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45-492: Because no new nuclear weapons have been developed by the United States since 1992, even its youngest weapons are at least 31 years old (as of 2024). Aging weapons can fail or act unpredictably in a number of ways: the high explosives that compress their fissile material can chemically degrade, their electronic components can suffer from decay, their radioactive plutonium / uranium cores are potentially unreliable, and

90-425: A Fissile Material Cutoff Treaty , the term fissile is often used to describe materials that can be used in the fission primary of a nuclear weapon. These are materials that sustain an explosive fast neutron nuclear fission chain reaction . Under all definitions above, uranium-238 ( U ) is fissionable, but not fissile. Neutrons produced by fission of U have lower energies than

135-599: A well-known curve in nuclear physics of atomic number vs. atomic mass number are more stable than others; hence, they are less likely to undergo fission. They are more likely to "ignore" the neutron and let it go on its way, or else to absorb the neutron but without gaining enough energy from the process to deform the nucleus enough for it to fission. These "even-even" isotopes are also less likely to undergo spontaneous fission , and they also have relatively much longer partial half-lives for alpha or beta decay. Examples of these isotopes are uranium-238 and thorium-232 . On

180-464: A chain reaction. U can be used as a source material for creating plutonium-239, which can in turn be used as nuclear fuel. Breeder reactors carry out such a process of transmutation to convert the fertile isotope U into fissile Pu. It has been estimated that there is anywhere from 10,000 to five billion years worth of U for use in these power plants . Breeder technology has been used in several experimental nuclear reactors. By December 2005,

225-454: A closed system an equilibrium would be reached, with all amounts except for lead-206 and U in fixed ratios, in slowly decreasing amounts. The amount of Pb will increase accordingly while that of U decreases; all steps in the decay chain have this same rate of 3 × 10 decayed particles per second per mole U. Thorium-234 has a mean lifetime of 3 × 10 seconds, so there is equilibrium if one mole of U contains 9 × 10 atoms of thorium-234, which

270-437: A completion date set for 2047. Both China and India have announced plans to build nuclear breeder reactors. The breeder reactor as its name implies creates even larger quantities of Pu or U than the fission nuclear reactor. The Clean And Environmentally Safe Advanced Reactor (CAESAR), a nuclear reactor concept that would use steam as a moderator to control delayed neutrons , will potentially be able to use U as fuel once

315-413: A few exceptions. This rule holds for all but fourteen nuclides – seven that satisfy the criterion but are nonfissile, and seven that are fissile but do not satisfy the criterion. To be a useful fuel for nuclear fission chain reactions, the material must: Fissile nuclides in nuclear fuels include: Fissile nuclides do not have a 100% chance of undergoing fission on absorption of a neutron. The chance

360-454: A fissile core works to reflect neutrons and to add inertia to the compression of the Pu charge. As such, it increases the efficiency of the weapon and reduces the critical mass required. In the case of a thermonuclear weapon , U can be used to encase the fusion fuel, the high flux of very energetic neutrons from the resulting fusion reaction causes U nuclei to split and adds more energy to

405-407: A fission nuclear reactor , uranium-238 can be used to generate plutonium-239 , which itself can be used in a nuclear weapon or as a nuclear-reactor fuel supply. In a typical nuclear reactor, up to one-third of the generated power comes from the fission of Pu, which is not supplied as a fuel to the reactor, but rather, produced from U. A certain amount of production of Pu from U

450-424: A half-life of hundreds of millennia, and this isotope does not reach an equilibrium concentration for a very long time. When the two first isotopes in the decay chain reach their relatively small equilibrium concentrations, a sample of initially pure U will emit three times the radiation due to U itself, and most of this radiation is beta particles. As already touched upon above, when starting with pure U, within

495-415: A high probability after capturing a low-energy thermal neutron is referred to as fissile . Fissionable materials include those (such as uranium-238 ) for which fission can be induced only by high-energy neutrons. As a result, fissile materials (such as uranium-235 ) are a subset of fissionable materials. Uranium-235 fissions with low-energy thermal neutrons because the binding energy resulting from

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540-533: A human timescale the equilibrium applies for the first three steps in the decay chain only. Thus, for one mole of U, 3 × 10 times per second one alpha and two beta particles and a gamma ray are produced, together 6.7 MeV, a rate of 3 μW. U atom is itself a gamma emitter at 49.55 keV with probability 0.084%, but that is a very weak gamma line, so activity is measured through its daughter nuclides in its decay series. U abundance and its decay to daughter isotopes comprises multiple uranium dating techniques and

