . While an ongoing rich research topic since at least the 1940s, no self-sustaining fusion reactor for any purpose has ever been built. Used by thermal reactors: In 2003, the French Commissariat à l'Énergie Atomique (CEA)

17622-535: The experiment. There were 49 scientists present. Although most of the S-1 Executive Committee was in Chicago, only Crawford Greenewalt was present, at Compton's invitation. Other dignitaries present included Szilard, Wigner and Spedding. Fermi, Compton, Anderson and Zinn gathered around the controls on the balcony, which was originally intended as a viewing platform. Samuel Allison stood ready with

17800-414: The fact that much greater amounts of energy were produced by the reaction than the proton supplied. Ernest Rutherford commented in the article that inefficiencies in the process precluded use of it for power generation. However, the neutron had been discovered by James Chadwick in 1932, shortly before, as the product of a nuclear reaction . Szilárd, who had been trained as an engineer and physicist, put

17978-484: The fast fission factor ε {\displaystyle \varepsilon } , the resonance escape probability p {\displaystyle p} , the probability of thermal non-leakage P T N L {\displaystyle P_{\mathrm {TNL} }} , the thermal utilization factor f {\displaystyle f} , and the neutron reproduction factor η {\displaystyle \eta } (also called

18156-564: The first nuclear fission experiment in the United States on 25 January 1939. Subsequent work confirmed that fast neutrons were indeed produced by fission. Szilard obtained permission from the head of the Physics Department at Columbia, George B. Pegram , to use a laboratory for three months, and he persuaded Walter Zinn to become his collaborator. They conducted a simple experiment on the seventh floor of Pupin Hall at Columbia, using

18334-642: The first reactor dedicated to peaceful use; in Russia, in 1954, the first small nuclear power reactor APS-1 OBNINSK reached criticality. Other countries followed suit. Heat from nuclear fission is passed to a working fluid coolant (water or gas), which in turn runs through turbines . In commercial reactors, turbines drive electrical generator shafts. The heat can also be used for district heating , and industrial applications including desalination and hydrogen production . Some reactors are used to produce isotopes for medical and industrial use. Reactors pose

18512-461: The fissile component, and, on 29 February 1940, Nier separated the first uranium-235 sample, which, after being mailed to Dunning at Columbia, was confirmed to be the isolated fissile material. When he was working in Rome, Italy, Fermi had discovered that collisions between neutrons and neutron moderators can slow the neutrons, and thereby make them more likely to be captured by uranium nuclei, causing

18690-407: The fission process generates heat, some of which can be converted into usable energy. A common method of harnessing this thermal energy is to use it to boil water to produce pressurized steam which will then drive a steam turbine that turns an alternator and generates electricity. Modern nuclear power plants are typically designed for a lifetime of 60 years, while older reactors were built with

18868-753: The fission reaction was not yet discovered, or even suspected. Instead, Szilárd proposed using mixtures of lighter known isotopes which produced neutrons in copious amounts. He filed a patent for his idea of a simple nuclear reactor the following year. In 1936, Szilárd attempted to create a chain reaction using beryllium and indium but was unsuccessful. Nuclear fission was discovered by Otto Hahn and Fritz Strassmann in December 1938 and explained theoretically in January 1939 by Lise Meitner and her nephew Otto Robert Frisch . In their second publication on nuclear fission in February 1939, Hahn and Strassmann used

19046-437: The fission reactions. Since the rate of release of these neutrons depends on fission events taking place some time earlier, there is a delay between any power spikes and the later criticality event. This time gives the operators leeway; if a spike in the prompt neutron flux is seen, they have several minutes before this causes a runaway reaction. If a neutron absorber, or neutron poison , is injected at any time during this period,

19224-424: The following formula: In this formula k eff is the effective neutron multiplication factor, described below. The six factor formula effective neutron multiplication factor, k eff , is the average number of neutrons from one fission that cause another fission. The remaining neutrons either are absorbed in non-fission reactions or leave the system without being absorbed. The value of k eff determines how

19402-529: The form of boric acid ) into the reactor to shut the fission reaction down if unsafe conditions are detected or anticipated. Most types of reactors are sensitive to a process variously known as xenon poisoning, or the iodine pit . The common fission product Xenon-135 produced in the fission process acts as a neutron poison that absorbs neutrons and therefore tends to shut the reactor down. Xenon-135 accumulation can be controlled by keeping power levels high enough to destroy it by neutron absorption as fast as it

