EVALUATION OF THE ENVIRONMENTAL RADIOACTIVITY AROUND DANGOTE CEMENT EXCAVATION SITE

CHAPTER TWO

2.0 LITERATURE REVIEW

The late 1800s and early 1900s were a period of intense research in to the new ‘nuclear’ ream of physics. In 1896 Henri Beiquerel found that a sample of uranium he was doing experiment with had a special property. After he was done with a series of experiments using the uranium, he put it in to a drawer with a photographic plate.

A photographic plate is a piece of glass covered in chemicals. It was used as the ‘film’ in old style cameral. Becquerel was surprised to find out later that the uranium had caused the plate to be fogged up, as if it had been exposed to light.

He correctly assumed that the uranium was emitting radiation similar to visible light. He was even able to show that a magnetic field seemed to change the direction that this invisible radiation traced. Shortly after this, Marine and Pierre Curie isolated two other radioactive elements, polonium and radium.

No matter what physical or chemical stresses they placed on these elements, they continued to emit radiation just like the uranium that Becquerel had used. Since nothing they did could stop the radioactivity, they believed that the radioactivity’s most come from deep within the atom, in what we could today call the nucleus. We now know that radioactivity actually results from the decay (disintegration) of an unstable nucleus.

This process fundamentally changes the nucleus of the element itself. The radiation that measure is evidence of events happening inside the nucleus. In many cases this will actually result in the element changing to a different element, a process called transmutation.

The reason these decays happen is because they result in more stable nuclei. Ernest Rutherford and others started studying the radiation that was emitted by these elements. He found three distinct form of radiation, originally divided up based on their ability to pass through certain materials and their defection in magnetic field.

Alpha (ά): could barely pass through a single a single sheet of paper defection as a positive particle in a magnetic field.

Beta (ß): can pass through about 3mm of aluminium, deflection as a negative particle in a magnetic field.

Gama (γ): can pass through several centimetres of LEAD! Not deflection in a magnetic field. (Henri Becquerel et al., 1800s).

2.1 Types of Radiation

The most common types of radiation include:

1. An alpha particle (ά)
2. Beta particle (ß)
3. Gamma rays (γ
4. X – rays

2.2.1 An Alpha particle

Certain radionuclide’s of high atomic mass number (U238, Ra226, Pu23q) decay by this emission of alpha particle, these particles are highly bound unit of two neutron and two proton each of helium nucleus (4tte nucleus) and have a positive change. Emission of an alpha particle from the nucleus result in a decrease of two unit of atomic number and four (4) mass number, they are emitted with discrete energies characteristics of the particle transformation from which they originated. All alpha particles from a particular radionuclide’s transformation will be having identical energies characteristics of alpha radiation are: Alpha radiation is a heavy, very short range particle and is actually an ejector of helium nucleus; most alpha radiation is not able to penetrate human skin.

Figure 1: Alpha decay (radioactive decay)

Alpha decay (radioactive decay): Occurs when the nucleus has too many protons in it and it wants to rid of them through excessive repulsion. This causes an alpha or helium particle to be released in order to stop the repulsion. With the particle leaving the nucleus it causes the element to decay or change. It causes the mass number to be four less and the atomic number two less than the element before, this causes a transmutation. α, is the symbol for alpha. The equation using an example of uranium is represented by:

………………………..2.1

2.2.2 A Beta Particles

A nucleus with an unstable ratio of neutron to protons may decay through the emission of a high speed electron called a beta particle this result as a change of one of the unit atomic number beta particles have a negative and the beta particles emitted by specific radionuclides will range from near zero up to a maximum value which is characteristics of beta radiation may travel several feet in air and moderately penetrating beta particle can penetrate human skin to the germinal layer where new skin cells are produced.

Figure 2: Beta decay (radioactive decay)

Beta decay (radioactive decay): occurs when a beta particle is emitted out of an atom. A beta particle is the same as an electron or a positron (positive electron). There are two types of beta decay, positive and negative. When a beta decay is positive (β+), it is emitting a positive electron, also known as a positron, when this is occurring it is accompanied by a neutrino. When a beta decay is negative (β−), it is emitting a normal electron, when this is occurring it is accompanied by an anti-neutrino. The neutrinos and antineutrinos do not have any neutrons or protons; they are simply there to make up for the energy “lost” through decay. When the decay occurs, the mass number and stays the same and the atomic number increases or decreases by one, this causes a transmutation.