585-448: A material for dry cask storage systems to store radioactive waste . The opposite of enriching is downblending . Surplus highly enriched uranium can be downblended with depleted uranium or natural uranium to turn it into low-enriched uranium suitable for use in commercial nuclear fuel. U from depleted uranium and natural uranium is also used with recycled Pu from nuclear weapons stockpiles for making mixed oxide fuel (MOX), which

630-450: A nuclear chain reaction in the correct setting. Under this definition, the only nuclides that are fissionable but not fissile are those nuclides that can be made to undergo nuclear fission but produce insufficient neutrons, in either energy or number, to sustain a nuclear chain reaction . As such, while all fissile isotopes are fissionable, not all fissionable isotopes are fissile. In the arms control context, particularly in proposals for

675-475: A system may be typified by either slow neutrons (i.e., a thermal system) or fast neutrons. Fissile material can be used to fuel thermal-neutron reactors , fast-neutron reactors and nuclear explosives . The term fissile is distinct from fissionable . A nuclide that can undergo nuclear fission (even with a low probability) after capturing a neutron of high or low energy is referred to as fissionable . A fissionable nuclide that can undergo fission with

720-431: Is 1.5 × 10 mole (the ratio of the two half-lives). Similarly, in an equilibrium in a closed system the amount of each decay product, except the end product lead, is proportional to its half-life. While U is minimally radioactive, its decay products, thorium-234 and protactinium-234, are beta particle emitters with half-lives of about 20 days and one minute respectively. Protactinium-234 decays to uranium-234, which has

765-617: Is an integral part of lead–lead dating , which is most famous for the determination of the age of the Earth . The Voyager program spacecraft carry small amounts of initially pure U on the covers of their golden records to facilitate dating in the same manner. Uranium emits alpha particles through the process of alpha decay . External exposure has limited effect. Significant internal exposure to tiny particles of uranium or its decay products, such as thorium-230, radium-226 and radon-222 , can cause severe health effects, such as cancer of

810-409: Is dependent on the nuclide as well as neutron energy. For low and medium-energy neutrons, the neutron capture cross sections for fission (σ F ), the cross section for neutron capture with emission of a gamma ray (σ γ ), and the percentage of non-fissions are in the table at right. Fertile nuclides in nuclear fuels include: Uranium-238 Uranium-238 ( U or U-238 )

855-418: Is not as effective as ordinary water for stopping fast neutrons . Both metallic depleted uranium and depleted uranium dioxide are used for radiation shielding. Uranium is about five times better as a gamma ray shield than lead , so a shield with the same effectiveness can be packed into a thinner layer. DUCRETE , a concrete made with uranium dioxide aggregate instead of gravel, is being investigated as

900-420: Is now being redirected to become fuel for nuclear reactors. This dilution, also called downblending, means that any nation or group that acquired the finished fuel would have to repeat the very expensive and complex chemical separation of uranium and plutonium process before assembling a weapon. Most modern nuclear weapons utilize U as a "tamper" material (see nuclear weapon design ). A tamper which surrounds

945-454: Is one of the most common radioactive isotopes used in radiometric dating . The most common dating method is uranium-lead dating , which is used to date rocks older than 1 million years old and has provided ages for the oldest rocks on Earth at 4.4 billion years old. The relation between U and U gives an indication of the age of sediments and seawater that are between 100,000 years and 1,200,000 years in age. The U daughter product, Pb,

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990-550: Is supported by the following experimental facilities: The data produced by the experiments carried out in these facilities is used in combination with the Advanced Simulation and Computing Program . Fissile In nuclear engineering , fissile material is material that can undergo nuclear fission when struck by a neutron of low energy. A self-sustaining thermal chain reaction can only be achieved with fissile material. The predominant neutron energy in

1035-437: Is the most common isotope of uranium found in nature, with a relative abundance of 99%. Unlike uranium-235 , it is non-fissile, which means it cannot sustain a chain reaction in a thermal-neutron reactor . However, it is fissionable by fast neutrons , and is fertile , meaning it can be transmuted to fissile plutonium-239 . U cannot support a chain reaction because inelastic scattering reduces neutron energy below

