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62-451: An operational license for the MAPLE I reactor was granted in 1999, and the reactor went critical for the first time in early 2000. MAPLE II followed in the fall of 2003. Problems with the reactors during the testing period, most notably an unexpected positive power co-efficient of reactivity , led to the cancellation of the project in 2008 and the shutdown of both reactors. With the completion of

124-427: 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

186-592: 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

248-738: A bomb. Originally planned to complete construction in 1999 and 2000, both reactors were instead completed in May 2000. An operational license was granted in August 1999 for the MAPLE I reactor, and extended to include the MAPLE II reactor in June 2000. Commissioning testing was begun immediately, with the MAPLE I achieving its first sustained reaction in February 2000, and MAPLE II following in October 2003. However, during testing, it

310-427: 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

372-594: 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

434-429: 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,

496-467: 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

558-511: A new facility would be needed to continue the production of medical isotopes. In the late 1980s, AECL began to acknowledge that continued isotope production would require the construction of a new reactor to replace capacity lost by the planned closing of the NRX in 1992, and the planned closing of the NRU early in the new millennium. Design work on a replacement, originally under the name "Maple-X10", began in

620-403: 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

682-474: 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

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744-401: 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

806-500: 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

868-411: 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

930-400: 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

992-426: 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,

1054-553: 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

1116-535: 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

1178-453: 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

1240-427: 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

1302-518: Is made necessary by the nature of medical isotopes; many have short half-lives , and must be used within a few days of production. With treatments being constantly carried out around the globe, an uninterruptible supply was essential. There had been some local opposition to the use of highly enriched uranium ( HEU ) in the reactor, as well as from activists in the United States who fear that the uranium could be stolen by terrorists and used to fabricate

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1364-513: 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

1426-424: 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

1488-413: 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

1550-412: 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

1612-423: 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

1674-554: The NRX reactor in 1947, AECL 's Chalk River Laboratories possessed the world's most powerful research reactor. While the large neutron fluxes available in the reactor led to advances in such fields as condensed matter physics and neutron spectroscopy, many experiments were carried out involving the production of new isotopes . The field of nuclear medicine developed when it was realized that some of these artificially created isotopes could be used to diagnose and treat many diseases, especially cancers. Pioneering medical work done in

1736-494: 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

1798-528: 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

1860-448: 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

1922-530: The commissioning and start-up of the reactors". In this statement, AECL indicated that they would move to further extend the licence of the operating NRU reactor to continue the production of medical isotopes. The statement left unclear what long-term direction AECL would take for its medical isotope production business. 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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1984-486: The cost of the entire unit used to perform the first cobalt-60 treatment was about $ 50,000. By way of contrast, it would cost $ 50,000,000 just to produce enough radium (which had been previously used as a therapy source) to perform the same procedure. With this promising start, AECL came to be a major world supplier of medical isotopes, using both the NRX reactor, and the NRU reactor, which came on-line in 1957. However, as these reactors began to age, it became clear that

2046-418: 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

2108-414: 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

2170-484: 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

2232-743: 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

2294-424: 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

2356-529: The late 1940s and early 1950s established cobalt-60 as a useful isotope, as the relatively high-energy gamma rays produced when it undergoes beta decay are able to penetrate the skin of the patient, and deliver a greater portion of the dose directly to the tumor. The high neutron efficiency of the NRX's heavy water -moderated design, coupled with the high neutron flux of the reactor, made it relatively inexpensive for AECL to produce medical-grade cobalt-60. For example,

2418-460: The late 1980s. As part of a restructuring taking place around the same time, the medical isotopes side of AECL was reorganized as Nordion in 1988. Work on the X10 project essentially ended at this point. Nordion company was purchased by MDS in 1991, and an agreement was reached between AECL and MDS Nordion that a new facility dedicated to the production of medical isotopes would be needed. A formal agreement

2480-408: 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

2542-446: 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

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2604-584: 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)

2666-482: 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

2728-399: 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

2790-415: The reactor in a settlement. The MAPLE facility was granted an extension on its operating license on 25 October 2007, which would permit operations until 31 October 2011. This (final) submission envisioned that the MAPLE I reactor would be operational in late 2008. On 16 May 2008, AECL released a statement announcing that the MAPLE program had been terminated, as it had become "no longer feasible to complete

2852-451: The reactors was markedly slowed. During the subsequent eight-year-long delay in the start of commercial production, the project significantly overran its budgeted cost. The original budget was $ 140 million, but by 2005 it had already cost $ 300 million. Disputes over responsibility for the overruns between AECL and MDS Nordion added a further layer of complexity to the process. After considerable negotiation, AECL assumed full responsibility for

2914-426: 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

2976-458: 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

3038-474: 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

3100-601: 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

3162-421: 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,

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3224-473: 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

3286-499: 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} }} ,

3348-421: 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

3410-421: 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

3472-403: 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

3534-420: Was in disagreement with the prediction of the modelling, and was a significant barrier to commissioning. A positive power co-efficient means that the reactor becomes more reactive when it heats up; in the case of an unplanned power spike, such a design can "run away" and potentially cause a meltdown . Consequently, significant efforts were made to resolve the outstanding issues, but progress towards commissioning

3596-513: 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

3658-424: Was noted that some of the emergency shut-off rods in the MAPLE I reactor could fail to deploy in certain demanding situations. This failure was ascribed to workmanship and design issues, and related to fine metal particles accumulating in the control rods' housing and interfering with their free movement. In addition, later testing found that the reactors have a positive power co-efficient of reactivity (PCR), which

3720-442: Was signed to begin the project in August 1996. Following a year-long environmental assessment, construction began in December 1997. The design that resulted involved a facility with two identical reactors, each capable of supplying 100% of the world's medical isotope demand. The second reactor would function primarily as a backup, to ensure that the supply of isotopes would not be interrupted by maintenance or unplanned shutdowns. This

3782-514: 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

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3844-465: 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

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