Antonio Rivera-Corder0 Drexel University Philadelphia, Pa.
The nuclear industry l h e nuclear power industry is a relatively new industry, in which rapid and important changes are taking place that will significantly affect the relation of the electric power industry to the nation's air pollution problem. The first of these changes is the rapid increase in the number of reactors to begin operation in the next few years. There are, in the US., 21 power generating and experimental reactors in operation, with a total gross generating capacity of about 3000 Mw(e). As of April 1, 1969, the Atomic Energy Commission (AEC) was considering applications for another 69 nuclear reactors, with a total gross generating capacity of about 53,000 Mw(e) . Conservative estimates indicate that about 25% of the total generating capacity of the country, equivalent to about 150,000 Mw(e)., will be nuclear by 1980. The second change of importance is the increasing size of nuclear power reactors. Of the 21 nuclear reactors now in operation, only two have a gross generating capacity of over 500 Mw(e)., and the maximum size is 800 Mw(e). Of the 69 reactors being considered by AEC, 59 have a gross generating capacity over 500 Mw(e)., 16 of over 1000 Mw(e)., and the maximum size is 1100 Mw( e ) , The third important change is the tendency of passing ownership of nuclear facilities from the federal government to the private sector. The first privately owned fuel processing plant in West Valley, N.Y., began operating in 1966, and a second, the Midwest Fuel Recovery plant in Illinois, was recently approved for construction by AEC. In addition, AEC is now considering passing ownership of gaseous diffusion plants to the private sector. Radioactive air pollution
The fact that radioactivity can cause damage to people, plants, and animals is well known, and, since its inception, the nuclear industry has been acutely aware of the potentially hazardous affects of its wastes. This has not always been the case for the 392 Environmental Science & Technology
other industries. Data on accepted standards for permissible concentrations of nonradioactive pollutants in the environment have always been controversial, in sharp contrast to the well established standards for radioactivity. In fact, probably greater scientific and technical effort has been brought to bear on radioactive air pollution than any other problem. Perhaps more money has been spent, on a comparable basis, to control the effluents of the nuclear industry than any other industrial waste. Aside from this, the question of whether fossil-fuel power plants discharge significant quantities of radioactivity has also been raised. Eisenbud and Petrow analyzed the fly ash produced by combustion of pulverized Appalachian coal and the combustion product discharged to the atmosphere by an oil burning plant of 1000 Mw. for radioactivity content. They concluded (Science, 144, 288, 1964) that when physical and biological properties of various radionuclides are taken into consideration, the conventional fossil-fueled plants discharge relatively greater quantities of radioactive material into the atmosphere than nuclear power plants of comparable size. Other authors have also studied or commented on radioactivity from fossil-fuel plants and it seems apparent that the amount of radioactivity released to the atmosphere from a nuclear power reactor is at least comparable to that released by a fossilfuel burning plant of similar size. Of course, a fair comparison of the amounts of radioactivity released should consider all activities required to produce a given quantity of power. As we shall see, there are other activities in the nuclear power production process which are potential sources of radioactive pollution. One of them, fuel processing, is more important from the point of view of the amount of radioactivity released to the environment than the reactor itself. Fuel fabrication
Airborne effluents from the nuclear fuel cycle vary in terms of volume,
concentration, and chemical composition, depending upon their origin. One way to visualize the different sources of air pollution from the nuclear industry is to consider the various steps in fuel processing. After being mined and concentrated, uranium is shipped to refineries for conversion to pure metal or an oxide of sufficient purity to be used in reactors. The refinery product is either shipped directly to the fuel-element fabrication plants, or converted to UF4 and transported to gaseous-diffusion plants for conversion to UF,, a volatile compound which can be enriched in its content of 235U. From the diffusion plant, the uranium-either in natural or enriched form, or as a metal oxide, or other compound-then goes to the fuel element fabrication plant. All these operations are potential sources of radioactive airborne dust. In mining, uranium, thorium, and their daughter products are sources of radioactivity, but the main problem is gaseous radon and its decay products. Equipment to clean the air is available, and forced ventilation through stacks is usually used also. As a result, the quantity of airborne dust released to the atmosphere is usually very small. Dilution in the atmosphere makes their concentrations insignificant relative to the amount of these gases normally present. The refining operations involve mechanical processing of dry powder of uranium compound. Since the majority of daughter products will have been removed earlier, airborne materials from the purification steps consist primarily of dust or fumes of uranium or thorium. These are controlled by conventional gas cleaning equipment. Similarly, the fuel fabrication steps are primarily mechanical, and the dust essentially consists of uranium or thorium compounds involved in the process. In these operations, as well as in refining, the relatively high economic value of the product material makes it unlikely that significant air contamination will occur in normal operation conditions, especially in the complex gaseous diffusion plants.
