**2. The key sources of soil contamination by radioactive pollutants**

**1. Introduction**

254 Soil Contamination - Current Consequences and Further Solutions

genic or terrestrial. The 3

of a certain tissue [3].

to the ionizing radiation.

Radioactivity is a phenomenon related to unstable atomic nuclei with excess of energy and/or mass, which spontaneously decompose emitting ionizing radiation in the form of electromag‐ netic waves (gamma rays) or streams of subatomic (alpha, beta, or neutron) particles [1]. The activity of a particular radioactive substance is characterized by the constant decay rate and the half‐life (t1/2—time taken for the activity of a given quantity of a radioactive substance to decay to half of its initial value), and it is a general rule of thumb that ten half‐lives are required for each radioisotope to be eliminated [2]. Since the half‐lives of various nuclei vary from seconds to billions of years [3], the time required for their total decay significantly differ as well.

Some radionuclides occur naturally in the environment, and their presence is either cosmo‐

the interaction of atmospheric gases with cosmic rays. On the other hand, the rocks, minerals, and consequently the soil, contain naturally occurring radioactive materials (NORM), charac‐ terized by a long half‐life periods [3]. The most important terrestrial radionuclides are 238U and 232Th decay series, as well as 40K. The world average values for soil activity coming from 226Ra,

The term radioactive contamination indicates the unintended or undesirable presence of radioactive substances on the surfaces or within solids, liquids, gases, or biota [5]. The origin of NORM is related to the formation of the planet; thus, their presence cannot be referred to as contamination. On the other hand, anthropogenic activities, related to the development of nuclear energy and its versatile use, have become important source of pollution. Since the middle of the last century, the radioactive contamination have appeared through the discharge of man‐made radionuclides, making the ionizing radiation one of the important ecological factors, in line with other types of soil degradation (physical, chemical, and biological) [6]. Even though the radioactive contamination of the environment is relatively rare, it requires a great attention because of extreme degrading effects of ionizing radiation on living tissues. The adverse effects are in correlation with the quantity of absorbed energy, the penetrating power of the radiation, the duration of the exposure, as well as with the reproduction rate of the cells

In terrestrial ecosystems, soil corresponds to the major receiving pool of emitted radionuclides. Given that the nutrient cycles and the flow of energy present links between the abiotic and biotic components of the ecosystem, soils contaminated with radionuclides lose their ability to produce good quality agricultural crops and thus can be classified as degraded [6]. The issues related to the degradation of radioactively contaminated soils are being considered as an exceptional type of chemical contamination, with the additional, specific features related

The transport and fate of radionuclides in the soil are governed by a number of factors and the effects of their interactions; therefore, the detection and comprehension of the retention mechanisms are of great importance for the selection, development, and application of ap‐ propriate remediation technologies. In this chapter, the following topics were summarized

232Th, and 40K are 32 Bq/kg, 45 Bq/kg, and 420 Bq/kg, respectively [4].

H, 7,10Be, 14C, 26Al, and 39Ar are the main radionuclides produced after

Contamination of the soil with the radioactive pollutants is an important origin of hazard for the environmental and health safety, as well as for the economy. Exploitation of the nuclear energy is a key source of pollution. Radiation can enter and affect the environment at any of the stages of the nuclear fuel cycle, starting with the excavation and processing of uranium ore, over production and recycling of the nuclear fuels, to the processing and disposal of radioactive wastes. The average uranium concentration in the earth crust is 2.8 mg/kg [7]. This radionuclide is contained with variable concentrations in the range of oxide, silicate, arsenate, vanadate, and phosphate minerals. Ores, processed by conventional uranium production methods, vary from reach (>20%, Canada) to very poor (0.01%, Namibia) [8]. Uranium is extracted from the ore matrix by hydrometallurgical process, and the final product, (the so‐ called yellowcake), used in the following steps of the nuclear fuel production typically contain 75–85% U3O8. Studies of the effect of uranium production process onto environmental pollution and the potential health risks have revealed elevated activities at cites around ore processing facilities and around old mines, in particular [9, 10]. Nowadays, almost half of world‐wide uranium mining, and most of the mining in the USA, Kazakhstan, and Uzbekistan, was conducted by *in situ* recovery (ISR) method [11]. This process is based on uranium leaching from the ore matrix, within the deposit. ISR is the most economically efficient method of uranium extraction; however, the associated risks include contamination of drinking‐water aquifer with uranium or other heavy metals [12]. At present, approximately 60.000 tonnes of uranium ore are mined annually to supply fuel for more than 430 nuclear reactors around the world, which provide approximately one‐eighth of the world's electricity [11].

Any material that is radioactive itself or is contaminated by radioactivity at levels greater than the quantities established by the competent authorities, and which cannot be of further use, is characterized as—radioactive waste. Within civil society, this kind of waste arises mainly from nuclear power production, but also from a variety of industries, medicine, agriculture, research, and education and other activities in which radioisotopes are used [13]. The radio‐ active wastes are being classified based on the level of radioactivity (low, medium, and high) and the half‐lives of the isotopes with predominant activity [14]. In the short‐lived waste, predominant activity is defined by radionuclides with t1//2 < 30 years, whereas the long‐lived wastes are characterized by isotopes with t1/2 > 30 years.

