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OCR for page 25
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
OCR for page 26
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.
OCR for page 27
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.
OCR for page 28
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
OCR for page 29
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.
OCR for page 30
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
OCR for page 31
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.
OCR for page 32
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.
OCR for page 33
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.
OCR for page 34
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
OCR for page 35
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).
OCR for page 50
50
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OCR for page 51
GUIDELINES FOR EXPOSURE TO TENORM
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OCR for page 52
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.
OCR for page 53
GUIDELINES FOR EXPOSURE TO TENON
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NATURAL RADIOACTIVITYAND RADIATION
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OCR for page 55
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
OCR for page 56
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
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(l MASH ~
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OCR for page 57
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
OCR for page 58
58
NATURAL RADIOACTIVITYAND RADIATION
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OCR for page 59
GUIDELINES FOR EXPOSURE TO TENORM
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OCR for page 60
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.
Representative terms from entire chapter:
cosmic radiation
