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Chapter III SOURCES OF IONIZING RADIATION AND POPULATION EXPOSURES I. Introduction 12 II. Natural Background Radiations 12 III. Medical Exposures 13 A. Medical and Dental Radiology 13 B. Diagnostic Use of Radiopharmaceuticals 14 C. Summary of Medical Exposures 14 IV. Nuclear Power 15 A. Projected Growth 15 B. Estimated Exposures 15 1. Uranium Mines 15 2. Uranium Mills and Fabrication Plants 15 3. Power Generating Plants 15 (a) Normal Operations 15 (b) Accident Conditions 16 4. Fuel reprocessing plants 16 C. Tritium 17 D. Krypton-85 17 V. Nuclear Explosions 17 A. Local Fallout from Atmospheric Tests 17 B. Local Exposure from Underground Tests 17 C. Worldwide Global Fallout 17 VI. Nuclear Ships 18 VII. Nuclear Rocket Development 18 VIII. Miscellaneous and Occupational Exposures 18 IX. Summary 18 References ........................ ......................,......,....... 19 11
Chapter III SOURCES OF IONIZING RADIATION AND POPULATION EXPOSURES I. Introduction The scale and nature of past and foreseeable exposures are reviewed in order to provide a focus for risk estimates and because radiation protection guidance should ultimately take into account all sources of exposure. This dis- cussion is not intended to be a critical, compre- hensive, or independent review, and is included only for information and convenience of refer- ence. The Special Studies Group, Office of Ra- diation Programs, Environmental Protection Agency is preparing a detailed review and their draft material (1), kindly made available to us, has served as a major basis for the present statement. Except as otherwise referenced, material in this chapter is drawn from the re- port of the Special Studies Group. Estimates of present and past exposures are thought to be accurate within a factor of two; projections have uncertainties probably within an order of magnitude. It is considered useful, neverthe- less, to have available some estimate of possi- ble exposures, indications of which processes are liable to produce various types of expo- sures, and particularly information as to changes that could be made to reduce exposures effectively. II. Natural Background Radiation External exposure from natural background comes primarily from cosmic radiation and ter- restrial gamma radiation. Dose equivalent rates from these sources vary because of alti- tude, latitude, and differences in amounts of natural radioactive material present in the earth. For example, cosmic radiation increases by a factor of three in going from sea level to 10,000 feet, and by 10 to 20% in going from 0° to 50° geomagnetic latitude. Internal exposures arise from body deposi- tion of radionuclides that have been inhaled or ingested. The major contributor from inhala- tion is radon and its daughter products, and from ingestion, potassium-40. Table 1 presents estimated total annual av- erage whole-body doses from natural radiation in the United States (2). The dose equivalent rate for cosmic radiation ranges from 38 mrem/ yr in Florida to 75 mrem/yr in Wyoming. Ter- restrial gamma dose rates range from 15 to 35 mrem/yr for the Atlantic and Gulf Coastal Plains to 75 to 140 mrem/yr for the Colorado Plateau. Taking into account shielding by building structures and overlying tissue, it has been calculated that the average gonadal dose is about 90 mrem/yr (2). Table 1 Estimated Total Annual Average Doses from Natural Radiation in the United States (Whole-body) Source Annual Doses Cosmic Rays Terrestrial Radiation External Internal Total 44 40 18 102 12
III. Medical Exposures The use of radiation in the healing arts is recognized today as the largest man-made com- ponent of radiation dose to the United States population. Contributions from this source in- clude medical diagnostic radiology, clinical nuclear medicine, radiation therapy, and occu- pational exposure of medical and paramedical personnel. A. Medical and Dental Radiology Estimates of the annual genetically signifi- cant dose (GSD) to the U.S. population from medical and dental radiology range from 18 to 136 mrems. This variation is influenced by rela- tive child expectancy, age and sex distribution of the subjects, usage distribution of specific examinations, and gonad dose per examination. On the basis of the study of a large segment of the population in 1964, the Public Health Serv- ice estimated an annual GSD of 55 mrems (3); it was further estimated that 80% of this dose was due to x-ray examinations of males above age 15 