High temperature gas cooled reactors

High Temperature Gas-Cooled Reactors. The high temperature gas-cooled reactor (HTGR) uses graphite as moderator, but has an unusual type of fuel (31,43). As produced by General Atomics of San Diego, California, the highly enriched (93%) fuel consists of coated spherical particles of diameter about 1 mm. As shown in Eigure 2, the kernel is a sphere of uranium dioxide, uranium carbide, or mixtures of these with siHcon or aluminum. Kernels are prepared by a series of chemical processes and heat treated. Several thin coatings are appHed, consisting of pyrolytic carbon or siHcon carbide, or a combination of the two. These layers prevent fission products from escaping from the kernel, even when the temperature is as high as 1000°C. SoHd fuel rods are fabricated from the coated particles and a carbon binder, and inserted into holes in large hexagonal graphite blocks. The prisms also have holes for passage of coolant. Stacks of blocks form the large core, measuring several meters in each direction. The core is located within a large prestressed concrete reactor vessel.  [c.213]

As previously stated, uranium carbides are used as nuclear fuel (145). Two of the typical reactors fueled by uranium and mixed metal carbides are thermionic, which are continually being developed for space power and propulsion systems, and high temperature gas-cooled reactors (83,146,147). In order to be used as nuclear fuel, carbide microspheres are required. These microspheres have been fabricated by a carbothermic reduction of UO and elemental carbon to form UC (148,149). In addition to these uses, the carbides are also precursors for uranium nitride based fuels.  [c.325]

In an entrained-bed reactor, the coal and oxidant are fed simultaneously at one end of the reactor and pass through the reaction zone together. A short residence time of less than a minute is typical for this type of reactor. Because of the short residence time the coal feed must be ground to a powder to ensure rapid reaction with the oxidant. Coal can be fed as a dry powder or slurried with water and sprayed ia the gasifier. These reactors operate at very high temperature, which eliminates all tars and Hquids from the product gas. Also, the coal ash is converted to a molten form that forms a glassy, granular slag when removed from the gasifier. Product gas cooling is accompHshed either with a combination of radiant and convection coolers or by direct quench with water. In the quench mode, elaborate heat recovery equipment is required for efficient operation. Several entrained-bed processes including Texaco, Dow, Shell, Koppers-Totzek, and Prenflo have been commercialized or demonstrated on a commercial scale.  [c.164]

Internal Heating. In many high pressure gas reactors, where for economic reasons it is essential to operate at a high temperature, special provision is made by heat exchange or direct cooling to keep the inner walls at a temperature low enough to minimise creep. One way of doing this is to allow the cool gas entering the reactor to flow between the inner surface of the vessel and an insulated layer on the outside of the reaction chamber. Thus the outer vessel, which has to withstand the internal pressure, is at a relatively low temperature, while the inner reaction chamber, which is maintained at a high temperature, is not subjected to an unbalanced pressure.  [c.86]

The Union Carbide gas-phase process is suitable for the production of both HDPE and LLDPE (41—45). A flow diagram is shown in Eigure 4 (46). The reactor is a tall cylindrical tower (height up to 25 m) with a length-to-diameter ratio of 7. It usually operates at pressures of 1.5—2.5 MPa (15—25 atm) and temperatures of 70—95°C. The reactor is filled with a bed of dry polymer particles vigorously agitated by a high velocity gas stream, a mixture of ethylene, a-olefin, nitrogen, and hydrogen, which is used for molecular weight control. The gas stream enters the reactor through a perforated distribution plate at the reactor bottom. Rapid ckculation serves two purposes, fluidization of the particle bed and the removal of polymerization heat. The unreacted gas stream enters an expanded disengagement zone at the top of the reactor, separates from the entrained polymer particles, and is then compressed, cooled, and recycled. In some cases, the gas mixture can be cooled below its dew point. Pine droplets of Hquid a-olefin are carried by the gas stream into the reactor where they rapidly evaporate (44). Operation in the condensing mode increases the heat removing capacity of the ckculating stream. Advances in condensing mode technology has greatly increased productivity of fluid-bed reactors (47).  [c.399]