……………………………….2.2

2.2.3 Gamma Rays

Gamma rays have the smallest wave length and the most energy of any other wave in the electromagnetic spectrum. These waves are generated by radioactive atoms and nuclear explosion.

Gamma rays can kill living cells, a fact which medicine uses to its advantage, using gamma rays to kill cancerous cell. Gamma rays travel to US across vast distance of the universe, only to be absorbed by the earth atmosphere (Angland, 2004).

Gamma rays are the most energetic form of light and are produced by the hottest region of the universe. They are also produced by such violent event as supernova explosion or the destruction of atom, and by less dramatic events, such as the decays of radioactive materials in space. Things like supernova explosion, neutron stars, pulsars and black are source of celestial gamma rays (Aharonium et al., 1997).

Figure 3: Gamma Decay (radioactive decay)

Gamma Decay (radioactive decay): Occurs when a gamma ray is emitted, a high energy photon is emitted by the nucleus, this does not result in transmutation. Here is an example of an equation:

…………………………………2.3

2.2.4 X – Ray

These are part of electromagnetic spectrum and are distinguished from gamma rays only by their source (orbit electron rather than the nucleus). X –Ray is emitted with discrete energies by electron as they shift orbits following certain types of nuclear decay purpose internal conversion in an isotope when the energy is transferred to an atomic origin electron that is then ejected with

Kinetic energy equal to the expected energy the atomic structure is filled by an external electronic electrons, resulting in the production of the X – rayshoruim – 178 is a good example of this type of disintegration characteristics of X – ray and gamma are electromagnetic spectrum differ only in their sources (Ernest Ratherford et al., 1900s).

2.3.1 Medical Application

Doctors and dentists use a variety of nuclear material and procedure to diagnose, monitor, and treat a wide assortment of metabolic processes and medical condition in humans. The most common of those medical procedures involves the use of X – rays, a typed of radiation that can pass through our skin when X – rayed, our bones and other structure cast shallows can be detected on pencil behind a piece of paper and holding the pencil and paper in front light. The shallow of the pencil is revealed because most light has enough to pass through the paper is revealed because most light has enough top pass through the paper, but the denser pencil stops all the light.

The different is the X – rays are invisible, so we need photographic film to ‘see’ them for us. This allows doctors and dentists to spot broken bones and dental problems. X – Rays and other forms of radiation also have a variety of therapeutic uses. When used in this way, they are most after intend to kill cancerous tissue, reduce the size of a tumour or reduce pain for example, radioactive iodine (specifically iodine – 131) is frequently used to treat thyroid cancer, a disease that strikes about 11,000 Americans every year (ICRP,1996). X – Ray machine called computed axial tomography (CAT) or computed tomography (CT) scanners. These instrument provide doctors with colour images that is how the shape and details of internal organs. This helps physicians locate and identify tumours, size anomalies, or other physiological or function organ problems.

2.3.2 Application in Agriculture

Radioisotopes can be used to help understand chemical and biological process in plant. This is true for two reasons;

1. Radioisotopes are chemically identical with other isotopes of the some element and be substituted in chemical reaction.
2. Radioactive forms of the element can be easily detected with Geiger counter or other such devices. Example;

A solution of phosphate, containing radioactive phosphorus – 32 is injected in to the root system of a plant. Since phosphorus – 32 behave identically to that phosphorus – 31, the more common and non – radioactive form of the element it is used by the plant in the same way. A Geiger counter is then used to detect the movement of the radioactive phosphorus – 32 through the plant. This information helps scientist understand the detailed mechanism of how plants utilized phosphorus to grow and reproduce. Phosphorus – 32 is injected in to the root of the plant and Geiger counter is used to determine the radioactivity in the plant.

2.3.3 Industrial Application

Radioactive elements are used in industry to study the defect in metals and welded pointed and to trace underground pipe leakage. The reading on the detector increases when a leak is detected as the radiation can escape through the hole in the pipe.