19580-424: The fuel rods. This allows the reactor to be constructed with an excess of fissionable material, which is nevertheless made relatively safe early in the reactor's fuel burn cycle by the presence of the neutron-absorbing material which is later replaced by normally produced long-lived neutron poisons (far longer-lived than xenon-135) which gradually accumulate over the fuel load's operating life. The energy released in

19758-435: The graphite. The entire pile was then canned by soldering sheet metal around it, and the contents heated above the boiling point of water to remove moisture. The result was a k of 0.918. In Chicago, Samuel K. Allison had found a suitable location 60 feet (18 m) long, 30 feet (9.1 m) wide and 26 feet (7.9 m) high, sunk slightly below ground level, in a space under the stands at Stagg Field originally built as

19936-440: The heavy cans with ease. The final result was a disappointing k of 0.87. Compton felt that having teams at Columbia University, Princeton University , the University of Chicago and the University of California was creating too much duplication and not enough collaboration, and he resolved to concentrate the work in one location. Nobody wanted to move, and everybody argued in favor of their own location. In January 1942, soon after

20114-408: The holes in the graphite in lieu of the uranium oxide pseudospheres. The process of filling the balloon with carbon dioxide would not be necessary, and twenty layers could be dispensed with. According to Fermi's new calculations, the countdown would reach 1 between the 56th and 57th layers. The resulting pile was therefore flatter on the top than on the bottom. Anderson called a halt after the 57th layer

20292-447: The idea of nuclear fission as a neutron source, since that process was not yet discovered. Szilárd's ideas for nuclear reactors using neutron-mediated nuclear chain reactions in light elements proved unworkable. Inspiration for a new type of reactor using uranium came from the discovery by Otto Hahn , Lise Meitner , and Fritz Strassmann in 1938 that bombardment of uranium with neutrons (provided by an alpha-on-beryllium fusion reaction,

20470-476: The major relevant contaminant was boron, both because of its concentration and its affinity for absorbing neutrons, confirming a suspicion of Szilard's. More importantly, MacPherson and Hamister believed that techniques for producing graphite of a sufficient purity could be developed. Had Fermi and Szilard not consulted MacPherson and Hamister, they might have concluded, incorrectly, as the Germans did, that graphite

20648-403: The materials for Fermi's new pile would be on hand before the new structure was completed. In early November, Fermi came to Compton with a proposal to build the experimental pile under the stands at Stagg Field. The risk of building an operational reactor running at criticality in a populated area was a significant issue, as there was a danger of a catastrophic nuclear meltdown blanketing one of

20826-450: The matter to my superior. But this would have been unfair. President Hutchins was in no position to make an independent judgment of the hazards involved. Based on considerations of the University's welfare, the only answer he could have given would have been—no. And this answer would have been wrong. Compton informed Groves of his decision at the 14 November meeting of the S-1 Executive Committee. Although Groves "had serious misgivings about

21004-414: The morning of 16 November 1942. The first layer placed was made up entirely of graphite blocks, with no uranium. Layers without uranium were alternated with two layers containing uranium, so the uranium was enclosed in graphite. Unlike later reactors, it had no radiation shielding or cooling system, as it was only intended to be operated at very low power. The work was carried out in twelve-hour shifts, with

21182-408: The neutron efficiency factor). The six-factor formula is traditionally written as follows: k e f f = P F N L ε p P T N L f η {\displaystyle k_{eff}=P_{\mathrm {FNL} }\varepsilon pP_{\mathrm {TNL} }f\eta } Where: In an infinite medium, the multiplication factor may be described by

21360-449: The normal nuclear chain reaction, would be too short to allow for intervention. This last stage, where delayed neutrons are no longer required to maintain criticality, is known as the prompt critical point. There is a scale for describing criticality in numerical form, in which bare criticality is known as zero dollars and the prompt critical point is one dollar , and other points in the process interpolated in cents. In some reactors,

21538-446: The nuclear chain reaction begins after increasing the density of the fissile material with a conventional explosive. In a gun-type fission weapon , two subcritical masses of fuel are rapidly brought together. The value of k for a combination of two masses is always greater than that of its components. The magnitude of the difference depends on distance, as well as the physical orientation. The value of k can also be increased by using

