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27-543: Diorit was an experimental nuclear research reactor at the Swiss Federal Institute for Reactor Research (EIR) in Würenlingen ( Canton of Aargau , Switzerland ) This nuclear reactor was operated by EIR from 1960 to 1977. Heavy water (D 2 O) was used as the moderator , as well as the coolant. The initially commissioned reactor had a thermal power of 20 MW. Natural uranium was initially used as fuel, which

54-409: A moderator is required to slow the neutron velocities and enhance fission. As neutron production is their main function, most research reactors benefit from reflectors to reduce neutron loss from the core. The International Atomic Energy Agency and the U.S. Department of Energy initiated a program in 1978 to develop the means to convert research reactors from using highly enriched uranium (HEU) to

81-583: A few companies that concentrate the key projects on a worldwide basis. The most recent international tender (1999) for a research reactor was that organized by the Australian Nuclear Science and Technology Organisation for the design, construction and commissioning of the Open-pool Australian lightwater reactor (OPAL). Four companies were prequalified: Atomic Energy of Canada Limited (AECL), INVAP , Siemens and Technicatom . The project

108-419: A high rate of spontaneous fission , which can cause a nuclear weapon to pre-detonate. This makes plutonium unsuitable for use in gun-type nuclear weapons . To reduce the concentration of Pu-240 in the plutonium produced, weapons program plutonium production reactors (e.g. B Reactor ) irradiate the uranium for a far shorter time than is normal for a nuclear power reactor . More precisely, weapons-grade plutonium

135-432: A neutron is absorbed by U-238, forming U-239, which then decays in a rapid two-step process into Pu-239. It can then be separated from the uranium in a nuclear reprocessing plant. Weapons-grade plutonium is defined as being predominantly Pu-239 , typically about 93% Pu-239. Pu-240 is produced when Pu-239 absorbs an additional neutron and fails to fission. Pu-240 and Pu-239 are not separated by reprocessing. Pu-240 has

162-561: A research reactor are used for neutron scattering , non-destructive testing, analysis and testing of materials , production of radioisotopes , research and public outreach and education. Research reactors that produce radioisotopes for medical or industrial use are sometimes called isotope reactors . Reactors that are optimised for beamline experiments nowadays compete with spallation sources . Research reactors are simpler than power reactors and operate at lower temperatures. They need far less fuel, and far less fission products build up as

189-416: Is fissile U-235, with the rest being almost entirely uranium-238 (U-238). They are separated by their differing masses . Highly enriched uranium is considered weapons-grade when it has been enriched to about 90% U-235. U-233 is produced from thorium-232 by neutron capture . The U-233 produced thus does not require enrichment and can be relatively easily chemically separated from residual Th-232. It

216-451: Is any fissionable nuclear material that is pure enough to make a nuclear weapon and has properties that make it particularly suitable for nuclear weapons use. Plutonium and uranium in grades normally used in nuclear weapons are the most common examples. (These nuclear materials have other categorizations based on their purity.) Only fissile isotopes of certain elements have the potential for use in nuclear weapons. For such use,

243-423: Is not clear that this has ever been implemented. The latter substances are part of the minor actinides in spent nuclear fuel . Any weapons-grade nuclear material must have a critical mass that is small enough to justify its use in a weapon. The critical mass for any material is the smallest amount needed for a sustained nuclear chain reaction. Moreover, different isotopes have different critical masses, and

270-545: Is not possible with the light water reactors most commonly used to produce electric power. In these the reactor must be shut down and the pressure vessel disassembled to gain access to the irradiated fuel. Plutonium recovered from LWR spent fuel, while not weapons grade, can be used to produce nuclear weapons at all levels of sophistication, though in simple designs it may produce only a fizzle yield. Weapons made with reactor-grade plutonium would require special cooling to keep them in storage and ready for use. A 1962 test at

297-477: Is obtained from uranium irradiated to a low burnup . This represents a fundamental difference between these two types of reactor. In a nuclear power station, high burnup is desirable. Power stations such as the obsolete British Magnox and French UNGG reactors, which were designed to produce either electricity or weapons material, were operated at low power levels with frequent fuel changes using online refuelling to produce weapons-grade plutonium. Such operation

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324-554: Is therefore regulated as a special nuclear material only by the total amount present. U-233 may be intentionally down-blended with U-238 to remove proliferation concerns. While U-233 would thus seem ideal for weaponization, a significant obstacle to that goal is the co-production of trace amounts of uranium-232 due to side-reactions. U-232 hazards, a result of its highly radioactive decay products such as thallium-208 , are significant even at 5 parts per million . Implosion nuclear weapons require U-232 levels below 50 PPM (above which

351-475: The Incident of 1967, the decision was made to swap out the reactor vessel . The conversion also served to transition from natural uranium fuel to enriched uranium oxide . Some workers were exposed to raised radiation levels during construction. Single doses were up to 1020 mrem , while the highest accumulated dose was 2600 mrem (26 mSv ). Operation of Diorit ceased in 1977. First plans for dismantling were made in

