Radionuclide Concentrations in Foor and the Environment - Chapter 2 docx

23

2

Radionuclide Sources

Jeffrey S. Gaffney and Nancy A. Marley

CONTENTS

2.1 Introduction 23
2.2 Primordial and Secondary Radionuclides 24
2.3 Cosmogenic Radionuclides 29
2.4 Anthropogenic Sources 31
2.5 Concluding Remarks 33
Acknowledgments 34
References 35

2.1 INTRODUCTION

We live on a planet that was created by the initial forces of the “big bang” and
continues to be affected by both natural events and human activities. The global
environment that surrounds us contains small amounts of radioactive (unstable)
elements or radionuclides (radioisotopes) that are derived from primordial, sec-
ondary, cosmogenic, and anthropogenic sources. Radionuclides in the air, soil,
water, and rocks that make up the Earth’s geosphere and atmosphere can be
transferred into the biosphere by many organisms and bioaccumulated in the food
chain. Indeed, the well-known uptake by living organisms of measurable amounts
of naturally produced radionuclides, such as

14

C, is used as a means of differen-
tiating living from “fossil” carbon. Most of the radioactivity to which we are
exposed daily comes from background natural sources commonly occurring in
our surrounding environment and the buildings in which we live.
Chapter 1 defines radionuclides and discusses the most common types of
ionizing radiation, namely

α

particles (energetic helium nuclei),

β

particles (ener-
getic electrons), and

γ

radiation (high-frequency, highly energetic electromagnetic
radiation). This chapter deals with the natural and anthropogenic sources of
radionuclides found in the environment. Addressing all of the more than 1,500
known radionuclides is beyond the scope of this chapter. We will focus on isotopic
species that are important contributors to overall radionuclide abundances in
various media, whose distributions in air, water, and soil are the topic of later
chapters. More detailed information can be found in more extensive books on
the sources of radionuclides, both natural and man-made [1].

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Radionuclide Concentrations in Food and the Environment

Traditionally radionuclides have been separated into three categories or types:
(1) primordial and secondary, (2) cosmogenic, and (3) anthropogenic. Primordial
radionuclides, such as uranium, thorium, and certain isotopes of potassium, have
very long lifetimes and were produced at or before the creation of planet Earth.
Secondary radionuclides are derived through radioactive decay of the long-lived
primordial parent nuclides. These decay products are commonly referred to as
daughters. Along with the parent sources, the daughters constitute radiogenic
decay families or “chains” that are an important source of natural radioactivity.
Cosmogenic radionuclides are formed by the interaction of cosmic rays with
Earth’s atmosphere or lithosphere, while anthropogenic radionuclides are formed
from human activities that create artificial radionuclides or enhance the levels of
certain radionuclides already present on Earth. In this chapter we discuss the
three types of radionuclide sources separately and highlight some of the more
important examples.

2.2 PRIMORDIAL AND SECONDARY RADIONUCLIDES

The primordial radionuclides have radioactive decay half-lives that are approxi-
mately Earth’s age or older (i.e., about 4 to 5 billion years). Primordial radio-
nuclides (and the radioactive decay products they produce) are an important
source of Earth’s radioactivity. These radionuclides play an important role in the
Earth’s processes. Indeed, primordial radionuclides, in particular a potassium
isotope of mass 40 (

40

K), have been suggested as a key source of long-term heat
in the Earth’s core over the past 4.5 billion years [2]. The human population is
exposed to radiation from primordial radionuclides directly, as a result of external
exposure, or through incorporation of these radionuclides into the body through
inhalation or ingestion. The primordial radionuclides present when the Earth was
formed that have half-lives less than 10

8

years have since decayed to undetectable
levels. Furthermore, the primordial radionuclides with half-lives greater than
10

10

years do not make significant contributions directly to background radiation
because their half-lives are long and their specific radioactivity levels are low.
However, they do contribute significantly to natural background levels of radio-
activity through their radioactive progeny or daughters, which often have much
shorter half-lives and lead to a chain of radioactive isotope production.
The primordial radionuclides compose a significant portion of the natural
radionuclides present on Earth because they are significantly long-lived and have
half-lives long enough to have been present at the beginning of the Earth’s
formation. Table 2.1 lists some of the more important primordial radionuclides
and their half-lives. Included are uranium and thorium isotopes having half-lives
on the order of 1 to 10 billion years.

