Decontamination & decommissioning | Soil remediation
Soil contamination in Belarus, 25 years later13 March 2012
The Fukushima accident has raised questions about the best methods for remediation of soil contaminated with radioactive caesium and strontium. This article discusses a number of historical and prospective methods to decontaminate soil in Belarus after the Chernobyl accident of 1986. By L. N. Maskalchuk
Radioactive contamination of a considerable amount of agricultural lands of Belarus, Russia, Ukraine and some European countries resulted from the Chernobyl accident in 1986 [1-4]. The process of the radioactive pollution of the earth’s surface after the Chernobyl accident occurred in three stages: the release the radioactive substances of the catastrophe, their spread and their fall. The products of the accident were fragments of the reactor, microparticles, and gases.
The greatest amount of fallout was close to the reactor in the territory of the Ukraine, Belarus and Russia; the three main areas with Cs-137 contamination greater than 1 Ci/km2 have been designated the Central, Gomel-Mogilev-Bryansk and Kaluga-Tula-Orel areas. The Central area is within about 100 km of the reactor, predominantly to the west and northwest. The Gomel-Mogilev-Bryansk contaminated area is centred 200 km north-northeast of the reactor at the boundary of the Gomel and Mogilev regions of Belarus and of the Bryansk region of the Russian Federation. The Kaluga-Tula-Orel area is in the Russian Federation, about 500 km to the northeast of the reactor.
The central focus occurred during the initial active stage of fallout. Two other foci were formed on 28-29 April 1986 due to fallout in rain. Rain is the most effective means of washing radioactive substances out of atmosphere. Most microparticles (diameter of some tens of microns) fall with rain. In spring 1986 it did not rain often. In those places where it did rain, there was a considerable precipitation of radioactive substances. Depending on their distance and direction from the accident, the spots have differing radionuclide content that is defined by the progress of the accident and the dynamics of the meteorological situation. The spots are strictly inhomogeneous; spot intensity can vary greatly, and spot intensity does not correlate with distance from Chernobyl. The dominant radionuclide within the spots is caesium-137 (half life: 30.17 years). Full information about the contamination of Europe with radioactive caesium is given in .
The data on the scale of contamination of the territory of the Russian Federation, Belarus and Ukraine as a result of the Chernobyl accident in 1986 is given in Table 1 [1-3]. The analysis of the radioactive contamination of Europe with caesium-137 shows that about 35% of the Chernobyl fallout of this radionuclide falls on the territory of Belarus [2, 4].
Before the accident the contamination of the territory of Belarus with caesium-137 made up from 1.5 kBq/m2 to 3.7 kBq/m2 in individual spots. After the Chernobyl accident caesium-137 content in soil exceeded 10 kBq/m2 on 136,500 km2 (66% of the area of Belarus). One of the criteria for the definition of contaminated areas according to current legislation is radioactivity exceeding 37 kBq/m2 (1 µCi/m2). A total of 23% of Belarussian territory was registered as contaminated by this measure, compared with 7% in Ukraine and 1.5% in the European part of Russia (see Table 1). The Belarussian contaminated area includes more than 3600 settlements, including 27 towns with a 1986 population of 2.2 million people, that is about one-fifth of the whole population of Belarus. The Gomel (1528 settlements), Mogilev (866) and Brest (167) regions turned out to be the most contaminated. The peak level of soil contamination with Cs-137 amounting to 60,000 kBq/m2 was observed in separate settlements both in the nearest (the Bragin district of the Gomel region) and in the farthest areas (the Cherikov district of the Mogilev region). Some 265,000 hectares with contamination density of Cs-137 above 1480 Bq/km2 (40 nCi/km2) and density of Sr-90 above 111 kBq/m2 (3 Ci/km2) have been excluded from agricultural usage [2-4].
In January 2004, the area of contaminated lands with Cs-137 of the level more than 37 kBq/m2 had declined to 41,100 km2 of Belarus as a result of natural radioactive decay (see Figure 2).
Maps of predicted Cs-137 contamination for 2016 and 2046 have been made by the Belarussian government department on liquidation of the consequences of accident at the Chernobyl NPP (Ministry of the Emergency Situation of Belarus) . By 2016 (see Figure 3) the area of the contamination of Belarus with Cs-137 content of 37 kBq/m2 and more will have decreased to 66% of its original size, and by 2046 to 42% of its original size (see Figure 4).