2.4 Sources of Radioactive Radiation

There are two types of radiation sources;

1. Natural sources
2. Artificial sources

2.4.1 Natural Sources

This is the spontaneous disintegration of the nucleus of an atom during which alpha particle, or gamma rays or a combination of any or all the three and heat (or energy) are released. The main source of natural occurring radiation of terrestrial origin and consist principally of accelerated charged particles from galactic and possibly extra galactic sources, and also from the sun (UNSCEAR, 2000).

2.4.2 Artificial Sources

In artificial sources, an ordinary material, not normally radioactive is made radioactive by bombarding it with radioactive particle. Examples are radiation from medical source, consumer product such as TVs.

2.5 Interaction of Radiation with Matter

Interaction between penetrating ionizing radiation and matter is not a simple process in which the primary photon changes to some other form of energy and effectively disappears. When X – ray and gamma rays are incident on matter, they produce secondary electrons interacts and produce ionization and excitations. Charged ionizing particle (particle and particles) have discrete tracks and they travel fairly well at defined distance before losing their kinetic energy that is experienced in a medium per unit length through which it interact is known as linear energy transfer (LET).

The mechanism by which charged particles losses its energy or is deflected from its original path include;

1. Elastic collision with electrons.
2. Elastic collision with nucleus
3. Inelastic collision with nucleus

Of all these mechanism, the predominant one is the inelastic collision with electron and nucleus. Between the energy range of 0.01 – 10 Mev, the predominant mechanism of interaction of the radiation of matter includes; photoelectric and caption effect (Storm et al., 1964).

2.5.1 Photoelectric Effect

Photoelectric effect is the process where electrons are ejected from a surface by the action of light (electromagnetic radiation). The process was discovered by Heinrich Hertz in 1887 in an attempt to explain the effect by classical electromagnetic field.

In 1905, Albert Einstein presented an explanation base on quantum concept of process for X – ray absorption up to energies of also dominant for atoms on high atomic numbers.

Equations

Einstein's formula relates the maximum kinetic energy (Kmax) of the photoelectrons to the frequency of the absorbed photons (ƒ) and the threshold frequency (ƒ0) of the photoemissive surface. (This formula is only approximate, however, and a minor correction is needed for the Compton Effect.)

Kmax = h(ƒ − ƒ0)………………………………………………………….2.4

This can also be written in another form, to the energy of the absorbed photons (E) and the work function (ϕ) of the surface

Kmax = E – ϕ……………………………………………………………2.5

Where the first term is the energy of the absorbed photons (E) with frequency (ƒ) or wavelength (λ)

And the second term is the work function (ϕ) of the surface with threshold frequency (ƒ0) or threshold wavelength (λ0)

The maximum kinetic energy (Kmax) of the photoelectrons (with charge e) can be determined from the stopping potential (V0).

When charge (e) is given in coulombs, the energy will be calculated in joules. When charge is given in elementary charges, the energy will be calculated in electron volts.

The rate (n/t) at which photoelectrons (with charge e) are emitted from a photoemissive surface can be determined from the photoelectric current (I).

Planck's constant

• h = 6.63 × 10−34 J s = 4.14 × 10−15 eV s .................................2.10
• hc = 1.99 × 10−25 J m = 1240 eV nm .......................................2.11

2.5.2 Compton Effect

Compton Effect (Compton scattering) is also known as incoherent scattering. It occurs when the incident ray photon eject from the atom. Relative energy and momentum are considered in this and therefore greater wavelength than the incident photon. Compton scattering is important for low atomic number specimens, at energies of 100kev – 10Mev. The absorption of radiation is mainly due to the Compton Effect.

Figure 4: Compton Scattering Equation

In his explanation of the Compton scattering experiment, Arthur Compton treated the x-ray photons as particles and applied conservation of energy and conservation of momentum to the collision of a photon with a stationary electron. Using the Planck relationship and the relativistic energy expression, conservation of energy takes the form.