Processing of radioactive waste may result in an accidental release of the radionuclides during characterization, segregation, transportation, treatment, and disposal. By the review of the inventory of fission products important in the case of accidental releases, it can be concluded that 89Sr, 90Sr/90Y, 91Sr, 92Sr, 95Zr, 97Zr, 103Ru/103mRh, 105Rh, 129mTe/l29Te, 131mTe/131Te, 132Te, 131–135J, 140Ba/ 140La, 134Ce, 144Ce/144Pr are important pollutants at the reactor stage; 90Sr, 125mTe/129Te, 131I, 134Cs, 137Cs may be released during fuel element transport; 90Sr, 95Zr/95Nb, 106Ru, 131I, 137Cs, 144Ce/144Pr, and actinides are important at the fuel reprocessing stage; 90Sr, 106Ru, 137Cs, and 144Ce/144Pr contamination may occur during fission product solidification, whereas leaching from the final disposal may result in soil contamination with 90Sr, 137Cs, and actinides [15]. In addition to fission products, several corrosion products may become significant soil pollutants. Namely, during nuclear reactor operation, most metallic surfaces oxidize and form a layer of corrosion film rich in oxides of structural elements. This layer is exposed to high pressures and temper‐ atures, where radionuclides are generated under the neutron activation [16]. Depending on the composition of the reactor materials and their trace elements, reactor type and design, thermal power, years of irradiation and shutdown period, the corrosion products and their relative proportions are different. The products of steel corrosion are 55Fe, 59Ni, 63Ni, 94Nb, 60Co, 39Ar, 54Mn, with the 60Co and 55Fe being the most important in the first 10 years following the closure of a reactor, and 63Ni, 94Nb, 108Ag in the next 50 years. Reinforced concrete's corrosion products are 3 H, 14C, 41Ca, 55Fe, 60Co, 152,154Eu, whereas 3 H, 14C, 152,154Eu originates from graphite. Considering these two groups of materials, 3 H becomes the most prominent after 10 years, and 14C, 41Ca, 152,154Eu after 50 years from the reactor shut‐down. Taking into account both fission and corrosion products, 10–20 years after the reactor shutdown the most abundant radionu‐ clides in contamination residues generally include 3 H, 60Co,55Fe, and 137Cs, whereas in the period 20–30 years, 63Ni, 137Cs, 60Co, and 90Sr generally prevail [16].

Another key source of soil contamination with radionuclides is nuclear weapons tests, particularly atmospheric, which have started in 1945 in the USA [17]. In the period 1945–1980, the power of USA atmospheric tests (428 megatons) was approximately equivalent of the size of 29,000 Hiroshima bombs [17]. Finally, in 1990, thanks to the moratorium signed by SSSR, UK and USA, nuclear testing was stopped. Atmospheric detonations produce radioactive debris of different particle size, which are partitioned in the tropo‐ and stratosphere and my precipitate over a period of a few minutes to 1 year, or longer [18]. The concern is especially focused onto released Pu isotopes, due to the high biological toxicity and long half‐lives of its relevant isotopes (e.g., 24.2 × 103 , 373 × 103 , 81 × 106 years, respectively, for 239Pu, 242Pu, and 244Pu) [19]. Furthermore, 137Cs, 90Sr, 241Am, and 131I are the released radioactive isotopes with major impact on the environment and irradiation of the human body [20]. The mentioned isotopes were predominantly found in most of the nuclear test sites worldwide, especially in western US soil [21, 22].

Nuclear accident are the events that led to significant consequences to people, the environment or the facility, such as the ones in Chernobyl (Ukraine, 1986) and Fukushima (Japan, 2011). These two events caused global contamination of the environment, including air, water, soil, and living organisms. Huge amounts of radioactive elements especially 131I, 137Cs, 90Sr and the sum activity of 239Pu and 240Pu were dispersed into environment [23]. Some 40% of Europe has been exposed to Chernobyl's 137Cs at a level 4–40 kBq/m2 [24]. The size of the disaster can be illustrated by the fact that the maximum radioactive contamination in the soil in the 1993 was found to be 3500 times higher than the level before Chernobyl accident.

Apart from uranium mining and related fuel cycle activities, the industrial sectors which generate technologically enhanced naturally occurring radioactive materials (TENORM) include the following: mining and combustion of coal, the oil and gas production, metal mining and smelting, production of mineral sands (rare earth minerals, titanium, and zirconium), phosphate fertilizer industry, building industry, and recycling [25–27]. The dose of radiation coming from primordial radionuclides (40K, 232Th, 235U, 238U, and the members of decay series), which are normally found in natural minerals and ores (uranium ore, coal, phosphate rock, monazite, bauxite, etc.), can be elevated in their by‐products and wastes such as phosphogyp‐ sum, fly ash, and red mud. Consequently, the releases from non‐nuclear industries represent a continuous source of soil contamination with natural radioactive elements, by spreading of dust from rock and solid wastes dump, as well as by the overflow of wastewater from treatment ponds. Furthermore, years of application of phosphate fertilizers enriched with TENORM may become a source of soil contamination. Depending on the contamination level, restriction of land use or the remediation measures may be necessary. Finally, soil contamination may also arise from less common sources such as incidents during use of radioisotopes in medicine, industry, and agriculture [28].

At 160 U.S. Department of Energy (DOE) sites with radioactive contamination, 137Cs, 226Ra, 238U, 238–242Pu, 60Co, 232Th, and 90Sr were detected as the key artificial and natural radionuclides [29].