and that 95% was due to abdominal examinations of both males and females. About 96% of the dose was contributed by radiogra- phy, 4% by fluoroscopy, and less than 1% by photofluorography. Estimates have been made of future doses. Data indicate that film sales between 1945 and 1965 increased at a rate of 5.4% per annum. A study in selected hospitals indicates that be- tween 1963 and 1968 the annual rate of dis- charges of patients in any diagnostic radiation category and in total diagnostic radiation cate- gories increased by 3.6% and 6.6%, respective- ly. Public Health Service studies show that the examination rate for medical diagnostic ra- diography increased from 61.8 examinations per 100 persons in 1964 to 68.5 examinations per 100 persons in 1970, about 2% increase per annum. In summary, it can be estimated that the per annum increase in the rate of delivery of radiographic procedures ranges between 1% and 4%. Other factors which are relevant to the pro- jected population dose from medical radiogra- phy include potential reductions which may be accomplished through improved collimation and radiographic techniques. Studies have shown that dose reductions of as much as 30% could be accomplished through improved tech- niques. This is inclusive of 10% of unnecessary radiation due to repetitive examinations. Gona- dal shielding is particularly important in the reduction of male gonad doses and data show that use of proper shields can reduce this dose by as much as 90%. Proper collimation can result in reductions of the genetically signifi- cant dose by about 65%. However, in practice, this is not accomplished easily, or carried out routinely, since it requires care in positioning the patient and x-ray tube as well as selection of proper cone for x-ray field size adjustment. Considering all of these factors, experts esti- mate that it appears reasonable that as much as a 50% reduction in the genetically signifi- cant dose from medical radiology might be pos- sible through improved technical and educa- tional methods. On the assumption that techni- cal improvements keep pace with the increased usage of radiation in medical radiology, it ap- pears reasonable to estimate that the mean per capita radiation dose within the next several decades will remain fairly stable. Estimates of specific organ doses are gener- ally not yet available for the United States. As a first step in estimating approximate relative somatic dose one might determine the mean dose in the center of the abdomen. This index, the "Abdomen dose" has been calculated from dose estimates, ovarian (for females) and "sim- ulated ovarian" (for males), weighted for their representation in the U.S. population. Because the entire population, regardless of age, is used, the data are not weighted for future child bearing potential, and the dose estimates are not sensitive to small variations in beam size and position, the difficulties encountered in de- termination of the GSD are reduced. Further- more, one is able to calculate dose estimates for the exposed population (i.e. considering only persons receiving x-ray examinations). The biological significance of the "abdominal dose" has not been evaluated. The unequal distribu- tion of body area exposed, the non-homogeneity of human tissue, and variations of dose with age all preclude such application. In 1970, based on preliminary information, the abdomi- nal dose due to medical radiography appears to have risen. This increase, due entirely to an elevation of the female per capita abdominal dose, requires elucidation. For the whole U.S. population, the annual per capita abdominal 13
doses for 1964 and 1970 are estimated to have been 61 and 72 mrem, respectively. Errors in the survey and the measurements from which these estimates were derived have not, as yet, been fully evaluated. Accordingly, it appears that the most prudent conclusion is that this index has remained relatively stable during this interval. Dental x-ray visit rates during 1961 and 1964 were similar, with a slight decrease during the latter year being attributable to sampling differences. Between 1964 and 1970, a gradual rise of about 4% per annum in the annual rate of dental x-ray visits was evident. Using a per capita dental x-ray visit rate of 0.27 and an average of five films per visit, the predicted somatic person-rem to the United States popu- lation from dental x rays in the year 2000 will be less than 0.2 x 106. B. Diagnostic Use of Radiopharmaceuticals An early estimate (1956) of