Conservation of momentum requires

..........................................................................2.12

Where p=E/c is used for the photon momentum. Squaring this equation using the scalar product gives

........................................2.13

Again using the Planck relationship and the relativistic energy expression:

..........................2.14

The energy conservation expression above can be squared to give

................................2.15

These two forms can be equated to give

..........................................2.16

which can be rearranged to

....................................................................2.17

and finally to the standard Compton formula:

....................................................................2.18

2.5.3 Pair production

Pair production is the creation of an elementary particle and its antiparticle, for example an electron and its antiparticle, the positron, a muon and antimuon, or a tau and antitau. Usually it occurs when a photon interacts with a nucleus, but it can be any other neutralboson, interacting with a nucleus, another boson, or itself. This is allowed, provided there is enough energy available to create the pair – at least the total rest mass energy of the two particles – and that the situation allows both energy and momentum to be conserved. However, all other conserved quantum numbers (angular momentum, electric charge, lepton number) of the produced particles must sum to zero – thus the created particles shall have opposite values of each other. For instance, if one particle has electric charge of +1 the other must have electric charge of −1, or if one particle has strangeness of +1 then another one must have strangeness of −1. The probability of pair production in photon-matter interactions increases with photon energy and also increases approximately as the square of atomic number.

Examples

γ + γ → e− + e+................................................................................2.19

In nuclear physics, this occurs when a high-energy photon interacts with a nucleus. The energy of this photon can be converted into mass through Einstein’s equation, E=mc2; where E is energy, m is mass and c is the speed of light. The photon must have enough energy to create the mass of an electron plus a positron. The rest mass of an electron is 9.11 × 10−31 kg (0.511 MeV), the same as a positron. Without a nucleus to absorb momentum, a photon decaying into electron-positron pair (or other pairs for that matter) can never conserve energy and momentum simultaneously.

2.6 SOIL RADIOACTIVE

When talking about soil radioactivity there are several situations to consider first, one can study the naturally occurring radioactivity materials (NORM) in undisturbed situation these include radionuclides, with long half –life, 232U and 40K as well as radionuclide with short tritium (3H), 14C and 7Be of these radionuclides. The ones that emit gamma ray are often studied to determine the extent the gamma radiation is close to the public in a particular location. High level of Uranium in the soil can indicate an increase risk from higher level of random in homes. Also, raw building materials need to be characterised so that they do not cause radiation problem with subsequent use, for example using monazite sand, which can be rich in thorium in buildings.

Finally, there are several situations where artificially produced radionuclide end up in the soil and researchers conduct research to determine the location and level of the radionuclide.

2.7 EFFECT OF RADIATION ON HUMAN HEALTH AND THE ENVIRONMENT

Radiation occurs when unstable nuclei of atom decay release particles. There are many different types of radiation. When these particles touch various organic materials such as tissues may and probably will be done. Radiation can cause burns, cancer and death. Radiation dosage is measured in rem which is the unit and these unit stands for roentgen equivalent in man. It represents the amount of radiation needed to produce a particular amount of dosage to living tissue. The total dose of rem determines how much a person suffers. At Hiroshima and Nagasaki, people received a dose rem at the instant of the explosion, then more from the surrounding and in limited areas, from fallout. Fallout is composed of radiation particles. Exposure to radiation makes our bodies produce fewer blood clothing agents, called blood platelets, increasing our risk of internal bleeding. Some experts say that approximately 50% of humans exposed to 450 rems will die, and 800 rems will kill virtually any one. Death is inevitable and will occur from between two days to a couple of week (UNSCEAR, 2000). Radiation effects are divided in to somatic effect and genetic effect. The former affect the function of cells and organs whereas the later affect the later generation.

2.7.1 Somatic Effects

Somatic effects are cell damages that pass on to succeeding cell generations. Radiation affects rate of cell division. Hastening and slowing embryonic tissue damage to cell membranes, mitochondria and cell nuclei result in abnormal cell function, (UNSEAR, 1993).

2.7.2 Genetic Effect

These are damages to genes and chromosome which affect future generation. Genetic information for production and function of new organs is continued in the chromosomes of germ cells. Thus health physicists estimate that about fifty severe heredity effects will occur in a group of one million time born children whose parent were both expose to radiation.

2.8 RADIATION PROTECTION WITH EMPHASIS OF THE SAFETY RECOMMENDATION BY ICRP

The relevant national authorities will often play a major role in selecting values for dose constraints and reference levels. Guidance on the selection process is provided in the revised Recommendations. This guidance takes account of numerical recommendations made previously by the Commission.