A solution containing about 25% DNT in methanol is pumped along with a Raney nickel slurry and enough hydrogen gas to complete the reaction through a series of reactors. There are three high pressure reactors plus an auxiHary reactor, each with a volume of approximately 450 L. The reactors are about 6 m long and 35 cm in diameter and are equipped with water cooling for temperature control. This reaction mixture is fed to the first reactor at a rate of approximately 2000 kg/h. The material from the first reactor is spHt and fed to the second and third reactors, which are mn in parallel. The temperature of the reactors is maintained at about 100°C and the hydrogen pressure at between 15,000 and 20,000 kPa (150 and 200 atm). After the reaction is completed, the pressure is reduced and the excess hydrogen removed in a Hquid —gas separator. The hydrogen is recycled to the beginning of the process and the product stream continues on through a series of catalyst removal steps. The recovered catalyst is also recycled with only a small amount, generally 0.1 to 0.3%, being lost in the process. After the catalyst is removed, the product is sent to a methanol column where the solvent is removed and recycled. The water is then removed in the dehydration column. The recovered water from this step contains volatile amines and other by-products and must be processed further before it can be recycled or discharged to the waste treatment plant. The product from the dehydration column can be used directiy or taken to a final TDA column where it is distilled to give a product which is more than 99% pure. The residue from the final column can be disposed of by incineration or processed to recover some of the amine, thereby improving the yield and reducing waste-disposal problems.  [c.260]

For many years the corrosion of uranium has been of major interest in atomic energy programmes. The environments of importance are mainly those which could come into contact with the metal at high temperatures during the malfunction of reactors, viz. water, carbon dioxide, carbon monoxide, air and steam. In all instances the corrosion is favoured by large free energy and heat terms for the formation of uranium oxides. The major use of the metal in reactors cooled by carbon dioxide has resulted in considerable emphasis on the behaviour in this gas and to a lesser extent in carbon monoxide and air.  [c.906]

Several appHcations of cryogenic temperatures are based on properties that vary more or less smoothly with temperature. The electrical conductivity of pure metals iacreases as the temperature is lowered, allowiag cryogenicaHy cooled, normal metals (aluminum, copper, or sodium) to be used for electromagnets (115) and ia electrical power distributioa (116). The vapor pressures of all substances decrease with decreasiag temperature at the helium boiling poiat, only helium itself retains any substantial vapor pressure. HeHum-cooled panels form cryopumps used for high capacity vacuum pumping systems or for smaller systems where very clean pumping is required (117). Even residual helium gas can be removed effectively by an adsorbent such as activated charcoal cooled to helium temperature. Large cryopumps are used to sustain vacuums ia the space-simulatioa chambers and ia the aeutral-hydrogea beam iajectors and plasma chambers of experimental hydrogen fusion reactors (118,119). The thermal noise ia electrical components decreases at low temperature. Many radio, and other radiation-detecting devices use neon or helium cooling to enhance their sensitivities (120).  [c.16]

The high carbon content of acetylene (92%) and its property of decomposing exothermically to carbon and hydrogen make it an attractive raw material for conversion to carbon. Acetylene black is made by a continuous decomposition process at an atmospheric pressure of 800—1000°C in water-cooled metal retorts lined with refractory. The process consists in feeding acetylene into the hot reactors. The exothermic reaction is self-sustaining and requites water cooling to maintain a constant reaction temperature. The carbon black-laden hydrogen stream is then cooled followed by separation of the carbon from the hydrogen tail gas. The tail gas is either flared or used as fuel. After separation from the gas stream acetylene black is very fluffy with a bulk density of only 19 kg/m (1.2 lb/fT). It is difficult to compact and resists pelletization. Commercial grades are compressed to various bulk densities up to 200 kg/m (12.5 Ibs/fU).  [c.547]

In this arrangement, in contrast to the previous approach, the coolant is kept from evaporating by maintaining it under an inert gas pressure higher than its vapor pressure. A centrifugal pump is used to achieve high circulation rates. Besides the previously mentioned three coolants (water, tetralin, and Dowtherm A), other nonvolatile heat transfer oils as well as molten salts or molten metals can be used. These coolants are advantageous primarily at higher temperatures where, even if exothermic reactions are conducted in laboratory reactors, the heat losses are usually more than the reaction heat generated. A simple temperature controller, therefore, can be used to keep the electric heater adjusted by sensing with a thermocouple immersed in the return line. At a high recirculation rate of the liquid coolant, the constant wall temperature can again be approximated, but not as well as with boiling-type cooling.  [c.41]