genetically sig- nificant dose from medical uses of radionuclides indicated a dose of 8 mrem per person per year, based on the total quantity of 131I and 32P shipped during the year. A subsequent analysis (1957) assuming a diagnostic examination rate of 150,000 to 200,000 per year using 131I, of which probably not more than 25,000 examina- tions were performed per year on patients be- low age 30, indicated the genetically significant dose would be equivalent to 0.004 mrem. An equal genetically significant dose was alleged to the accrued through other diagnostic radio- nuclide procedures for a total annual GSD of 0.008 mrem (1). More recent information yielded an estimat- ed total accumulated gonad dose of 195,000 rem (sum of doses to all exposed individuals) from all diagnostic radiopharmaceutical procedures to all age groups. Again, if one assumes that 12.5% of the individuals receiving these proce- dures are below age 30, and that 50% of the total population is also below age 30, then the estimated annual GSD is 0.26 mrem from the diagnostic use of radiopharmaceuticals. Studies of radiopharmaceuticals indicate increases in the rate of administrations of be- tween 15 and 20% per year in the mid-1960,s. More recent information based on sales of radi- opharmaceuticals indicated an annual increase of 25% per year. It appears judicious to esti- mate that in the 1960,s the use of radiopharma- ceuticals increased fivefold during the 10-year period and that an increase of sevenfold may be experienced in the next 10 years. Thereafter, it is difficult to make predictions, especially in terms of dose since technical changes are likely to play a large role in dose reduction in so rap- idly changing a field. Assuming no technical changes, and the growth pattern indicated above, it is expected that the whole-body dose to the United States population in 1980 from diagnostic use of radiopharmaceuticals will be 3.3 million person-rem. Even with a slowing of the rate of increased use of radiopharmac- euticals the accrued whole-body man-rem could easily reach 15% of the total somatic dose from all man-made souces by the year 2000. Improvements in equipment have led to de- creased dosage requirements in thyroid func- tion tests and kidney scans, and the substitu- tion of radionuclides yielding lower patient exposure have already reduced total body and kidney doses per procedure. Even with these improvements, in the 4-year period 1964 to 1968, one institution reports that the average whole-body dose per patient increased from 100 mrem to 160 mrem due to the increased use of radiopharmaceuticals. C. Summary of Medical Exposures Published literature shows that the Geneti- cally Significant Dose to the U.S. population from medical and dental radiation in 1964 was 55 mrem (3). A preliminary estimate of the val- ue for 1970 is 36 mrem (4); however, the uncer- tainty surrounding the GSD results for 1964 and 1970 have not as yet been calculated. Estimates indicate that the annual per capita "abdominal dose" from these sources to the whole population was 72 mrem in 1970. It ap- pears that the mean "abdominal dose" for the U.S. population as determined in 1964 and 1970 has remained relatively stable during the inter- val. As with the GSD, because the uncertainty has not been calculated, the magnitude of whatever change may have occurred has not been determined. The main contributor of the total dose from medical exposures is diagnostic x-radiation, the contribution from dental radia- tion and radiopharmaceuticals being far lower. Radiation therapy in the treatment of cancer was estimated to contribute an additional 5 14
mrem to the GSD annually. Based on informa- tion presently available, it appears that the mean per capita radiation dose from diagnostic medical radiology could remain stable in future years, if technical improvements keep pace with increased usage rates. IV. Nuclear Power A. Projected Growth There are great pressures for growth of the nuclear power industry. These include: demand for electricity, potential shortages of certain types of fossil fuels, and the effects of present modes of producing electricity on health and the environment. The extent to which the nucle- ar industry will grow depends upon technology development, establishment of its health and environmental costs, and public acceptance. As a point of departure the Special Studies Group of the Environmental Protection Agency (1) has assumed that nuclear capacity in the Unit- ed States will increase from 