Environmental Geosciences

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Environmental Geosciences

• Human Interactions with the Environment

Radioactive Compounds
Andrea Koschinsky
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Radioactive compounds
Radioactivity Atoms are either stable or
radioactive . The nucleus of a stable atom does
not change in form over time. A radioactive
isotope (radioisotope) has an unstable nucleus.
In 1980, Choppin and Ryberg stated in their
book Nuclear Chemistry , "We can conclude that
nuclear stability is favored by even numbers of
protons and neutrons." Therefore, it is the ratio
of neutrons to protons that determines the
stability of an atom. Stability for light
elements exists when the number of protons to
neutrons is about equal. However, for heavier
atoms, the nucleus is stable when the ratio of
neutrons to protons is from 1 to 1.5.
Choppin, G.R. and Ryberg, J. Nuclear
Chemistry Theory and Applications. New York
Pergamon Press, 1980.
Plot of the number of neutrons versus the number
of protons in stable nuclei. As the atomic number
increases, the neutron-to-proton ratio of the
stable nuclei increases. The stable nuclei are
located in the shaded area of the graph known as
the belt of stability. The majority of
radioactive nuclei occur outside this belt. All
nuclei with 84 or more protons (atomic number 84)
are radioactive .
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Radioactive compounds
Radioactivity A radioactive nucleus undergoes
change by emitting different forms of radiation,
either particles or photons . (A photon is a
"particle of light" that produces electromagnetic
radiation.) These changes are known as
radioactive decay, and the phenomenon is known as
radioactivity. Through radioactive decay, the
unstable nucleus is rearranged to become more
stable, until the proton-to-neutron ratio falls
within the belt of stability.
The basic unit of measure for radioactivity is
the number of atomic decays per unit time. In the
SI system, it is the Becquerel (Bq), defined as
one decay per second. An older measure is the
curie (1 Ci 3.7x1010 Bq). The units used to
describe the dose or energy absorbed by a
material exposed to radiation, are dependent upon
the type of radiation and the material. X-ray or
g-radiation absorbed by air is measured in
Roentgens (R ). The dose absorbed by any
material, by any radiation, is measured in rad (1
rad 100 erg/g with 1 erg 10-7 J of absorbed
energy). The SI equivalent is the gray (1 Gy
100 rad). The historical unit to describe
biological damage is rem (roentgen-equivalent-man)
and the SI unit is sievert (1 Sv 100 rem).
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Radioactive compounds
Radiations from radioactivity There are three
types of radiations corresponding to three types
of radioactivity. alpha radioactivity
corresponds to the emission of a helium nucleus,
a particularly stable structure consisting of two
protons and two neutrons, called an a particle.
beta radioactivity corresponds to the
transformation, in the nucleus - either of a
neutron into a proton, beta -radioactivity,
characterised by the emission of an electron e-
- or of a proton into a neutron, beta
radioactivity, characterised by the emission of
an anti-electron or positron e. It only appears
in artificial radioactive nuclei produced by
nuclear reactions. gamma radioactivity , unlike
the other two, is not related to a transmutation
of the nucleus. It results in the emission, by
the nucleus, of an electromagnetic radiation,
like visible light or X-rays, but more energetic.
gamma radioactivity can occur by itself or
together with alpha or beta radioactivity.
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Radioactive compounds
Radioactivity Two other phenomena, nuclear
fusion and nuclear fission, involve fusing and
splitting nuclei, respectively. Fusion occurs
naturally on the stars. Fission occurs, for the
most part, in a nuclear reactor and (on rare
occasions) in natural deposits of heavy elements.
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Radioactive compounds
Half-Life Radioactivity can be described in
terms of half-life . The term was coined by
Ernest Rutherford. A half-life is defined as the
amount of time required for one-half of the atoms
of a radioactive substance to decay into another
form. For example, if you have one pound of an
isotope of polonium-210 and the half-life is 138
days, after 138 days, only a half pound of the
original amount remains after 276 days, only
one-fourth of the original isotope is left. The
polonium-210 has changed (decayed) into atoms of
lead-206. Only one-eighth will remain after 414
days and one-sixteenth after 552 days. Every
radioisotope has a characteristic half-life.