21716-584: The ones that are a result of radioactive decay of fission fragments are called "delayed neutrons". The fraction of neutrons that are delayed is called β, and this fraction is typically less than 1% of all the neutrons in the chain reaction. The delayed neutrons allow a nuclear reactor to respond several orders of magnitude more slowly than just prompt neutrons would alone. Without delayed neutrons, changes in reaction rates in nuclear reactors would occur at speeds that are too fast for humans to control. The region of supercriticality between k = 1 and k = 1/(1 − β)

21894-581: The opportunity for the nuclear chain reaction that Szilárd had envisioned six years previously. On 2 August 1939, Albert Einstein signed a letter to President Franklin D. Roosevelt (written by Szilárd) suggesting that the discovery of uranium's fission could lead to the development of "extremely powerful bombs of a new type", giving impetus to the study of reactors and fission. Szilárd and Einstein knew each other well and had worked together years previously, but Einstein had never thought about this possibility for nuclear energy until Szilard reported it to him, at

22072-406: The physics of radioactive decay and are simply accounted for during the reactor's operation, while others are mechanisms engineered into the reactor design for a distinct purpose. The fastest method for adjusting levels of fission-inducing neutrons in a reactor is via movement of the control rods . Control rods are made of so-called neutron poisons and therefore absorb neutrons. When a control rod

22250-478: The pile was dismantled and moved to Site A in the Argonne Forest, now known as Red Gate Woods . There the original materials were used to build Chicago Pile-2 (CP-2). Instead of being spherical, the new reactor was built in a cube-like shape, about 25 feet (7.6 m) tall with a base approximately 30 feet (9.1 m) square. It was surrounded by concrete walls 5 feet (1.5 m) thick that acted as

22428-482: The possibility of a self-propagating series or "positive feedback loop" of these reactions. The specific nuclear reaction may be the fission of heavy isotopes (e.g., uranium-235 , U). A nuclear chain reaction releases several million times more energy per reaction than any chemical reaction . Chemical chain reactions were first proposed by German chemist Max Bodenstein in 1913, and were reasonably well understood before nuclear chain reactions were proposed. It

22606-503: The possibility of creating a nuclear chain reaction with uranium, but initial experiments were unsuccessful. In order for a chain reaction to occur, fissioning uranium atoms had to emit additional neutrons to keep the reaction going. At Columbia University in New York, Italian physicist Enrico Fermi collaborated with Americans John Dunning , Herbert L. Anderson , Eugene T. Booth , G. Norris Glasoe , and Francis G. Slack to conduct

22784-423: The possibility of nuclear weapons, and encouraging the development of a program that could result in their creation. With the help of Eugene Wigner and Edward Teller , he approached his old friend and collaborator Albert Einstein in August 1939, and convinced him to sign the letter, lending his prestige to the proposal. The Einstein–Szilard letter resulted in the establishment of research into nuclear fission by

22962-406: The possible existence of impurities in graphite, and the procurement of graphite of a purity that had never been produced commercially. National Carbon, a chemical company, had taken the then unusual step of hiring MacPherson, a physicist, to research carbon arc lamps, a major commercial use for graphite at that time. Because of his work studying the spectroscopy of the carbon arc, MacPherson knew that

23140-463: The power plant is limited by the life of components that cannot be replaced when aged by wear and neutron embrittlement , such as the reactor pressure vessel. At the end of their planned life span, plants may get an extension of the operating license for some 20 years and in the US even a "subsequent license renewal" (SLR) for an additional 20 years. Even when a license is extended, it does not guarantee

23318-457: The practicality of an atomic bomb. For this report, he worked with Fermi on calculations of the critical mass of uranium-235. He also discussed the prospects for uranium enrichment with Harold Urey . Niels Bohr and John Wheeler had theorized that heavy isotopes with odd atomic mass numbers were fissile . If so, then plutonium-239 was likely to be fissile. In May 1941, Emilio Segrè and Glenn Seaborg produced 28 μg of plutonium-239 in

23496-399: The presence of a neutron moderator like heavy water or high purity carbon (e.g. graphite) in the absence of neutron poisons , which is even more unlikely to arise by natural geological processes than the conditions at Oklo some two billion years ago. Fission chain reactions occur because of interactions between neutrons and fissile isotopes (such as U). The chain reaction requires both

