Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
2 Natural Radioactivity and Radiation This chapter describes the behavior of selected natural radionuclides in the environment, the sources and variability of natural radiation, and the doses received by humans. Its purpose is to provide background information for discussions of the mechanisms by which exposures to natural sources can be increased by technologic activities, that is, can become exposures to TENORM. A more detailed account of natural radiation can be found in Eisenbud and Gesell (1997), which was used as a guide to prepare parts of this chapter. Natural radiation comprises cosmic radiation and the radiation arising from the decay of naturally occurring radionuclides. The natural radionuclides include the primordial radioactive elements in the earth's crust, their radioactive decay products, and radionuclides produced by cosmic-radiation interactions. Primordial radionuclides have half-lives comparable with the age of the earth. Cosmogenic radionuclides are produced continuously by bombardment of stable nuclides by cosmic rays, primarily in the atmosphere. Humans are exposed to natural radiation from external sources, which include radionuclides in the earth and cosmic radiation, and by internal radiation from radionuclides incorporated into the body. The main routes of radionuclide intake are ingestion of food and water and inhalation. A particular category of exposure to internal radiation, in which the bronchial epithelium is irradiated by alpha particles from the short-lived progeny of radon, constitutes a major fraction of the exposure from natural sources. In most places on the earth, natural radiation from external sources varies within about a factor of 4; but in some localities, the variation is greater because of abnormally high or low soil concentrations of radioactive minerals. Cosmic radiation alone varies by about a factor of 2 over the range of elevation that encompasses most of the world's population (0-2,000 m) and to a much smaller degree with latitude because of the variation in the earth's magnetic field. Particularly high concentrations of radioactive minerals in soil have been 25
26 NO TURAL R24DIOA CTIVITY AND RADIA TION reported in Brazil, India, and China. Variations of radon concentrations in buildings are responsible for the largest variations in doses received by the public from natural internal sources. NATURALLY OCCI)~RING RADIONUCLIDES The origin of the primordial natural radionuclides of the earth is associated with the phenomenon of nucleosynthesis in stars (Fowler 1967~. The fact that the uranium, thorium, and actinium decay chains are found in nature is directly related to the very long half-lives of the parents of these chains. The absence of the neptunium decay chain is due to the lack of sufficiently long- lived members of this chain; complete decay of the parent radionuclides and their progeny has already occurred. Naturally occurring radionuclides with long half-lives that are not members of decay chains also exist in relatively high isotopic abundance. For purposes of discussion, the naturally occurring radionuclides are divided into those which occur singly (tables 2.1 and 2.2) and those which are components of three chains of radioactive elements. The uranium chain (table 2.3) originates with MU; the thorium chain (table 2.4), with 232Th; and the actinium chain (table 2.5), with 235U. Each table shows the nuclide, half-life, and principal radiations associated with each important branch of the chain. Minor branches, (less than 1%) and natural fissions are not listed, nor do they make any important contribution to the radiation dose from these chains. Tables 2.1 and 2.2 also show typical concentrations in various environmental media. 2In nature, 235U and a few other nuclides of uranium and thorium undergo fission spontaneously or as a result of interactions with neutrons that originate in cosmic rays or other natural sources. The half-life of 235U owing to spontaneous fission is 10~5-10~6 y, so decay by this process is at a rate less than 10-7 of that due to alpha-particle emission.
GUIDELINES FOR EXPOSURE TO TENORM Table 2.1 Radionuclides Induced in Earth's Atmosphere by Cosmic Raysa 27 Radio- Half- Major Target nuclide life Radiations Nuclides Typical Concentrations, Bq/kg Air (troposphere) Rain Water Ocean Water ~°Be 1,600,00 ,B N. O -- -- 2 x 10-8 Oy 26Al 716,000 ~Ar -- -- 2 x y 1 0-'° 36C1 300,000 ,B Ar -- -- 1 x 10-5 y 8'Kr 229,000 K x rays Kr -- -- - y 14C 5730 Y ,B N,O - 5 x 10-3 32si 172y ~Ar -- -- 4 x 10-7 39Ar 269 y ~Ar -- -- 6 x 10-8 3H 12.33 y ,B N,O 1.2x10-3 7x10-4 22Na 2.60 y ,B+ Ar lx10 ~2.8x10-4 35S 87.51 d ,B Ar 1.3 x 10-4 7.7x 10-3 107x 10-3 7Be 53.29 d ~N. O 0.01 0.66 - 37Ar 35.0 d K x rays Ar 3.5 x 10-s 33p 25.3 d ~Ar 1.3 x 10-3 32p 14.26 d ~Ar 2.3 x 10-4 - 28Mg 20.91 h ~Ar 24Na 14.96 h ,B Ar -- 3.0x10-3 5.9 x 10-3 38s 2.84 h ,B Ar -- 6.6x 10-2 21.8 x 10-2 aAdapted from NCRP (1987a) and NuDat online database maintained by Brookhaven National Laboratory, September 9, 1997.
28 Table 2.1 (continued) NATURAL RADIOACTIVITYAND RADIATION Radio- Half- Major Target nuclide life Radiations Nuclides Typical Concentrations, Bq/kg AirRain Water Ocean (troposphere)Water 31 18F 2.62 h ,B Ar 1.83 h p+ Ar -- -- - 39C1 55.6 m ~Ar -- 1.7 x 10-i 8.3 x 10-~ 38C1 37.24 m ~Ar -- 1.5 x 10-~ 25 x 10-~ 34mcl 32.0 m ,B + Ar
GUIDELINES FOR EXPOSURE TO TENORM Table 2.2. Nonchain Primordial Radionuclidesa Radionuclide Half-life, y Major Radiations Typical Crustal Concentration, Bq/kg 40K 1.28 x 109 0, y sov 1.4 x 10~7 4.75 x 10'° 87Rb 't3Cd 9 x 1O's 630 2 x 10-5 70 <2 x 10-6 ~sIn 6x 10'4 ~2x 10-5 123Te 1.24 x 10'3 x rays 2 x 10-7 ~3sLa 1.05 x 10t' ~, ~y 2 x lo-2 ·42ce >5x 10~6 ~<1 x 10-5 i44Nd 2.29 x 10~5 a 3 x 10-4 '47Sm 1.06x 10" a 0.7 .s2Gd 1.08 x 10'4 a 7 X 10-6 174Hf 2.0 x 10~5 a 2 x 10-7 ~76Lu 3.73 x 10~° ,B, 1t 0~04 ~S7Re 4.3 x 10~° ,B 1 x 10-3 '90Pt 6.5 x 10" a 7 x 10-8 29 aAdapted from NCRP (1987a) and NuDat online database maintained by Brookhaven National Laboratory, September 9, 1997.