feature
and air pollution Flow diagram of the nuclear industry
uranium
Reactors
The actual releases of radioactive product during normal operation of nuclear power plants depend upon the design of the fuel system and the cooling system of the plant. The principal types of nuclear power plants presently in operation in the U.S. are the pressurized water reactor ( PWR) and boiling water reactor ( B W R ) . These two reactor types will probably continue to dominate the industry, at least until the fast breeder reactor becomes economically competitive with water reactors. For the purpose of considering their air pollution potential, reactors may be classified by the relationship between the cooling system and the atmosphere: direct cycle and the indirect cycle types. In the first type the coolant flows through the reactor core to remove heat and then is discharged to the atmosphere; direct cycle cooling systems are used in boiling water reactors. In these plants, the steam flows directly from the reactor to the turbine, after which it is condensed and returned to the reactor. In the second type, the indirect cycle, the coolant circulates, but the vapor is used directly to drive the energy-recovery system, and any entrained gas is discharged through the turbine condenser. The indirect cycle plants correspond to the present pressurized water reactors systems. Under normal conditions, wastes generated by PWR and BWR fall into two categories: Induced radionuclides, produced by neutron bombardment of sub-
stances contained in the primary coolant. Fission products which enter the primary coolant, either because uranium was present as a contaminant in the surface of the component parts or because of a leak in the fuel element. These radionuclides exist in gaseous form, as dissolved solids, and, to a lesser extent, as suspended solids. The induced activities that occur in the primary coolant will depend on the materials of construction of the core, pressure vessel, pump, piping, and other components in contact with the water. The induced activities also result, to a lesser extent, from impurities ordinarily present in water, because they are removed by water treatment before the coolant is introduced into the reactor. Corrosion and activation products are a potential source for release of radioactive material during routine operation, but the major problem is in maintaining the integrity of the fuel and cladding so that fission products do not escape to the coolant and then to the wastes or atmoshpere. During irradiation the structural changes that occur cause the fuel to swell, and cracks and breaks occur in the fuel material. All these changes tend to stress the fuel cladding so that eventually the clad may break or lose its integrity. Failure may occur in the form of pinholes or larger ruptures. Of the products generated in nuclear fission, three are discharged to the environment in whole or in part and are therefore of particular significance in environmental protection.
These radionuclides are iodine, krypton, and tritium. Radioactive iodine exists in a number of highly volatile forms which escape from nuclear facilities much more readily than nonvolatile materials such as strontium or the rare earths. It may reach man through the human food chain. Fortunately, the most important isotope, iodine-131, has a half-life of only 8 days, so that it decays during normal handling and storage processes. The other volatile radionuclides of importance are sjKr and 3H. In the direct cycle plant (BWR),as discussed before, steam flows directly from the reactor to the turbine, after which it is condensed and returned to the reactor. A continuous demineralization system is used to remove most of the fission products in the coolant. In these plants, some of the gaseous and volatile radioactive materials carried with the steam are carried with the gases from the turbine condenser and exhaust to the atmosphere. The phase separation at the surface of the boiling water is an efficient method of decontaminating the steam, except for the noble gases. For normal operation conditions, separation factors of 500 to 1000 have been estimated for iodine. The majority of the untroublesome fission products remain in the system and are removed by ion exchange. About 99% of the radiolytic and fission gases are removed continuously with relatively large volumes of air through the steam turbine air ejector. Normally, about 20 to 30 minutes of delay occur before release. In most pressurized water reactors the coolant is first filtered to remove suspended radionuclides. The water then passes through cation and anion exchange resins. If necessary, the coolant can be passed through a gas stripper in which the water is percolated over plates across which a countercurrent stream of steam is passed. This removes any dissolved gases such as air, fission gases, and hydrogen. These reactors operate with a closed primary coolant circuit, and gases are removed only when the coolant is withdrawn or as a result of leakages. Volume 4, Number 5, May 1970 393
Safe operation of nuclear reactors with respect to air pollution has been demonstrated. In a review of the operating experiences of the Dresden I, Big Rock Point, Humboldt Bay, Elk River, Indian Point I, and Yankee Power Station reactors, J. 0. Blomeke and F. F. Harrington found that the maximum average release of gaseous activation products and noble gases ranged from 0.7 mc./sec. at Yankee to 35,000 mc./sec. at Big Rock plant. With respect to the maximum allowable limits, the releases varied from 0.0026% of the limit at Indian Point to as much as 28% of the limit at Humboldt. Release of halogens and particulates in the gaseous waste range from 2 X mc./sec. at Indian Point to nearly 1.2 mc./sec. at Big Rock Point, corresponding to about lO-j% and 30% of the respective limits. Fuel processing