6000 megawatts in 1970 to 800,000 megawatts in 2000. Associated with this increase there is a postulated 25-fold increase in uranium mining and milling, a 15- fold increase in fuel fabrication facilities, and establishment of about 15 commercial fuel re- processing plants compared to one now in exist- ence. For purposes of dose projections, the Spe- cial Studies Group has also assumed that a lim- it of 5 mrem per year per reactor at the site boundary will be met. B. Estimated Exposures 1. Uranium mines The mining of uranium, while increasing the amount of uranium and its radioactive decay products accessible to man, has not been found to cause measurable increases in environmen- tal radioactivity outside the immediate vicinity of the mines. The primary problem in under- ground uranium mines is known to be the inert gas radon 222 and its short-lived daughters; physical attachment of the daughters to air- borne dust particles is probably the most sig- nificant process. It is possible that long-term effects of later daughters, especially bismuth- 210, lead-210, and polonium-210, could be of importance as the latter two appear to move readily through the biosphere. Whereas these operations are not considered as contributors to general environmental contamination there have been serious health problems among un- derground uranium miners and these matters are discussed in detail in a later chapter. 2. Uranium mills and fabrication plants The extraction of uranium from ore produces by-products or tailings and waste that can constitute a source of environmental radiocon- tamination, primarily 226Ra and its decay prod- ucts. 226Ra is rather insoluble in water but does dissolve slowly and enters the biosphere espe- cially in water and aquatic biota. Deposition in crops from irrigation water has been observed. The location of uranium mills in sparsely popu- lated areas and appropriate control of tailings and liquid waste on restricted areas can pre- vent population exposures from this source. Tailings have been used in the past for public construction in certain communities. Presumably this will not happen again. Fuel fabrication can be and is carried out in such a way as not to increase levels of radioac- tivity in the environment. 3. Power generating plants (a) Normal Operations The principal radionuclides in present reac- tor effluents are 3H, 58Co, 60Co, 85Kr, 89Sr, 90Sr, 131I, 131Xe, 133Xe, 134Cs, 137Cs, and 140Ba. The amounts released depend greatly upon the type and design of the reactor. Gaseous and volatile nuclides such as 85Kr, 131Xe, and 133Xe contrib- ute to external gamma dose as a result of im- mersion; the others contribute to the dose ex- ternally by surface deposition and internally via the food chain. The most important stack discharges from reactors are radionuclides of noble gases in the elemental form, tritium as water vapor, and iodine in a variety of forms such as elemental or organic compounds. The noble gases are metabolically inert in contrast to the tritium and iodine compounds. Liquid wastes from reactors contain tritium as HTO and activation radionuclides of iron, cobalt, nickel, and zinc. The latter may be oxidized or complexed to affect solubility. 15
K-and when Liquid Metal Fast Breeder Reac- tors (LMFBR) become commonplace, other ra- dionuclides will become sources of environmen- tal contamination including 239Pu, 238Pu, 241Pu, 24Na, and 22Na. Normal release rates are expected to be low but the implications of han- dling large amounts of radioactivity and of accidents will have to be taken into account. External dose rates from power reactor ef- fluents have been estimated by computer mod- els assuming whole-body gamma doses of 5 mrem/year at each reactor boundary. The an- nual average dose to the U.S. population was estimated to be 0.002 mrem for the year 1970 and 0.17 mrem for the year 2000. Predictions could be high because of improvement in tech- nology or could be low if plant performance deteriorates with time; also no account is taken in this calculation of the possibility of local exposures due to accidents. Consideration of internal exposures leads to two conclusions: (a) the principal radionuclide will be 131I; (b) internal doses will be much low- er than the overall external gamma radiation doses. (b) Accident Conditions Nuclides escaping from reactors under de- sign basis accident conditions may be classified as volatile or non-volatile. The former include noble gases, iodine and tritium, whose behavior is as previously