30 Table 2.3 Uranium-238 Chaina NATURAL RADIOACTIVITYAND RADIATION Nuclide Historical Name Half-life Major Radiations 23su Uranium I 4.47 x 109 y a, < 1% y aData from NuDat online database maintained by Brookhaven National Laboratory, September 9, 1997. Minor branches, <1%, not shown. 234Th Uranium X, 24.1 d ,B 234mpa Uranium X2 1.17 m 0, <1% y 234u Uranium II 2.46 x 105 y a, < 1% 230Th Ionium 7.54 x 104 y a, < 1% 226Ra Radium 1600 y a, y 222Rn Emanation 3.82 d a, < 1% y 2~8po Radium A 3.10 m a, < 1% ~y 214pb Radium B 26.8 m ,B, y 2~4Bi Radium C 19.9 m 0, ~ 2~4po Radium C 164.3 ~s a, < 1% ~y 2~0Pb Radium D 22.3 y ,B, y 2~0Bi Radium E 5.01 d 2~0po Radium F 138.4 d a, < 1% ~y 206pb Radium G Stable None
GUIDELINES FOR EXPOSURE TO TENORM Table 2.4 Thorium-232 Chaina Nuclide Historical Name Half-life Major Radiations 232Th Thorium 1.41 x 10l° y a, <1%y 228Ra Mesothorium I 5.75 y 0, <1 228Ac Mesothorium II 6.15 h ,B, ~ 228Th Radiothorium 1.91 y a,~y 224Ra Thorium X 3.66 d a, ~ 220Rn Emanation 55.6 s a, <1% y 2,6po Thorium A 0.145 s a, <1% 2l2pb Thorium B 10.64 h ,B, ~ 2~2Bi Thorium C 1.01 h a,y 212po (64%) 208TI (36%) Thorium C' / 0.300 ms I a / ,B, Thorium C'' 3.05 m 208pb Thorium D Stable None 31 aData from the NuDat online database maintained by Brookhaven National Laboratory, September 9, 1997. Minor branches, <1%, not shown.
32 Table 2.5 Uranium-235 (Actinium) Chaina NA TUR,4L RADIOA CTIVI7YAND RADIA TION Nuelide Historical Name Half-life Major Radiations 235u Aetinouranuim 7.04 x 1Os y a, 23lTh Uranium Y 1.06 d 0, y 23lpa Protoaetinium 3.28 x 104 y a, y `^ Aetinium 21.77y ~,<1%Y 227Th 223Fr Radioaetinium / 18.72 d / a, y 1 ~, (98~62%) (1.38%) Aetinium K 22.0 m 223Ra Aetinium X 11.44 d a, 2~9Rn Aetinon 3.96 s a, 2.5po Aetinium A 1.78 ms a, < 1% 2'~Pb Actinium B 36.1 m 0,y 2"Bi Actinium C 2.14 m a, ~y 2o7Tl Actinium C' 4.77 m 0, e 1% 2o7pb Actinium D Stable None aData from NuDat online database maintained by Brookhaven National Laboratory, September 9, 1997. Minor branches, <1%, not shown.
GUIDELINES FOR EXPOSURE TO TENORM 33 The three chains of radioactive elements and the long-lived primordial nuclide potassium-40 account for much of the external background radiation dose from radionuclides to which humans are exposed. Of the 22 nuclides identified as cosmogonic (table 2.1) only two, carbon-14 and tritium (3H), are of any consequence from the perspective of dose to humans. Only two of the 15 nonchain primordial nuclides, 40K and rubidium-87, are of particular interest (table 2.2~. Uranium and thorium can be concentrated in rocks by igneous and sedimentary processes (Bliss 1978~. Where uranium and thorium concentrations are high enough, rocks constitute ores to industrial societies. In the western United States, uranium ores have been extensively mined and milled to produce nuclear fuels. The biogeochemical behavior of a radionuclide in a given decay chain can be expected to vary with atomic number (that is, the element). For example, in the uranium decay chain, isotopes of uranium, thorium, radium, radon, and other elements occur. Chemically they range from an inert gas (radon) to a readily sorbed, tetravalent cation (thorium). Those properties determine the fate of the radionuclides in fuel and mineral processing, their transport in soil or surface disposal environments, and ultimately Heir biologic availability and uptake; a knowledge of their behavior is essential for defining source terms and assessing doses. Regulations for controlling exposure of the public to radionuclides are often dose-based. Because the doses result from interaction of humans with radionuclides contained in environmental media air, water, soil, and biota a knowledge of the behavior of naturally occurring radionuclides in these media is needed (Landa 1980~. It is important to know: · The different mobilities of the various radionuclides in the decay chains. · How technologic processes have changed the physical and chemical form of radionuclides and the release rates of radionuclides to the various media. · How naturally occurring radioactive materials evolve with time (weathering reactions). . The concentrations and physical and chemical forms of the radionuclides. The following sections discuss the naturally occurring radionuclides that are potentially important contributors to human exposure to TENORM.
34 NATURAL RADIOACTIVITYAND RADIATION Other natural radionuclides that are contributors to background radiation dose but not necessarily to exposure to TENORM are discussed for completeness, but in less detail. Uranium The primordial uranium found ubiquitously in nature consists of two isotopes with mass numbers of 235 and 238. In the earth's crust, 23sU constitutes 99.27% of the uranium by mass, and 235U, the parent isotope of the actinium chain, 0.72%. 234U, a shorter-lived member of the 238U chain, is usually in radioactive equilibrium or near-equilibrium with the parent isotope. Geochemistry Oxidation-reduction processes play a major role in the occurrence and behavior of uranium in aqueous environments. The dominant uranium valence states that are stable in geologic environments are the uranous (U4+) and uranyl (U6+) states, the former being far less soluble. Uranium transport generally occurs in oxidizing surface water and groundwater as the uranyl ion, UO22+, or as uranyl fluoride, phosphate, or carbonate complexes. UO22+ and uranyl fluoride complexes dominate in oxidizing, acidic waters, whereas the phosphate and carbonate complexes dominate in near-neutral and alkaline oxidizing waters, respectively. Hydroxyl, silicate, organic, and sulfate complexes might also be important, the sulfate complex being important especially in mining and milling operations that use sulfuric acid as a leaching agent. Maximum sorption of uranyl ions on natural materials (organic matter; iron, manganese and titanium oxyhydroxides; zeolites, and clays) occurs at pH 5.0-8.5. The sorption of uranyl ions by such natural media appears to be reversible; for uranium to be "fixed" and thereby accumulate, it requires reduction to U4+ by the substrate or by a mobile phase, such as H2S. Occurrence and Doses Uranium is found in all rocks and soils. Typical concentrations in the more prevalent types of rock and average concentrations in the earth's crust and in soil are listed in table 2.6. In the common rock types, the uranium concentrations range from 0.5 to 4.7 ppm, corresponding to activity concentrations for 238U of 7-60 Bq/kg (0.2-1.6 pCi/g). The overall effect of soil development results in an average soil concentration of uranium less than the average rock concentration. Some ores mined and processed for nonradioactive materials can produce residues with elevated concentrations of radionuclides. A well-known example is phosphorus ore, which contains uranium at up to 120 ppm and has also been used as a commercial source of uranium (NCRP 1993b). Natural materials that contain uranium at over 500 ppm are considered to be uranium ores. Uranium also occurs in air, water, and food and so is present in human tissues. The average annual intake of uranium from all dietary sources is about
GUIDELINES FOR EXPOSURE TO TENORM 35 Table 2.6 Rangesa and averages of concentrations of 40K, 232Th, and 238U in Typical Rocks and Soilsb Material 40K 232Th 238u %total K Bq/kg ppm Bq/kg ppm Bq/kg Igneous rocks Basalt 0.8 300 3-4 10-15 0.5-1 7-10 (crustal) 1.1 300 2.7 10 0.9 10 Mafic Salic 4.5 1400 20 80 4.7 60 Granite(crustal) >4 >1000 Sedimentary rocks 17 70 3 40 Shale 2.7 800 12 50 3.7 40 Sandstones Clean quartz <1 <300 <2 <8 <1 <10 2? 400? 3-6? 10-25? 2-3? 40? Dirty quartz 2-3 600-900 2? <8 1 -2? 10-25? Arkose Beach sands <1 <300 6 25 3 40 Carbonate 0.3 70 2 8 2 25 rocks All rocky 0.3-4.5 70-1400 2-20 7-80 0.5-4.7 7-60 Continental 2.8 850 crust Soil 1.5 400 10.7 44 2.8 36 9 37 1.8 22 l aExamples of materials outside ranges can be found, but quantities are relatively small. bAdapted from NCRP (1987a).