232

Th, one of the most abundant of the
primordial radionuclides, has a half-life of 1.4

×

10

10

years and is found at
concentrations of 1.5 to 20 ppm in most crustal rocks [1].

238

U, another abundant
primordial radionuclide, is typically found at concentrations in the low parts per
million in minerals and rocks. Both

232

Th and

238

U are concentrated in coals and
peats, indicating that the bioaccumulation of these species has occurred over long

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Radionuclide Sources

25

periods of time. The humin and humic materials that are known to produce coals
and peat are strong chelating agents for these and other radionuclides [3]. Indeed,
the first discoveries of radioactivity and the isolation of important radionuclides
by the Curies and other pioneers in this area came in work with pitchblende and
peat known to be enriched in radionuclides through the interaction of organic
materials with rocks and minerals containing radioactive isotopes and elements.
Uranium was identified as an element by the German chemist Martin Klaproth,
who isolated it from samples of pitchblende in 1789. It was not until 1841 that
uranium was isolated in metallic form by the French chemist Eugene-Melchior
Peligot. Most of the early interest in this element grew from its ability to add
color to ceramics and paints. In 1896 the applied physicist Antoine Henri
Becquerel reported that all uranium salts are radioactive. This work led to his
sharing the 1906 Nobel Prize in physics with Pierre and Marie Curie for the
discovery of spontaneous radioactivity [4]. The three naturally occurring isotopes
of uranium are

234

U,

235

U, and

238

U.

238

U, by far the most abundant of the three,
has a half-life of 4.47

×

10

9

years. Thus about half of its original primordial level
at Earth’s formation remains. In comparison,

235

U is fairly depleted from its
original levels, having passed through more than six half-lives since Earth’s origin.
These two isotopes are both primordial, but

234

U, having a much shorter half-life,
would have essentially disappeared from the planet after more than 18,000 half-
life periods since its formation. However,

234

U is a good example of a secondary
radionuclide, as it is produced in small quantities by the radioactive decay of the
parent

238

U (see Figure 2.1). As we discuss later,

235

U and other isotopes that are
fissionable by neutrons have played an important role in anthropogenic radio-
nuclide production.
The uranium isotopes are all radioactive, and their decay produces a number
of secondary radioactive elements that continue to decay until they reach stable
nuclei. This decay chain of radionuclides is commonly referred to as the uranium
decay series. Similarly thorium, another primordial isotope with a long half-life,
also has a decay series that leads to the formation of numerous naturally occurring

TABLE 2.1
Some Important Primordial Radionuclides

Radionuclide Half-life (Years)
Estimated Abundance
in Crust (ppm)

40

K 1.38

×

10

9

2–3

87

Rb 4.8

×

10

10

3–9

138

La 1.1

×

10

11

1

×

10

2

to 2

×

10

2
147

Sm 1.1

×

10

11

0.5–1

187

Re 4.0

×

10

10

3–5

×

10

4
232

Th 1.4

×

10

10

1–20

235

U 7.0

×

10

8

0.3

×

10

2

to 3

×

10

2
238

U 4.5

×

10

9

0.5–5

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Radionuclide Concentrations in Food and the Environment

secondary radionuclides. Thus the key primordial radionuclides of uranium and
thorium decay to many other radioactive isotopes that occur in the environment
at different levels of abundance, depending on their own decay rates and those
of their parents. Figure 2.1 and Figure 2.2 show the decay schemes for primordial

238

U and

232

Th, respectively. Figure 2.3 shows the decay processes for

235

U. Only
the major pathways are shown in these figures, with the significant

γ

emitters
highlighted in bold type. More detailed information on the isotopic decay processes,
including minor pathways, can be obtained from the

Table of Isotopes

[5–7].
Other primordial isotopic species on the Earth’s surface include

40

K, which
has a half-life of 1.28

×

10

9

years. Potassium is quite an abundant element,
composing more than 2% of the Earth’s crustal mass. Of that amount, about
1.0

×

10

–4

(0.01%) is

40

K atoms.

40

K can decay by

γ

emission (11% of the decay
pathway) to give

40

Ar, and this is the basis for the potassium/argon methods used
to age date very old rocks, meteorites, etc.

40

K can also emit a

β

particle and lead
to the formation of

40

Ca (89% of the decay processes). Because of its ubiquity
and biological uptake,

40

K is the most significant natural source of radioactivity
ingested by humans.