Exposures arising in such circumstances are referred to by the Commission as potential exposures. Potential exposures are not planned but they can be anticipated. The designer and the user of a source must therefore take actions to reduce the likelihood of a potential exposure happening, such as assessing the probability of an event and introducing engineering safeguards commensurate to this probability. Recommendations for planned exposure situations are substantially unchanged from these provided in Publication 60 and subsequent publications. The dose limits for occupational and public exposures for practices are retained for application to regulated sources in planned exposure situations.

Radiological protection in medicine includes the protection not only of patients but also of individuals exposed to radiation whilst caring for or comforting patients, and volunteers involved in biomedical research. The protection of all of these groups requires special consideration. The Commission’s Recommendations for radiological protection and safety in medicine are given in Publication 73 (ICRP, 1996a) which has been further elaborated in a series of publications. The recommendations, guidance and advice in these publications remain valid and are summarized in the present recommendations and in Publication 105 (ICRP, 2007b) which was drafted by ICRP Committee 3 to support these Recommendations.

The Commission anticipates that although the revised Recommendations do not contain any fundamental changes to the radiological protection policy, these recommendations will help to clarify application of the system of protection in the plethora of exposure situations encountered, thereby further improving the already high standards of protection.

On 21 March 2007, the Main Commission of the International Commission on Radiological Protection (ICRP) approved these revised Recommendations for a System of Radiological Protection which formally replace the previous Recommendations issued in 1991 as Publication 60 (ICRP, 1991b) and update the additional guidance on the control of exposure from radiation sources issued since Publication 60. These revised Recommendations consolidate and develop the previous Recommendations and guidance. The Commission has prepared these Recommendations after two phases of international public consultation, one in 2004 and one in 2006, on draft Recommendations. By following this policy of transparency and involvement of stakeholders, the Commission is anticipating a clearer understanding and wider acceptance of its Recommendations.

The major features of the present Recommendations are:

2.9 RADIOACTIVITY FROM MINING AND PROCESS SITES

2.9.1 Uranium Mines

Most uranium ore is mined in open pit or underground mines. The uranium content of the ore is often between only 0.1% and 0.2%. Therefore, large amounts of ore have to be mined to get at the uranium. In the early years up until the 1960's uranium was predominantly mined in open pit mines from ore deposits located near the surface. Later, mining was continued in underground mines. After the decrease of uranium prices since the 1980's on the world market, underground mines became too expensive for most deposits; therefore, many mines were shut down.

2.9.2 Waste Rock

Waste rock is produced during open pit mining when overburden is removed, and during underground mining when driving tunnels through non-ore zones. Piles of so-called waste rock often contain elevated concentrations of radioisotopes compared to normal rock. Other waste piles consist of ore with too low a grade for processing. The transition between waste rock and ore depends on technical and economic feasibility.

Figure 5: Uranium concentrations in Rock

All these piles threaten people and the environment after shut down of the mine due to their release of radon gas and seepage water containing radioactive and toxic materials.

Waste rock was often processed into gravel or cement and used for road and railroad construction. VEB Hartstein Werke Oelsnitz in Saxony has processed 200,000 tonnes of material per year into gravel containing 50 g/t uranium. Thus, gravel containing elevated levels of radioactivity was dispersed over large areas.

2.9.3 Heap Leaching

In some cases uranium has been removed from low-grade ore by heap leaching. This may be done if the uranium contents are too low for the ore to be economically processed in a uranium mill. The leaching liquid (often sulfuric acid) is introduced on the top of the pile and percolates down until it reaches a liner below the pile, where it is caught and pumped to a processing plant.

During leaching, piles present a hazard because of release of dust, radon gas and leaching liquids. After completion of the leaching process, a longterm problem may result from naturally induced leaching if the ore contains the mineral pyrite (FeS2), as with the uranium deposits in Thuringia, Germany) or Ontario, Canada. Then, access of water and air may cause continuous bacterially induced production of sulfuric acid inside the pile, which results in the leaching of uranium and other contaminants for centuries and possibly permanent contamination of ground water.