36 NATURAL RADIOACTIVITY21ND RADIATION 13 Bq (350 psi) (NCRP 1987b). The intake of uranium from tap water can be a small or large fraction of the total intake depending on concentrations in local water supplies (Hess and others 1985). In the United States, the typical concentration of uranium in skeleton (wet weight) is about 8 mBq/kg (0.2 psi/kg) (NCRP 1987b). Lung, kidney, and bone receive the highest annual doses of radiation from uranium, estimated at 11, 9.2, and 6.4 ,uSv (1.1, 0.92, and 0.64 mrem), respectively, for US residents. The decay products of uranium, particularly radium and its decay products, are more important than uranium itself with respect to dose to humans from both external and internal exposures (NCRP 1987a). Radium-226 Radium-226 and its decay products, members of the uranium chain, are responsible for a major fraction of the internal dose received by humans from the naturally occumng radionuclides (IAEA 1990). 226Ra is an alpha-particle emitter that decays, with a half-life of 1600 y, to radon-222, which has a half-life of 3.82 d (table 2.3). The decay of 222Rn is followed by the successive disintegration of a number of short-lived alpha-particle- and beta- particle-emitting progeny. After six decay steps, in which radionuclides that range in half-life from 1.6 x 10-4 to 26.8 men are produced, 2~0Pb is produced; it has a half-life of 22.3 y. This nuclide decays through 2'0Bi to produce Typo, which decays by alpha-particle emission to stable 206Pb. Radium itself adds little to the gamma-ray activity of the environment, but it does so indirectly through its gamma- ray-emitting decay products. Geochemistry Radium exhibits only the +2 oxidation state in solution, and its chemistry resembles that of barium. Radium forms water-soluble chloride, bromide, and nitrate salts. The phosphate, carbonate, selenate, fluoride, and oxalate salts of radium are slightly soluble in water, whereas radium sulfate is relatively insoluble in water (Ksp = 4.25 x 10-~ at 20° C). Radium in uranium ore is only slightly soluble in H2SO4 but is highly soluble in HC1 and HNO3, presumably because of the greater solubility of RaCl2 and Ra (NON than of RaSO4. The hydrated ion of radium is the smallest in the alkaline earth series, so it would tend to be preferentially retained by ion exchange. In alkaline solutions, anionic complexes of radium with organic ligands, such ethylenedia- mine tetraacetic acid (EDTA) and citric acid, are known to occur. Means and others (1978) suggest that EDTA mobilization might be responsible for elevated concentrations of radium seen in water and soil sampled around a radioactive- waste disposal trench at the Oak Ridge National Laboratory burial ground.
GUIDELINES FOR EXPOSURE TO TENORM 37 Radium does not form discrete minerals but can coprecipitate with many minerals, including calcium carbonate, hydrous ferric oxides, and barite (Basted. Radium can be sorbed by clay minerals, colloidal silicic acid, manganese oxides, and organic matter. Although radium (unlike uranium) has only a single valence state, the dissolution or precipitation of sorbing phases, such as barite and ferric hydrous oxides, under changing oxidation-reduction conditions can influence its mobility. Groundwaters low in sulfate but high in ionic strength, calcium, and barium are conducive to the transport of radium. Leaching data suggest that uranium mill tailings in the environment can constitute a long-term source of radium contamination of surface water and groundwaters that are in contact with them. The same is probably true of other NORM wastes in which 226Ra is associated with sparingly soluble minerals, such as BaSO4. Occurrence and Doses 226Ra is present in all rocks and soils in variable amounts. In nature, 226Ra is generally in rough equilibrium with 238U, so the concentrations compiled for 238U in table 2.6 can be taken as a good guide to the expected range for 226Ra. The radium contents of soils can show considerable spatial variability, both locally and regionally. These are the result of differences in parent materials and in soil-forming factors such as climate and weathering time. Soil-development processes can lead to substantial redistribution of macro-constituents, such as iron, and of trace elements and radionuclides, such as radium, in the soil profile, thereby introducing variations in distribution with depth, as well as location. The distribution of radium in uncontaminated, surface soils of the United States was investigated on a statewide-scale by Myrick and others (1981) in a study done in support of Department of Energy (DOE) remedial action programs dealing with fuel-cycle NORM. Individual 226Ra measurements ranged from about 8.5 to 160 mBq/g (0.23 to 4.2 pCi/g). The state average 226Ra measurements ranged from about 24 mBq/g (0.65 pCi/g) in Alaska to 56 mBq/g (1.5 pCi/g) in Kentucky, Nevada, New Mexico, and Ohio. Relative arithmetic standard deviations for the state averages ranged from 12 to 158%. The areal and cross-sectional variations that one might expect to see on smaller scales are exemplified in data presented by Meriwether and others (1995) and Van den Bygaart and Protz (1995), which show two-fold differences in 226Ra concentration between surface horizons at different sampling sites and between surface and subsurface horizons at a given site. Spatial variability and other issues associated with soil sampling at sites that are potentially contaminated with radioactivity are discussed in detail in the Multi-Agency Radiation Survey and Site Investigation Manual (Nuclear Regulatory Commission/EPA 1996~. The radium content of surface waters (4-19 Bq/m3, 0.1-0.5 pCi/L) is lower than that of most groundwaters (Hess and others 1985~. Surveys of water supplies in many states (Cothern and Lappenbusch 1984) showed that the
38 NATURAL RADIOACTIVITY AND RADIATION Environmental Protection Agency (EPA) limit for total radium of 0.2 kBq/m3 (5 pCi/L) was exceeded in many communities that obtain water from groundwater, including communities of about 600,000 in Illinois, Iowa, Missouri, and Wisconsin. About 75% of the supplies that exceeded 0.2 kBq/m3 (5 pCi/L) were in two areas of the United States: the Piedmont and coastal plain areas of the Middle Atlantic states, and the north central states of Minnesota, Iowa, Illinois, Missouri, and Wisconsin. The concentration of 226Ra was in some cases as high as 0.93 kBq/m3 (25 pCi/L), with 228Ra concentrations up to about 0.63 kBq/m3 (17 pCi/L). EPA (1991a) later conducted a random survey (stratified by system size) of radionuclides in 1,000 drinking-water supply systems that obtain water from ground water. For 226Ra, 3.4 million persons were probably exposed to over 0.2 kBq/m3 (5 pCi/L), and 890,000 to over 0.74 kBq/m3 (20 pCi/L). The corresponding numbers are 1.3 million and 164,000 for 228Ra. Persons consuming water that contains 226Ra at 0.2 kBq/m3 (5 pCi/L) at 2 L/d would receive an annual effective dose equivalent of about 50 ,uSv (5 mrem). Radium is chemically similar to calcium and is absorbed from the soil by plants