When a reatcor core has reached the end of its useful life, only a small amount of 235U will have been consumed by fission and an additional amount of zSgU will have been transformed to 239Pu. The core, with its inventory of fission products, must then be removed from the reactor and transported to a chemical processing plant where uranium and plutonium are recovered for use in new fuel elements. This is usually accomplished by a chemical separation process in which the uranium mixture is dissolved in acid and the uranium is then extracted with solvents. In newly proposed processes, the uranium will be volatilized by converting it to uranium hexafluoride and the plutonium and fission products left will be treated 394 Environmental Science & Technology
separately. Various other methods are in use, and additional new methods are being developed. The greatest potential danger of environmental contamination at a fuel reprocessing plant arises from the enormous amount of radioactivity in spent fuel. Even though the fuel is stored for periods ranging from 100 days to one year or longer, to provide time for radioactive decay, a large quantity of highly radioactive material is still present when the fuel reprocessing begins. About 99.9% of all nuclear wastes arises through the reprocessing of reactor fuel. The offgases from a radiochemical plant are generally divided into categories depending on the nature of its radioactivity and the means necessary for its removal. From the standpoint of contamination, the most serious is generally the dissolver offgas, into which large quantities of radioactivity are released during the fuel dissolution. In this operation, the noble gases and part of the I31I left after the decay period are released. The chief noble gas is s3Kr which has a half-life of 10.76 years. If the cooling time is of the order of 100 days, significant quantities of 133Xe will also be released. Although 8jKr emits only soft beta radiation and little gamma radiation, the 135Xe radiation has a significant gamma component and can deliver external radiation to individuals in the environs. Vessel and laboratory offgases also contain considerable activity. The cell or canyon offgas is much greater in volume than any of the other offgases, but contains relatively little of the activity since activity enters the cell offgas only through leaks in the equipment or spills.
The gases evolved from a fuel processing plant are usually contaminated with such chemicals as nitric acid and organic solvents, as well as with fission products, depending upon the particular process employed. Since fuel processing requires treating effluent gases to remove minute quantities of radioactive materials, it removes many of the nonradioactive components as well. It would be very difficult to remove micrograms of radioactivity without, at the same time, significantly reducing the amount of vapor and acid mists. Thus, air pollution from such chemicals is not a problem in reprocessing plants. The operation of air-cleaning equipment depends on the mechanisms of direct interception, gravitational settling, initial impact, centrifugal force, particle diffusion, or electrostatic attraction to cause the particles to move to the collecting surface. The most useful equipment in radiochemicalseparation plants consists of paper, bag, sand, and glass-fiber filters; electrostatic precipitators; wet scrubbers; and porous-metal or graphite filters. Each type of equipment has different characteristics, and equipment suitable for each application must be selected carefully, since, in radiochemical separation plants, there are problems of paramount importance not found in other industries, namely. efficiency requirements and health problems. For example, in filtering contaminated gaseous streams great care must be taken in changing filter media, since exposure of maintenance crews must be considered. No particulates shaken loose during filter changing can be allowed to escape, in subsequent use of the system, to sur-
rounding areas. A large fraction of contaminated offgases are filtered through high efficiency paper filters designed to remove more than 99.97% of 0.3 p diameter particles. This efficiency is seldom needed or used in other industrial filtering operations. The most critical problem of air pollution in the nuclear industry is probably the release to the atmosphere of s5Kr from fuel reprocessing plants. As the nuclear power industry grows, this condition may become a limiting factor in fuel plant design and location. Many authors have presumed that, eventually, the discharge of noble gases would he prohibited during the reprocessing of nuclear fuel. Location of the fuel-reprocessing plant in site of favorable weather conditions and the use of high stacks are possible methods to reduce 85Kr emissions below permissible quantities. If 85Kr exposure cannot be reduced to these levels, as in unfavorable weather conditions at a large plant site, alternative choices of action might include: closing the plant during period of restrictive meteorological conditions, storing the plant offgas during restrictive meteorological conditions, disposing the offgas to porous underground media storage, and separating 85Kr from the majority of the offgas and storing it for future dispersal or decay. Methods of 85Kr removal from gas streams can be classified into the following process: room temperature absorption
Antonio Rivera-Corder0 is on the staff o f the University of Puerto Rico and is a research associate at the Puerto Rico Nuclear Center. Currently on leave f r o m there, he is studying f o r a Ph.D. in environmental engineering at Drexel University. He received his B.S. in industrial engineering in 1964 and an M.S. in nuclear engineering in 1968, both from the University of Puerto Rico. From 1964-66 he was with Caribe General Electric, Znc., at Palmer, P.R.