This amount of time varies from millionths of a
second to billions of years, depending upon
the particular isotope.
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Radioactive compounds
The following tables give examples of some
well-known radioisotopes, the type of radiation
emitted, and their half-lives.
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Radioactive compounds
Half-Life Biological molecules contain
significant amounts of carbon, including both
carbon-12 and carbon-14 isotopes. The age of
biological material can be determined based on
the carbon-14 decay rate. Since the half-life of
carbon is relatively long, this method can only
be applied to objects from a few hundred to
50,000 years old. Carbon-14 is incorporated into
our body tissues due to the amount of carbon-14
in our food. The intake of carbon-14 stops when
we die. The approximate age of an individual
organism can be estimated if the ratio of
carbon-14 to carbon-12 is known in a similar
organism today.
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Radioactive compounds
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Radioactive compounds
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Radioactive compounds
Natural radioactivity and its distribution The
exposure to natural sources is caused by two
different sources A. Cosmic radiation that
impacts on the Earth, and its intensity depends
on geographical positions on the Earth. B.
Natural radionuclides that are normally present
in the environment. These nuclides can be divided
into three groups according to their origin
Cosmogenic nuclides that are generated by the
nuclear reactions during the interaction between
cosmic radiation and stable isotopes, especially
in the atmosphere (for example, a well-known 14C
isotope is generated by the reaction 14N(n,p) --gt
14C ). The original primordial nuclides that
originated in the early stages of the universe
are even now present on the Earth due to their
long half-lives (gt108 years) in a significant
quantity (e.g. 238U, 235U, 232Th, 40K, 87Rb
etc.). Many other nuclides that were early
generated decayed due to their short half-lives,
and these isotopes are not detectable any more.
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Radioactive compounds
The original radionuclides disintegrate to the
secondary radionuclides and form the decay
series. There are four well-known decay series,
i.e., uranium-radium decay chain (starting from
238 U), thorium decay chain (starting from 232
Th), actinium decay chain (starting
from 235 U), and neptunium decay chain (starting
from 237 Np). The last two groups of natural
radionuclides originate from the Earth, and these
are called " terrestrial ". From the point of
view of human exposure, only some natural
radionuclides are important. The external
exposure is mainly caused by 226 Ra (or by
uranium), 232 Th and 40 K which can be found in
rocks and soil on the earth's surface (the
thickness of the layer is some few tens of
centimetres). The dose rate that originates from
terrestrial nuclides is about 0.057 mGy/hr, (this
is the mean value on the Earth), the maximum
values have been measured on monazite sand in
Guarapari, Brazil (up to 50 mGy/hr and in Kerala,
India (about 2 mGy/hr), and on rocks with a high
radium concentration in Ramsar, Iran (from 1 to
10 mGy/hr). From the point of view of internal
exposure, radon ( 222 Rn), thoron ( 220 Rn), and
their decay products prevail.
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Radioactive compounds
Natural radioactivity and its danger to
humans Nowadays, fear of the population of
radioactivity is focused on artificial radiation
sources, especially on nuclear facilities. Most
people do not suspect that the greatest exposure
to the population is caused by natural sources.
Human bodies have been always exposed to natural
radiation, and to a certain extent, this exposure
has been unavoidable. Some groups of the
population on the Earth are exposed to radiation
doses that are by one to two orders higher than
the global mean value of radiation doses.
Attention has been devoted since the turn of the
1980s to the highest exposures to the population
that are caused by indoor radon. In some houses
in the Czech Republic, radon concentrations that
entered from the soil were as high as the ten
times the value of the limit value of the radon
concentrations in uranium mines, and the annual
doses of the population were more than a hundred
times higher than the dose mean value in the
population.
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Radioactive compounds
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Radioactive compounds
Some natural radiation sources are affected by
human activities, and it is reasonable to control
them. The examples are as follow remedial
measures during the construction of new buildings
or the reconstruction of the existing buildings
remedial measures to reduce the exposure to the
population from underground water sources with
the higher concentration of natural
radionuclides and control of the natural
radionuclides released to the environment during
industrial activities.