1080-514: Is unavoidable wherever it is exposed to neutron radiation . Depending on burnup and neutron temperature , different shares of the Pu are converted to Pu , which determines the "grade" of produced plutonium, ranging from weapons grade , through reactor grade , to plutonium so high in Pu that it cannot be used in current reactors operating with a thermal neutron spectrum. The latter usually involves used "recycled" MOX fuel which entered

1125-531: Is what is meant when various agencies refer to their work as "science-based"). It also involves the manufacture of additional plutonium " pits " to replace ones of unknown quality, and finding other methods to increase the lifespan of existing warheads and maintain a credible nuclear deterrent . Most work for stockpile stewardship is undertaken at United States Department of Energy national laboratories , mostly at Los Alamos National Laboratory , Sandia National Laboratories , Lawrence Livermore National Laboratory ,

1170-737: The National Ignition Facility (NIF). Such facilities have been deemed necessary under the program since President Bill Clinton signed the Comprehensive Test Ban Treaty (CTBT) in 1996, The US Senate never ratified the CTBT. President Obama initiated a broad effort to modernize U.S. nuclear forces, which the Congressional Budget Office estimates will require approximately $ 494 billion to complete. The stockpile stewardship program

1215-604: The Nevada Test Site , and Department of Energy productions facilities, which employ around 27,500 personnel and cost billions of dollars per year to operate. The Stockpile Stewardship and Management Program is a United States Department of Energy program to ensure that the nuclear capabilities of the United States are not eroded as nuclear weapons age. It costs more than $ 4 billion annually to test nuclear weapons and build advanced science facilities, such as

1260-428: The U isotope, and even low-enriched uranium (LEU), while having a higher proportion of the uranium-235 isotope (in comparison to depleted uranium), is still mostly U. Reprocessed uranium is also mainly U, with about as much uranium-235 as natural uranium, a comparable proportion of uranium-236 , and much smaller amounts of other isotopes of uranium such as uranium-234 , uranium-233 , and uranium-232 . In

1305-422: The decay products are present, at least transiently, in any uranium-containing sample, whether metal, compound, or mineral. The decay proceeds as: The mean lifetime of U is 1.41 × 10 seconds divided by ln(2)  ≈ 0.693 (or multiplied by 1/ln(2) ≈  1.443), i.e. ca. 2 × 10 seconds, so 1 mole of U emits 3 × 10 alpha particles per second, producing the same number of thorium-234 atoms . In

1350-537: The yield and to fallout of such weapons. Fast fission of U tampers has also been evident in pure fission weapons. The fast fission of U also makes a significant contribution to the power output of some fast-neutron reactors . No fission products have a half-life in the range of 100 a–210 ka ... ... nor beyond 15.7 Ma In general, most actinide isotopes with an odd neutron number are fissile. Most nuclear fuels have an odd atomic mass number ( A = Z + N =

1395-401: The "yield" of the weapon. Such weapons are referred to as fission-fusion-fission weapons after the order in which each reaction takes place. An example of such a weapon is Castle Bravo . The larger portion of the total explosive yield in this design comes from the final fission stage fueled by U, producing enormous amounts of radioactive fission products . For example, an estimated 77% of

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1440-574: The 10.4- megaton yield of the Ivy Mike thermonuclear test in 1952 came from fast fission of the depleted uranium tamper . Because depleted uranium has no critical mass, it can be added to thermonuclear bombs in almost unlimited quantity. The Soviet Union 's test of the Tsar Bomba in 1961 produced "only" 50 megatons of explosive power, over 90% of which came from fusion because the U final stage had been replaced with lead. Had U been used instead,

1485-578: The absorption of a neutron is greater than the critical energy required for fission; therefore uranium-235 is fissile. By contrast, the binding energy released by uranium-238 absorbing a thermal neutron is less than the critical energy, so the neutron must possess additional energy for fission to be possible. Consequently, uranium-238 is fissionable but not fissile. An alternative definition defines fissile nuclides as those nuclides that can be made to undergo nuclear fission (i.e., are fissionable) and also produce neutrons from such fission that can sustain

1530-451: The isotopes used by thermonuclear weapons may be chemically unstable as well. Since the United States has also not tested nuclear weapons since 1992, this leaves the task of its stockpile maintenance resting on the use of simulations (using non-nuclear explosives tests and supercomputers , among other methods) and applications of scientific knowledge about physics and chemistry to the specific problems of weapons aging (the latter method

