Humans have used radioactivity for one hundred years, and through its use, added to the natural inventories. A few human produced or enhanced nuclides are given in Table 13.2.
The basic sources of radioactive wastes in environment are the nuclear fuel cycle, mining activities, medical and laboratory facilities, nuclear weapon testing, building materials and nuclear industry.
Given below are the details of each source and its impact on human health and environment:
Source # 1. Nuclear Fuel Cycle:
Uranium, as it is mined from the earth’s crust, is not directly useable for power generation. Much processing must be carried out to concentrate the fissile isotope U-235 before uranium can be used efficiently to generate electricity.
More so than other energy resources such as coal, oil and natural gas, uranium has its own distinctive and very complicated fuel cycle. This is called the ‘Nuclear Fuel Cycle’—activities carried out to produce energy from nuclear fuel as shown in Fig. 13.2. There are several steps in the nuclear fuel cycle—mining and milling, conversion, enrichment, and fuel fabrication. These steps are known as the ‘front end’ of the cycle.
Once uranium becomes ‘spent fuel’ (after being used to produce electricity), the ‘back end’ of the cycle follows. This may include temporary storage, reprocessing, recycling, and waste disposal. This is where radioactive wastes are a major issue.
Radioactive wastes occur at all stages of the nuclear fuel cycle, the process of producing electricity from nuclear materials.
(i) Residual Materials form the ‘Front End’ of the Fuel Cycle:
The annual fuel requirement for a 1000 MWe light water reactor is about 25 tonnes of enriched uranium oxide. This requires the mining and milling of some 50,000 tonnes of ore to provide 200 tonnes of uranium oxide concentrate (U3O8) from the mine.
At uranium mines, dust is controlled to minimise inhalation of radioactive minerals, while radon gas concentrations are kept to a minimum by good ventilation and dispersion in large volumes of air. At the mill, dust is collected and fed back into the process, while radon gas is diluted and dispersed to the atmosphere in large volumes of air.
Residual wastes from the milling operation contain the remaining radioactive materials from the ore, such as radium. These wastes are discharged into tailings dams, designed to retain the remaining solids and prevent any seepage of the liquid. Eventually the tailings may be put back into the mine or they may be covered with rock and clay, then revegetated.
The tailings are around ten times more radioactive than typical granites, such as used on city buildings. If someone were to live continuously on top of the Ranger tailings they would receive about double their normal radiation dose from the actual tailings (i.e., they would triple their received dose). With in situ leach (ISL) mining, dissolved materials other than uranium are simply returned underground from where they came.
Uranium oxide (U3O8) produced from the mining and milling of uranium ore is only mildly radioactive, most of the radioactivity in the original ore remains at the mine site in the tailings.
Turning uranium oxide concentrate into a useable fuel has no effect on levels of radioactivity and does not produce significant waste. First, the uranium oxide is converted into a gas, uranium hexafluoride (UF6), as feedstock for the enrichment process.
Then, during enrichment, every tonne of uranium hexafluoride becomes separated into about 130 kg of enriched UF6 (about 3.5% U-235) and 870 kg of ‘depleted’ UF6 (mostly U-238). The enriched UF6 is finally converted into uranium dioxide (UO2) powder and pressed into fuel pellets, which are encased in zirconium alloy tubes to form fuel rods.
Depleted uranium has few uses, though with a high density (specific gravity of 18.7) it has found uses in the keels of yachts, aircraft control surface counterweights, anti-tank ammunition and radiation shielding. It is also a potential energy source for particular (fast neutron) reactors.
It is when uranium is used in the reactor that significant quantities of highly radioactive wastes are created. More than 99% of the radioactivity produced during the fission reaction is retained in the fuel rods. The balance is within the reactor structure.
About 25 tonnes of spent fuel is taken each year from the core of a 1000 MWe nuclear reactor. The spent fuel can be regarded entirely as waste, or it can be reprocessed. Whichever option is chosen, the spent fuel is first stored for several years under water in large cooling ponds at the reactor site. The concrete ponds and the water in them provide radiation protection, while removing the heat generated during radioactive decay.
Source # 2. Mining Activities:
Minerals and mineral products are the backbone of most industries and some form of mining or quarrying is carried out in virtually every country in the world. Mining has important economic, environmental and labour and social effects in the countries or regions where it is carried out, and beyond. In many developing countries, mining accounts for a significant proportion of GDP and often, for the bulk of foreign exchange earnings and foreign investment.