Although there are a number of publications dealing with the basic chemistry of ethylene polymerisation under high pressure, little information has been made publicly available concerning details of current commercial processes. It may however be said that commercial high polymers are generally produced under conditions of high pressure (1000-3000 atm) and at temperatures of 80-300°C. A free-radical initiator such as benzoyl peroxide, azodi-isobutyronitrile or oxygen is commonly used. The process may be operated continuously by passing the reactants through narrow-bore tubes or through stirred reactors or by a batch process in an autoclave. Because of the high heat of polymerisation care must be taken to prevent runaway reaction. This can be done by having a high cooling surface-volume ratio in the appropriate part of a continuous reactor and in addition by running water or a somewhat inert liquid such as benzene (which also helps to prevent tube blockage) through the tubes to dilute the exotherm. Local runaway reactions may be prevented by operating at a high flow velocity. In a typical process 10-30% of the monomer is converted to polymer. After a polymer-gas separation the polymer is extruded into a ribbon and then granulated. Film grades are subjected to a homogenisation process in an internal mixer or a continuous compounder machine to break up high molecular weight species present.  [c.208]

The processes utilize catalysts in the presence of substantial amounts of hydrogen under high pressure and temperature to react the feedstocks and impurities with hydrogen. The reactors are nearly all fixed-bed with catalyst replacement or regeneration done after months or years of operation often at an off-site facility. In addition to the treated products, the process produces a stream of light fuel gases, hydrogen sulfide, and ammonia. The treated product and hydrogen-rich gas are cooled after they leave the reactor before being separated. The hydrogen is recycled to the reactor. The off-gas stream may be very rich in hydrogen sulfide and light fuel gas. The fuel gas and hydrogen sulfide are typically sent to the sour gas treatment unit and sulfur recovery unit. Catalysts are typically cobalt or molybdenum oxides on alumina, but can also contain nickel and tungsten.  [c.91]

After completing die cracking reactions in die tubular reactors, die gaseous mixture flows to a liquid quench tower where die gas temperature is lowered enough to stop die cracking reactions. Oil or water can be used as die cooling media. Transfer line heat e.xchangers can be used to recover die heat contained in die product gas, and diis energy can be used to produce high pressure steam.  [c.629]

Nuclear Applications. The strength of graphite at high temperatures and its behavior with respect to products of nuclear fission/bision make it a suitable material for nuclear moderators and reflectors, materials of constmction, and thermal columns in various reactors. Since its use in the first nuclear reactor, CP-1, constmcted in 1942 at Stagg Field, University of Chicago, many thousands of metric tons of graphite have been used for this purpose. Uranium—graphite moderators were used in the Calder Hall reactors for power generation. The advanced gas-cooled reactors (AGR), the high temperature gas-cooled reactors (HTGR), the molten salt breeder reactors (MSBR), and Hquid metal fueled reactors (LMFR) all use graphite moderators. The thermal stabiHty, resistance to corrosion, and high thermal conductivity of graphite make it a most suitable moderator material for consideration in advanced design, high temperature, atomic energy efficient nuclear reactors.  [c.513]

The metal is a source of nuclear power. There is probably more energy available for use from thorium in the minerals of the earth s crust than from both uranium and fossil fuels. Any sizable demand from thorium as a nuclear fuel is still several years in the future. Work has been done in developing thorium cycle converter-reactor systems. Several prototypes, including the HTGR (high-temperature gas-cooled reactor) and MSRE (molten salt converter reactor experiment), have operated. While the HTGR reactors are efficient, they are not expected to become important commercially for many years because of certain operating difficulties.  [c.174]

