Energy and the Environment Fall 2012 Instructor: Xiaodong Chu ： Office Tel.: 81696127.

Energy and the Environment Fall 2012 Instructor: Xiaodong ChuOffice Tel.: Flashbacks of Last Lecture Nuclear reactors Boiling water reactors Pressurized water reactors Gas-cooled reactors Breeder reactors Nuclear-Fueled Power Plants: Nuclear Fuel Cycle mininguranium thoriumore extraction of the useful uranium concentrate gasification to UF 6 enrichment of 235 U conversion to metallic uranium or oxide of uranium fuel rod fabrication retrieval of spent fuel reprocessing of spent fuel fuel waste disposal The nuclear fuel cycle starts from mining uranium (or thorium) ore ( ( ) ), through extraction of the useful uranium concentrate ( ), gasification to UF 6 (UF 6 ), enrichment of 235 U ( ), conversion to metallic uranium or oxide of uranium ( ), fuel rod fabrication ( ), loading of a reactor ( ), retrieval of spent fuel ( ), reprocessing of spent fuel ( ), and finally fuel waste disposal ( ) Nuclear-Fueled Power Plants: Nuclear Fuel Cycle Mining and refining Rich ores may contain up to 2% uranium, medium-grade ores 0.5 1%, and low-grade less than 0.5% Large deposits are found in Australia, Kazakhstan, Canada, South Africa, Namibia, Brazil, and Russia Because uranium deposits are always associated with decay products of uranium, these ores can be radioactive, and workers protection must start at the mining phase Nuclear-Fueled Power Plants: Nuclear Fuel Cycle Mining and refining leached The ore is crushed and leached ( ) with sulfuric acid, through which uranium, together with some other metals, dissolves yellow cake precipitated Uranium oxide with the approximate composition U 3 O 8, called yellow cake ( ), is precipitated ( ), dried, and packed into 200-liter drums for shipment, from which the radiation is negligible radiumbismuth tailings heap The solids remaining after leaching with acid may contain radioactive isotopes of radium ( ), bismuth ( ), and lead and these solids are pumped as a slurry to the tailings heap ( ), also called tailings dam clay The tailings must be covered with clay ( ) or other impenetrable material to protect humans and animals from radiation exposure Nuclear-Fueled Power Plants: Nuclear Fuel Cycle Gasification and enrichment Most reactors need uranium enriched with 235 U hydrogen fluoride gas uranium oxide uranium hexafluorideUF 6 sublimates The uranium concentrate is treated with hydrogen fluoride gas ( ) and the uranium oxide ( ) is converted to uranium hexafluoride UF 6 ( ), which is a white solid that sublimates ( ) to a vapor at a pressure of 1 atmosphere at 56 C gaseous diffusion Enrichment can be achieved by gaseous diffusion ( ) using membranes with small pores ( ) and the rate of diffusion is a function of pressure and temperature and is also a function of the molecular diameter of the diffusing gas 235 UF 6 has a slightly smaller diameter than 238 UF 6 so that 235 UF 6 diffuses faster than 238 UF 6 By forcing the gases to pass many membranes, almost any degree of enrichment can be achieved Nuclear-Fueled Power Plants: Nuclear Fuel Cycle Gasification and enrichment gaseous centrifuge Enrichment plants have been built that work on the principle of a gaseous centrifuge ( ) When the uranium gases are spun very fast in a centrifuge, the heavier 238 UF 6 spreads toward the edges of the centrifuge while the lighter 235 UF 6 concentrates toward the center The enrichment depends on the rate of revolutions and the time spent in the centrifuge Gaseous centrifuges are less energy-consuming than diffusion plants and require lower capital investment Nuclear-Fueled Power Plants: Nuclear Fuel Cycle Gasification and enrichment Development is in progress in various countries on laser enrichment ( ) of uranium In this process, metallic uranium is vaporized in an oven and a stream of vaporized uranium atoms exits the oven port A laser beam with a very narrow wavelength band is shone unto the atomic stream to differentially excite only 235 U to a higher electronic state, but not the heavier isotope The excited atom is then ionized with ultraviolet light and the ionized 235 U is collected on a negatively charged plate Nuclear-Fueled Power Plants: Nuclear Fuel Cycle Gasification and enrichment After enrichment, the UF 6 gas or metallic uranium is converted to uranium dioxide UO 2 This is a ceramic-like material that is fabricated into pellets The pellets are loaded into fuel rods Nuclear-Fueled Power Plants: Nuclear Fuel Cycle Spent fuel reprocessing and temporary waste storage In boiling and pressurized water reactors the fuel stays in the reactor for 23 years After that period, the fission reaction has slowed down, with a decline in steam and electricity production At that time, the fuel rods have to be replaced with fresh ones The retrieved fuel rods emit a high level of radiation because of the radioactive fission products and other neutron-activated isotopes that have accumulated in the spent fuel rods Nuclear-Fueled Power Plants: Nuclear Fuel Cycle Spent fuel reprocessing and temporary waste storage dry casks The extracted fuel rods are stored in the containment vessel of the power plant, in steel and concrete-walled water pools or dry casks ( ) Once the radioactive level of the spent fuel has declined sufficiently to be handled