FIGURE 2.1

Uranium 238 decay, showing the main paths for the production of various
radionuclides. Clear arrows indicate

β

decay and gray arrows are

α

processes. Half-lives
for the decay processes are indicated inside the arrows. The major

γ

emitters are in bold
letters. For complete radioactive decay processes, refer to

Table of Isotope

s and updates
[5–7].
4.7 × 10
9
y
234

24 d 4.2 m
238
U
234
Pa
234
U
2.4
× 10
5
y
3.8 d
1.6 × 10
3
y
7.7 × 10
4
y
222
Rn
226
Ra

218
Po
230

214
Pb
3 m
214
Bi
214
Po
210
Pb
20 m
6.4 × 10
−5
s
210
Bi
22.3 y
5 d
210
Po
138 d
27 m
206
Pb

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Radionuclide Sources

27

A very important and widespread secondary radionuclide is

222

Rn. This noble
gas, with a half-life of 3.8 days, is produced from the longer-lived

226

Ra (half-
life 1,600 years) formed by the decay of

238

U (see Figure 2.1). As a gas,

222

Rn
can diffuse through the crustal material into the atmosphere, where it can be
transported over continental regions. Its decay products attach themselves to fine
atmospheric aerosols in the respirable size range. The dominant secondary radi-
onuclide in this chain is

210

Pb, which has a half-life of 22.3 years. The fine aerosol

210

Pb and its daughters

210

Bi (half-life 5 days) and

210

Po (half-life 138 days) have
been used to estimate the residence times of submicron aerosols in the environ-
ment [8–10].

222

Ra and its progeny have been of particular concern as environ-
mental hazards, particularly in homes and buildings where air infiltration rates
can be low. Significant

222

Rn from ground-source uranium parents (see Figure 2.1)
can concentrate in the lower levels of buildings (cellars, basements, etc.) and lead
to potential inhalation risks in indoor environments [11].

210

Pb is another very ubiquitous secondary radionuclide that is formed from

238

U decay via

222

Rn (see Figure 2.1). Because it attaches itself to fine aerosols

FIGURE 2.2

Thorium 232 decay, showing the main paths for the production of various
radionuclides. Clear arrows indicate

β

decay and gray arrows are

α

processes. Half-lives
for the decay processes are indicated inside the arrows. The major

γ

emitters are in bold
letters. For complete radioactive decay processes, refer to

Table of Isotopes

and updates
[5–7].
1.4 × 10
10
y
228
Ra
5.8 y 6.1 h
228
Ac
232

228

1.9 y
56 s
0.15 s
3.7 d
212
Pb
216
Po
220
Rn
224
Ra
11 h
212

Bi
212
Po
208
Tl
61 m
(64%)
3.1 m
61 m
(36%)
3.1
× 10
−7
s
208
Pb

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28

Radionuclide Concentrations in Food and the Environment

in the lower to mid troposphere once it is produced from the gaseous

222

Rn,

210

Pb
can spread over significant distances. Indeed, a significant amount of

210

Pb is
usually present in the upper sections of soil cores because of the atmospheric
deposition of

210

Pb (half-life 22.3 years). Concentrations of

210

Pb usually decrease
as a function of distance downward in soil cores, gradually diminishing from the
surface to a fairly constant level within about 1 m of the surface. The background
level of

210

Pb in subsurface soils is due to

222

Rn decay as the gas diffuses from
soil and rock matrices. This background level of

210

Pb is considered to be sup-
ported by the local soil environment. The “excess”

210

Pb found in the soil closer
to the surface is due to

222

Rn gas that is dispersed through the lower atmosphere
and decays to produce

210

Pb, which becomes attached to fine aerosol particles
and is deposited on soil surfaces by wet and dry deposition. The presence of this
“excess”

210

Pb in surface soils due to atmospheric deposition has been useful for
estimating soil sedimentation rates and erosion rates in many environments [12–17].
While many of the primordial and cosmogenic radionuclides are concentrated
in Earth’s lithosphere, significant amounts of

14

C,

238

U, and other radionuclides
are found in the oceans as well. These accumulations are due to the equilibration
of

14

CO

2

with ocean waters and the dissolution of minerals into fresh and ocean
waters from rocks and soil erosion. Many primordial and secondary radionuclides
are also found in fresh surface waters and groundwaters at low concentrations.