23674-497: The process might be self-perpetuating. Szilard proposed using mixtures of lighter known isotopes which produced neutrons in copious amounts, and also entertained the possibility of using uranium as a fuel. He filed a patent for his idea of a simple nuclear reactor the following year. The discovery of nuclear fission by German chemists Otto Hahn and Fritz Strassmann in 1938, and its theoretical explanation (and naming) by their collaborators Lise Meitner and Otto Frisch , opened up

23852-572: The reactor fleet grows older. The neutron was discovered in 1932 by British physicist James Chadwick . The concept of a nuclear chain reaction brought about by nuclear reactions mediated by neutrons was first realized shortly thereafter, by Hungarian scientist Leó Szilárd , in 1933. He filed a patent for his idea of a simple reactor the following year while working at the Admiralty in London, England. However, Szilárd's idea did not incorporate

24030-416: The reactor will continue to operate, particularly in the face of safety concerns or incident. Many reactors are closed long before their license or design life expired and are decommissioned . The costs for replacements or improvements required for continued safe operation may be so high that they are not cost-effective. Or they may be shut down due to technical failure. Other ones have been shut down because

24208-436: The reactor will shut down. Consequently, the reaction can be controlled with electromechanical control systems such as control rods . Compton felt this delay was enough to provide a critical margin of safety, and allowed Fermi to build Chicago Pile-1 at Stagg Field. Compton later explained that: As a responsible officer of the University of Chicago, according to every rule of organizational protocol, I should have taken

24386-437: The reactor's output, while other systems automatically shut down the reactor in the event of unsafe conditions. The buildup of neutron-absorbing fission products like xenon-135 can influence reactor behavior, requiring careful management to prevent issues such as the iodine pit , which can complicate reactor restarts. There have been two reactor accidents classed as an International Nuclear Event Scale Level 7 "major accident":

24564-520: The relationship between his pile and Volta's. Another grant, this time of $ 40,000, was obtained from the S-1 Uranium Committee to purchase more materials, and in August 1941 Fermi began to plan the building of a sub-critical assembly to test with a smaller structure whether a larger one would work. The so-called exponential pile he proposed to build was 8 feet (2.4 m) long, 8 feet (2.4 m) wide and 11 feet (3.4 m) high. This

24742-426: The release of neutrons from fissile isotopes undergoing nuclear fission and the subsequent absorption of some of these neutrons in fissile isotopes. When an atom undergoes nuclear fission, a few neutrons (the exact number depends on uncontrollable and unmeasurable factors; the expected number depends on several factors, usually between 2.5 and 3.0) are ejected from the reaction. These free neutrons will then interact with

24920-470: The remaining materials posed no danger to public health. The successful test of CP-1 not only proved that a nuclear reactor was feasible, it demonstrated that the k factor was larger than originally thought. This removed the objections to the use of air or water as a coolant rather than expensive helium. It also meant that there was greater latitude in the choice of materials for coolant pipes and control mechanisms. Wigner now pressed ahead with his design for

25098-458: The result was a low-powered steam explosion from the relatively small release of heat, as compared with a bomb. However, the reactor complex was destroyed by the heat, as well as by ordinary burning of the graphite exposed to air. Such steam explosions would be typical of the very diffuse assembly of materials in a nuclear reactor, even under the worst conditions. In addition, other steps can be taken for safety. For example, power plants licensed in

25276-433: The rods was simply a variable resistor , controlling an electric motor that would spool the clothesline wire over a pulley that also had two lead weights attached to ensure it would fail-safe and return to its zero position when released. About two layers were laid per shift. Woods' boron trifluoride neutron counter was inserted at the 15th layer. Thereafter, readings were taken at the end of each shift. Fermi divided

25454-481: The same analysis. This discovery prompted the letter from Szilárd and signed by Albert Einstein to President Franklin D. Roosevelt , warning of the possibility that Nazi Germany might be attempting to build an atomic bomb. On December 2, 1942, a team led by Fermi (and including Szilárd) produced the first artificial self-sustaining nuclear chain reaction with the Chicago Pile-1 experimental reactor in

25632-491: The size of a hypothetical bomb. The successful use of graphite as a moderator paved the way for progress in the Allied effort, whereas the German program languished partly because of the belief that scarce and expensive heavy water would have to be used for that purpose. The Germans had failed to account for the importance of boron and cadmium impurities in the graphite samples on which they ran their test of its usability as

