Environmental Geosciences. Human Interactions with the Environment. Radioactive Compounds. Andrea Koschinsky. Radioactive compounds. Radioactivity
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Human Interactions with theEnvironment
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 .
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).
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.
Nuclear fusion (a thermonuclear reaction ) is a process in which two nuclei join, forming a larger nucleus and releasing energy . Nuclear fusion is the energy source which causes stars to shine, and hydrogen bombs to explode.
U fission chain reaction: along with barium and krypton, three neutrons are released during the fission process. These neutrons can hit further U-235 atoms and split them, releasing yet more neutrons.Radioactive compounds
Two other phenomena, nuclear fusionand 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.
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.
has a characteristic half-life.
This amount of time varies from
millionths of a second to billions
of years, depending upon the
The following tables give examples of some well-known radioisotopes, the type of radiation emitted, and their half-lives.
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.
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) --> 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 (>108 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.
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.
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.
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 stablein 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.
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.
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
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.
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.
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.
The geochemistry of uranium.
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.
HLW = high-level waste
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.
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%).
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.
The Russian Ministry of Atomic Energy is the owner of the world’s 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.
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 1970’s 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. Sellafield’s gas discharges of Krypton can be measured in Miami.
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.
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.
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.