FIGURE 2.3

Uranium 235 decay, showing the main paths for the production of various
radionuclides. Clear arrows indicate

β

decay and gray arrows are

α

processes. Half-lives for
the decay processes are indicated inside the arrows. The major

γ

emitters are in bold letters.
For complete radioactive decay processes, refer to

Table of Isotopes

and updates [5–7].
7.0 × 10
8
y
231

25 h
235
U
231
Pa
227
Ac
22 y
(99%)
4 s
11 d
19 d
219
Rn

223
Ra
215
Po
227

211
Pb
8 ×
10
−4
s
211
Bi
207
Tl
207
Pb
36 m
2.1 m 4.8 m
223
Fr
22 y (1%)
22 m
3.3 × 10
4
y

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Radionuclide Sources

29

These are typically chemically bound or chelated by dissolved organic substances,
principally humic and fulvic acids, that can limit the bioavailability of these
materials in the natural environment [3].

2.3 COSMOGENIC RADIONUCLIDES

Cosmogenic radionuclides are formed by interactions of highly energetic cosmic
particles with Earth’s atmosphere and surface that lead to the formation of radio-
active isotopes [18]. Some important cosmogenically produced radionuclides and
their lifetimes are shown in Table 2.2.
Galactic cosmic rays that are capable of nuclear interactions lead to the direct
formation of radionuclides and also generate secondary particles, particularly
neutrons, that can result in the production of important radionuclides including

3

H,

7

Be,

10

Be,

14

C, and

22

Na. Most of these interactions occur in Earth’s atmo-
sphere, particularly in the stratosphere and upper troposphere. However, some
minor production of radioisotopes also occurs at the Earth’s surface (e.g.,

10

Be,

26

Al, and

21

Ne) and their presence is an important indicator of cosmic ray activity.
The differences in production rates in the atmosphere and surface are primarily
due to the strong attenuation of many cosmic particles and secondary particles
by Earth’s atmosphere, with the result that more high-energy particle interactions
occur in the upper atmosphere than at the surface, where the flux is smaller.
Production rates of many cosmogenic radionuclides depend on incoming
cosmic particle intensities, which can be affected by Earth’s magnetic field or
can vary due to solar activity (e.g., sunspots or solar flares). The variations in the
cosmic radiation fluxes lead to some temporal and spatial variability in the pro-

duction of these radionuclides. Thus relatively short-term variations (on the order
of days) and seasonal variations can result from solar events and changes in
cosmic ray intensities. In addition, variability in latitudinal production arises
because Earth’s magnetic field can focus incoming cosmic rays, leading to more
significant production at higher latitudes. The northern lights (the aurora borealis)

TABLE 2.2
Some Important Cosmogenically Produced
Radionuclides and Their Half-Lives

Radionuclide Half-Life Major Source

3

H 12.3 years Atmospheric N, O

7

Be 53.3 days Atmospheric N, O

10

Be 1.5

×

10

6

years Atmosphere N, O, surface O

14

C 5.73

×

10

3

years Atmospheric N

26

Al 7.1

×

10

5
years Surface Si, meteorites
36
Cl 3.1 × 10
5
years Surface Ca, K
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30 Radionuclide Concentrations in Food and the Environment
are evidence of the increased production of radionuclides and the atmospheric
effects of cosmic rays on the atmosphere at higher latitudes.
Cosmic radiation consists primarily of highly energetic particles, including
α particles and neutrons. Their effects in the upper atmosphere lead to nuclear
interactions with nitrogen and oxygen atoms and molecules resulting in the
production of
14
C and
7
Be and other radionuclides, directly or via secondary
neutron interactions. These same types of nuclear reactions can occur during
aboveground nuclear tests that release energetic particles into the atmosphere.
These reactions produce secondary particles (neutrons, protons, etc.) that can
generate the same radionuclides as normal cosmic radiation exposures. Thus,
during the 1950s, significant amounts of “bomb carbon” (
14
C produced from
aboveground nuclear tests) were produced, along with other radionuclides that
will be discussed later in this chapter, by the same processes that occur naturally.
14
C is one of the more important natural radionuclides, being produced in the
atmosphere by cosmic particle bombardment of nitrogen atoms. Once formed,
the atomic
14
C is rapidly oxidized to carbon dioxide in the upper atmosphere.
The
14
C-labeled carbon dioxide, quite a stable molecule, is mixed from the upper
atmosphere down into the troposphere, where it is taken up by plants during

