Radioactive Elements
Exposure to ionizing radiation in high doses, as for example the radiation to which human beings were exposed in the atomic explosions in the cities of Hiroshima and Nagasaki in Japan, or the nuclear disaster in Chernobyl in the Ukraine, involves proven health risks. It is accepted that these risks exist, although at a low probability, even when a person is exposed to low doses of radiation.It is customary to classify the natural radiation to which people are exposed into three categories:
- Cosmic Radiation - which emanates from the outer universe and whose strength varies from place to place on the earth's surface, depending on altitude, the magnetic field and other factors. The average effective radiation dose that people absorb from cosmic radiation is estimated at about 0.3 - 0.4 millisievert per year at sea level. At an altitude of 10 -12 km., (the altitude at which most passenger airplanes fly), the rate of cosmic radiation dosage is higher than these values by a factor of 15 - 20.
- Ground Radiation - its source is natural radioactive isotopes that are found in the earth's crust, such as potassium 40 (40K), radium 226 (226 Ra), and Thorium 232 (232Th), which are conveyed in the air by means of dust or radon gas, penetrate the food chain and impact people also by means of construction materials whose source is the ground. The average external radiation dose to people from these sources reaches about 0.5 millisievert per year, with values ranging between 0.3 to 1.5 millisievert per year, depending on location and type of structure.
- Internal Radiation - its source is radioactive substances that penetrate the human body by way of the breathing passages or the digestive system. The effective internal radiation dose that is obtained from potassium 40 and the other radioisotopes in our bodies is about 0.2 millisievert per year. In addition, we are exposed to an internal effective radiation dose of about 0.8 - 1.2 millisievert per year, as a result of breathing radon gas and its daughters.
In Summary - The overall natural radiation to which the population of the Earth is exposed is on an order of magnitude of 1.5 - 3 millisievert per year on average, depending on geographic latitude, altitude above sea level, presence of uranium and thorium deposits in the ground, and the construction materials in the nearby environment. In certain areas on the Earth's surface, the cumulative effective radiation dose from natural background radiation can also reach 5 millisievert per year and even 8 millisievert per year.
According to current estimates (detailed surveys have not yet been performed), the population in Israel is exposed to natural radiation that causes an average effective dose of 1.8 to 2 millisievert per year.
Like all rocks, coal also contains low concentrations of natural radioactive materials in general and specifically radioisotopes potassium 40 (40K) , radium 226 (226Ra), and thorium 232 (232Th). As a result of the combustion process, the concentrations of these radioisotopes increase significantly in the ash (7 to 12 times). The concentrations of the activities of these isotopes in the coal vary depending on the source of the coal excavation, but in general they are within the range of tens to hundreds of Becquerel per kilogram.(Bq/kg).
The concentrations of radionuclides in coal ash in Israel from various sources are as follows:|
Source
|
K40
|
Ra226
|
Th232
|
|
South Africa
|
154 - 181
|
179 - 230
|
176 – 246
|
|
Colombia
|
380 - 564
|
85 - 103
|
63 - 65
|
|
Australia
|
195 - 209
|
106 - 121
|
83 - 115
|
|
Indonesia
|
400 - 436
|
109 - 128
|
98 - 124
|
The concentrations in ash depend, among other things, also on the ash particle size and the efficiency of the combustion process. The concentrations are in general higher in fly ash than in bottom ash that are produced from the same coal and in the same combustion process.
In contrast to the environmental and economic advantages inherent in the utilization of coal ash in the construction industry, in agriculture and in infrastructures, these applications also involve certain difficulties. The application of coal ash in the construction industry is liable to cause a potential increase, although small, of exposure of the population to ionizing radiation whose source is in the natural radioactive materials that are present in the coal ash in concentrations that are relatively higher than those in regular soil.
In this context it should be noted that the emanation (the relative portion of radon that is created in the construction material and that diffuses away from the product into the room space) from construction materials in Israel that contain coal ash is generally smaller by 5 to 10 times the emanation from similar materials, having similar density, structure and geometric dimensions, that do not contain coal ash. The source of the difference is in the crystal structure of the product. Whereas in regular construction materials the radon is free to be emitted from the aggregate and to migrate in the spaces in the solid material matrix in the construction product, ash is made of glassy material, the product of combustion at a temperature that is above 1,500 ºC that prevails in the power plant furnace. Thus, the radon that is created in the material is trapped, and the compaction of the material from which it is made, being a fine powder having a particle diameter of tens of microns and less, decreases the size of the spaces that enable the movement of gases within the particle and away from it.
The authorized governmental agencies in Israel (Ministry of National Infrastructures, the planning and construction authorities, and the Ministry of the Environment) condition the permits for various coal ash applications on compliance with the requirements of the International Standard for Protection from Radiation (IAEA 1996), that is, compliance with the justification and optimization requirements, as established by the International Commission for Radiological Protection (ICRP 1991), and receipt of data regarding the risks from radiation that are involved in these applications (hereinafter in a separate context).
Construction Applications of Coal Ash
The use of coal ash in the construction industry (production of blocks and concrete) is dependent on compliance of these construction products with the requirements of Israel Standard 5098, which deals with the limitation of the composition of natural radioactive materials in construction products. The maximum radiation dosage supplement whose source is coal ash that is permitted for a member of the public according to the standard, compared to the normative reference source, is 0.3 mSv/y. This value is part of the maximum effective dosage supplement for a person of 1 mSv/y from all sources that can be controlled (including radiation in medical diagnostic and treatment procedures).