The physicochemical properties of the soil determine the influence of radioactive isotopes in soil, in terms of isotope migration and uptake by plants. As a result of long-term research in the radioactive contamination zones of Belarus and other countries, it is possible to single out the following characteristics: the maintenance of organic substances, granulometric composition of the soil, acidity, availability of potassium, and the presence of alumosilicates, aluminium oxide and iron.
Clay minerals play a major role in the availability of caesium in soils, particularly micaceous clay minerals, the most common of which in temperate zones is illite [5, 6]. These clays have highly-selective sorption sites, which are located at the wedge-shaped edges of clay particles. The selectivity of adsorption is inversely proportional to the radius of hydrated ions and hydration energy, and decreases in the sequence Cs+ > Rb+ > NH4+ > K+. If sufficient numbers of potassium ions are absorbed, the minus charge on the internal surface is neutralized, the repulsive forces are reduced and there is a collapse of adjacent layers, immobilizing potassium ions and Cs-137.
Mobility of Sr-90 in soils (half-life: 28.18 years) occurs mainly due to isomorphic substitution of calcium or magnesium in minerals containing calcium and magnesium, such as calcite (CaCO3) and dolomite (CaMg(CO3)2).
Carrying out agrochemical and agricultural measures on radioactive contaminated soils in Belarus has lowered the amount of Cs-137 and Sr-90 in received production, reducing the economic damage by several times and lowering the dose loadings on the population [2, 4].
At the initial stage after Chernobyl accident in the Republic of Belarus various countermeasures were applied to decrease in radioactive contamination of agricultural production. There have been two stages in carrying out of protective measures on the radioactive contaminated soils of Belarus: the first, between 1986-1991, and the second, from 1992 and up to the present [2, 3].
In the first stage, lands where it was impossible to have agricultural production with tolerable radionuclide content were excluded from agricultural usage. In productive areas, crops that tended to accumulate large amounts of radionuclides were excluded from agricultural rotation. In addition, liming of acid soils, and application of raised doses of potassium and phosphoric fertilizers, and organic fertilisers, were carried out across all productive areas. Depending on climatic and soil conditions, these countermeasures decrease the contamination of agricultural products by several times. The positive effect is reached by decreasing the availability of Cs-137 and Sr-90 to plants, and by increasing the productivity of agricultural crops, the so-called ‘dilution effect’.
In the second stage, countermeasures aimed at further optimizing economic activities on the contaminated lands were carried out, including specialization of agricultural production and carrying out different agrochemical measures based on the level of contamination and special features of the specific local soil properties. The characteristics of the current methods for rehabilitation of radioactive contaminated soils and their efficacy of application on soddy-podzolic soils of the Russian Federation, Belarus and Ukraine are shown in Table 2 [2-4, 7-11].
Thus, the protective measures which were carried out in Belarus in some cases have lowered radiocaesium contamination of agricultural products on average from 2 to 10 times. Radiostrontium content in foodstuff has decreased by two times. However, availability of Sr-90 remains high and it is tending to increase [2-4].
In spite of substantial improvement of the radiation situation, and successes achieved on rehabilitation of the contaminated territories, the issues of the safety of residents and ecologically-sound agricultural production on radioactively-contaminated soils in Belarus remain unsolved [2-4, 12].
Now the main problems are concentrated in rural regions of Belarus, and in farms and holdings where poor, overmoistened, sandy and peaty soils prevail. These soils are characterized by the high transfer factor of Cs-137 and Sr-90 into plants. In these places, radionuclides accumulate in the human body through consumption of vegetables and fruits that are cultivated by the family household, and also through consumption of berries and mushrooms collected in contaminated forests.
That is why these sources made up 70% of internal irradiation of the Belarussian population in 2010, compared with 90% in 1986 [2-4]. By comparison, internal irradiation rates in Ukraine in 2006, depending on region, made up 70-95% of total dose .
The radioisotope reduction countermeasures shown above have not been efficient enough.
Some of the countermeasures are not ecologically safe. Especial concern is caused by regular application of raised doses of mineral fertilizers, that have in a number of cases lead to decrease of soil fertility and contamination of groundwater.
Another problem in the Belarussian private sector is the quality of agricultural products. In many cases these products are not competitive in the market even though their radionuclide content is below permissible levels . Considering scales of agricultural soil contamination in Belarus, Russia and Ukraine, economic aspects of the developed technologies and its application should be prioritized. Large-scale protective countermeasures can be defined by the economic efficiency of their application. For example, the application of cow manure is an effectual measure of decreasing of radionuclide contamination on Belarussian agricultural lands. But it is hampered by several technical difficulties. First, there is no large quantity of cow manure. Second, it is necessary to apply pure cow manure, that is, to bring in manure from uncontaminated areas. But because there is no industrial production of organic fertilizers (cow-manure recycling) in Belarus, its transportation and application in an untreated state is expensive and technically difficult.