25810-647: The small number of officials in the government who were initially charged with moving the project forward. The following year, the U.S. Government received the Frisch–Peierls memorandum from the UK, which stated that the amount of uranium needed for a chain reaction was far lower than had previously been thought. The memorandum was a product of the MAUD Committee , which was working on the UK atomic bomb project, known as Tube Alloys , later to be subsumed within

25988-401: The square of the radius of the pile by the intensity of the radioactivity to obtain a metric that counted down to one as the pile approached criticality. At the 15th layer, it was 390; at the 19th it was 320; at the 25th it was 270 and by the 36th it was only 149. The original design was for a spherical pile, but as work proceeded, it became clear that this would not be necessary. The new graphite

26166-601: The surrounding medium, and if more fissile fuel is present, some may be absorbed and cause more fissions. Thus, the cycle repeats to produce a reaction that is self-sustaining. Nuclear power plants operate by precisely controlling the rate at which nuclear reactions occur. Nuclear weapons, on the other hand, are specifically engineered to produce a reaction that is so fast and intense it cannot be controlled after it has started. When properly designed, this uncontrolled reaction will lead to an explosive energy release. Nuclear weapons employ high quality, highly enriched fuel exceeding

26344-421: The system. The neutrons that occur directly from fission are called prompt neutrons, and the ones that are a result of radioactive decay of fission fragments are called delayed neutrons. The term lifetime is used because the emission of a neutron is often considered its birth , and its subsequent absorption or escape from the core is considered its death . For "thermal" (slow-neutron) fission reactors,

26522-477: The term uranspaltung ( uranium fission) for the first time and predicted the existence and liberation of additional neutrons during the fission process, opening up the possibility of a nuclear chain reaction. A few months later, Frédéric Joliot-Curie , H. Von Halban and L. Kowarski in Paris searched for, and discovered, neutron multiplication in uranium, proving that a nuclear chain reaction by this mechanism

26700-499: The timing of these oscillations. The effective neutron multiplication factor k e f f {\displaystyle k_{eff}} can be described using the product of six probability factors that describe a nuclear system. These factors, traditionally arranged chronologically with regards to the life of a neutron in a thermal reactor , include the probability of fast non-leakage P F N L {\displaystyle P_{\mathrm {FNL} }} ,

26878-421: The two nuclear experimental results together in his mind and realized that if a nuclear reaction produced neutrons, which then caused further similar nuclear reactions, the process might be a self-perpetuating nuclear chain reaction, spontaneously producing new isotopes and power without the need for protons or an accelerator. Szilárd, however, did not propose fission as the mechanism for his chain reaction since

27056-421: The typical prompt neutron lifetime is on the order of 10 seconds, and for fast fission reactors, the prompt neutron lifetime is on the order of 10 seconds. These extremely short lifetimes mean that in 1 second, 10,000 to 10,000,000 neutron lifetimes can pass. The average (also referred to as the adjoint unweighted ) prompt neutron lifetime takes into account all prompt neutrons regardless of their importance in

27234-403: The uranium milling process) into a gaseous form. This gas is known as uranium hexafluoride , which is created by combining hydrogen fluoride , fluorine , and uranium oxide. Uranium dioxide is also present in this process and is sent off to be used in reactors not requiring enriched fuel. The remaining uranium hexafluoride compound is drained into metal cylinders where it solidifies. The next step

27412-412: The uranium to fission. Szilard suggested to Fermi that they use carbon in the form of graphite as a moderator. As a back-up plan, he considered heavy water . This contained deuterium , which would not absorb neutrons like ordinary hydrogen, and was a better neutron moderator than carbon; but heavy water was expensive and difficult to produce, and several tons of it might be needed. Fermi estimated that

27590-424: The water for the steam turbines is boiled directly by the reactor core ; for example the boiling water reactor . The rate of fission reactions within a reactor core can be adjusted by controlling the quantity of neutrons that are able to induce further fission events. Nuclear reactors typically employ several methods of neutron control to adjust the reactor's power output. Some of these methods arise naturally from

27768-567: The wisdom of Compton's suggestion", he did not interfere. James B. Conant , the chairman of the NDRC, was reported to have turned white. But because of the urgency and their confidence in Fermi's calculations, no one objected. Chicago Pile-1 was encased within a balloon so that the air inside could be replaced by carbon dioxide . Anderson had a dark gray balloon manufactured by Goodyear Tire and Rubber Company . A 25-foot (7.6 m) cube-shaped balloon

