Proc. Nat. Acad. Sci. USA Vol. 68, No. 8, pp. 1923-1930, August 1971 Electric Power from Nuclear Fission MANSON BENEDICT Department of Nuclear Engineering, Massachusetts Institute of Technology, Cambridge, Mass. 02139 Contributed to Symposium on Energy for the Future, April 26, 1971 An abundant supply of electric energy generated at low cost United States through the joint efforts of industry and with minimal adverse environmental effects is essential to government, and their use throughout the world today has civilized society. Generation of electricity by nuclear fission reduced the cost of electricity and paid important dividends is capable of meeting all three of these requirements: abundant in enhanced U.S. prestige and favorable foreign trade. supply, low cost, and minimal environmental effect. My purpose is to describe the types of nuclear reactors used The pressurized water reactor for electric generation in the United States today; the advan- The principle of the pressurized-water nuclear power plant is tages, deficiencies, and problems of today's reactors; and the illustrated in Fig. 1. Fuel rods for this reactor consist of steps that need to be taken to realize the full potential of uranium dioxide, enriched to about 3% in 2"5U, hermetically nuclear fission as a source of electricity. sealed in tubes of a zirconium alloy. This zirconium tubing Limited time does not permit description of all reactor constitutes the first barrier against escape of the highly types under development. Attention is focused on light- radioactive fission products that form in the fuel as the end water and fast-breeder reactors, the principal types in use or product of the nuclear reaction. Assemblies of these zir- under development today in the United States. Heavy-water conium-clad fuel rods are mounted in a heavy-walled steel reactors and molten-salt reactors, which may also play a role pressure vessel and are surrounded by flowing water entering in the future, are not dealt with. at a temperature of around 540'F and leaving around 600'F, The type of nuclear reactor used throughout the world held at a pressure of around 2250 psi to prevent boiling. The today for electric power generation obtains most of its energy water slows down neutrons produced in fission and increases from slow-neutron fission of the scarce isotope of uranium, their probability of reacting with 236U to such an extent that 285U, which occurs in natural uranium only to the extent of the uranium fuel constitutes a critical mass capable of sus- 1 part in 140. To obtain the full potential of nuclear energy it taining a nuclear-fission chain reaction. To hold the chain will be necessary to develop effective means for utilizing the reaction at a steady rate, a variable amount of neutron- abundant isotope, 238U, which makes up the remaining absorbing boron is used in the reactor, partly as movable con- 99.3% of natural uranium. The most promising type of trol rods and partly as boric acid dissolved in the water. reactor for this purpose is the fast-breeder reactor, in which Pressurized water is pumped through the reactor by the 288U is converted to plutonium, which then undergoes fission circulating pump, past the gas-cushioned pressurizer which with fast neutrons. holds the pressure constant, and through the steam generator. The breeder reactor will provide the world with electric There heat is transferred from the primary pressurized water energy for thousands of years, far beyond the capability of at 600'F and 2250 psi to secondary water boiling at a lower all fossil fuels-coal, oil, and gas-now known or likely to be pressure of around 720 psi to make steam at around 506'F. discovered. Already, nonbreeding reactors in operation in The steam flows through a turbine driving an electric many parts of the United States and elsewhere in the world are generator and then passes to the condenser where it is con- generating electricity at a cost as low as the cost of electricity densed at subatmospheric pressure. The condensate is re- from fossil fuels in the same place. Breeder reactors, under turned to the steam generator by the condensate pump. development in many countries, are likely to generate elec- In the condenser, heat from the condensing steam is trans- tricity at an equally low cost. ferred through cooling coils to cooling water at a pressure above atmospheric, which leaves the condenser at a tempera- LIGHT-WATER REACTORS ture typically 20'F warmer than the incoming water. In some Turning first to today's power reactors, the slow-neutron, plants, this cooling water is drawn from the ocean or other nonbreeding, 2"5U-consuming kind, the reactors used pre- natural sources; in others, it is recirculated through cooling dominantly in the United States are of the light-water type. towers. Disposal of the heat contained in this warm