Mining, processing, and the use of coal, natural gas, phosphate rock, and rare earth deposits result in the concentration and release or disposal of large amounts of low-level radioactive material. Processing rare-earth containing ores produces concentrated waste high enough in radioactivity to be disposed of as low- level radioactive waste.
During mining, the uranium and its decay products buried deep in the earth are brought to the surface, and the rock containing them is crushed into fine sand. After the uranium is chemically removed, the sand is stored in huge reservoirs. The leftover piles of radioactive sand or uranium tailings contain over a dozen radioactive materials, which are all extremely harmful to all life forms on earth. Uranium mining is known to be hazardous.
Apart from the usual risks of mining, uranium miners worldwide have experienced a much higher incidence of lung cancer and other lung diseases. There are several studies indicating an increased incidence of skin cancer, stomach cancer and kidney disease among uranium miners. Uranium is the heaviest metal that occurs in nature. It is an unstable material that gradually breaks apart or ‘decays’ at the atomic level. Any such material is said to be radioactive.
As uranium slowly decays, it gives off invisible bursts of penetrating energy called ‘atomic radiation’. It also produces more than a dozen other radioactive substances as by-products. These unstable by-products, having little or no commercial value, are known as ‘uranium decay products’. They are discarded as waste when uranium is mined. One of them is a toxic radioactive gas called radon. The others are radioactive solids.
i. Uranium and its decay products buried deep in the earth are brought to surface.
ii. Radon gas produced in the mine causes lung cancer.
iii. Leftover piles of materials or ‘uranium tailings’ contain over a dozen radioactive materials.
There is no perfect storage of these radioactive materials to prevent them from finding their way into the soil, water, plants, animals, fish and humans.
Uranium tailings constantly produce large amounts of radon gas through the decay of radium in the tailings. This gas can travel thousands of kilometres with a light breeze in just a few days. As it travels, it continually deposits solid radon progeny on the ground, water and vegetation below. Radon also dissolves readily in water and can be transported by ground-water into wells and streams. Radioactive radon gas decays, producing seven radioactive decay products called ‘radon progeny’. These solid radioactive materials attach themselves to tiny dust particles and droplets of water vapour floating in the air.
Source # 3. Medical and Laboratory Facilities:
Radioisotopes are used extensively in medical facilities, biomedical research laboratories, and to a lesser extent in other types of laboratories (Table 13.3). Clinical use of radioisotopes is expanding rapidly in such areas as cancer treatment and diagnostic testing. The lack of waste management plans at many of these facilities results in frequent misclassification of materials as radioactive. Relatively large doses of isotopes, frequently powerful gamma emitters with short half-lives, are used in clinical procedures. Medical generators include hospitals and clinics, research facilities, and private medical offices.
More than 120 million medical procedures using radioactive materials are conducted annually in the United States. Large doses of radiation directed specifically at a tumour are used in radiation therapy to kill cancerous cells, and thereby often save lives (usually in conjunction with chemotherapy or surgery). Much larger doses are used to kill harmful bacteria in food, and to sterilise bandages and other medical equipment. Radiation has become a valuable tool in our modern world.
Source # 4. Nuclear Weapons Testing:
The term “nuclear testing” encompasses all experiments in which special nuclear material (or a stimulant) is placed in contact with high explosives, which are then detonated, or with a propellant, which is ignited. Tritium (3H) and several isotopes of iodine, cesium and strontium are found in the environment largely because of nuclear testing. The use of nuclear devices in weapons is the primary cause of radioactive fallout, although the nuclear accident at Chernobyl and various volcanic eruptions have also contributed. Nuclear test explosion is an experiment involving the detonation of a nuclear weapon.
Motivations for testing generally are broken into the categories:
i. ‘Weapons related’ (verifying that a weapon works, or examining exactly how it works)
ii. ‘Weapons effects’ (how weapons behave under various conditions, and how structures behave when subjected to weapons).
Often, though, testing has also been a demonstration of the possessing nation’s military and scientific strength.
Nuclear weapons tests are generally classified-as being either ‘atmospheric’ (in or above the atmosphere), ‘underground,’ or ‘underwater.’ Of these, underground testing contained in deep shafts poses the least health risk in terms of fallout. Atmospheric testing which comes in contact with the ground or other materials poses the highest risk. Nuclear weapons have been tested by dropping them from planes (an ‘airdrop’), from the tops of towers, hoisted from balloons, on barges at sea, attached to the bottom of ships, and even shot into outer space by rockets. A radioactive fireball tops the smoke column from a nuclear weapon test.