and passed up the food chain to humans. Because the radium in food originates in soil and the radium content of soil is variable, the radium content of foods varies. In addition, it is reasonable to expect that such chemical factors as the amount of exchangeable calcium in the soil will determine the rate at which radium is absorbed by plants. From radiochemical analyses of food, Fisenne and Keller (1970) determined the daily 226Ra intake by inhabitants of New York City and San Francisco at 0.07 and 0.03 Bq (1.7 and 0.8 psi), respectively. That difference is not reflected in the difference in 226Ra content of human bone between the two cities (Fisenne and others 1981), which suggests an uncertainty of at least a factor of 2 in the relationship between intake and body burden. There is, however, an association between 226Ra concentration in bone and the 226Ra concentration in drinking water in the midwestern United States (NCRP 1987a). The National Council on Radiation Protection and Measurements (NCRP 1984c) estimates an average dietary intake of 0.05 Bq/d (1.3 pCi/d). Worldwide, the 226Ra content of adult skeletons ranges from about 0.3 to 3.7 Bq (8 to 100 psi), and the population-weighted average skeletal content is 0.85 Bq (23 psi) (NCRP 1984c), which corresponds to annual equivalent doses of 170 pSv (17 mrem) to cortical and trabecular bone, 90 pSv (9 mrem) to the bone lining cells, 15 ,uSv (1.5 mrem) to the red marrow, and 3 pSv (0.3 mrem) to soft tissues. Thorium The only primordial isotope of thorium is thorium-232. Like uranium, it is ubiquitous in nature. Shorter-lived isotopes of thorium occur in all three of
GUIDELINES FOR EXPOSURE TO TENORM 39 the natural decay chains. Geochemistry In aqueous systems, only the Th4+ oxidation state is known to exist. Th4+ undergoes hydrolysis in aqueous solutions above pH 2-3 and is subject to extensive sorption by clay minerals and humic acid at near- neutral pH. At near-neutral pH and in alkaline soils, precipitation of thorium as a highly insoluble hydrated oxide phase and coprecipitation with hydrated ferric oxides can, with sorption reactions, be important mechanisms for the removal of thorium from solution. Because of sorption and precipitation reactions and the low solution rate of thorium-bearing minerals, thorium concentrations in natural waters are generally low. At low pH, such as in an acid-leach uranium mill, thorium becomes more soluble. Acid-leach milling might dissolve 30-90% of the thorium in the ore. Acidic effluents (pH 2.5) from uranium mills in the Grants Mineral Belt of New Mexico contain 230Th at 5.6-6.3 MBq/m3 (150,000-170,000 pCi/L). The solubilized thorium can be precipitated if the acidic effluent is neutralized by contact with natural media or by process additions of limestone to the waste solutions. The high inventory of soluble 230Th in such an effluent made it the radionuclide of greatest mobility when a dam at a New Mexico uranium mill failed in 1979, sending effluent down an arroyo (Weimer and others 1981~. Similarly, under acidic conditions at some uranium mills, 230Th has been shown to have migrated considerably deeper into the subsoil than 226Ra (DOE 1993b). Occurrence and Doses Typical concentrations of 232Th in the more prevalent rock classes, the crustal average, and the soil average are listed in table 2.6. 232Th concentrations range from 2 to 20 ppm in the common rock types, corresponding to activity concentrations of 8 to 80 Bq/kg (0.18 to 22 pCi/g). Like 238U, 232Th has markedly higher concentrations in some parts of the world. Because of its specif~c activity and low mobility, except in the low-pH situations mentioned previously, 232Th is normally present in biologic materials only in insignif~cant amounts. The mean concentration in 25 vegetable samples (Linsalata 1994) was 0.67 + 0.81 mBq/l~g (0.018 + 0.022 psi/kg). Thorium was found in the highest concentrations in pulmonary lymph nodes and lungs; this indicates that the principal source of human exposure is inhalation of suspended soil particles (Ibrahim and others 1983; Wrenn and others 1981~. Because thorium is removed from bone very slowly, the concentrations of both 230Th (which is found in the 238U decay chain) and 232Th were found to increase with age. Average concentrations of 232Th in major tissues reported by NCRP (1987a) indicated that the highest concentrations (wet weight) were in lung and cortical bone, at 20 and 12 mBq/kg (0.5 and 0.3 psi/kg), respectively. The external dose rate due to gamma radiation from the thorium chain is usually somewhat greater than that from the uranium chain and arises primarily from the decay products rather than from 232Th itself. The internal
40 NATURAL RADIOACTIVITY AND RADIA TION dose from the 232Th chain is due primarily to 228Ra and its decay products (NCRP 1987a), which are discussed in the next section. Radium-228 Radium-228 is a member of the 232Th chain. Although 228Ra and 226Ra commonly occur in soil and water in about a 1:1 ratio, 228Ra has not been systematically measured in food and water on a scale comparable with that of 226Ra. NCRP (1987a) estimates that the daily intake of 228Ra is about 0.04 Bq (1 psi), which can be compared to its 226Ra estimate of 0.05 Bq (1.3 psi). Where elevated concentrations of 226Ra have been noted in drinking water, 228Ra concentrations are often comparable (Hess and others 1985; Gilkeson and others 1984~. The geochemistry of 228Ra is essentially identical with that of 226Ra.228Ra and its decay products are estimated to contribute annual dose equivalents of 300 pSv (30 mrem) to cortical bone, 84 pSv (8.4 mrem) to trabecular bone, 120 pSv (12 mrem) to the bone lining cells, 22 pSv (2.2 mrem) to the red marrow, and 1.S ,uSv (0.15 mrem) to soft tissues (NCRP 1987a). Radon Radium-226 decays by alpha-particle emission to 222Rn, which has a half-life of 3.82 d. Radium-224, which is a member of the 232Th chain, decays by alpha-particle emission to 55.6 s 220Rn. Radon-219 is a member of the 23Su chain and decays most rapidly, having a half-life of 3.96 s. Radon is a noble gas; it occurs as nonpolar, monatomic molecules and is inert for practical purposes. The 3.82-d 222Rn isotope has a greater opportunity than the nuclei of shorter-lived radon isotopes to escape to the atmosphere. The mechanisms by which 222Rn is transported from soil into the atmosphere have been treated extensively by Tanner (1992; 1980; 1964~. When the parent radium decays in rock or soil, the resulting radon atoms recoil and some of them come to rest in geologic fluids, most likely water in the capillary spaces. Some of the radon in soil water enters soil gas, primarily by diffusion, and then becomes more mobile. Radon reaches the atmosphere when soil gas at the surface exchanges with atmospheric gas. A less important mechanism is diffusion from soil gas to atmospheric gas. The concentration of 222Rn in typical soil gas is 4-40 kBq/m3 (102 - 103 pCi/L), several orders of magnitude higher than 222Rn concentrations found in the outdoor atmosphere. Gesell (1983) reviewed the reported data from various parts of the United States and found that the annual average outdoor 222Rn concentration ranged from 0.6 Bq/m3 (0.016 pCi/L) in Kodiak, AK, to 28 Bq/m3 (0.75 pCi/L) in Grand Junction, CO, a location with elevated soil radium concentrations. Data from the United States and several other countries indicate that the average