described. Nonvolatile nuclides of importance include all fission products, many activation products, uranium, and plu- tonium. Inhalation with dependency on particle size will probably be more important than ex- posure via environmental contamination. In an LMFBR the initial inhalation problem will arise from plutonium with long-lived 22Na be- coming subsequently of more concern. For a light water thermal power reactor, the major catastrophic event which could result in the release of large quantities of radioactive materials to the environment would be the loss of coolant accident. Although the absence of water moderator would stop the fission process, the decay heat due to the inventory of radioac- tive material could result in a meltdown of the reactor core. Most power reactors have provisions for an emergency core cooling system, but questions have been raised (5-8) as to the ability of such systems to prevent a meltdown. If there were a loss of coolant accident or if the cooling system were damaged by sabotage, the effectiveness of the emergency cooling system would be critical. Unless this system provides adequate cooling water in a very short time, the fuel in the reac- tor core would certainly melt and the molten material would break through the containment vessel. If this were to happen, substantial quantities of gaseous and volatile and nonvola- tile fission products would escape to the atmos- phere in the form of a radioactive cloud, and could cause considerable radiation exposure to people downwind from the reactor site. The total number of individuals who could receive serious damage in a single accident of this type, if it should occur, is likely to be considera- bly larger than from all of the predicted design- basis accidents. This Committee is not competent to estimate the frequency of such accidents, to assess the severity of effects that could occur, or to rec- ommend the extent of preventive measures that should be taken. It seems clear, however, that considerable effort and expenditures are war- ranted to reduce these risks and will, in fact, be required if the nuclear power industry is to develop in conformance with protection of the public that is required, taking into account the design and engineering needs commensurate with the potential for damage. 4. Fuel reprocessing plants From fuel reprocessing plants, the most im- portant gaseous effluents include tritium as HTO, noble gases especially 85Kr, elemental iodine, organic iodides, and probably such forms as NOI and HIO. Short-cooled fuels may contain 131I, all fuels will contain 129I. Liquid effluents from reprocessing contain radionu- clides of cesium and strontium, tritium as HTO, other long-lived fission products including the rare earths and possibly plutonium. Cesium and strontium are relatively soluble and meta- bolically available. The rare earths, zirconium, and niobium tend to be insoluble and metaboli- cally inert. Dose calculations for population exposures from nuclear fuel reprocessing are greatly dependent on the assumptions made. It has been estimated by the Special Studies Group that the average annual whole-body dose to the 16
U.S. population from fuel reprocessing was 0.0008 mrem in 1970 and will be 0.2 mrem in the year 2000; the tissues of major exposure are the respiratory lymph nodes, the thyroid gland, and skin. Skin and whole-body doses from i^3Xe and thyroid doses from 131I could be reduced by longer decay times before reprocessing; 129I would still need to be removed as well as 85Kr if possible. Improved particulate removal could reduce the dose to the respiratory lymph nodes. C. Tritium Tritrium and krypton-85 should be assessed on a basis of worldwide production because of their distribution patterns. Tritium is distrib- uted throughout the surface waters of the world and is of concern to man through any exposure involving water. The sources and re- lative contributions to the world inventory in 1970 expressed as megacuries of tritium are roughly as follows: reactor produced, 0.5 to 1; naturally produced, 10 to 102; nuclear explo- sions, about 103. Tritium from the nuclear pow- er industry is not expected to reach levels equal to those resulting from past weapons-tests un- til about the year 1990. The annual dose from worldwide tritium is estimated to beJUM mrem/ person in 1970 and 0.03 mrem/pefsorT in the year 2000. In 1970, the concentrations of triti- um in the oceans and surface and ground wa- ters in the U.S. typically ranged from 0.2 - 1.5 