photosynthesis. As herbivores and omnivores ingest plants for food, the
14
C is
carried throughout the food chain, ultimately labeling all living things on the
surface of the planet. With a half-life of 5.73 × 10
3
years, the abundance of
14
C
has been used to differentiate recent carbon present in samples from “fossil”
carbon derived from petroleum that is hundreds of millions of years old and is
quite “dead” with regard to
14
C content [19].
14
C is also the basis for carbon
dating of organic artifacts in archeology.
Cosmic particle-driven neutron spallation reactions near the Earth’s surface
can lead to the formation of some important radionuclides that have been used for
geochronology, such as
10
Be,
26
Al, and
36
Cl. Estimation of the production rates of
cosmogenic nuclides requires an understanding of the cross sections for the nuclear
reactions, along with estimates of cosmic ray fluxes that vary with geomagnetic
latitude and altitude. Modeling that incorporates experimentally derived cross
sections for gases and minerals has been used to estimate radionuclide production

rates [20,21]. These production rates are then compared with direct measurements
to evaluate the estimated results and also to probe past cosmic ray activity by
examining the variance and concentrations of surface radionuclides of various
lifetimes. Since the first measurements of
14
C by Willard Libby and coworkers
[22], these cosmogenic isotopes have been used for geochronology, becoming
important tools for the “dating” of events in geochemistry and geomorphology.
An extraterrestrial source for some of the heavier cosmogenic nuclides such
as
26
Al is meteoric material that strikes the atmosphere or Earth’s surface. Cross
sections for atmospheric production of this radioisotope are small, because
26
Al
is largely produced from argon, which composes only 1% of the atmosphere by
volume. In contrast, the production of
26
Al can be quite high on the mineral
surfaces of meteors because of the higher cosmic ray exposures in space. This
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Radionuclide Sources 31
difference has led to the use of
26
Al to evaluate meteoritic material deposition on
the Earth’s surface and to measure ice surface ages in the Antarctic [23,24].
There are indeed many trace-level cosmogenically produced radionuclides
besides the ones we have discussed here, including
18

F,
22
Na,
24
Na,
31
Si,
32
Si,
32
P,
33
P,
35
S,
37
Ar,
38
Cl,
38
Mg,
38
S,
39
Ar,
39
Cl, and
80
Kr, as well as stable radionuclides
like

3
He [1]. Many of the cosmogenic radioisotopes with longer half-lives are
difficult to measure with conventional radiochemical counting methods. Because
they have low radioactivity levels, they are measured directly by using accelerator
mass spectrometry methods that have enhanced sensitivity and speed. Cos-
mogenic radioisotopes have been used to estimate surface ages of the Earth
because their general production rates have remained fairly constant over time.
Examination of the surface concentrations of the longer-lived radionuclides
clearly indicates that Earth’s surface has been exposed to cosmic radiation for
millions of years, at a minimum. These data have been used as an effective
argument against the concept of a much shorter time for Earth’s creation that has
been put forth by some creationist philosophies.
2.4 ANTHROPOGENIC SOURCES
Most of the radionuclides present on Earth are from primordial or cosmogenic
sources, as noted above. During the early 1930s, a series of events that would
change history and the world we live in began in the physics and chemistry
communities. Following Enrico Fermi’s lead in exploring the interactions of
heavy nuclei with neutrons, Otto Hahn and Fritz Strassman attempted to make
heavier elements (transuranics) by bombarding uranium with neutrons. They were
able to identify the production of
141
Ba, which was correctly explained by Lise
Meitner and Otto Frisch [25] as a fission product of
235
U. Soon, Niels Bohr and
others recognized that the release of very large amounts of energy from nuclear
fission might be useful for both peaceful and military applications. Letters from
Bohr to Einstein and from Einstein to President Franklin Roosevelt ultimately led
to the initiation of the Manhattan Project in the U.S. in June 1942 [26].
As part of the Manhattan Project, a group led by Enrico Fermi began to build