GUIDELINES FOR EXPOSURE TO TENORM 41 concentrations of 222Rn in outdoor air can normally be taken to be (19 Bq/m3 (0.1-0.5 pCi/L). NCRP (1987a) compiled results from 14 studies of outdoor 222Rn concentrations in the United States and found a similar range of 18 Bq/m3 (0.1-0.5 pCi/L), except for Colorado Springs, where the mean for five sites was 44 Bq/m3 (1.2 pCi/L). Several investigators have determined that the highest concentrations are observed in the early hours and the lowest in the late afternoon, when the concentrations are about one-third the highest morning ones (see for example, UNSCEAR 1982; Gold and others 1964~. Over the course of a year, 222Rn concentrations tend to peak in the fall or winter months and have minimums in the spring. This variation is consistent with the pattern of atmospheric turbulence, which tends to be greater in spring. Because the decay products of 222Rn and 220Rn are electrically charged when formed, they tend to attach themselves to dusts, which are normally present in the atmosphere. If the radioactive decay products of radon are not removed by mechanisms other than radioactive decay, the parents and their various decay products will achieve radioactive equilibrium. The growth of the 222Rn decay products approaches an equilibrium in about 2 h; beyond that, fiercer growth in the activity of the nuclide chain is slowed by the presence of 22.3-y Pub, which, in the short term, acts as a nearly stable nuclide. Wilkening (1952) found that the 222Rn decay products tend to distribute themselves on atmospheric dust in a manner that depends on the particle size of the dust, and that the bulb of the activity is contained on particles having diameters less than 0.035 ~m. When air that contains 222Rn or 220Rn in partial or total equilibrium with its decay products is inhaled, the inert gases are largely exhaled immediately. However, some of the dust particles will be deposited in the respiratory system. Additional radon decay products will be deposited with each breath until radioactive equilibrium is reached, at which point the amount of activity deposited per unit time equals the amount eliminated from the lungs by the combination of physiologic clearance and radioactive decay. In the case of 222Rn in equilibrium with its decay products, the total energy dissipation in the lungs derived from the decay products is about 500 times greater than that derived from decay of the 222Rn itself. The dosimetry of radon and its decay products is discussed in Chapter 8. Indoor Radon In confined spaces, especially those bounded by radon-emitting materials, 222Rn concentrations can be orders of magnitude higher than outdoors. Examples include underground mines (especially uranium mines), caves, and structures, especially one- or two-story homes. One of the surprising developments in recent years has been the finding that in many homes the concentration of 222Rn (and its decay products) is so high as to pose potential risks far greater than those posed by other pollution hazards that have
42 NATURAL RADIOACTIVITYAND RADIATION attracted attention. Reviews of indoor 222Rn can be found in Nazaroff and Nero (1988), Nero and others (1990), and Eisenbud and Gesell (1997~. The indoor 222Rn problem exists mainly in homes because the 222Rn originates primarily in the soil, which has its greatest effect on one- or two-story buildings. The building materials themselves are a minor source of 222Rn, compared with soil, except when the materials contain relatively high concentrations of radium and have sufficient permeability and porosity to allow 222Rn to escape. That is true, for example, if gypsum board or another building material has been manufactured as a byproduct of phosphate-fertilizer production (Lettner and Steinhausler 1988; Paredes and others 1987~. 222Rn can enter the indoor atmosphere in a number of ways, including advection and diffusion from soil, diffusion from construction materials, infiltration with outdoor air, emanation from water, and presence in natural gas (UNSCEAR 1988~. In EPA's draft report on diffuse NORM waste (EPA 1993b), a diffusion model is used to estimate indoor radon concentrations on the basis of 226Ra concentrations in waste on which a house was built. The model incorporates a one-dimensional version of Fick's law to estimate radon diffusion from soil through concrete of different densities. However, there is evidence that diffusion of 222Rn is a minor pathway compared with the advection of soil gases directly through breaches in the foundation as a result of slight pressure differentials that can result from atmospheric pressure changes, temperature differentials, or wind velocity. For example, the UN Scientific Committee on the Effects of Atomic Radiation (UNSCEAR 1988) has shown that advection typically accounts for 75% of the radon that enters a reference house, whereas diffusion accounts for only 3%. Steinhausler (1975) has shown that meteorologic factors in particular can influence indoor concentrations of 222Rn and its decay products. The approximate contributions of various sources to the indoor 222Rn concentrations of single-family dwellings and apartments are given in table 2.7 (Nero 1988; Nero and others 1986~. Several efforts to estimate the US national distribution of indoor 222Rn have been made (Marcinowski and others 1994; White and others 1992; Cohen 1991; Cohen 1989; Alter and Oswald 1987; Cohen 1986; Nero and others 1986), but the most current representative US survey of indoor 222Rn is the National Residential Radon Survey (Marcinowski and others 1994; EPA 1992b). From this survey, the average national 222Rn concentration was found to be 46 Bq/m3 (1.25 pCi/L) and the median, 25 Bq/m3 (0.67 pCi/L). The average and median 222Rn concentrations in each of the 10 EPA regions are shown in f~gure 2.1. Regionally, the Midwest and Intermountain West have the highest indoor 222Rn concentrations, averaging about twice the national average, whereas the Nor~west has the lowest. Figure 2.2 shows the distribution of 222Rn
GUIDELINES FOR EXPOSURE TO TENOR31 ~4 s" o ._ o C) o ~q o .~ o C~q s: o ._ ._ o C~ ._ o C) ._ 3 oc U. ~ E m :^ ._ 00 ~ c~ v, A o r-, ~ ~ A o ° ° ° o o o o o o == ~ m =~ - > ~ ·0 0 c~ 43 U3 U3 Ct C~ o U. .= Ct - ~: ~D C) U. o .~ ·E . ~: o V C~ C,~ _~._ U. - U. o C) _ s: o Z ~ C~ oo ~ o C~ z ~ o s~ =.O ¢ ¢ ~ D
44 NA TURAL RADIOS CTIVITYAND RADIA TION o0 O- '/~ ~0 to t-~] Boar Rain any..,-. C7`- am\ C_ , . ..... _ ~1~ ~ ~U1~_ - . ~ V .£ _ - o · _ ¢ ;^ - Ct ._ · _ - - r~ e' o ~ O O · - U] ~ 5- 5_ _ O ~ o C ° ·~ ~ V Cot A - to - ._ U. o . - e a' o ~ - ~ co -= Ail ~ ~ ¢ it am .= ~ - ~ o - ~ ~ - ~ m Ct Ct . a> .= U. v