nanocuries/liter. D. Krypton-85 Krypton-85, a noble gas, is distributed throughout the atmosphere and is a source of exposure to man both externally and through inhalation. Production of krypton-85 naturally by cosmic rays and artifically by weapons deto- nations is very low compared to production by the nuclear power industry. The world invento- ry from nuclear explosives is calculated to be about 3 MCi; reactors are already producing more than 10 megacuries/yr. The concentration of 85Kr in air has been es- timated as 15 pCi/m3 for 1970. The estimated annual whole-body doses to the U.S. population from worldwide distribution of 85Kr are 0.0004 mrem/person in 1970 and 0.04 mrem/person in the year 2000. Skin doses are calculated to be about 50 times greater and lung doses twice as great as whole body (*oses. V. Nuclear Explosions A. Local Fallout from Atmospheric Tests As an example of local fallout, calculations have been made for exposures in the vicinity of the Nevada Test Site for the period September 15, 1961 to September 15, 1962. This was the period of resumption of atmospheric nuclear tests following the moratorium of 1958. The exposures are estimated as 47 mrem external gamma dose to a population of 18,000; 10 mrem whole body dose from i37Cs to a population of 792,000; 9 mrem to the thyroids of the same population. These sum to a total of 8766 per- son-rem to the whole body. B. Local Exposure from Underground Tests As examples of exposures from underground tests and applications of nuclear devices, men- tion is made of two specific events. During the "Gnome" test (December 10, 1961) some venting occurred to produce mainly a gaseous effluent. Radioactivity was detected only within about 10 miles of the test site. External gamma dose from the cloud gave a total of 30 person-rem to a population of about 45,000; there were no in- ternal radiation exposures as judged by analy- sis of environmental samples. During the gas production phase of the "Gas- buggy" test, radioactivity in which only triti- um, 14C and 85Kr were detected, was released from the well. No radioactivity was detected beyond 10 miles from the well and there are no populated sites within that area. Experimental programs have progressed to some extent along lines of excavation, gas stim- ulation, recovery of oil from shale, mineral re- covery, underground storage, waste and water management, and use of geothermal energy. There is no basis for protection of population exposures from such activities and estimates would need to be made on an individual basis. C. Worldwide Global Fallout Worldwide fallout is mainly a result of large- scale high-yield atmospheric tests conducted by the U.S. and U.S.S.R. prior to 1963; during 4«9-7«7 O - 7J - 3 17
the past several years the relatively small tests conducted by the French and Chinese have maintained an annual fallout deposition that has been relatively constant. The total annual whole-body doses from glo- bal fallout in mrem/person ranged from 13 in 1963 to 4.0 in 1969. As an example, the contri- butions to the total dose of 4.0 mrem/person in 1969 were: 0.9 mrem external gamma; 2.1 mrem from 90Sr; 0.4 mrem from 137Cs; 0.6 mrem from 14C. It should be emphasized that these are average values and actual values could vary by more than factors of 2 because of variation in fallout and diet. If the rate and type of test- ing from 1965-1970 continues, the annual dose is calculated to reach 4.9 mrem/person in the year 2000. VI. Nuclear Ships In 1970, the United States had in operation 96 nuclear powered vessels, 92 submarines and 4 surface ships. About 0.024 Ci of radioactivity were released in liquid wastes in 1970 including 187W, siCr, isi Hf, 59Fe, sspe, 95Zr, 182T1, 54Mn,58Co,60Co. VII. Nuclear Rocket Development From 1959 through 1969, 31 nuclear reactor rocket engine tests were conducted at the Nu- clear Rocket Development Station. External gamma doses were too low to be significant. Thyroid doses from 131I and 133I were calculat- ed to be about 3 mrem to a population of about 740,000 or about 2100 person-rem during the 10-year period. VIII. Miscellaneous and Occupational Radia- tion The contribution of miscellaneous radiation sources such as television, consumer products, and air transport, to average whole-body doses are as follows: 2.0 and 2.6 mrem/yr for 1960 and 1970, respectively. Projected doses are 2.1 mrem for 1980 and 1.1 mrem for 1990 and 2000. The extent of occupational exposures to radia- tion is grossly estimated as follows: total per capita