The energy released from a nuclear weapon comes in four primary categories:
i. Blast—40-60% of total energy.
ii. Thermal radiation—30-50% of total energy.
iii. Ionizing radiation—5% of total energy.
iv. Residual radiation (fallout)—5-10% of total energy.
The amount of energy released in each form depends on the design of the weapon, and the environment in which it is detonated. The residual radiation of fallout is a delayed release of energy, while the other three forms of energy release occur immediately.
The damage from each of the three initial forms of energy release differs with the size of the weapon. Thermal radiation drops off the slowest with distance, so the larger the weapon the more significant the impact of this effect. Ionizing radiation is strongly absorbed by air, so it is only dangerous by itself for smaller weapons. Blast damage falls off more quickly than thermal radiation but more slowly than ionizing radiation.
The energy released by a nuclear weapon is generally measured by the explosive power of an equivalent amount of trinitrotoluene, known as the weapon’s yield. The yield of nuclear weapons may be rated as equivalent to several kilotons or megatons of TNT. The first fission weapons has yields measurable in the tens of kilotons, while the largest practical hydrogen bombs have yields around 20 megatons.
Source # 5. Building Materials:
Building materials contribute a considerable amount of in-door radioactivity. All building materials that originate from minerals always contain a certain amount of radionuclides. These are mainly potassium, uranium, thorium and the radionuclides that are created as their radioactive decay chains. Of these, the most significant is radium (Ra-226). Building materials, derived from soil and rock, also have substantial amount of radium and act as the source of indoor radon.
The World Health Organization (WHO) as well as the US Environmental Protection Agency (EPA) have classified radon as a known human carcinogen. The Ra-226 presence in building materials causes exposure to persons living in dwellings—either by inhalation of radon daughters that decay from radium and release from the building material to indoor air, or by hard gamma radiation that releases from the building material as a consequence of the radioactive decay of the natural radionuclides present.
The soil and bedrock beneath houses are the main sources of indoor radon. In larger structures, the building materials may contribute a greater share to the indoor radon concentration, but the absolute contribution is usually small. However, certain materials have been found to constitute unusually large sources of radon, and in such cases the building materials may be the source of unacceptably high indoor radon concentrations. Other building materials of natural origin containing enhanced levels of 226Ra include granites, some clay bricks and tuff.
Brick tends to have slightly higher 226Ra concentrations but significantly lower radon emanation rates than concrete. Red bricks are thought to have higher uranium content, probably because of fixation by iron, than lighter coloured or silica bricks. Because they are more commonly used on the exterior of buildings, their contribution in indoor radon would be even lower. Clay bricks contain typically 1.4 ppm of radium whereas granite bricks have an elevated concentration of 2.4 ppm.
Gypsum has been evaluated as a radon source and radon emanation rates have been found to be very low except where phosphate by-products have been added to make phosphosgypsum. In the phosphate milling process, the waster products are enriched in 226Ra.
In recent years, wastes from different industries have been used in the building industry. Fly ash from coal-fired power plants is used as an additive in cement and by-product gypsum from the phosphate industry is utilized in plasterboards and in concrete. These wastes contain a higher than average concentration of 226Ra, and in some cases, such building materials may be the significant sources of indoor radon. Fly ash has been found to have enhanced levels of radium, upto several hundreds Bq kg–1, but only a few per cent can be added to concrete. Also, comparative studies showed that the radon emanation from concrete mixed with fly ash was lower than that for the same concrete without fly ash. The concentration of uranium in fly ash varies between 172 Bq kg–1 to 1599 Bg kg–1 in India.
Hazardous waste like phosphogypsum (PG) is a byproduct of the fertilizer industry, PG is used liberally by the construction industry. In India, PG is used to make construction material such as gypsum plaster, gypsum ceiling tiles, boards, panels and marbles/blocks. Phosphate rock is mined extensively around the world, mainly for use in fertilizers. Phosphogypsum contains radium-226, which decays to radon gas.