27946-425: Was a key step in the Chernobyl disaster . Reactors used in nuclear marine propulsion (especially nuclear submarines ) often cannot be run at continuous power around the clock in the same way that land-based power reactors are normally run, and in addition often need to have a very long core life without refueling . For this reason many designs use highly enriched uranium but incorporate burnable neutron poison in

28124-502: Was apparent that the proposed facilities would be too extensive for the site, and it was decided to build the pilot plant elsewhere. The subcritical piles posed little danger, but Groves felt that it would be prudent to locate a critical pile—a fully functional nuclear reactor—at a more remote site. A building at Argonne to house Fermi's experimental pile was commenced, with its completion scheduled for 20 October. Due to industrial disputes, construction fell behind schedule, and it became clear

28302-483: Was discovered in Schermerhorn Hall . The pile was built in September 1941 from 4-by-4-by-12-inch (10 by 10 by 30 cm) graphite blocks and tinplate iron cans of uranium oxide. The cans were 8-by-8-by-8-inch (20 by 20 by 20 cm) cubes. When filled with uranium oxide, each weighed about 60 pounds (27 kg). There were 288 cans in all, and each was surrounded by graphite blocks so the whole would form

28480-403: Was hesitant to add a third secret project. Before leaving for Chicago, Fermi's team made one last attempt to build a working pile at Columbia. Since the cans had absorbed neutrons, they were dispensed with. Instead, the uranium oxide, heated to 250 °C (480 °F) to dry it out, was pressed into cylindrical holes 3 inches (7.6 cm) long and 3 inches (7.6 cm) in diameter drilled into

28658-492: Was in an impromptu code: Compton: The Italian navigator has landed in the New World. Conant: How were the natives? Compton: Very friendly. On 12 December 1942, CP-1's power output was increased to 200 W, enough to power a light bulb. Lacking shielding of any kind, it was a radiation hazard for everyone in the vicinity, and further testing was continued at 0.5 W. Operation was terminated on 28 February 1943, and

28836-466: Was indeed possible. On May 4, 1939, Joliot-Curie, Halban, and Kowarski filed three patents. The first two described power production from a nuclear chain reaction, the last one called Perfectionnement aux charges explosives was the first patent for the atomic bomb and is filed as patent No. 445686 by the Caisse nationale de Recherche Scientifique . In parallel, Szilárd and Enrico Fermi in New York made

29014-567: Was now producing 30 short tons (27 t) a month. Metallic uranium also began arriving in larger quantities, the product of newly developed techniques. On 25 June, the Army and the Office of Scientific Research and Development (OSRD) had selected a site in the Argonne Forest near Chicago for a plutonium pilot plant; this became known as " Site A ". 1,025 acres (415 ha) were leased from Cook County in August, but by September it

29192-788: Was officially started by the Generation ;IV International Forum (GIF) based on eight technology goals. The primary goals being to improve nuclear safety, improve proliferation resistance, minimize waste and natural resource utilization, and to decrease the cost to build and run such plants. Generation V reactors are designs which are theoretically possible, but which are not being actively considered or researched at present. Though some generation V reactors could potentially be built with current or near term technology, they trigger little interest for reasons of economics, practicality, or safety. Controlled nuclear fusion could in principle be used in fusion power plants to produce power without

29370-463: Was opened in 1956 with an initial capacity of 50 MW (later 200 MW). The first portable nuclear reactor "Alco PM-2A" was used to generate electrical power (2 MW) for Camp Century from 1960 to 1963. All commercial power reactors are based on nuclear fission . They generally use uranium and its product plutonium as nuclear fuel , though a thorium fuel cycle is also possible. Fission reactors can be divided roughly into two classes, depending on

29548-403: Was placed in charge of the plutonium project. Its objectives were to produce reactors to convert uranium to plutonium, to find ways to chemically separate the plutonium from the uranium, and to design and build an atomic bomb. It fell to Compton to decide which of the different types of reactor designs the scientists should pursue, even though a successful reactor had not yet been built. He proposed

29726-439: Was placed. When completed, the wooden frame supported an elliptical-shaped structure, 20 feet (6.1 m) high, 6 feet (1.8 m) wide at the ends and 25 feet (7.6 m) across the middle. It contained 6 short tons (5.4 t) of uranium metal, 50 short tons (45 t) of uranium oxide and 400 short tons (360 t) of graphite, at an estimated cost of $ 2.7 million. The next day, 2 December 1942, everybody assembled for