2.9.4 In Situ Leaching

With the in situ leaching technology, a leaching liquid (e.g. ammonium-carbonate or sulfuric acid) is pumped through drill- holes into underground uranium deposits, and the uranium bearing liquid is pumped out from below. This technology can only be used for uranium deposits located in an aquifer in permeable rock, confined in non-permeable rock.

Figure 6: In situ leaching

In situ leaching gains importance with a decrease in price of uranium. In the USA, in situ leaching is often used. In 1990, in Texas alone in situ leaching facilities for uranium were operated at 32 sites. In Saxony, Germany, an underground mine converted to an underground in situ leaching mine was operated until end of 1990 at Königstein near Dresden. In the Czech Republic, the in situ leaching technology was used at a large. The advantages of this technology are:

• The reduced risk for the employees from accidents and radiation;
• The lower cost; and
• No need for large tailings piles.

The disadvantages are:

• The risk of leaching liquid excursions beyond the uranium deposit and subsequent contamination of ground water;
• The unpredictable effects of the leaching liquid on the host rock of the deposit;
• the production of some amounts of waste sludge and waste water when recovering the leaching liquid; and
• The impossibility of restoring natural conditions in the leaching zone after finishing the leaching operation.

After finishing the in situ leaching, the waste sludge must be dumped in a final deposit and the ore zone aquifer must be restored to pre-leaching conditions. Ground water restoration is a very protracted and troublesome process, which is not yet completely understood. It is still impossible to establish pre- leach levels for all parameters.

2.9.5 Mining of the Ore

Ore mined in open pit or underground mines is crushed and leached in a uranium mill. A uranium mill is a chemical plant designed to extract uranium from ore. It is usually located near the mines to limit transportation. In the most cases, sulfuric acid is used as the leaching agent, but alkaline leaching is also used. As the leaching agent not only extracts uranium from the ore, but also several other constituents like molybdenum, vanadium, selenium, iron, lead and arsenic, the uranium must be separated out of the leaching solution. The final product produced from the mill, commonly referred to as "yellow cake" (U3O8 with impurities), is packed and shipped in casks.

When closing down a uranium mill, large amounts of radioactively contaminated scrap is produced, which have to be disposed in a safe manner. In the case of Wismut's Crossen uranium mill, to reduce cost some of the scrap is intended to be disposed in the Helmsdorf tailings, but there it can produce gases and thus threaten the safe final disposal of the sludge.

2.9.6 Uranium Mill Tailings Deposits

Uranium mill tailings are normally dumped as a sludge in special ponds or piles, where they are abandoned. The largest such piles in the US and Canada contain up to 30 million tonnes of solid material. In Saxony, Germany the Helmsdorf pile near Zwickau contains 50 million tonnes, and in Thuringia the Culmitzsch pile near Seelingstädt 86 million tonnes of solids.

The amount of sludge produced is nearly the same as that of the ore milled. At a grade of 0.1% uranium, 99.9% of the material is left over. Apart from the portion of the uranium removed, the sludge contains all the constituents of the ore. As long lived decay products such as thorium-230 and radium-226 are not removed, the sludge contains 85% of the initial radioactivity of the ore. Due to technical limitations, all of the uranium present in the ore cannot be extracted. Therefore, the sludge also contains 5% to 10% of the uranium initially present in the ore. In addition, the sludge contains heavy metals and other contaminants such as arsenic, as well as chemical reagents used during the milling process.

Mining and milling removes hazardous constituents in the ore from their relatively safe underground location and converts them to a fine sand, then sludge, whereby the hazardous materials become more susceptible to dispersion in the environment. Moreover, the constituents inside the tailings pile are in a geochemical disequilibrium that results in various reactions causing additional hazards to the environment. For example, in dry areas, salts containing contaminants can migrate to the surface of the pile, where they are subject to erosion. If the ore contains the mineral pyrite (FeS2), then sulfuric acid forms inside the deposit when accessed by precipitation and oxygen. This acid causes a continuous automatic leaching of contaminants.

Radon-222 gas emanates from tailings piles and has a half-life of 3.8 days. This may seem short, but due to the continuous production of radon from the decay of radium-226, which has a half-life of 1600 years, radon presents a long-term hazard. Further, because the parent product of radium-226, thorium-230 (with a half-life of 80,000 years) is also present, there is continuous production of radium-226.