Uranium forms a continuous range of solid solutions with molybdenum at high temperarnres, and it is possible by alloying to remove the phase change which limits the upper temperature of operation of metallic fuel elements. However the absorption of thermal neutrons by molybdenum is unacceptably high for gas-cooled reactors, but viable for use in fast neuUon reactors. This fuel element undergoes fission of together with the formation of Pu from which also undergoes fission with fast neutrons to produce 2-3 neutrons per fission event. A prototype reactor functioned satisfactorily with stainless steel cladding and cooling with a sodium-potassium liquid alloy. Attempts to make larger commercial reactors of this type have not yet been successful. The modern developments of commercial nuclear reactors have all been centred on oxide fuels, which will be considered later.  [c.195]

Nuclear Reactors. The first-generation of gas-cooled fission reactors used coolant temperatures of only ca 400°C which permitted the use of inexpensive carbon dioxide as the coolant gas. Eor improved thermal efficiency, second-generation reactors were designed to use gas-coolant temperatures up to 790°C, temperatures at which only helium offers the necessary chemical stabiHty, inertness, high heat-transfer rates, low aerodynamic pressure losses, and low neutron cross section ( He only) (127). Although gas-cooled reactors cl aim certain operating advantages and relative immunity from loss-of-coolant accidents, only one commercial power generating reactor of this type (Eort St. Vraine, Colorado) is in operation (128).  [c.16]

Nuclear Steam Generators. There are three basic types of nuclear heat sources. Most common are pressurized water reactors having steam generators (Fig. 21). In this system, the nuclear reactor heats water in a high pressure loop, typicaHy 14.1 to 17.2 MPa (2050—2500 psia). The water is circulated through tubes in a steam generator. On the outside of the tubes, water is boiled to steam which goes to the steam turbine. In recirculating steam generators, the steam is saturated. In once-through steam generators, the steam has about 28°C (50°F) superheat. In boHing-water reactors, the second type, the nuclear heat is used to boil the feedwater directly. In gas-cooled reactors, the third type, the gas takes the same role as the pressurized water in a pressurized-water reactor (PWR) and transfers the heat to the steam generator. Typical nuclear turbine inlet conditions range from 5.0 to 7.3 MPa (725 to 1059 psia) at saturation temperatures of264—289°C (507—552°F) and approximately 0.25% moisture.  [c.358]

The major difficulty with these reactors is in the outside recycle pump, especially at high temperatures. Reciprocating pumps require seal rings, and these cannot take the high temperature needed for most reactions. If the recycle gas is cooled down before entering the compressor, it must be reheated before it enters the reactor again. This makes them complicated in construction and excessive in cost.  [c.46]

OUgomeriza.tlon, Dimerization and trimerization are used to produce supplemental quantities of 92+ octane-blending constituents of gasoline. In the UOP HexaH process, about 80% conversion of propylene to hexane and small amounts of nonene and dodecene are obtained in fixed-bed reactors. Low concentrations of the Hquefied petroleum gas (LPG) feed are used because the reactions are highly exothermic. The reactor effluent is water cooled and unreacted LPG is recycled to adjust propylene feed concentrations. High purity propylene is a by-product. Existing tubular reactors used to produce polygasoline, largely nonene, can be retrofitted for this lower pressure and temperature process (105) (see Liquefied petroleum gas).  [c.526]

Azelaic Acid. This acid is produced by the Emery Group of Henkel Corporation in Cincinnati, Ohio in multimilHon kg quantities. The process that is currendy used is based on the ozonolysis of oleic acid (from grease or tallow) foUowed by the decomposition of the ozonide with oxygen. Oleic acid [112-80-1] and pelargonic acid [112-05-0] are fed into an ozone absorber countercurrent to a continuous dow of oxygen gas containing about 2% ozone. The pelargonic acid serves as a solvent or diluent to help moderate the reaction. Since the reaction is highly exothermic the ozone absorber is cooled and usuaUy maintained at a temperature of 25—45°C. The reaction mixture is then fed into reactors maintained at about 100°C and sparged with oxygen gas where the ozonides are decomposed and oxidized rapidly. Manganese salts are used to catalyze the oxidation of any aldehydes that were formed to acids. The product at this stage contains pelargonic, azelaic, and palmitic and stearic acids that were present in the feed and high molecular weight materials such as esters and dimer that were formed during ozonization.  [c.62]

See pages that mention the term High temperature gas cooled reactors : [c.479]   
Carbon materials for advanced technologies (1999) -- [ c.0 ]