Krypton plant

Del y for Dec y. Nuclear power plants generate radioactive xenon and krypton as products of the fission reactions. Although these products ate trapped inside the fuel elements, portions can leak out into the coolant (through fuel cladding defects) and can be released to the atmosphere with other gases through an air ejector at the main condenser. 

The latest of three ethylene recovery plants was started in 1991. Sasol sold almost 300,000 t of ethylene in 1992. Sasol also produces polypropylene at Secunda from propylene produced at Sasol Two. In 1992 Sasol started constmction of a linear alpha olefin plant at Secunda to be completed in 1994 (40). Initial production is expected to be 100,000 t/yr pentene and hexene. Sasol also has a project under constmction to extract and purify krypton and xenon from the air separation plants at Sasol Two. Other potential new products under consideration at Sasol are acrylonitrile, acetic acid, acetates, and alkylamines. 

Krypton and Xenon from Huclear Power Plants. Both xenon and krypton are products of the fission of uranium and plutonium. These gases are present in the spent fuel rods from nuclear power plants in the ratio 1 Kr 4 Xe. Recovered krypton contains ca 6% of the radioactive isotope Kr-85, with a 10.7 year half-life, but all radioactive xenon isotopes have short half-Hves. 

Commercially pure (< 99.997%) helium is shipped directiy from helium-purification plants located near the natural-gas supply to bulk users and secondary distribution points throughout the world. Commercially pure argon is produced at many large air-separation plants and is transported to bulk users up to several hundred kilometers away by tmck, by railcar, and occasionally by dedicated gas pipeline (see Pipelines). Normally, only cmde grades of neon, krypton, and xenon are produced at air-separation plants. These are shipped to a central purification faciUty from which the pure materials, as well as smaller quantities and special grades of helium and argon, are then distributed. Radon is not distributed commercially. 

Adsorption of Radionuclides. Other appHcations that depend on physical adsorption include the control of krypton and xenon radionuchdes from nuclear power plants (92). The gases are not captured entirely, but their passage is delayed long enough to allow radioactive decay of the short-hved species. Highly rnicroporous coconut-based activated carbon is used for this service. 

Delay for Decay A process for trapping radioactive gases (e.g., xenon, krypton, iodine) from nuclear power plants until their radioactivities have decayed to acceptable levels. Activated carbon is the usual adsorbent, and the gases are first dried with a zeolite. 

Air separation industry, U.S., 27 754 Air-separation plants, 27 359, 750-751 Air-separation units, krypton and xenon recovery from, 2 7 362 Air-slaked lime, 15 26 Air slaking, 25 43 Air sparging... 

Kroll process, 13 84-85 15 337 17 140 in titanium manufacture, 24 851-853 Kroll zirconium reduction process, 26 631 KRW gasifier, 6 797-798, 828 Krypton (Kr), 17 344 commercial, 17 368t complex salts of, 17 333-334 doubly ionized, 14 685 hydroquinone clathrate of, 14 183 in light sources, 17 371-372 from nuclear power plants, 17 362 physical properties of, 17 350 Krypton-85, 17 375, 376 Krypton compounds, 17 333-334 Krypton derivatives, 17 334 Krypton difluoride, 17 333, 336 uses for, 17 336... 

ISOTOPES There are a total of 37 isotopes of krypton. Six of these are stable Kr-78, Kr-80, Kr-82, Kr-83, Kr-84, and Kr-86. The isotope Kr-78 has such a long half-life (0.9x1 years) that it is considered stable even though it contributes only 0.35% to the natural krypton in the Earth s atmosphere. All the others are radioactive, man-made by-products of nuclear power plants and radioactive isotopes with half-lives ranging from 107 nanoseconds to 2.29x 10+ years. 

Most krypton produced in commercial scale comes from air. Krypton and other inert gases are obtained from air by a distdlation-hquefaction process. Different types of air-separation plants varying in design are known for commercial production of nitrogen, oxygen, and inert gases (See Hehum). 

Krypton also may be recovered from spent fuel rods of nuclear power plants. It is produced, along with xenon, in fission of uranium and plutonium. This process, however, is not a major source of krypton, and the recovered gas also contains radioactive Kr-85 isotope. 

First, the uranium-235 changes into uranium-236, a very unstable isotope. Then, the uranium-236 undergoes fission, breaking apart and producing two daughter atoms—barium-142 and krypton-91, plus several neutrons. In a nuclear power plant, the heat generated during these reactions is captured and used to produce electricity. 

Argon, neon, krypton, and xenon are all produced commercially as byproducts from large cryogenic air separation plants. The distillation of liquid air is normally performed in the double-column arrangement (Fig. 1). The rare gases are produced in side columns operated in conjunction with the standard double-column plant. 

Krypton and xenon have high boiling points relative to oxygen and tend to accumulate in the liquid oxygen sump of the upper column of the main plant. 

Research on removal of noble gases by permeation method with dimethyl silicon membranes was carried out in Oak Ridge National Laboratory [160]. On the basis of experimental work, the calculations for different industrial cascades separating krypton and xenon from the space of molten salt and sodium cooled breeder reactor or from the off gas from a plant processing spent reactor fuel were performed. 

There is no evidence that krypton is harmful to humans, other animals, or plants. 

Nuclear power plants use nuclear fission to generate power. The first nuclear fission reaction discovered involved uranium-235. As you can see in Figure 25-16, when a uranium-235 nucleus is struck by a neutron, it undergoes fission. Barium-141 and krypton-92 are just two of the many possible products of this fission reaction. In fact, scientists have identified more than 200 different product isotopes from the fission of a uranium-235 nucleus. 

Separation. — Krypton tins never been obtained except from the atmosphere, A convenient method of separation consists in passing a slow stream of dry oxygen, which has I men materially enriched by fractionation in a liquid air plant, through a tube cooled with liquid air, In this way krypton and xenon together with a little argon condense ns a liquid or solid. 

A Purex plant processes 1 MT of fuel from a PWR whose content of radioactivity is given in Table 8.7. Air discharged from the plant stack contains 5 percent of the tritium and 1 percent each of the krypton and iodine in the fuel. The highest radionuclide concentration to which humans are exposed is one one-thousandth the concentration in stack effluent. 

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