IGA

22. J. R. CLARK and H. A. POHL, Earth Disposal of Radioactive Wastes in Sanitary Engineering Aspects of the Atomic Energy Industry, A Sponsored by the AEC and the Public Health Service, Held at the Taft Engineering Center, Cincinnati, Ohio, December 6-9, 1955, Report TID-7517 (Pt. 1a), pp. 162-171, Division of Reactor Deve and Public Health Service, October 1956.

23. R. OVERSTREET, Comments on the Burial of Radioactive Wastes in A Conference on Radioactive Isotopes in Agriculture, Held on Januar and 14, 1956, Michigan State University, East Lansing, Michigan, Report TID-7512, pp. 81-85, Argonne National Laboratory, Janua 24. R. E. BROWN, M. W. McCONIGA, and P. P. RowE, Geological and logical Aspects of the Disposal of Liquid Radioactive Wastes, in Engineering Aspects of the Atomic Energy Industry, A Seminar S by the AEC and the Public Health Service, Held at the Robert A. Taft H ing Center, Cincinnati, Ohio, December 6-9, 1955, USAEC Report T (Pt. 1b), pp. 413-424, Division of Reactor Development and Publi Service, October 1956.

25. W. F. MERRITT and P. J. PARSONS, Sampling Devices for Water and Disposal of Radioactive Wastes, Proceedings Conference, Monac Vol. 2, p. 331, International Atomic Energy Agency, Vienna, Austr

DISPOSAL TO THE AIR ENVIRONMENT*

Most human activities leading toward a better standard of result in some contamination of the atmosphere. In this nuclea it is impossible to prevent some release of radioactive materials it is therefore the degree rather than the act of contamination b nuclear energy industry which must be considered. The goal attain a discharge to the air environment that is consistent wit health and welfare of the general public and yet is satisfactor economic progress.

There is a finite volume of atmosphere available to the integ activities of the total world population, a volume computed 1.5 X 1018 cubic meters on the assumption that maximum diff conditions would carry contamination to an altitude of 3000 m. a fraction of this total volume is available, however, for diluti airborne wastes in any geographical area. The physical, met logical, and topographical factors that affect atmospheric disp have combined to create air-pollution problems in many urban industrial areas. In areas where air pollution is a recognized pro tons of chemicals are discharged daily from various industrial o tions. Since the specific activity of significant radioisotopes high, however, micrometeorological conditions that could receive of chemical pollution without any problems could be overloaded a few milligrams of fission products.† Conversely, although a slight mass of material may create unsatisfactory atmospheric ditions, it is physically and economically impossible to remov last vestiges of radioactive material from process and off-gas stre Although the atomic energy industry does not usually work with

*Richard D. Coleman, Chief, Field Evaluation of Aerospace Nuclear Sys Division of Radiological Health, U. S. Public Health Service, Las Vegas, assisted in the preparation of this chapter.

† As of Mar. 1, 1961, the planned U. S. Power Reactor Program 2 totaled Mw. Since the fissioning of 1 g of uranium produces about 1 Mw(t) of e and 2.68×10' curies of fission products,3 it would require the release and 1 geneous mixing of all the fission products produced by the U. S. Power Pr in 1 day to raise the concentration level of the world's atmospheric volum the 168-hr maximum permissible concentrations (MPC). The total mass of fission products would be less than 9 lb.

W

+

masses of radioactive material, some of the materials reach the a phere as wastes. Therefore, if a sound nuclear industry is developed, a balance must be attained between the health pro and the economic advantages of atmospheric dispersal.

6-1 SOURCES

6-1.1 Mining and Milling Operations

Atmospheric contamination can arise from any number of so within the atomic energy industry. The first steps in the na flow line of nuclear materials are the mining and the milling of ura and thorium ores. Although the naturally occurring parent iso have recognized toxic values, the major radioactive hazards from their decay products, radium and mesothorium, and their re tive decay chains. In the mining and the milling phases, the a pheric contamination comes from the dusts and mists that are erated mechanically. Within enclosures such as mines and rooms, the outward diffusion of the inert gases can create u problems, but this type of activity has never been detected at I of concern in the open air.

The concentrated product (the uranium or thorium ore) is fu refined and processed in the mills. The atmospheric contami from these plants are akin to those of any advanced metallu processing and fabricating activities. Dusts, corrosive mists gases, and other aerosols are generated which usually have greater che toxicity than radiological toxicity. When feed materials are essed, the decay products are eliminated, and the uranium and rium isotopes are essentially daughter free. Processed uranium thorium are in more-concentrated form than the ore, however, a given dust loading would contain a higher proportion of these materials than a comparable loading in the mining and milling a ities where the bulk of the material is stable. These areas have trol and evaluation problems, but from the public standpoint the of relatively local concern.