378-455: The U-233 is considered "low grade"; cf. "Standard weapon grade plutonium requires a Pu-240 content of no more than 6.5%." which is 65,000 PPM, and the analogous Pu-238 was produced in levels of 0.5% (5000 PPM) or less). Gun-type fission weapons would require low U-232 levels and low levels of light impurities on the order of 1 PPM. Pu-239 is produced artificially in nuclear reactors when

405-588: The U.S. Nevada National Security Site (then known as the Nevada Proving Grounds) used non-weapons-grade plutonium produced in a Magnox reactor in the United Kingdom. The plutonium used was provided to the United States under the 1958 US–UK Mutual Defence Agreement . Its isotopic composition has not been disclosed, other than the description reactor grade , and it has not been disclosed which definition

432-434: The concentration of fissile isotopes uranium-235 and plutonium-239 in the element used must be sufficiently high. Uranium from natural sources is enriched by isotope separation , and plutonium is produced in a suitable nuclear reactor . Experiments have been conducted with uranium-233 (the fissile material at the heart of the thorium fuel cycle ). Neptunium-237 and some isotopes of americium might be usable, but it

459-448: The critical mass for many radioactive isotopes is infinite, because the mode of decay of one atom cannot induce similar decay of more than one neighboring atom. For example, the critical mass of uranium-238 is infinite, while the critical masses of uranium-233 and uranium-235 are finite. The critical mass for any isotope is influenced by any impurities and the physical shape of the material. The shape with minimal critical mass and

486-497: The earliest. In part this is because the development of reliable LEU fuel for high neutron flux research reactors, that does not fail through swelling, has been slower than expected. As of 2020 , 72 HEU research reactors remain. While in the 1950s, 1960s and 1970s there were a number of companies that specialized in the design and construction of research reactors, the activity of this market cooled down afterwards, and many companies withdrew. The market has consolidated today into

513-461: The early 80s, while the actual decision was made in 1994. During decommissioning of the reactor the following amounts of radioactive waste accrued: 250 t steel, 120 t concrete, 5.4 t aluminium and alloys, as well as 45 t graphite. The burnt out fuel rods were stored in a CASTOR 1c Diorit dry cask storage container until they were transported to the central interim storage site (ZZL) of Zwilag . Research reactor The neutrons produced by

540-469: The fuel is used. On the other hand, their fuel requires more highly enriched uranium , typically up to 20% U-235 , although some use 93% U-235; while 20% enrichment is not generally considered usable in nuclear weapons, 93% is commonly referred to as " weapons-grade ". They also have a very high power density in the core, which requires special design features. Like power reactors, the core needs cooling, typically natural or forced convection with water, and

567-453: The procurement of nuclear weapons, even for a state whose army limits itself to defence only. " In 1967 the reactor produced a partially melted fuel rod , which contaminated the reactor hall. Considerably raised levels of radioactivity were also measured in the River Aare . Subsequently, the complete primary heavy water cooling system had to be decontaminated by chemical milling . Following

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594-425: The smallest physical dimensions is a sphere. Bare-sphere critical masses at normal density of some actinides are listed in the accompanying table. Most information on bare sphere masses is classified, but some documents have been declassified. At least ten countries have produced weapons-grade nuclear material: Natural uranium is made weapons-grade through isotopic enrichment . Initially only about 0.7% of it

621-522: The use of low enriched uranium (LEU), in support of its nonproliferation policy. By that time, the U.S. had supplied research reactors and highly enriched uranium to 41 countries as part of its Atoms for Peace program. In 2004, the U.S. Department of Energy extended its Foreign Research Reactor Spent Nuclear Fuel Acceptance program until 2019. As of 2016, a National Academies of Sciences, Engineering, and Medicine report concluded converting all research reactors to LEU cannot be completed until 2035 at

648-537: Was awarded to INVAP that built the reactor. In recent years, AECL withdrew from this market, and Siemens and Technicatom activities were merged into Areva . A complete list can be found at the List of nuclear research reactors . Research centers that operate a reactor: Decommissioned research reactors: Weapons grade No fission products have a half-life in the range of 100 a–210 ka ... ... nor beyond 15.7 Ma Weapons-grade nuclear material

675-405: Was later changed to enriched uranium . The 2 m (6.6 ft) long, aluminium and nickel cased fuel rods were produced by AMF Atomics Canada Ltd . Heavy water ( deuterium ) has a particularly good neutron economy , which in turn makes for good production of high-quality weapons-grade plutonium . However, Diorit was only ever used for civil research purposes and weapons-grade plutonium

702-642: Was never extracted from it. This didn't stop the Swiss Army from thinking of itself as an emerging nuclear power during the Cold War , as historian Jürg Stüssi-Lauterburg found in a study about previously-classified meeting notes. The army's aspirations for nuclear armament during some phases of the Cold War were at least somewhat public. For example, a major from Solothurn wrote the following in an anniversary publication, in 1957: " ... military considerations therefore compel

729-486: Was used in describing the material this way. The plutonium was apparently sourced from the Magnox reactors at Calder Hall or Chapelcross. The content of Pu-239 in material used for the 1962 test was not disclosed, but has been inferred to have been at least 85%, much higher than typical spent fuel from currently operating reactors. Occasionally, low-burnup spent fuel has been produced by a commercial LWR when an incident such as

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