1575-763: The only breeder reactor producing power was the 600-megawatt BN-600 reactor at the Beloyarsk Nuclear Power Station in Russia. Russia later built another unit, BN-800 , at the Beloyarsk Nuclear Power Station which became fully operational in November 2016. Also, Japan's Monju breeder reactor, which has been inoperative for most of the time since it was originally built in 1986, was ordered for decommissioning in 2016, after safety and design hazards were uncovered, with

1620-474: The original neutron (they behave as in an inelastic scattering ), usually below 1  MeV (i.e., a speed of about 14,000  km/s ), the fission threshold to cause subsequent fission of U , so fission of U does not sustain a nuclear chain reaction . Fast fission of U in the secondary stage of a thermonuclear weapon, due to the production of high-energy neutrons from nuclear fusion , contributes greatly to

1665-477: The other hand, other than the lightest nuclides, nuclides with an odd number of protons and an odd number of neutrons (odd Z , odd N ) are usually short-lived (a notable exception is neptunium-236 with a half-life of 154,000 years) because they readily decay by beta-particle emission to their isobars with an even number of protons and an even number of neutrons (even Z , even N ) becoming much more stable. The physical basis for this phenomenon also comes from

1710-495: The pairing effect in nuclear binding energy, but this time from both proton–proton and neutron–neutron pairing. The relatively short half-life of such odd-odd heavy isotopes means that they are not available in quantity and are highly radioactive. According to the fissility rule proposed by Yigal Ronen, for a heavy element with Z between 90 and 100, an isotope is fissile if and only if 2 × Z − N ∈ {41, 43, 45 } (where N = number of neutrons and Z = number of protons ), with

1755-433: The purpose of electricity production without necessitating the development of fuel enrichment capabilities, which are often seen as a prelude to weapons production . U is also used as a radiation shield – its alpha radiation is easily stopped by the non- radioactive casing of the shielding and the uranium's high atomic weight and high number of electrons are highly effective in absorbing gamma rays and X-rays . It

1800-454: The radioactive heat produced within the Earth. The U decay chain contributes six electron anti-neutrinos per U nucleus (one per beta decay ), resulting in a large detectable geoneutrino signal when decays occur within the Earth. The decay of U to daughter isotopes is extensively used in radiometric dating , particularly for material older than approximately 1 million years. Depleted uranium has an even higher concentration of

1845-528: The range where fast fission of one or more next-generation nuclei is probable. Doppler broadening of U's neutron absorption resonances , increasing absorption as fuel temperature increases, is also an essential negative feedback mechanism for reactor control. Around 99.284% of natural uranium 's mass is uranium-238, which has a half-life of 1.41 × 10 seconds (4.468 × 10 years, or 4.468 billion years). Due to its natural abundance and half-life relative to other radioactive elements , U produces ~40% of

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1890-428: The reactor containing significant amounts of plutonium . U can produce energy via "fast" fission . In this process, a neutron that has a kinetic energy in excess of 1  MeV can cause the nucleus of U to split. Depending on design, this process can contribute some one to ten percent of all fission reactions in a reactor, but too few of the average 2.5 neutrons produced in each fission have enough speed to continue

1935-405: The reactor is started with Low-enriched uranium (LEU) fuel. This design is still in the early stages of development. Natural uranium, with 0.711% U , is usable as nuclear fuel in reactors designed specifically to make use of naturally occurring uranium, such as CANDU reactors . By making use of non-enriched uranium, such reactor designs give a nation access to nuclear power for

1980-560: The total number of nucleons ), and an even atomic number Z . This implies an odd number of neutrons. Isotopes with an odd number of neutrons gain an extra 1 to 2 MeV of energy from absorbing an extra neutron, from the pairing effect which favors even numbers of both neutrons and protons. This energy is enough to supply the needed extra energy for fission by slower neutrons, which is important for making fissionable isotopes also fissile. More generally, nuclides with an even number of protons and an even number of neutrons, and located near

2025-558: The yield of the Tsar Bomba could have been well above 100 megatons, and it would have produced nuclear fallout equivalent to one third of the global total that had been produced up to that time. The decay chain of U is commonly called the " radium series " (sometimes "uranium series"). Beginning with naturally occurring uranium-238, this series includes the following elements: astatine , bismuth , lead , polonium , protactinium , radium , radon , thallium , and thorium . All of

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