The concentrations of typical radionuclides in the raw materials that are used in construction in Israel are as follows:
|
Rock
|
K40
|
Ra226
|
Th232
|
|
Limestone
|
4 - 9
|
2 - 17
|
1
|
|
Dolomite
|
5 - 263
|
7 - 53
|
1 - 11
|
|
Basalt
|
231 - 419
|
10 - 22
|
10 - 21
|
|
Pebbles
|
9 - 425
|
10 - 22
|
1 - 14
|
|
Gypsum
|
30
|
13
|
1
|
|
Dune Sand
|
50
|
5
|
3
|
|
Fossil Sand
|
148
|
21
|
13
|
|
Cement 42.5
|
105 - 212
|
29 - 69
|
7 - 41
|
The results of radionuclides concentration tests that were conducted on concrete samples that were produced from selected dolomite and fossil sand quarries having no coal ash, with the addition of 150 kg of South African coal ash (relatively rich in radionuclides) indicate that the addition of coal ash to concrete complies with the limitation according to which the standard was established, compared to concrete as a reference source:
|
Construction Product
|
K40
|
Ra226
|
Th232
|
mSv/y
|
|
Concrete without Ash
|
46 - 57
|
33 - 34
|
8 - 10
|
0.45 – 0.47
|
|
Concrete with Ash
|
52 - 68
|
33 - 44
|
11 - 16
|
0.47 – 0.62
|
Agricultural Applications of Coal Ash
Coal bottom ash has structural and textural properties that qualify it to be used as a component for growth beds for plants in agriculture and gardening. The possible uses are: growth beds in containers and buckets in greenhouses and planting containers in nurseries, as a substitute for Tuff. The ash can also be used as a fill material for lawn infrastructures at sports fields and public parks and as a floor covering material for animals: cowsheds, stables and sheep and cattle pens and chicken coops.Estimates were performed at the Nuclear Research Center - Soreq that indicated that the radiation dosage supplement for workers and the public that is associated with these applications will be smaller than 40 microsievert per year, which is a supplement of about 2% above the natural background radiation. This supplement is on an order of magnitude that was called a trivial dose by the International Commission for Radiological Protection, and it is exempt from regulation and control.
Tests of edible plants that were grown on a bed that contains coal ash indicated that no significant supplement was obtained to the concentration of natural radionuclides in the agricultural product, compared with a similar product grown on a bed of Tuff. The equivalent dosage supplement to a population that is nourished only from such crops, based on extreme assumptions, is on the order of magnitude of the trivial dosage according to international guidelines.
Coal Ash Applications in Infrastructures
Coal ash is used as a structural fill material in roads and various infrastructures, including infrastructures for residential buildings. A survey of international professional literature concerning the environmental conditions and standards for using coal ash in infrastructures that was conducted by the Nuclear Research Center indicated that there is no significant radiological problem as a result of using coal ash in infrastructures. Further, no significant environmental problem was found due to the penetration of heavy metals and radioactive materials into the groundwater or into the food chain as a result of the use of coal ash in infrastructures. Tests that were conducted by the Radiation Branch of the Ministry of the Environment indicated that the coverage of coal ash embankments, which are an infrastructure for roads and fill materials, using a layer of earth or asphalt/concrete, lowered the exposure of the population to natural background levels.The use of coal ash in the preparation of infrastructures for construction of residential buildings was performed in Israel according to the following details: filling of a depression in the ground having an area of about 154 dunams with a layer of coal bottom ash having a thickness of about 4.5 meters, and addition of a layer of regular soil having a thickness of about 1.5 meters above the layer of coal ash (the purpose of the soil layer was to reduce the majority of the supplemental radiation that was caused by the coal ash). The radiation dosage supplement expected for the residents of the buildings that will be built on this infrastructure will be completely negligible (even according to stringent calculations) and will amount to only about 1 microsievert per year. The construction of basements that are intended for residential space in the layer of coal ash is conditioned on the creation of an absorption casing from soil that provides a separation between the ash and the wall of the basement.
The principles for the protection of workers and the public from ionizing radiation
The 1991 recommendations of the ICRP (International Commission for Radiological Protection) for protection of workers and the public from ionizing radiation, similar to the previous recommendations of 1977, are founded on three fundamental principles - justification, optimization of radiation protection and individual dose limits. These principles are briefly summarized as follows:
- Justification
This principle refers to the criteria that will be used as the basis for decisions by authorities to permit the use of a specific technique or activity that is associated with exposure of people (workers or the general public) to ionizing radiation. The criteria for examining the justification are determined according to risk - benefit considerations, whereas the rule is that an activity should not be permitted unless it provides a net benefit to the individual and/or to the public. - Optimization
The second principle is the principle of optimization that states that the activity that is permitted should be performed with an effort to reduce the exposure as much as possible (ALARA - As Low as Reasonably Achievable), while taking into account the economic and social factors. The basis for optimization of means for protection from radiation is the estimate of the overall damage to health that may be caused by the specific activity and a comparison of this damage to the cost of the means for protection from radiation that must be implemented, in order to reduce the exposure. - Limitation of Individual Radiation Dose
According to this principle, care must be taken to ensure that the exposure of individual people from all radiation sources (which use was justified and for which optimal protective means were used) must not exceed, under any expected circumstances, the limitations that will be established.