After about 1 million years, the radioactivity of the tailings and thus its radon emanation will have decreased so that it is only limited by the residual uranium contents, which continuously produces new thorium-230.

If, for example, 90% of the uranium contained in an ore with 0.1% grade was extracted during the milling process, the radiation of the tailings stabilizes after 1 million years at a level 33 times that of uncontaminated material. Due to the 4.5 billion year half-life of uranium-238, there is only a minuscule further decrease.

2.9.7 Potential hazards from uranium mill tailings

Radionuclides contained in uranium tailings emit 20 to 100 times as much gamma-radiation as natural background levels on deposit surfaces. Gamma radiation levels decrease rapidly with distance from the pile.

The radium-226 in tailings continuously decays to the radioactive gas radon-222, the decay products of which can cause lung cancer. Some of this radon escapes from the interior of the pile. Radon releases are a major hazard that continues after uranium mines are shut down. The U.S. Environmental Protection Agency (EPA) estimates the lifetime excess lung cancer risk of residents living nearby a bare tailings since radon spreads quickly with the wind, many people receive small additional pile of 80 hectares at two cases per hundred radiation doses. Although the excess risk for the individual is small, it cannot be neglected due to the large number of people concerned. EPA estimates that the uranium tailings deposits existing in the United States in 1983 would cause 500 lung cancer deaths per century, if no countermeasures are taken. Tailings deposits are subject to many kinds of erosion. Due to the long half-lives of the radioactive constituents involved, safety of the deposit has to be guaranteed for very long periods of time.

After rainfall, erosion gullies can form; floods can destroy the whole deposit; plants and burrowing animals can penetrate into the deposit and thus disperse the material, enhance the radon emanation and make the deposit more susceptible to climatic erosion. When the surface of the pile dries out, the fine sands are blown by the wind over adjacent areas. The sky has darkened from storms blowing up radioactive dust over villages located in the immediate vicinity of Wismut's uranium mill tailings piles. Subsequently, elevated levels of radium-226 and arsenic were found in dust samples from these villages.

Seepage from tailings piles is another major hazard. Seepage poses a risk of contamination to ground and surface water. Residents are also threatened by radium-226 and other hazardous substances like arsenic in their drinking water supplies and in fish from the area. The seepage problem is very important with acidic tailings, as the radionuclides involved are more mobile under acidic conditions. In tailings containing pyrite, acidic conditions automatically develop due to the inherent production of sulfuric acid, which increases migration of contaminants to the environment.

Tailings dams are often not of stable construction. In most cases, they were made from sedimentation of the coarse fraction of the tailings sluge. Some, including those of Culmitzsch and Trünzig in Thuringia, were built on geologic faults. Therefore, they are subject to the risk of an earthquake. As the Thuringian tailings deposits are located in the center of an area of earthquake risk in the former GDR, they suffer a risk of dam failure. Moreover, strong rain or snow storms can also cause dam failures. It is of no surprise that again and again dam failures have occurred. Some examples are:

• 1977, Grants, New Mexico, USA: spill of 50,000 tonnes of sludge and several million liters of contaminated water.
• 1979, Church Rock, New Mexico, USA: spill of more than 1000 t of sludge and about 400 million liters of contaminated water.
• 1984, Key Lake, Saskatchewan, Canada: spill of more than 100 million liters of contaminated liquids.

Occasionally, because of their fine sandy texture, dried tailings have been used for construction of homes or for landfills. In homes built on or from such material, high levels of gamma radiation and radon were found. The U.S. Environmental Protection Agency (EPA) estimates the lifetime excess lung cancer risk of residents of such homes at 4 cases per 100.