a uranium-based reactor that they hoped would demonstrate the potential for a
controlled chain reaction starting with
235
U. On December 2, 1942, the first self-
sustained chain reaction, using enriched uranium oxide moderated by graphite
rods, was achieved at the University of Chicago’s Stagg Field stadium. This initial
experiment demonstrating controlled nuclear fission led to the development of
atomic weapons and nuclear industries in medicine and energy [26]. It also was
the dawn of development of many radionuclides produced by humans for widely
ranging uses including nuclear reactors, nuclear medicine, and nuclear weapons.
Nuclear fission is the process by which neutrons produce chain reactions in
a nuclear reactor. When a fissionable nucleus is hit by a thermal or slow neutron,
the nucleus can interact with the neutron and divide (fission) into two smaller
nuclei, releasing neutrons and energy that initiate the splitting of more fissionable
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32 Radionuclide Concentrations in Food and the Environment
atoms, leading to a chain reaction.
235
U is the most abundant naturally available
isotope that can undergo fission. Gaseous diffusion and other methods are used
to enrich and separate the small amount of
235
U (0.72% natural abundance) from
the predominantly
238
U found in nature. For most nuclear reactors, such as the
light-water reactors, the enrichment required for a sustained nuclear reaction is
approximately 10-fold. The more significant enrichment of
235

U required for
atomic weapons is a difficult and expensive task.
In a nuclear reactor, the chain reaction with
235
U releases energy and neutrons
and produces a number of side products, including isotopes of plutonium from
neutron capture by
238
U in the fuel rods.
239
Pu produced through exposure of
238
U
to neutrons is also fissionable. Both
235
U and
239
Pu have been used in nuclear
reactors and in atomic weapons. Indeed, the first atomic weapons used in World
War II, “Little Boy” and “Fat Man,” were bombs that used
235
U and
239
Pu,
respectively. Other fissionable materials, including
233
U and
232
Th, could conceiv-
ably be used in nuclear fuel cycles, although currently

235
U and
239
Pu are the main
fuels used.
239
Pu is produced from
238
U by neutron irradiation, usually in
235
U/
238
U
“breeder” reactors.
Fission reactions lead to the formation of many isotopes (both stable and
radioactive) from a wide variety of elements, as many fragment combinations are
possible and do occur. For
235
U, the addition of one neutron would lead to two
fission nuclei of 118 mass units if the process gave two equally sized nuclei.
However, the fission reaction leads mostly to fragments of unequal sizes. For the
case of
235
U, the major fission products are
137
Cs and
90
Sr. During aboveground
testing of atomic bombs, a significant amount of anthropogenic radionuclides
was released into the stratosphere and upper troposphere. This material became

attached to particulate matter in the atmosphere and was deposited worldwide as
“radioactive fallout.”
Early on, bombardment of
238
U with neutrons was considered the only source
of
238
Pu,
239
Pu,
240
Pu, and
241
Pu because plutonium was not a known natural
radionuclide until its discovery in 1940 by Glenn Seaborg and colleagues. The
15 known isotopes of plutonium are mostly short-lived. The most important of
these, as noted above, is
239
Pu, which is fissionable and has a long half-life (2.4 ×
10
4
years). Not until the early 1970s did discovery of the remains of a natural
fission reactor system in the Oklo district of Gabon, Africa, provide evidence that
plutonium production could occur naturally [27–29]. The Oklo area is very high
in uranium. Analysis of mines there yielded anomalous isotopic data indicating
that neutron chain reaction events might have occurred under natural water medi-
ation of the deposits. Furthermore, very low levels of
239
Pu produced more
recently by normal neutron capture in uranium cores were measured in samples

from the site [27,29]. Although these results demonstrate that natural production
of
239
Pu is possible, it is safe to say that most of the plutonium currently present
on Earth came from anthropogenic sources.
Many of the anthropogenic radionuclides produced from nuclear power or
nuclear bomb tests have reasonably short half-lives with the exception of
239
Pu.
Some other anthropogenic radionuclides include
131
I, which has a half-life of
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Radionuclide Sources 33
8 days and is used in the treatment of thyroid cancer. Space limitations allow us
to mention only some of the more important members of this large group. Since
the cessation of aboveground testing, a number of the shorter-lived radioisotopes
have already decreased and eventually will be lost; these include
90
Sr and
137
Cs,
which have half-lives of approximately 30 years. In addition, a number of radio-
isotopes, including
3
H,
11
C,
13

