Enriched uranium

• 1. Introduction

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  Enriched uranium starts with a lopsided count. Of every thousand uranium atoms pulled from the ground, only about seven are uranium-235. That rare atom is the only primordial nuclide found in nature in appreciable quantity that fissions readily with slow, thermal neutrons. Nearly everything else in the mix is uranium-238, along with a trace of uranium-234 amounting to roughly five parts in a hundred thousand.

  Seven in a thousand is the share that enrichment sets out to raise, and the result feeds two very different worlds: civil nuclear power and nuclear weapons. The questions are practical ones. How do engineers sort atoms that behave almost identically in chemistry? How pure must the result be for a power station, a naval reactor or a bomb? And what happens to the uranium that is left behind?

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• 2. From Ore To Gas

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  Ore from a uranium mine may hold as little as 0.1 percent uranium, so the first task is concentration. Miners reach it underground or in an open pit, depending on how deep it lies. A milling stage then uses chemical processes to pull the metal out of the rock. The product is a concentrated uranium oxide called yellowcake, which is roughly 80 percent uranium.

  Yellowcake still cannot go straight into most reactors, so a conversion step turns it into one of two forms. Uranium dioxide fuels the reactor types that need no enrichment. Uranium hexafluoride, which can be enriched, serves the majority of reactor types. A few designs, among them the CANDU and the RBMK, manage on natural uranium because they use a graphite or heavy water moderator.

  Spent fuel from light-water reactors offers a second supply, known as reprocessed uranium, or RepU. It typically holds slightly more uranium-235 than the natural metal, enough that CANDU reactors could burn it. It also carries uranium-236, which swallows neutrons and therefore demands higher enrichment. Uranium-236 also breeds neptunium-237, one of the more mobile and troublesome radionuclides in a deep geological repository. Traces of other transuranic elements and fission products call for careful monitoring during fuel fabrication and reactor operation.

  The hexafluoride form carries a nickname among engineers, who call it hex for short. Both commercial enrichment processes begin with it.

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• 3. Rungs Of Enrichment

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  A range of 3.5 to 4.5 percent uranium-235 is what most reactors need, and the lower rungs of enrichment are built around it. Low-enriched uranium, or LEU, means anything below 20 percent. Light-water reactors, the most common power reactors in the world, run on 3 to 5 percent. Slightly enriched uranium sits lower still, under 2 percent.

  High-assay LEU, called HALEU, covers 5 to 20 percent and is called for by many small modular reactor designs. Fresh LEU for research reactors usually runs from 12 to 19.75 percent. That top figure is the one used to replace HEU fuels when a research reactor is converted.

  At 20 percent and above, uranium counts as highly enriched, or HEU. Nuclear weapons use it, as do naval propulsion reactors, some research reactors and certain specialized reactor designs. About 2,000 tonnes of it exist in the world.

  Fast neutron reactors need cores with about 20 percent or more fissile material. The Fermi-1 commercial fast reactor prototype ran on HEU at 26.5 percent. Naval reactors often hold at least 50 percent uranium-235, though they typically stay at or below 90 percent.

  Molybdenum-99, the parent of the technetium-99m generators used in medicine, is made with significant quantities of HEU. At the far end of the scale, physicists have taken uranium past 97 percent enrichment for criticality experiments.

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• 4. Critical Mass

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  Little Boy, the bomb dropped on Hiroshima in 1945, held 64 kg of uranium enriched to 80 percent. Its core sat inside a neutron reflector, so at the moment of explosion it amounted to almost 2.5 critical masses. Every nuclear explosive uses a reflector, and it cuts the required mass dramatically.

  Weapons-grade uranium means 85 percent or more uranium-235. At that level the critical mass is about 50 kg, which at normal density forms a sphere roughly 17 cm across. Implosion, fusion boosting and tamping, which uses inertia to slow the expansion of the fissioning core, push designs below one bare-sphere critical mass.

  China's first nuclear test, Project 596, used a highly enriched uranium core of about 15 kg. That is a sphere only 11.4 cm in diameter.

  Dilution makes everything harder, because uranium-238 inhibits the runaway chain reaction. In theory an implosion design could work at just 20 percent, a level called weapon-usable. It would need hundreds of kilograms of material, which makes it impractical. Lower the percentage further and the critical mass for unmoderated fast neutrons climbs rapidly. At 5.4 percent it would take an infinite mass.

