Energy from Uranium by J. CHEM. EDUC. Staff
Realities of Nuclear Power Energy from nuclear sources amounted to about 4.5% of US. energy production in 1977, and is expected to increase from about 7.0% in 1980 to 22% by 1990.Nearly allcommercial nuclear energy produced is used in the generation of electricity. Nuclear fuel consumption for electricity generation is expected to increase from 2.8 quadrillion Btu (1 Btu 3 0.25 kcal) in 1974 to 23.8 quadrillion Btu in 1990. More than 40% of the electricity generated in this country in 1990 is expected to come from nuclear powered facilities. At present, there are 66 nuclear power plants operating in the U.S.. and 158 additional nlanta are either under construction or on order. Twenty-one foreign countries have operating nuclear power plants, and 20 others have made firm commitments to develop commercial nuclear power. Outside this country 481 nuclear power plants are either in operation, under construction, on order o; planned. Present best estimates of electricity production costs indicate that in the mid-1980s nuclear power generation will he 38% less expensive than coal-fired generation, and 58% less expensive than oil-fired generation.
Fission and Fissionable Material The process hy which energy is produced in nuclear power reactors is known as fission. In essence. fission involves splitting atoms of heavy elements such as uranium or plutonium into fraements with the release of considerable amounts " of energy. Fission is not a spontaneous process; it is initiated when neutrons bombard and are captured bv certain isoto~es of uranium and plutonium, chieflykanium235 (U-235) and 233 (U-233) and plutonium 239 (Pu-239). In addition to splitting atoms and releasing energy, fission results in release of one or more neutrons for each atom split-on the average 2.4 neutrons are produced for each U-235 atom undergoing fission. Some of these neutrons can, in turn, bombard and be captured by other fissionable atoms, releasing more heat, producing more neutrons and creating a continuously sustained or "chain reaction," The fission reaction for U-235 can be represented by where X and Y are atomic particles with masses in the range 85-110 for X and 125-150 for Y. More than 90% of the energy
released in fission is in the form of kinetic enerm associated with the X and Y fragments. This energy is trksferred by collision to nearby particles. The fragments X and Y usually are radioactive and spontaneously-disintegrate liberating alpha and beta particles and gamma rays. High speed neutrons also are released, and unless some provision is made to capture them, most will be lost from the system. Of the three fissionable isotopes suitable for nuclear power reactors (U-235, U-233 and Pu-239), only U-235 occurs in nature in usable quantities. Natural uranium contains about 0.7% U-235. The most abundant isotope in natural uranium-U-238-does not undergo fission. However, it can be converted to fissionable Pu-239 by a series of steps that begin with bombardment by and capture of a neutron
Uraninm-233 can he produced from the relatively abundant element, thorium, by the reaction g2~h+An-g3Th+4kU : 9!. "~.-, In any nuclear reactor using natural uranium as a fuel, a portion of the U-238 is converted to Pu-239. Some of this plutonium will react by fission and some will be left in the spent reactor fuel. This unreacted plutonium represents a future supply of nuclear fuel, but it is radioactive, very chemically reactive, and can be used to make nuclear weapons. At present, the reprocessing of spent nuclear fuel to recover plutonium is not allowed in the United States.
Nuclear Power Reactors The Atomic Enerm -. Commission developed five types .. of nuclear power reactors: the pressurized-water reactor, the boiling water reactor, the homogeneously fueled reactor, the sodium-cooled fast-neutron breeder reactor, and the sodium-cooled thermal neutron reactor operating at high temperatures. Canada has developed a heavy-water-moderated reactor, and several European countries are working on high pressure helium moderated reactors. The pressurized-water reactor originally was conceived for powering nuclear ships, but it has proved to he the best commercial reactor for electricity generation. More kilowatt hours of electric energy have been generated by pressurized-water reactors than by all other types combined.
Volume 56. Number 2,February 1979 / 119
I
ontainment Structure
Steam
Generator Turbine Generator
Figure 1. Diagram of a pressurized water reactor for the generation of electricity by nuclear fission Note that the pressurized water that makes contact with the reactor core ts isolated from the water used to produce steam and todrive the turbine.
How Electricity is Generated Except in hydroelectric power plants, heat is required tu produce electric it,^. In fossil-fueled electric plants, the heat source is the burning of coal, natural gas or oil. In nuclear powered plants, the heat is prnduced by splitting uranium atoms in fission reactions. Once the heat is vrtrduced IIV fission. the neneratinr.. process . i$the same as i n I ' ~ A I I pI;f~its.1Iru1 t t ~ r ~Wi s, I I C ~i ~ l t