6-1.2 Reactor Operations

3

When fuel is fissioned to sustain a chain reaction, large amoun heat are created, and many curies of radioactivity are formed. activity arises primarily from the residue of uranium fission. Ac ing to Glasstone approximately 2.68X10 curies of radioact are formed per day for each megawatt of thermal power. Thus 3600 Mw of power reactors projected for the United States will duce about 108 curies of fission products daily. Fission prod from fuel are generally contained within a metal-clad solid max but occasionally the cladding is breached, and certain fission prod

cal work. In effect the coolant acts as a barrier to the gross re of fission products.

In reactor operations neutrons that are not captured in the fu available for activation of other materials. They can induce act in the coolant or in the surrounding support and structural mate which may be eroded and then carried by the coolant. In additi the activation products from the structure, the coolant may co impurities that would become activated when exposed to the ne flux of the reactor. This is particularly true of air-cooled rea where large amounts of Ar11, a short-lived inert gas, and smaller tities of several other nuclides, such as C14, P32, and Na2, are for Activity in the coolant stream may be released to the atmos through coolant leaks and spills, through special vents design degas liquid coolants and remove foreign gases in closed systems, direct atmospheric discharge in open systems. The major f products released by operating reactors are the inert gases, kry and xenon, and the halogens, which are in the vapor phase at oper temperatures. From the public-health standpoint, the longnuclides of current concern are Sroo, the daughter of Kr90, and Cs1 daughter of Xe137. In both cases the precursors have half-lives w though short, may be long enough to permit their diffusion thr one or more barriers before they decay completely. The time bet formation and release may vary with reactor design from one to se half-lives of the inert-gas nuclides.

In general, before discharge to the atmosphere, radioactive mat produced in reactor fuels must pass through several barriers, have large retention factors. First is the solid matrix in which f products are usually formed; second is the cladding of the fuel eler third is the coolant and its confinement system; and last, especial power reactors, is a containment vessel. Should radioactive ma pass the first three barriers, it can be treated before release t atmosphere.

In addition to activity released under operating conditions, res are the indirect source of other radioactive materials that may be airborne. One is the deliberately produced radionuclides forme irradiating selected stable elements with reactor neutrons. nuclides are processed and distributed for beneficial uses. Durin processing steps some airborne radioactive materials may be rele Another source of airborne contamination is the processing of fuel and blankets for recovery of enriched uranium and pluto Since 100% burnup is prevented by fission-product poisoning as

of its original fuel. The high cost of enriched uranium makes it economical to recover the unused fraction. A similar process is used as a step in the production of plutonium. Such processing is generally delayed for at least 100 days following removal of the fuel from the reactor to permit decay of isotopes with short-to-moderate half-lives. During the dissolution and the recovery operations, a certain amount of airborne contamination is generated by agitation, transfer, and chemical manipulation. Little loss of material occurs during processing, except for small amounts lost through vent-gas systems, leaks, and spills. Whenever radioactive material escapes or is released from the processing system, it becomes a potential atmospheric contaminant. Most chemical-separation processes use solvent extraction, and the first-cycle raffinate, which contains the majority of fission products as impurities, is stored. Any system that handles materials of this type must be vented. The formation of gas by gamma decomposition and by other processes, such as self-heating, generates radioactive aerosols that can escape from storage tanks. Therefore most vent gases are cleaned by suitable filtration or other methods prior to release.

Extensive investigations are under way to find ways of fixing the high-level wastes that are being stored as corrosive liquids. The present storage concept is considered "temporary." When satisfactory methods are developed, the fixation processes, such as adsorption on clay with subsequent vitrification or fluidized-bed calcination, will become sources for potential atmospheric contamination.

The radioactive pollutants most prevalent in the vicinity of fuelreprocessing facilities are the longer lived isotopes of krypton, xenon, iodine, strontium, zirconium-niobium, ruthenium-rhodium, cesium, barium-lanthanum, and cerium. The inert gases, krypton and xenon, are of concern only as sources of external gamma exposure when humans are in the immediate vicinity of the release plume. Iodine may be of concern both for external gamma and for inhalation exposure under special circumstances, but its greatest threat is from deposition on cattle feed and subsequent concentration in the thyroids of infants. The other isotopes may be minor threats in inhalation exposure and external gamma exposure if a subject is immersed in the release plume, but their greatest potential is through accumulation in the environment and eventual contamination of the food chain.

6-2 DIFFUSION PREDICTION

Since atmospheric dilution is the more economical method of handling many airborne contaminants, a number of approximations have