  Later American weapons usually put plutonium-239 in the primary stage. The secondary often uses HEU of 40 to 80 percent, together with the fusion fuel lithium deuteride. In a large weapon, the higher critical mass of less-enriched uranium is actually an advantage, because the core can hold more fuel when it explodes.

  Uranium-238 is not fissile, yet fast neutrons above 2 MeV, such as those released by D-T fusion, can still split it.

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• 5. Sorting Near Twins

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  Uranium-235 is only about 1.26 percent lighter than uranium-238, and that sliver of mass is all that separates the two. Both isotopes belong to one element, so their chemistry is nearly identical. Engineers work with uranium hexafluoride instead of atomic uranium, which shrinks the relative mass difference between molecules even further. They accept that cost for two reasons. Fluorine has only one naturally occurring isotope, and the compound can be handled as a gas at suitable temperatures.

  A single stage of separation changes the mix only slightly, so plants chain many stages into a cascade. Each stage passes a slightly enriched stream forward and sends a slightly depleted stream back to the stage before it.

  The German word Urantrennarbeit, meaning uranium separation work, gives the industry one of its measures. Separative work describes how much separation a process performs, and it depends on the concentrations of the feed, the product and the depleted tails. It is not energy. The same amount of separative work takes different amounts of energy depending on the technology. It is counted in separative work units, abbreviated SWU, and also in kg SW or kg UTA.

  To make one kilogram of LEU at 3.6 percent, a plant might leave 0.3 percent uranium-235 in the tails. That needs approximately 8 kilograms of natural uranium and 4.5 SWU. Cut the tails to 0.2 percent and the figures become 6.7 kilograms of natural uranium but nearly 5.7 SWU. The two costs move in opposite directions, so operators lean on whichever input is cheaper. When natural uranium is cheap, they leave more uranium-235 in the tails. Converting hexafluoride to metal loses a further 0.3 percent.

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• 6. Machines Left Behind

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  The Manhattan Project built the first large-scale enrichment plants, using gaseous diffusion and electromagnetic isotope separation. The electromagnetic machine, the calutron, was a production-scale mass spectrometer. Metallic uranium was vaporized and ionized, then accelerated and bent by magnetic fields onto collection targets. A true calutron is a multistage device arranged in a large oval around a powerful electromagnet. It supplied some of the uranium-235 for Little Boy.

  At the S-50 plant in Oak Ridge, Tennessee, wartime engineers tried heat. Liquid uranium hexafluoride was placed between two concentric vertical pipes, with the inner pipe heated and the outer one cooled. Lighter uranium-235 molecules drifted toward the hot wall and heavier uranium-238 molecules toward the cold one. Convection carried the enriched material upward, and taller columns gave better separation. The plant made feed for the electromagnetic process, then lost out to gaseous diffusion.

  Oak Ridge also gave weapons-grade uranium its codename, oralloy, short for Oak Ridge alloy.

  Gaseous diffusion pushes hexafluoride gas through semi-permeable membranes. Throughout the Cold War it was the dominant technique, and in 2008 it still produced about 33 percent of enriched uranium. By 2011 it was judged obsolete, and the Paducah plant in the United States ceased operating in 2013 as the last commercial diffusion plant in the world.

  Other routes never reached that scale. The aerodynamic Becker jet nozzle, developed by E. W. Becker and colleagues, and the vortex tube both behave like non-rotating centrifuges. The Uranium Enrichment Corporation of South Africa deployed the Helikon cascade for high-volume low enrichment. Its Pelsakon cascade was a semi-batch design for low-volume high enrichment. A consortium led by Industrias Nucleares do Brasil built a demonstration nozzle plant. All of these methods demanded large amounts of energy and removal of waste heat, and none is still in use.

  Chemical separation stopped at the pilot stage. The French CHEMEX process exploited a tiny difference in how the isotopes change valency, using immiscible aqueous and organic phases. The Asahi Chemical Company in Japan developed an ion-exchange variant on a resin column.

  The plasma separation process tuned ion cyclotron resonance, with superconducting magnets, to energize only uranium-235. France's version, RCI, saw its funding drastically cut in 1986 and was suspended around 1990. The technique still finds use for separating stable isotopes.

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• 7. Centrifuges And Lasers

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  A separation factor of 1.3 per stage, against 1.005 for gaseous diffusion, explains why centrifuges won. That gap works out to about one-fiftieth of the energy requirement. The gas centrifuge spins a large number of cylinders in series and parallel. Heavier molecules containing uranium-238 move to the outside wall, while lighter uranium-235 molecules gather nearer the center. Centrifuges now produce close to 100 percent of the world's enriched uranium.