GUIDELINES FOR EXPOSURE TO TENORM CO CO 45 lo Cal A lo CD 1 coil lo 1 o O O Cal L ~ _ (D _ ~ fir Lo ~ L Jar ~ L CY) ~ ~ C~ C~ o o o o CO UD >1~°~S 6U!Sn°H S n ~o ~ua~aa - Cd o Cd ;> Cd - - ._ Q - o ._ a) C) o C) o ~5 CO o U. ._ U. V) ._ ~ ¢ r~ ~ o ~ o . ° =, D _- ._ ~ m - Cd ~o 11 C~ V _ ; - U) ._ ;, ~ C~
46 NA TUBAL DIVA CTIVITYAND INDIA TION concentrations for the entire country. About 6% of the residences surveyed had 222Rn concentrations over 150 Bq/m3 (4 pCi/L). Applied nationally, that implies that 5.8 million residences have 222Rn concentrations exceeding 150Bq/m3 (4pCi/L). The results from the Nero and others (1986) review, the Cohen (1986) study, and the carefully designed National Residential Radon Survey (EPA 1992b) are similar, suggesting that the distribution of indoor 222Rn in the United States is reasonably well characterized. World indoor 222Rn concentrations do not necessarily follow the pattern seen in the United States. They are often higher in Scandinavian countries, such as Denmark (NIRH 1987), where the average of the summer and winter 222Rn concentrations is 93 Bq/m3 (2.5 pCi/L). A very low result was seen in Australia in a nationwide 222Rn survey of homes (Langroo and others 1991~. On the basis of limited measurements in a few buildings (Turk and others 1986; Cohen and others 1984) and reasoning that multistory buildings with forced ventilation would be less likely to reach high 222Rn concentrations (Nero 1988), 222Rn concentrations in commercial and industrial structures were generally believed to be much lower than those in residences. However, they might warrant reconsideration. High 222Rn has been found in underground workplaces in Germany (Schmitz and Fritsche 1992~. Scott (1992) identified 86 buildings at seven DOE sites that might exceed the EPA action level of 150 Bq/m3 (4 pCi/L) for residences. That amounts to 2.8% of the 3,100 structures surveyed, about half the percentage of US residences estimated to exceed 150 Bq/m3 (4 pCi/L). Natural underground caves have limited ventilation and are bounded by rock and soil capable of emanating radon into the air. Radon is also carried into caves by water. 222Rn concentrations are typically much higher in caves than outdoors (WiLkening and Watkins 1976~. In a study of caves operated by the US National Park Service, Yarborough (1980) identified numerous locations in several caves with radon greater than 7.5 kBq/m3 (200 pCi/L). Radon in Groundwater 222Rn dissolved in potable water is another source of human exposure, mainly because the 222Rn is released from solution at the tap and enters the home atmosphere (Nazaroff and others 1987; Watson and Mitsch 1987; Cross and others 1985; Prichard and Gesell 1983; Gesell and Prichard 1975~. Water supplies ordinarily make only a small contribution to the indoor 222Rn concentration but can be the predominant source in areas where the 222Rn content of groundwater is unusually high. Studies in Maine and Colorado have shown 222Rn in water to be an important contributor in some dwellings (Lawrence and others 1992; Hess and others 1981~. In the 1992 Lawrence and others study, performed in Colorado, estimates of the concentration of indoor radon attributed to radon in the domestic water supply depended on assumptions regarding the fraction of radon emanating from the water and on dwelling ventilation rates. The averages of the
GUIDELINES FOR EXPOSURE TO TENORM 47 concentrations of indoor radon attributed to radon in the domestic water supply for the 28 houses studied were 20 Bq/m3 (0.54 pCi/L) with assumptions that minimized radon attributable to water and 48 Bq/m3 ~ 1.3 pCi/L) with assumptions that maximized radon attributable to water. The highest estimated value of indoor radon attributed to radon in the domestic water supply for a single dwelling was 310 Bq/m3 (8.4 pCi/L). The proportion of total indoor radon concentration attributable to radon in water was estimated to range up to 77%. Continuous measurements in a single house demonstrated a strong correlation between water use and indoor radon concentration. Lead-210 and Polonium-210 Lead-210 is a 22.3-y beta-particle emitter separated from its antecedent 222Rn by six short-lived alpha-particle and beta-particle emitters (see table 2.3~. The longest-lived radionuclide between 222Rn and 2~0Pb is 2~4Pb, which has a half-life of only 26.8 min. 2~0Pb decays to 138.4-d typo via the intermediate 2~0Bi, which has a 5 d half-life (see table 2.3~. Thus, after the decay of 3.82-d 222Rn in the atmosphere, 2~0Pb is produced rapidly, but its long half-life allows little to decay in the atmosphere before it precipitates to the earth's surface, . . mam. By in ram or snow. The 2~0Pb content of the atmosphere has been found to vary from 0.2 to 1.5 mBq/m3 (5 x 10-3 to 40 x 10-3 pCi/m3), with the lowest values at such island stations as San Juan, PR, and Honolulu, HI, and the highest values in the interior of the United States (NCRP 1987a). The mean residence time of dust suspended in the troposphere is about 15 d, so there is little time for typo to be formed in suspended dust, and the concentration of typo near ground level is smaller than that of 2~0Pb. For purposes of estimating dose in the United States, NCRP (1987a) has adopted nominal ground-level concentrations for 2~0Pb and typo of 0.7 and 0.07 mBq/m3 (20 x 10-3 and 2 x 10-3 pCi/m3), respectively. On the basis of few measurements, Fisenne (1993) estimated that 2~0Pb concentrations indoors are about one-fourth those outdoors. NCRP (1987a) has estimated that the mean dietary intake of 2~0Pb is about 0.05 Bq/d (1.4 psi/d) and that the typo content of the standard diet is an average of 1.3 times that of Pub. Food and water ingestion is a more important contributor to blood 2'0Pb than inhalation. In the United States, 2~0Pb and its decay products are estimated to contribute an annual equivalent dose of 1,400 ,uSv (140 mrem) to cortical and trabecular bone, 700 ,uSv (70 mrem) to the bone lining cells, and 140 pSv (14 mrem) to the red marrow and soft tissues (NCRP 1987a). In two population groups, 2~0Pb and typo concentrations are apt to be higher than average: cigarette-smokers and people who eat substantial quantities of caribou from northern lands.2~0Pb and typo are believed to enter tobacco by being deposited on tobacco leaves from the atmosphere (Martell 1974~. When