2.9.8 Concepts for tailings disposal

In most cases, uranium mill tailings are disposed in some form or another, to limit contaminant release into the environment. There are, however, two known exceptions, where the tailings were simply released into the environment without any control:

• At the Bukhovo mill in Bulgaria, the slurries were dumped in a glen from 1947 to 1958; the finer particles flew into a nearby river. During heavy precipitation, the dumped material was spread over a wider area, contaminating an agricultural area of 120 hectares. Gamma dose rates about a hundredfold background were detected on the surface. After 1958, the most severely contaminated areas were fenced. But the fences deteriorated later, and the areas were in part reused for agriculture. Excessive radium concentrations of up to 1077 Bq/kg were found in cereals grown on these areas.
• At the Mounana mill in Gabon, more than 2 million tonnes of uranium mill tailings were released into a creek from 1961 to 1975, resulting in the formation of large deposits on the valley floor (view details).

The obvious idea of bringing the tailings back to where the ore has been taken from, does not in the most cases lead to an acceptable solution for tailings disposal. Although most of the uranium was extracted from the material, it has not become less hazardous, quite to the contrary. Most of the contaminants (85% of the total radioactivity and all the chemical contaminants) are still present, and the material has been brought by mechanical and chemical processes to a condition where the contaminants are much more mobile and thus susceptible to migration into the environment. Therefore, dumping the tailings in an underground mine cannot be afforded in most cases; there, they would be in direct contact with groundwater after halting the pumps.

The situation is similar for deposit of tailings in former open pit mines. Here also, immediate contact to ground water exists, or seepage presents risks of contamination of ground water. Only in the case of the presence of proven impermeable geologic or man-made layers can the contamination risk to ground water be prevented. An advantage of in-pit deposition is relatively good protection from erosion.

In France and Canada, on the other hand, the concept of dumping the tailings in former open pits in groundwater is pursued or proposed at several sites in recent years. In this case, a highly permeable layer is installed around the tailings, to allow free groundwater circulation around the tailings. Since the permeability of the tailings themselves is lower, it is anticipated (by the proponents) that nearly no exchange of contaminants between tailings and groundwater takes place. A similar method is being tested in Canada for the disposal of uranium mill tailings in lakes (called "pervious surround disposal"). Recent proposals even deny the necessity of an artificial permeable layer around the tailings, since the surrounding rock would provide a high enough permeability.

In most cases, tailings have to be dumped on the surface for lack of other options. Here, the protection requirements can more easily be controlled by appropriate methods, but additional measures have to be performed to assure protection from erosion.

2.9.9 Standards for Uranium Mill Tailings Management

In the early years of uranium mining after World War II, the mining companies often left sites without any clean up after the ore deposits were exhausted: often, in the United States, the mining and milling facilities were not even demolished, not to mention reclamation of the wastes produced; in Canada, uranium mill tailings were often simply dumped in one of the numerous lakes.

The untenability of this situation was for the first time recognized by U.S. legislation, which defined legal requirements for the reclamation of uranium mill tailings in 1978 (UMTRCA). On the basis of this law, regulations were promulgated by the Environmental Protection Agency (EPA: 40 CFR 192) and the Nuclear Regulatory Commission (NRC: 10 CFR 40). These regulations not only define maximum contaminant concentrations for soils and admissible contaminant releases (in particular for radon), but also the period of time, in which the reclamation measures taken must be effective: 200 - 1000 years. The reclamation action thus not only has to assure that the standards are met after completion of the reclamation work; but for the first time, a long-term perspective is included in such regulations. A further demand is that the measures taken must assure a safe disposal for the prescribed period of time without active maintenance. If these conditions cannot be met at the present site, the tailings must be relocated to a more suitable place. Considering the actual period of time the hazards from uranium mining and milling wastes persist, these regulations are of course only a compromise, but they are a first step, at least. Regulations for the protection of groundwater were not included in the initial legislation; they were only promulgated in January 1995.

Last but not least, public involvement is given an important role in planning and control of the reclamation action. Based on these regulations, various technologies for the safe and maintenance-free confinement of the contaminants were developed in the United States during subsequent years. The reclamation efforts also include the decontamination of homes in the vicinity built from contaminated material or on contaminated landfills.

2.9.10 Reclamation of Uranium Mill Tailings Deposits

To reclaim a uranium mill tailings pile according to principles of a safe long-term isolation, detailed investigations have to be performed in advance to assess the site. If the tailings pile presents an immediate hazard, then intermediate protective measures can be taken in parallel, such as installation of a cover against windblown dust, or collection of seepage waters. These measures, however, should not conflict with long-term measures to be taken later.