  The World Nuclear Association estimates modern plants need roughly 40-50 kWh per SWU, compared with about 2,400-2,500 kWh for diffusion. Cost follows. A centrifuge SWU runs around 100 dollars, about 40 percent cheaper than the older method. The industry calls this second-generation technology.

  The Zippe-type centrifuge adds heat at the bottom of the rotating cylinder. The resulting convection currents carry uranium-235 upward, where scoops collect it. Urenco uses this design commercially, and Pakistan used it in its nuclear weapons program.

  Laser enrichment is billed as a third generation. Atomic vapor laser isotope separation tunes lasers to ionize uranium-235 atoms and no others. A negatively charged plate then collects the positive ions. Molecular laser isotope separation aims an infrared laser at hexafluoride, exciting molecules that contain uranium-235. A second laser frees a fluorine atom, and the resulting uranium pentafluoride precipitates out.

  SILEX, short for separation of isotopes by laser excitation, is an Australian development that also uses hexafluoride. The American company USEC acquired and then relinquished commercialization rights. In 2006, GE Hitachi Nuclear Energy signed a commercialization agreement with Silex Systems. GE Hitachi built a demonstration test loop and announced plans for a first commercial facility. Details of the process are classified under agreements among the United States, Australia and the companies involved.

  In August 2011, Global Laser Enrichment, a GE Hitachi subsidiary, applied to the Nuclear Regulatory Commission for a permit. In September 2012, the commission issued a license. The company had not yet decided whether the plant would pay for itself. Laser enrichment is projected at approximately 30 dollars per SWU, less than a third of the centrifuge price. More than 20 countries worked on laser separation in the 1990s and 2000s, with very limited success.

  Proliferation worries trail the technology. A laser plant needs less than 25 percent of the space of typical separation plants. It also needs only the energy that would power 12 typical houses, which puts it below the detection threshold of existing surveillance. The American Physical Society petitioned the commission to require a formal proliferation review before any such plant is built. It called the technology a "game changer". Princeton physicist Ryan Snyder provided the foundational proliferation assessment.

  In 2025, Global Laser Enrichment submitted a license application for the proposed Paducah Laser Enrichment Facility, and the commission accepted it for detailed review. Centrifuges remain the only commercial process.

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• 8. Leftovers And Downblending

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  Depleted uranium is the name for the uranium-238 left after enrichment. It is considerably less radioactive than natural uranium but still very dense. Radiation shielding and armor-penetrating weapons are its uses.

  Downblending runs the process in reverse. Surplus HEU is diluted into LEU fit for commercial fuel. The HEU feed can carry unwanted isotopes. Uranium-234 concentrates during enrichment but stays well below 1 percent. Uranium-236 is a reactor byproduct that absorbs neutrons without fissioning, so it acts as a neutron poison. HEU reprocessed from reactors that made weapons material, at about 50 percent uranium-235, may hold as much as 25 percent uranium-236. That leaves about 1.5 percent in the blended LEU, so the uranium-235 level has to be raised to compensate.

  Because of those contaminants, the blendstock is sometimes slightly enriched uranium at typically 1.5 percent rather than natural or depleted uranium. Natural or depleted blendstock could push the product past the ASTM specifications for nuclear fuel. For that reason downblending does little to shrink the stockpiles of depleted uranium.

  The Megatons to Megawatts Program turned ex-Soviet weapons-grade HEU into fuel for American commercial power reactors. From 1995 through mid-2005 it recycled 250 tonnes of HEU, enough for 10,000 warheads. The goal was 500 tonnes by 2013. Russian warhead decommissioning supplied about 13 percent of the world's enriched uranium requirement in the years leading up to 2008.

  In 1996, the American government declared 174.3 tonnes of HEU surplus military material. The United States Enrichment Corporation helped process a portion of it. The U.S. HEU Downblending Program took material mainly from dismantled warheads and turned it into LEU for power plants.

  Fourteen countries are known to operate enrichment facilities: Argentina, Brazil, China, France, Germany, India, Iran, Japan, the Netherlands, North Korea, Pakistan, Russia, the United Kingdom and the United States. Belgium, Iran, Italy and Spain hold investment interests in the French Eurodif plant, and Iran's stake entitles it to 10 percent of the output. Libya and South Africa once had enrichment programs, though Libya's facility never operated. It has also been claimed that Israel runs a program at the Negev Nuclear Research Center near Dimona.

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