29904-595: Was predicted to be around 1.04, thereby achieving criticality. Leona Woods was detailed to build boron trifluoride neutron detectors as soon as she completed her doctoral thesis. She also helped Anderson locate the required large number of 4-by-6-inch (10 by 15 cm) timbers at lumber yards in Chicago's south side . Shipments of high-purity graphite arrived, mainly from National Carbon, and high-purity uranium dioxide from Mallinckrodt in St Louis, Missouri, which

30082-508: Was purer, and 6 short tons (5.4 t) of very pure metallic uranium began to arrive from the Ames Project at Iowa State University , where Harley Wilhelm and his team had developed a new process to produce uranium metal. Westinghouse Lamp Plant supplied 3 short tons (2.7 t), which it produced in a rush with a makeshift process. The 2.25-inch (5.7 cm) metallic uranium cylinders, known as "Spedding's eggs", were dropped in

30260-404: Was reconfigured to become Chicago Pile-2 (CP-2). There, it was operated for research until 1954, when it was dismantled and buried. The stands at Stagg Field were demolished in August 1957 and a memorial quadrangle now marks the experiment site's location, which is now a National Historic Landmark and a Chicago Landmark . The idea of a chemical chain reaction was first suggested in 1913 by

30438-418: Was responsible for instrumentation. They also fabricated the control rods , which were cadmium sheets nailed to flat wooden strips, cadmium being a potent neutron absorber, and the scram line, a manila rope that when cut would drop a control rod into the pile and stop the reaction. Richard Fox, who made the control-rod mechanism for the pile, remarked that the manual speed control that the operator had over

30616-430: Was somewhat unusual, but the Manhattan Project's AAA priority rating ensured prompt delivery with no questions asked. A block and tackle was used to haul it into place, with the top secured to the ceiling and three sides to the walls. The remaining side, the one facing the balcony from which Fermi directed the operation, was furled like an awning. A circle was drawn on the floor, and the stacking of graphite blocks began on

30794-522: Was the case of the Fukushima Daiichi nuclear disaster . In such cases, residual decay heat from the core may cause high temperatures if there is loss of coolant flow, even a day after the chain reaction has been shut down (see SCRAM ). This may cause a chemical reaction between water and fuel that produces hydrogen gas, which can explode after mixing with air, with severe contamination consequences, since fuel rod material may still be exposed to

30972-573: Was the first to refer to "Gen II" types in Nucleonics Week . The first mention of "Gen III" was in 2000, in conjunction with the launch of the Generation IV International Forum (GIF) plans. "Gen IV" was named in 2000, by the United States Department of Energy (DOE), for developing new plant types. More than a dozen advanced reactor designs are in various stages of development. Some are evolutionary from

31150-486: Was too large to fit in the Pupin Physics Laboratories. Fermi recalled that: We went to Dean Pegram, who was then the man who could carry out magic around the University, and we explained to him that we needed a big room. He scouted around the campus and we went with him to dark corridors and under various heating pipes and so on, to visit possible sites for this experiment and eventually a big room

31328-465: Was understood that chemical chain reactions were responsible for exponentially increasing rates in reactions, such as produced in chemical explosions. The concept of a nuclear chain reaction was reportedly first hypothesized by Hungarian scientist Leó Szilárd on September 12, 1933. Szilárd that morning had been reading in a London paper of an experiment in which protons from an accelerator had been used to split lithium-7 into alpha particles , and

31506-465: Was unsuitable for use as a neutron moderator. Over the next two years, MacPherson, Hamister and Lauchlin M. Currie developed thermal purification techniques for the large scale production of low boron content graphite. The resulting product was designated AGOT graphite (" Acheson Graphite Ordinary Temperature") by National Carbon. With a neutron absorption cross section of 4.97 mbarns , the AGOT graphite

31684-403: Was used to drill 3.25-inch (8.3 cm) holes in the blocks for the control rods and the uranium. A hydraulic press was used to shape the uranium oxide into "pseudospheres", cylinders with rounded ends. Drill bits had to be sharpened after each 60 holes, which worked out to be about once an hour. Graphite dust soon filled the air and made the floor slippery. Another group, under Volney C. Wilson,

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