dose - 0.8 mrem/yr; mean dose per work- er - 200 mrem/yr; number of workers - about 3/4 million; total person-rem/yr - 0.16 million. Most of this dose was incurred through the use of ionizing radiation in the practice of medicine and dentistry. If the projections made by the Special Studies Group (1) are accepted, increas- es in nuclear power production over the next several decades are expected to increase the average per capita dose by only about 0.1 mrem/yr. IX. Summary Table 2 presents a summary of estimates of whole-body radiation dosesJ It is clear that major contributors to radiation dose are natu- ral background and medical applications. By far the greatest portion of man-made radiation dose to the U. S. population is due to exposure accrued during medical diagnostic procedures. Medical diagnostic radiology accounts for at least 90% of the total man-made radiation dose to which the U.S. population is exposed. This is at least 35% of the total radiation dose from all sources (including natural radioactivity). On the average, according to the EPA report (1) the contribution from the developing nuclear power industry is expected to contribute a pop- ulation dose of less than 1% of natural back- ground. Therefore, in order to ascertain the radiation effects on the population of nuclear power development, risk estimates must be made for levels of a few millirem per year. It must be emphasized that the estimated radiation doses as presented in summary form, although the best now available, must be re- garded for what they are - rougrjt estimates subject to uncertainties of projection and engi- neering performance. To assess the effects of the radiation expo- sure it is important also to take into considera- tion the higher doses that might be received by sub-sets of the population; such matters have to be dealt with on an individual case basis. In addition, there are some areas of uncertainty in regard to nuclear power which cannot be iExcept for medical diagnostic radiation, which is based on the abdominal dose. 18
Table 2 Summary of Estimates of Annual Whole-Body Dose Rates in the United States (1970) Source Average Dose Ratei (mrem/yr) Annual Person-Rems (in millions) Environmental Natural Global Fallout Nuclear Power Subtotal 102 4 0.003 106 20.91 0.82 0.0007 21.73 Medical Diagnostic Radiopharmaceuticals Subtotal Occupational Miscellaneous 72** 1 73 0.8 2 14.8 0.2 15.0 0.16 0.s TOTAL 182 37.4 iNote: The numbers shown are average values only. For given segments of the population, dose rates considerably greater than these may be experienced. **Based on the abdominal dose. assessed by this Committee. These include: (a) engineering matters such as possibilities of failure of plants to meet anticipated levels of performance or deterioration of plant perform- ance with time, (b) large scale management of radioactive wastes, (c) probabilities and effects of catastrophic accidents, (d) possibilities of sabotage and diversion of fissionable material. REFERENCES (1) "Estimates of Ionizing Radiation Doses in the United States, 1960-2000," Special Studies Group, Office of Ra- diation Programs, Environmental Protection Agency, Rockville, Maryland (1972). Draft, June 1971. (2) Oakley, Donald T., "Natural Radiation Exposure in the United State," Doctoral Thesis, School of Public Health, Harvard University, Boston, Massachusetts (1972). (3) Public Health Service, "Population Dose from X-rays, U.S., 1964," PHS Publication No. 2001, Washington, D.C. (1969). (4) Gitlin, J. N., 1972. Preliminary dose estimates from the U.S. Public Health Service 1970 X-ray exposure study. Paper presented at the 49th Annual Meeting of the American College of Radiology, Miami Beach, Florida, April 6,1972. (5) Ford, Daniel F., Kendall, Henry W., and MacKenzie, James J., "A Critique of the AEC,s Interim Criteria for Emergency Core-cooling Systems," Nuclear News, Vol. 15, No. 1, pages 28-35 (January, 1972). (6) "Theoretical Possibilities and Consequences of Major Accidents in Large Nuclear Power Plants," U.S. Atomic Energy Commission, Report WASH-740 (1957). (7) Howells, H., and Dunster, H. J., "Environmental Moni- toring in Emergencies," Chapter 20, pages 151-161, in "Environmental Surveillance in the Vicinity of Nuclear Facilities," Proceedings of a Symposium Sponsored by the Health Physics Society, Charles C. Thomas, Publish- er, Springfield, Illinois (1970). (8) Weinberg, Alvin M, "The Moral Imperatives of Nucle- ar Energy," Nuclear News, Vol. 14, No. 12, pages 33-37 (December, 1971). 19