48 NA TURAL RADIOS CTI CITY AND RADIA TION the tobacco is smoked, the 2'0Pb and typo are volatilized and inhaled, and this results in blood concentrations of these nuclides about one-third higher than in nonsmokers. Caribou feed on lichens, which absorb trace elements in the atmosphere, including 2~0Pb and Typo. The tissue content of these nuclides in Lapps in northern Finland, who subsist on caribou, was about 12 times that of residents of southern Finland, where normal Scandinavian dietary regimes exist (Persson 1972; Kauranen and Miettinen 19694. Potassium-40 Of the three naturally occurring potassium isotopes, only 40K is unstable, having a half-life of 1.3 x 109 y. It decays by beta-particle emission to calcium-40 (89%) and by electron capture to argon-40 (11%) and produces 1.46-MeV gamma rays after electron-capture decay. Potassium-40 is present at 0.0117% by mass in natural potassium, thereby imparting a specific activity of about 30 kBqlkg (800 pCi/g) of potassium. Representative values of the total potassium content of rocks, as summarized in table 2.6, indicate a wide range of values, from 0.3% to 4.5% for various rock types. That corresponds to an activity concentration range of 90 to 1,400 Bq/kg (2.5 to 37 pCi/g). Some basalts and sands are low in potassium, whereas granites and other basalts are high. It has been estimated that about 110 TBq (3,000 Ci) of 40K is added annually to the soils of the United States in the form of fertilizer (Guimond 1978~. Seawater contains 40K at about 11 kBq/m3 (300 pCi/L). Because of its relative abundance and its energetic beta-particle emission (1.3 MeV), 40K is the predominant radioactive component in common foods and human tissues. It is important to recognize that the potassium content of the body is under homeostatic control and is little influenced by environmental variations. The dose from 40K in the body is therefore reasonably constant. A person who weighs 70 kg contains about 140 g of potassium, most of which is in muscle. From the specific activity of potassium, it follows that the 40K content of the human body is around 4 kBq (0.1 psi). NCRP (1987a) has estimated that this radionuclide delivers an annual dose of 0.18 mSv (18 mrem) to the soft tissues and 0.14 mSv (14 mrem) to bone. However, Paschoa and others (1992) have questioned the conventional dosimetry of 40K and other nuclides that decay by electron capture because the intracellular dose from Auger electrons, which have energies of a few thousand electron volts, has not been considered. Rubidium-87 The primordial beta-em~tting radionuclide 87Rb, with a half-life of 4.75 x 10~° y, is present in the environment and in human tissues at low
GUIDELINES FOR EXPOSURE TO TENORM 49 concentrations. Estimates of the average annual effective dose equivalent from 87Rb are 3-6 ,uSv (0.3-0.6 mrem) (UNSCEAR 1988; NCRP 1987a). Induced Radionuclides Some radionuclides that exist on the surface of the earth and in the atmosphere have been produced by the interaction of cosmic rays with atmospheric nuclei. The two most important of these induced radionuclides, tritium (3H) and 14C, are only minor dose contributors relative to the primordial radionuclides discussed in previous sections. Some of the properties of these radionuclides and the extent to which they have been reported in various media are listed in table 2.1. It is estimated that the annual dose from 14C iS 30 pSv (3 mrem) to the skeletal tissues of the body and 10 pSv (1 mrem) to the soft tissues. The annual average dose from 3H of natural origin is estimated (NCRP 1987a) at 0.01 ,uSv (1 prom). NATURAL SOURCES OF EXTERNAL IONIZING RADIATION The dose received from external sources of ionizing radiation originates in cosmic rays and photon-emitting radionuclides in the earth's crust (terrestrial sources). Terrestrial Sources of External Radiation The major terrestrial sources of gamma radiation are 40K and nuclides of the 238U and 232Th chains. The relationship between soil concentration of radionuclides and dose was developed originally by Hultqvist (1956) and by Beck (1980; 1975~. Tables relating external dose rates to the concentrations and distributions of many radionuclides have been published as Federal Guidance Report No. 12 by Eckerman and Ryman (1993~. The annual effective dose equivalent 1 m above soil that has uniformly distributed typical concentrations of each of the three major sources of terrestrial radiation is shown in table 2.8. For a hypothetical, unshielded individual residing full-time on a potassium chloride salt-flat, the maximum annual external dose would be about 5.4 mSv (540 mrem). Extensive measurements of the natural gamma-radiation background in a number of cities throughout the United States show (figure 2.3) that natural radiation exposure rates from terrestrial sources in the United States vary from less than 1 to about 20 pRJh (Beck 1966~. The temporal variation is illustrated by a series of measurements performed by the Environmental Measurements
50 ~- a 1 'C . ~ . . ~ S a. rat - e_ ~ = o eg o O ~ .z ^ . ~ ~ m O ~ 0 ~ O 0 ~ "^ : O 0 ~ - m ._ O ._ O ~1 O O O ^ F E .E F~ O o ~ O
GUIDELINES FOR EXPOSURE TO TENORM Cal [ o = o ~ no c, ~o . Ct _ ~° ~fi Ct V: . . . . . . o o CO UD o o ~ Cal suo!~oo1 Jo ~eq~nN 51 .= V) sit .s ~a' o ~ a) o Cal U) o Ct Ct o o ED .b Cal a> ~ . a _ Cal ~ ~ m O 5- ._ rT. ~ G
52 NATURAL RADIOACTIVITY AND RADIATION Laboratory at its rural background monitoring station in Chester, NJ (Klemic 1996~. Figure 2.4 shows short-term variations and the effects on dose rate of diurnal variations in radon concentration, soil moisture, and rainout of radon decay products. Diurnal variations in radon concentration are caused by diurnal changes in atmospheric stability. Rainout of radon decay products briefly increases the dose rate, whereas accumulated soil moisture decreases it as a result of attenuation of the gamma-ray flux. Figure 2.5 shows long-term variations, which are influenced mostly by the attenuating effects of soil moisture and snow cover. In addition to calculations and direct ground-level measurements of external dose, measurements can be made with sensitive gamma-ray detectors in aircraft (IAEA 1991~. Many such surveys have been made, either to explore for uranium or to provide information about the radiation in the vicinity of proposed nuclear facilities. The data were analyzed by Oakley (1972), who estimated the population dose distribution in the United States. The data are grouped by geographic region: (1) the Atlantic and Gulf coastal plain, for which the mean annual absorbed dose is 0.23 mGy (23 mrad); (2) a portion of the eastern slope of the Rocky Mountains, where the annual absorbed dose averages 0.9 mGy (90 mrad); and (3) the remainder of the United States, where the average annual absorbed dose is 0.46 mGy (46 mrad). Cosmic Radiation The primary radiation that originates in outer space and impinges isotropically on the top of the earth's atmosphere consists of 87% protons, 11% alpha particles, about 1% nuclei of elements of atomic number 4-26, and about 1% electrons of very high energy. An outstanding characteristic of the cosmic radiation is that it is highly penetrating, with a mean energy of about 10~° eV and maximum energy of as much as 102° eV. The primary radiation predominates in the stratosphere above an altitude of about 25 km (NCRP 1987a). Most cosmic radiation originates outside the solar system. However, the solar component is important outside the atmosphere after flares associated with sunspot activity that follows an 11-y cycle. The interactions of the primary particles with atmospheric nuclei produce electrons. gamma rays, neutrons, plans and muons. At sea level, muons account for about 80% of the cosmic-radiation charged-particle flux, and electrons account for about 20%. The neutron flux is comparable with the electron flux.