According to a report by the United Nations Scientific Commission for Emissions due to Atomic Radiation, the radium concentration is as high as 46 picocurie per gramme (pCi g–1) in the phosphate rock from Morocco, while that of India has only 4 pCi g–1 (a picocurie is one-trillionth of a curie, a unit for measuring radioactivity). Exposure to it can increase the risk of developing cancer. Apart from radioactivity, there are trace metals such as arsenic, lead, cadmium, fluoride, zinc, antimony and copper that the EPA believes may pose environmental and human health hazards.
Granite and marble, like other naturally occurring substances as air, water, minerals etc., contain tiny but measurable quantities of radioactive elements in different proportions. Granites from India and Egypt have very low emanation rates, and are, by far, the best in the world in this regard. Uranium, thorium and their daughter products like radium, polonium, radon, thoron, are generally present.
The amount of radioactivity emitted by the rocks varies from place to place. It is estimated that on an average, in each kilogram of granite, about 60 to 70 atoms of uranium and about 10 atoms of thorium and their associated elements emit radiation every second.
Further, close to 1200 atoms of potassium give out radiation each second. Radiation emission due to uranium and potassium in marble (which is derived from limestone) is about one tenth of what is found in granite.
But, the radiation emission due to thorium in marble, is almost the same as in granite. The radioactivity levels in granite and marble are too small to be of any concern. All construction materials, in general, give off a small amount of radon gas, which is radioactive.
The gas can accumulate in poorly ventilated buildings, usually found in countries with very cold climate. Inhaling the accumulated radon could give rise to a small degree of radiation exposure. Therefore, several western countries have prescribed the acceptable limits for radon emanation from the building materials. A few of common building materials and their estimated levels of uranium, thorium and potassium are given in Table 13.4.
Due to relatively long half-life and lack of chemical activity, radioactive decay of 226Ra, the gaseous daughter 226Rn is formed in the building materials. Radon being a noble gas, diffuse from the room surface material and from the subsoil below the building into the room air where it and its daughters are available for inhalation by the room occupants. 222Rn diffuses from the place of its origin into air filled pores of building materials and diffuses into the room air (Table 13.5).
The rate of exhalation into room air from surrounding walls can be influenced by the finish of the wall surface, plastic paints, thick washable wall paper or metal paneling which reduce the amount of 222Rn entering the room atmosphere. The radon exhalation will also vary with the atmospheric pressure. It is reported that a sudden drop in the atmospheric pressure in an unventilated room will cause an increase in the radon exhalation due to sucking effect of the falling pressure.
Stranden et al., (1979) found that a daily drop in atmospheric pressure of 1 mm of Hg will result in an increase of the radon exhalation of about 5-7%. However, the amount of passive exchange between air masses indoor and outdoor will determine to what extent this increased exhalation will result in an increased content of 222Rn. If there is no exchange with air masses of lower radioactivity, indoor radioactivity will decrease only due to radioactive decay and, for the daughter products, due to the attachment to interior surface such as walls and furniture (Table 13.6).
The rate of exhalation from the soil under the building is important due to the vertical concentration gradient of the radioactivity within the building. The 222Rn concentration in cellars 7or rooms on the lower basement is usually higher than in rooms on top floors.
Radon concentration of the indoor air is influenced by several factors, which can be divided into two groups:
i. Locally dependent factors, i.e., building materials, location of the building, locations of rooms inside the building, and
ii. Time dependent factors i.e., exhalation from the surrounding wall and from the soil underneath the building, ventilation condition, radon content in the outdoor air and meteorological parameter (atmospheric pressure, humidity, wind and temperature).
Source # 6. Nuclear Industry:
It is apparent that the nuclear industry is creating a major challenge to the environment. Thermal wastes from the current generation of large, nuclear power generating stations are immense in quantity and can cause drastic changes in the bodies of water involved.
If the past performance of the nuclear industry is taken as representative of what to expect, there will be vast quantities of radioactive wastes discharged both at nuclear power plant (reactor) sites and at fuel-fabricating and fuel reprocessing plants. Even without the catastrophic releases of radioactive wastes, which would result from a major accident, the introduction of these radioactive wastes in normal operation poses a major threat not only to the current generation but also to the future of the human race.
Perhaps the genetic effects which would result from the wastes of the nuclear industry can be tolerated and, perhaps, they cannot. It is a dangerous game to play, trading potential death and mutation for minor immediate economic gain. There is no reason why the radioactive wastes cannot be contained, but as usual it costs more to contain wastes than to dump them into the nearest waters or into the atmosphere.