on charcoal, silica gel, or molecular sieves, low temperature absorption on charcoal or molecular sieves, cryogenic distillation and scrubbing, preferential extraction by liquids, perm-selective silicone rubber membranes in a diffusion cascade, and thermal diffusion. These processes are at various stages of development, and most can remove more than 90% of the 85Kr. Other sources
Other sources of air pollution by radioactive materials from the nuclear industry are: radioisotope usage, disposal of radioactive wastes, and peaceful uses of nuclear explosives such as the Plowshare projects. Important industrial applications of radioisotopes include measurements of density and coating thickness, supplementing Xray machines in testing by radiography, control of catalyst flow rate, as tracing elements, as leak detectors, and many other applications. Air pollution from the industrial usages is generally not a problem because of the small quantities involved. In fact, the amount of materials that can he obtained for these purposes are controlled by the federal government, and most of the radioactive elements available are transported and used in adequately sealed containers. Liquid radioactive waste is normally disposed of by decay storage and, thus, represents no air pollution problem; solid wastes are normally buried. Plowshare projects may represent an important source of air pollution in the future. Possible applications of nuclear explosives include removal of earth cover to expose ore to open pit mining, the construction of storage water basins, the digging of canals and harbors, increasing oil production through fracturing, crushing ores underground to permit mining or in situ leaching, and other applications. Use of fission devices for these operations might release large quantities of dust and radioactivity to the atmosphere. Other techniques
Meteorological conditions can be advantageously used for reducing air pollution from the nuclear industry. The science of meteorology enters the nuclear energy field in two distinct phases in the life of any facility4uring the choice of site and adaptation of plant design and procedure to local meteorological characteristics, and during routine plant operations. Me-
teorological factors of plant sites are evaluated to determine the suitability of the plant environment to receive radioactive discharges under accidental as well as normal operating conditions. The objectives of a site meteorological analysis are: * Determining the local weather characteristics which infiuence the transport and dilution of airborne radioactivity. * Statistically describing the occurrence of certain weather phenomena. * Identifying irregularities in weather or meteorologic behavior. Average yearly meteorologic conditions are normally used for calculating dispersion and concentration of operational wastes releases. Since the total quantities of gases produced in nuclear facilities are much less than in most nonnuclear industries, continuous discharge to the atmosphere is not necessary. Gases can be normally stored for radioactive decay until meteorological conditions are adequate for dilution and dispersion in the atmosphere. A unique feature of the nuclear power field, closely related to protection standards, is extensive monitoring of radiation and radioactivity as a routine part of plant operations. Normally, every facility has a health physics division which is primarily concerned with detection and measurement of radiation fields. Surveys are made for both the plant and the general environment to determine if any changes are occurring because of the plant operation. The nuclear energy industry has been devoting a great effort toward understanding and control of air pollution. Over the years the AEC has sponsored a number of research programs to develop nuclear techniques for pollution studies. These studies have resulted in the development of such things as several instruments and analytical methods for determining SO, in air, an ozone monitor, a gauge to measure the amount of suspended solids in a flowing stream, and high efficiency and temperature filters. Radioactive tracers, in addition to these well established medical applications, are used in many other ways, especially in meteorology, for derivations of air fiow diffusion rates and studies of condensations, precipitations, evaporation, and hydrology, and for studies of atmospheric elect~city.Applications of nuclear techniques to this field seem unlimited. Volume 4, Number 5, May 1970 395