N,
14
C,
15
O,
99
Tc,
123
I,
125
I, and
131
I, are produced
by the use of special equipment (nuclear reactors, cyclotrons, etc.) developed in
the high-energy physics community and used routinely in nuclear medicine [30].
Some of the more important anthropogenic radionuclides are listed in Table 2.3,
along with their half-lives.
2.5 CONCLUDING REMARKS
It should be noted that the sources of radioactivity to which the average person is
exposed during a lifetime are dominated by the natural sources (i.e., the primordial
and secondary radionuclides and the cosmogenic radionuclides). Anthropogenic
material accounts for a small fraction of naturally occurring radioactivity. As
estimated by the National Council on Radiation Protection and Measurement
[31], exposure to radiation from artificial sources is only about 18% of the total
human exposure in the U.S., and most of that exposure is from medical x-rays,
nuclear medicine, etc. Exposures due to nuclear energy and fallout are responsible
for less than 1% of the background exposure (see Figure 2.4). Many fallout
materials have fairly short half-lives, and their levels are decreasing rapidly.
Aboveground testing largely stopped when the U.S., the USSR, and the U.K.
signed a nuclear test ban treaty on August 5, 1963. France and the Republic of

China continued to test nuclear weapons aboveground and in the oceans until
1996, but these tests were few in comparison with the aboveground tests of the
U.S. and USSR. Although it has not been approved by the U.S., a comprehensive
test ban treaty drawn up by 37 nations in 1996 is likely to prevent the kind of
aboveground nuclear testing that led to the dispersal of significant amounts of
radionuclides globally, barring a nuclear war.
TABLE 2.3
Some Anthropogenically Produced Radionuclides
and Their Half-Lives
Radionuclide Half-life (Years) Major Source
3
H 12.3 Atmospheric weapons tests, reactors
14
C 5.73 × 10
3
Atmospheric weapons tests
90
Sr 2.9 × 10
1
Reactors, weapons tests
99
Tc 2.1 × 10
599
Mo decay, nuclear medicine
129
I 1.6 × 10
7
Reactors, weapons tests
137
Cs 3.0 × 10

1
Reactors, weapons tests
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© 2007 by Taylor & Francis Group, LLC
34 Radionuclide Concentrations in Food and the Environment
Most of the radionuclides to which we are currently exposed in our foods
and the environment come primarily from the naturally occurring primordial,
secondary, and cosmogenic sources. Only during catastrophic events such as the
1986 Chernobyl reactor disaster or the testing of an atomic device aboveground
are releases of anthropogenic radionuclides significant on a regional or global
scale. Other sources of radionuclide release are tied to the nuclear energy industry
in the transport of fuel rods and the reprocessing and disposal of spent fuel rods.
In many instances spent uranium fuel rods are allowed to sit in interim cooling
sites (ponds or air-cooled containers) so that the short-lived radionuclides pro-
duced during fission and neutron release can decay before the rods are reprocessed
with the extraction of
239
Pu and unburned
235
U. Alternatively, the longer-lived
radionuclide waste is placed in a geologic repository like the Yucca Mountain
site. Other sources of radioactivity include medical radioactive sources used for
nuclear medicine, typically operated in hospitals in major cities. These are local-
ized sources and are not considered in this chapter. As they are of potential
concern as sources of radioactivity, they are strongly regulated by various federal
government agencies, as well as by the International Atomic Energy Agency.
ACKNOWLEDGMENTS
The submitted manuscript has been created by the University of Chicago as
operator of Argonne National Laboratory under contract no. W-31-109-ENG-38
with the U.S. Department of Energy. The U.S. government retains for itself, and

FIGURE 2.4 Relative radiation exposure estimates for the U.S. population, based on
estimates from Curtis et al. [29].
“222Rn and daughters
(inhalation)”
55%
“40 K, 14 C (ingestion)”
11%
“Soils and rocks”
8%
“Cosmic rays”
8%
“Fallout”
<1%
“Nuclear plants”
<1%
“Job related”
<1%
“Nuclear medicine (x-ray, etc.)”
15%
“Household products (smoke
detectors, TVs, etc.)”
3%
DK594X_book.fm Page 34 Tuesday, June 6, 2006 9:53 AM
© 2007 by Taylor & Francis Group, LLC
Radionuclide Sources 35
others acting on its behalf, a paid-up, nonexclusive, irrevocable worldwide license
in said article to reproduce, prepare derivative works, distribute copies to the
public, and perform publicly and display publicly, by or on behalf of the govern-
ment. The authors’ work is supported by the U.S. Department of Energy, Office
of Science, Office of Biological and Environmental Research under contract no.

W-31-109-ENG-38.
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