by remote control and proper shielding, it should be taken away to a permanent disposal site Nuclear-Fueled Power Plants: Nuclear Fuel Cycle Spent fuel reprocessing and temporary waste storage In some countries, after due decline of radioactivity, the spent fuel is reprocessed About 96% of the original uranium in the fuel is still present, although it contains less than 1% 235 U Another 1% of the uranium has been converted to 239 Pu leached The spent rods are leached ( ) in acid dissolve Uranium and plutonium dissolve ( ) and are separated chemically from the rest of dissolved elements The recovered uranium is sent back to the enrichment facilities The recovered plutonium is mixed with natural uranium and made into fresh fuel, called mixed oxide (MOX) Nuclear-Fueled Power Plants: Nuclear Fuel Cycle Permanent waste disposal The largest problem facing nuclear power plants is the permanent disposal of spent fuel The level of radioactivity of the spent fuel declines about tenfold every hundred years After about 1000 years the level reaches that of the original ore from whence the fuel (uranium) was extracted Nuclear-Fueled Power Plants: Nuclear Fuel Cycle Permanent waste disposal geologic formations The only practical way of disposing of the waste would be in stable geologic formations ( ) known not to suffer from periodic earthquakes and where the water table ( ) is either absent or very deep beneath the formation No country has yet solved the permanent disposal problem of radioactive waste from nuclear power plants Nuclear-Fueled Power Plants: Fusion A large amount of energy is evolved when light nuclei fuse together The sum of the masses of the nuclei that fuse (the left-hand side of the equations) is not exactly the sum of the masses of the fused nucleus plus the mass of the ejected neutron or proton (the right- hand side of the equations) The mass deficit appears as the evolved energy The ejected neutrons or protons collide with surrounding matter so that their kinetic energy is converted into sensible heat Nuclear-Fueled Power Plants: Fusion It would be desirable to perform fusion reactions under controlled conditions, so that the evolved heat energy can be transferred to a coolant working fluid, which in turn would drive a turbo-machinery The advantages of fusion-based power plants are threefold deuterium tritium The raw material or fuel available for fusion reactors is almost unlimited, because deuterium ( ) is a natural isotope of hydrogen to the extent of 1 deuterium atom in 6500 hydrogen atoms although tritium ( ) is not found in nature, but can be manufactured from an isotope of lithium in the following reactionc4d The fusion reactions would produce a minimal amount of radioactivity There is no spent fuel waste from which ingredients could be extracted for fabricating fission nuclear weapons Nuclear-Fueled Power Plants: Fusion electrical repulsion force The difficulty of achieving a controlled fusion is in overcoming the electrical repulsion force ( ) of the positively charged nuclei To overcome the repulsive force, the colliding nuclei must have a kinetic energy comparable to a temperature of tens of million degrees plasma state At such temperatures, atoms are completely dissociated into positively charged nuclei and free electrons, the so-called plasma state ( ) For the release of significant amounts of energy, many nuclei must collide so that the plasma needs to be confined to a small volume at a high pressure Attempts of controlled fusion processes have been conducted since the 1950s break-even point So far limited success has been achieved; here, success means that an equal or greater amount of energy is released as was consumed in the fusion experiment, the so-called break-even point ( ) Nuclear-Fueled Power Plants: Fusion Magnetic confinement Magnetic confinement ( ) plasma confinement Most approaches to plasma confinement ( ) and inducement to fusion rely on magnetic field confinement The magnetic field is created inside cylindrical coils in which a current flows The cylindrical coils form a circle, so that the magnetic field has the shape of a toroid (doughnut) The plasma particles travel in helical revolutions along the magnetic field lines Nuclear-Fueled Power Plants: Fusion Magnetic confinement Magnetic confinement ( ) Nuclear-Fueled Power Plants: Fusion Magnetic confinement Magnetic confinement ( ) Joint European Torus (JET) using tokamak ( ) reactors Nuclear-Fueled Power Plants: Fusion Laser fusion ( ) A fuel (e.g., a mixture of deuterium and tritium) is contained in a small (1-mm) sphere made of glass or steel and the mixture is irradiated with several high-intensity laser beams The sphere is compressed and heated by implosion and the glass or steel outer layer evaporates The temperature of the content gases increases to tens of millions degrees, and the pressure rises to thousands of atmospheres Laser beams of megajoules of energy are employed in very short pulses, a billionth of a second long The power at the center of the sphere reaches 110 15 W Unfortunately, the laser system operates at about 1% efficiency, so that 99% of the input energy does not result in light emission Higher-efficiency lasers pulsing at a higher rate will be necessary for break-even power production, let alone for a commercial power plant