From the point of view of internal exposure,
potassium 40K. is also a significant nuclide. The
potassium concentration in the human body is
nearly stable in all persons at a level of about
55 Bq/kg, which corresponds to the annual
effective dose of 0.17 mSv. Because of internal
exposure, attention should be devoted to the
following isotopes radium 226 Ra and 228 Ra,
uranium 238 U and 234 U, polonium 210 Po and lead
210 Pb. Great differences may appear in nuclide
uptake (and also in corresponding doses) for
individual persons or the groups of the
population. With the exception of the inhalation
of radon and its decay daughters that contribute
to the highest doses to the population, the
uptake by ingestion, in general, is much higher
than that by inhalation. From the point of view
of the exposure to population, the contribution
of the cosmogenic nuclides (not cosmic radiation)
is negligible.
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Radioactive compounds
Humans are exposed every day to radioactive
elements that occur naturally in the environment.
Gamma and alpha radiation emitted by
radioactive elements in rocks and soils,
especially those that decay quickly (such as
radon), pose a health risk. This radiation is
implicated in cancers of the lung, bone, and of
other organs. The health threat posed by uranium
alone primarily as a heavy-metal chemical poison
similar to arsenic. This is known as
chemotoxicity and is implicated in kidney
disease. Radon is especially dangerous because
it is a gas and can easily enter the lungs.
Although natural concentrations of these
radioactive materials are usually less than
established threshold health values, human
activity often inadvertently exposes us to
radionuclides at dangerous levels. The
Environmental Protection Agency (EPA) estimates
that as much as 30 percent of the public
drinking-water supply in the United States
exceeds their recently-established maximum
contaminant levels for radon. An even greater
percentage of private water supplies, unregulated
by EPA, may contain elevated levels of
radioactive materials.
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Radioactive compounds
Radionuclides are present in all rocks in varying
amounts, and they are easily mobilized in the
environment. The high geochemical mobility of
radionuclides in the environment allows them to
move easily and to contaminate much of the
environment with which humans come in contact.
Uranium, in particular, is easily mobilized in
ground water and surface water. As a result,
uranium and its decay product, radium, enter the
food chain through irrigation waters, and enter
the water supply through ground-water wells and
surface-water streams and rivers. The health
risks to humans are real, but the level of risk
involved is not clearly defined because we do
not yet know enough about the distribution and
concentration of these radionuclides.
Uranium frequently and preferentially
concentrates in wetland environments where
uranium-rich rocks occur. Concentrations of
uranium in dead and decaying organic material in
wetlands is a potential threat to the health of
humans and to wetland habitats. Although many
wetlands serve as natural filters protecting
surface waters from uranium contamination,
disturbances of wetlands from such events as
hurricanes, dredging, draining, road building,
and water recovery, may allow uranium to become
mobile, contaminating water which is subsequently
consumed by humans and other animals.
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Radioactive compounds
Uranium contaminates surface waters in many
irrigated lands. In irrigated areas along the
Arkansas River Valley in southeastern Colorado,
uranium and salts are actively leached from
marine shales. These contaminated, saline,
irrigation waters eventually return to the river
where uranium levels increase to concentrations
as high as 200 parts per billion and, because of
the accompanying high salinity, wetlands in this
area are not trapping uranium.
In other much-publicized wetland areas such as
the Kesterson Wildlife Refuge in California,
uranium and selenium contamination is responsible
for wildlife death and deformities. Many other
irrigation systems in semi arid areas that drain
farmland on marine shales face similar stresses
on water quality.
Uranium in surface waters of the Arkansas River
Valley, SE Colorado, April 1991. Irrigation
waters taken from the upstream parts of the river
are used to flood fields where they leach uranium
and salts from the rock and soils. Much of this
water is then returned to the tributary streams
and the main river.
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Radioactive compounds
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Radioactive compounds
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Radioactive compounds
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Radioactive compounds
The geochemistry of uranium. Dissolved uranium
complexes in water with dissolved fluorides,
phosphates, and carbonates. When phosphate
precipitates from water uranium goes with it. As
a result, for example, uranium is a serious
contaminant in phosphate fertilizers that are
ubiquitous in crop farming. As irrigation water
containing uranium is used, fertilizers that also
contain uranium serve to compound the potential
toxicity. Although most crops resist uptake of
radioactive materials in their leafy
(above-ground) components, those crops whose
roots are consumed (such as potatoes, peanuts,
carrots), are susceptible to contamination by
uranium. Geochemical sampling and detailed
geological mapping are essential early steps to
knowing where irrigation water, contaminated by
underlying rocks or by fertilizers may be a
problem.