GUIDELINES FOR EXPOSURE TO TENON a) o a, ~ o o ~ , ~ U) ~ a,' ° CL Q 1x _ CD o ·_ ·_ - E :3 ·_ I \ ~ I o I ~ I o 3' ~ ' o 11 ~ ~ , o . I Cal ~ I =o o ;' I ~ no' it' ~ ~ , . . . . . . . ~ .. . . . . . . . o ~ o Us o ~ o ~ Cal ~ ~ o o CD ~ on . . ~ . . (mu) aped So 53 Ct _ ~ 4~ ._ UD O ~ A ~ ~ ·m ~ u, ~ == cO ~ ~ ~ ~ 3 o ~ . - .~ ~ _^ ~ _ ~ =, C~ . · o o ~ ~ ~ ~ ~ ~ ° o ~ 3 ~ ~ ~ c o _ R ~ c, ~ 3 ~ .= o ~ ~o Cd o ~ ~ ~ ~ ~ C=~! ~ o -c Y ;: ~ C U.- . ~ ~ 3 , ~ o C~
54 :_2 ~. ~. , ,, ! i , ., . . . i : : 1 ! 1 : : 1 : . :C - . 1 : : 1 : ! . I 1 1 1 ~ 1 1 1 1 1 1 1 1 1 1 1 1 1 ~ .1 o o o o o C ~o oc) CO ~ C ~C~ NATURAL RADIOACTIVITYAND RADIATION ., ! ~ ., Lf) 1 w) · ~ C~; ~, .o i . ~ ~oe~ . i q : od · ·1 i i . 1 : : . i i - o ~ o ~o o b° o ~o p . ~o goo oo .o ,~ - _ C~ _ CD _ ~ ~ ~o _ ~ . ,~b ~ .~' Oo - `2 C~ ·( ~ · <' · o*< oc) oo oc) CD CO ao C~ LO oo CS) ~ ~ en C~> _ ~ o _ oc) _ oc) C5) .e o ~ ° .~C]Ol ~- ~ : ~ o_ ~ . ·[g ~ · ~ ~ o o o o oo CO Il/ASU - o · C~ I.4 ~ o ~ Cq ~; ._ .= ~ ~_` - CO ~o =,£.~e ~ A_ ~ .s ~ o o 3= ~ ~ d ,~ o ~ 'e ~ ~ ~ ~ o c': ~ ~ .= o c~ ~ - o ~ .N ·S uO.u ~ ~ ~ u, cd ~ ~ ~ ~'X ~ ~ - a~ ~ ~ c~ d o ~ o
GUIDELINES FOR EXPOSURE TO TENORM 55 The dose from cosmic radiation is markedly affected by elevation. The annual cosmic-ray dose equivalent is about 0.29 mSv (29 mrem) at sea level. For the first few kilometers above the earth's surface, the cosmic-ray dose rate doubles for each 2,000-m increase in altitude (figure 2.6~. With the development of high-altitude aircraft and manned space flight, the dose from primary cosmic radiation attracted interest (O'Brien and McLaughlin 1972; Curtis and others 1966), which continues to the present (NCRP 1995; Reitz and others 1993; NCRP 1989b). A transcontinental flight has been estimated to result in a dose of about 0.025 mSv (2.5 mrem), or 0.05 mSv (5 mrem) per round trip (NCRP 1987a). Air crews who work an exceptionally heavy schedule (1,100 h/y) can receive annual doses of 0.3-9 mSv (30-900 mrem), depending on the routes flown (O'Brien and others 1992~. Once or twice during the 11-y cycle, a giant solar event can deliver dose equivalents at very high altitudes (15-25 kin) of 10-100 mSv/h (1-10 rem/h), with a peak as high as 500 mSv (5 rem) during the first hour (Upton and others 1966~. During a well-documented solar flare in February 1956, dose rates in excess of 1 mSv/h (100 mrern/h) existed briefly at altitudes as low as 10,000 m (Schaefer 1971~. SUMMARY OF HUMAN EXPOSURES TO NATURAL IONIZING RADIATION The annual effective dose equivalent received by persons living in areas of normal background radiation is estimated at 2.4 mSv (240 mrem) for the world population (UNSCEAR 1988~. The annual external effective dose equivalent is estimated at 0.36 mSv (36 mrem) from cosmic sources and 0.41 mSv (41 mrem) from terrestrial radiation. 222Rn and its short-lived decay products contribute about 40% of the total effective dose equivalent. The natural sources of dose are shown in more detail in table 2.9. A somewhat larger total annual dose of 3 mSv (300 mrem) is estimated for residents of the United States and is shown in detail in table 2.10 (NCRP 1987a). The US estimates are 0.27 mSv (27 mrem) for cosmic sources and 0.28 mSv (28 mrem) from terrestrial radiation. The major difference between the two estimates, however, is the average effective dose equivalent due to 222Rn, which is 55% of the total in the US estimate but 40% of the total for the UNSCEAR estimate. That is a difficult quantity to estimate, because world average 222Rn concentrations are not well known and several models are used to convert 222Rn exposure to lung dose (chapter 8~. The population distribution of external dose in the United States from terrestrial and cosmic sources combined is shown in figure 2.7 and is seen to range over a factor of about 4. The variation in radon exposure would be
56 NATURAL RADIOACTIVITYAND RADIATION ABSORBED DOSE RATE IN AIR ( mGyly ) 0 0 O 0 _ . . ~I I _ TIT~ 1 1. 1_.- 111.] 1 1 1 ~ I 0 0 up call ad _ 0 0 (l MASH ~ 31~8 lN31VAlt)03 3soa 3nssl1 cO u) c o J fir lo ~ ~ o ~ Ho ·i% o CHID In -I ~ . ~ ~ ~ qua ha.,= 'e ~ ;^ l .e Do ~ ~ ~ o ~ ~ - ~ o ~ ~ ~ o cd 4 ~ - ~ =. v . o c) ~ - ~ ~ .~3 ~ `,, U. o ~ ~ ~ cd ~ c) ~ ·~ m cq ~ o - c~ 'e ~ o ~ ~^ o ~ ~ ~ ~ c) cd ~ ~ 3 · o ~o - ~ .= ~ 3o ·= ~ - CO ~; o ·£ V C) ~ ~, o _ Ct oo
GUIDELINES FOR EXPOSURE TO TENORM Table 2.9 Estimated Effective Dose Equivalents From Natural Sources in Normal Regionsa Source 57 Annual Effective Dose Equivalent mrem mSv External Internal Total External Internal Total -- 0.36 0.015 0.015 Cosmic, including neutrons Cosmogenic nuclides Primordial nuclides 40K 87Rb 238U chain 238U ~ 234U 230Th 226Ra 222Rn ~ 214Pb 2l0Pb ~ 210po 232Th chain 232Th 228Ra = 224Ra 220Rn = 208Pb Total (round) aAdapted from UNSCEAR (1988). 36 -- 36 0.36 1.5 1.5 18 -- 0.6 0.5 0.7 0.7 110 12 16 0.3 1.3 16 160 33 0.6 0.5 0.7 10.7 110 12 0.3 17.3 16 240 0.15 0.1 0.18 0.006 0.005 0.007 0.007 1.1 -- 0.12 0.16 0.013 0.006 0.005 0.007 0.107 0.12 0.003 0.003 0.173 0.16 0.16 0.8 1.6 2.4 80
58 NATURAL RADIOACTIVITYAND RADIATION ~e o ._ e. o U. C~ o CJ ._ C} ._ U. o CJ - C) C, a: ._ - o . - E~ Ct ._ o C, ._ C) - ¢ C) ~4 Ct C) au C~ o V) V) 0 oo o. ~ 0 0 0 0 o 0~ _` V - U' 0~ C~· _ ~0 ~._ Ct~ 0c> ._ _~ CtU) 0 3v o oo o o 5 Ct o U. Ct 3 CD ._ - o ·_ Ct s~ ._ C~ o ra O .~ Ct ·_ Ct - Ct .s U. r~ ~ r°~ ._ ~ ~: C ~ $- o .O _ U. ~ O O V E Ct oo C~ V Z ~o C~
GUIDELINES FOR EXPOSURE TO TENORM ! I 1 1 N 00 . ~9 - - l ! - ~o N N ~ (SUo!ll!U~) U°!~lnd°d 1~) 0 59 U. Ct a' · ~ so, En a_ N CS to IL a) 00 ° 8 ~5 to N to to o - o .e U. o C) Ct - Ct .~ U. - Ct U. o - Ct au a - Ct o o o . ~. · ~ =\ _1 Cal fi ~ ~ Ct o ~ o ·= ~
60 NATU~L ~DIOACTIVI~AND RADIATION expected to be nearly proportional to the distribution of indoor radon levels in figure 2.2, which implies a range of factor of more than 20. CONCLUSIONS The main conclusions drawn from the foregoing review are as follows: · All natural media earth, air, water, and biota, including humans are radioactive to some degree, and the concentrations of radionuclides in these media are highly variable, both between and within media. · Humans receive radiation exposure from natural sources outside and inside the body, averaging about 1 mSv (100 mrem) per year in the United States. · Humans receive radiation exposure from radon averaging about 2 mSv (200 mrem) per year in the United States. · Doses received by humans from sources of natural radiation in the environment are quite variable, with a range of a factor of about 4 for external sources except radon and about 20 for radon. As a practical matter, the implications of existing levels and the variability of natural radionuclides and doses received by humans should receive careful consideration as efforts to regulate TENORM are contemplated.