water In these reactors ordinary water, under pressure and at without adverse effect on the environment is one of the temperatures up to 600'F, is used both as coolant to transport problems of all steam-electric plants, nuclear as well as the heat released in fission and as moderator to slow down the fossil. In light-water nuclear plants, however, about 50% fast neutrons initially produced in fission. There are two more warm water must be handled than in an efficient fossil- principal types of light-water reactor: the pressurized water fuel plant, because the thermal efficiency of the water-cooled reactor, used in around 60% of the light-water reactor installa- nuclear plant is only 32.5% due to its relatively low steam tions, and the boiling-water reactor, used in about 40%. temperature of 506'F, compared to a fossil-fueled plant with Both of these reactor types were developed initially in the a thermal efficiency over 40% obtainable from steam tempera- 1923 Downloaded by guest on September 23, 2021 1924 N. A. S. Symposium: Energy for the Future Proc. Nat. Acad. Sci. USA 68 (1971) CONTAINMENT PRESSURIZER 2250 POUNDS/SQ. IN SHELL 6000F STEAM *~ ~~.' ~ ~~~~-n ~o ki lKinc, ,Cn Mt REACTOR 7aFUUNUS/bU IN PRESSURE VESSEL 5060F TURBINE _ GEN _ t STEAM WATER SUBATMOS. COOLING WATER PRESSURE ,TO OCEAN 30 POUNDS/SO IN FUEL -- CONDENSER RODS 4O ..FRO OCEAN POU/SU s N CONDENSED WATER SUBATMOS PRESSURE FIo. 1. Schematic diagram of pressurized-water nuclear power plant. ture around 1000°F. The environmental impact of warm When the reactor is shut down for refueling and the pressure water from a nuclear plant can be dealt with satisfactorily vessel is opened, precautions as stringent as deemed necessary either by siting on a natural body of water with adequate can be taken to concentrate, package, and confine radioactive heat-absorption capacity, such as the ocean or a large river, materials present in the water or pressurized gases. or by use of cooling towers. The cost of heat disposal from a I have dwelt this long on the many barriers against escape water-cooled nuclear plant is, however, somewhat higher than of radioactivity because it is important to realize that, al- from a fossil-fueled plant. though nuclear power plants contain enormous amounts of A more significant potential environmental aspect of a radioactivity release of radioactivity to the environment, from nuclear power plant is the enormous amount of radioactivity them can be controlled to any degree desired, but with in- contained in it. Water-cooled nuclear power reactors are pro- creasing cost. All U. S. nuclear power plants are monitored by vided with many barriers against escape of radioactivity, the U. S. Public Health Service, and have been found to add which have kept releases to insignificant levels, and can be to the environment only a minute fraction of the amount of provided with even more elaborate safeguards if these should radioactivity naturally present (1). be required. As an illustration, precautions taken to minimize radioactive releases from a pressurized water reactor will be The boiling-water reactor described. The boiling-water reactor differs from the pressurized-water Most of the radioactivity is in the form of fission products reactor mainly in that the primary water in the reactor is held contained in the uranium dioxide fuel. Some of the fission at a lower pressure, around 1000 psi, and is allowed to boil in products are refractory oxides, insoluble in water. Others, the reactor. Steam and water flowing past the fuel are sepa- however, are volatile or water-soluble, and a small fraction of rated, with the water being recirculated and the steam them appear in the primary water whenever zirconium tubes flowing directly to the turbine, after which it is condensed and leak, as they sometimes do. The primary water also carries returned to the reactor. The boiling-water reactor needs no corrosion products made radioactive by neutron activation. separate steam generator, as the reactor itself performs this The radioactive content of the primary water is kept low by function. A boiling-water power plant has about the same continuous purification by filtration and ion exchange. thermal efficiency as a pressurized water plant. Escape of radioactivity from the primary water is prevented by the leak-tight pressure vessel and piping system within Comparison of nuclear power plants which this water circulates. with fossil-fueled plants Even if the primary water system should unexpectedly At present, 17 light-water nuclear power plants, with a total leak, there are further barriers against escape of radioactivity. generating capacity of 7,000,000 kW, are in operation in the The primary system is completely housed within a steel-and- United States and 108 plants, with a total capacity of close concrete containment shell.