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Radioactive compounds
The geochemistry of uranium.
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Radioactive compounds
The geochemistry of uranium Long-Term Uranium
Migration in Agricultural Field Soils Following
Mineral P Fertilization. Jacques, D., Simunek,
J., Mallants, D., Van Genuchten, M.T. 2005.
Environmental Remediation Conference Proceedings.
In 10th International Conference on
Environmental Remediation and Radioactive Waste
Management. Sept. 4-8, 2005. American Society of
Mechanical Engineers. 8 P. Technical Abstract
To preserve soil fertility, organic and mineral
fertilizers are often applied to agricultural
fields. Mineral fertilizers such as phosphates
and super phosphates contain a certain amount of
long-lived alpha activity due to 238-U, 230-Th,
amongst others. The fate of U in soil systems is
quite complex. Since U forms aqueous complexes
with soil organic matter, nitrate, phosphate, and
carbonate, amongst others, U migration may be
influenced by their cycles in the soil.
Furthermore, surface complexation onto the soil
solid phase strongly influences the fate of U in
the soil profile, whereby U-surface complexation
competes with the adsorption of protons and other
cations.
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Radioactive compounds
HLW high-level waste
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Radioactive compounds
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Radioactive compounds
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Radioactive compounds
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Radioactive compounds
The global dispersion and deposition of debris
from atmospheric nuclear weapons is by far the
largest source of artificial radioactivity
released into the global environment.
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Radioactive compounds
The injection and partitioning of radioactive
debris in the atmosphere can be estimated from
the location and yield of each test. The total
fission energy is largely divided between the
equatorial Pacific (43) and the polar North
(43).
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Radioactive compounds
Other important sources of artificial
radioactivity in the marine environment include
the dumping of nuclear waste, effluent discharges
form nuclear fuel cycle and nuclear weapons
production, accidental releases from land-based
nuclear installations, and other accidents and
losses at sea involving nuclear material.
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Radioactive compounds
The Russian Ministry of Atomic Energy is the
owner of the worlds largest nuclear waste
stockpile. Estimates of releases to the
environment through the end of 1996 exceed 6.3 x
104 PBq compared with less than 100 PBq in the
US. Ca. 97 of the radioactive waste entering
the environment has been disposed of by
underground injection or discharged into surface
waters.
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Radioactive compounds
Sellafield nuclear fuel processing facility
discharges two million gallons of radioactive
water into the Irish Sea every day. This includes
a cocktail of over 30 alpha, beta and gamma
radionuclides. Radioactive discharges in the
1970s were 100times those of today. As a result
of these discharges, which include around half a
ton of plutonium, the Irish Sea has become the
most radioactively contaminated sea in the world.
Caesium-137 and Iodine-129 from Sellafield have
spread through the Arctic Ocean into the waters
of northern Canada and are having a bigger impact
on the Arctic than the Chernobyl accident.
Sellafields gas discharges of Krypton can be
measured in Miami.
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Radioactive compounds
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Radioactive compounds
Depth distribution of radionuclides in the ocean
is controlled by their geochemical behaviour Cs
and Sr are conservative elements the profile
indicates the input from the surface. Pu is a
particle-reactive element that is actively
transported to greater water depth with sinking
particles. The mid-depth maximum may indicate
(biogeochemical) recycling.
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Radioactive compounds
Behaviour of transuranium nuclides Because of
the variability of inputs both spatially and
temporally, transuranium inventories in the
oceans are not uniformely mixed, and activity
concentrations vary widely between different
marine zones.
Various factors are important for transuranium
behaviour - Physical and chemical forms of
nuclides - Physico-chemical properties of these
nuclides in seawater - Subsequent physical,
chemical and biological processes in the marine
environment.
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Radioactive compounds
During the 48 years history of sea disposal, 14
countries have used more than 80 sites to dispose
approximately 85 PBq (2.3 Mci) of radioactive
waste.
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Radioactive compounds
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