Results of much research on overcoming the consequences of Chernobyl accident have shown that decreasing the mobility of Cs-137 and Sr-90 in soil by fixing them in a soil-absorbing complex (rehabilitation) is sufficiently effective, ecologically safe and has the greatest prospects for prevention of migration of radionuclides in biological systems and nutritive chains .
We propose rehabilitation of radioactive contaminated soils in Belarus by applying natural raw materials (sapropels) [14-17] and industrial wastes (such as hydrolyzed lignin, clay-salt slimes, phosphogypsum) [17-24] as a sorbent of Cs-137 and Sr-90. These materials have a fair price, they are technically easy to apply, and the treatment is effective.
There are more than 10,000 lakes in Belarus containing more than 2.6 billion tons of sapropels. The advantages of sapropels as a raw material are their high organic matter content, high humic acids, high cation-exchange capacity, high amounts of nutrients (nitrogen, phosphorus, potassium), and optimal acidity (pH KCl)= 4.5-6.5.
Hydrolyzed lignin is a byproduct of the chemical industry produced in large volumes. It is a wood processing product containing organic matter (90-95%). Clay-salt slimes are wastes of the potassium industry produced as a result of sylvinite ore reprocessing at the factories of the Belaruskali Plant. They consist of 20-30% soluble components (KCl, NaCl), with the remainder insoluble residue (quartz, feldspar and hydromuscovite (illite)) [17, 18, 20-24].
Results and discussion
The physicochemical and sorption properties of various types of sapropels and industrial wastes were studied in ISTC project #3189 (execution period – 2005-2009, performer – GI SPA ‘Typhoon’ (Russia), FGI RRC KI (Russia) and SSO JIPNR ‘Sosny’ NAS of Belarus) .
To develop an optimal composition, samples have been combined in various proportions to produce 10 organomineral amendments (OMAs) (see Table 3, below). Initial components used are silica sapropel (SS) from the lake of Chervonoe, Gomel region, hydrolyzed lignin (HL) from the Retchica factory and clay-salt slimes (CSS) from the Belaruskali factory) [17-23]. Quantitative parameters of selective sorption of Sr-90 and Cs-137 by OMA samples are also shown in Tables 3 and 4.
Table 4 shows that the fraction of CSS in OMA dictates their ability to sorb and fix Cs-137. The OMA with most CSS has the highest value of RIP(K)ex and the OMA with the least amount of CSS has the lowest value. However, the comparative analysis of research results shows that using CSS above 10 mass % is inappropriate in preparing OMA with optimum sorption properties for simultaneous extraction of Cs-137 and Sr-90, because larger proportions of CSS tend to decrease of Sr-90 sorption properties. Another negative factor in using large proportions of CSS is the considerable content of water-soluble sodium chloride, which can create unfavourable conditions for growth and development of plants .
In addition to the mechanism of selective sorption, another mechanism of radionuclide fixation was observed in these OMAs. Silica sapropel and (neutralised) hydrolyzed lignin have a well-developed micro-, meso- and macro-structure. In sapropel, it is determined by the ratio of the organic and mineral matter; in hydrolyzed lignin by the morphology of its organic matter. The chemical bond holding the adsorbed cations (Cs-137 and Sr-90) increases because the diameter of the cation is close to the size of the pores and cross section of channels in the structure of OMAs.
The choice of a prospective OMA with various contents of sapropel, hydrolyzed lignin, clay-salt slimes was carried out on the basis of aex/CEC for Sr-90 and aex/RIP(K) for Cs-137. Analysis of tables 3 and 4 shows that the best samples for simultaneous sorption and fixation of Cs-137 and Sr-90 from soil solution are OMA 1-1 and OMA 1-3.
A new approach for soil rehabilitation
In Belarus, the task remains to develop the new organizational, agrochemical, agro-technical measures and technologies for growing of ecologically safe agricultural products in private plots, farming enterprise and in the public sector.
Among the most urgent tasks for Belarus in 2011-2015 and up to 2020 [4, 12] are:
supporting agricultural production on the contaminated soils, corresponding to the national and international radiological standards
supporting work to ultimately recover lands taken out of use for radiation safety with an aim of future economic activity.
However, the large scale of contamination requires further work aimed not only to minimize the consequences of the Chernobyl accident, but also to ensure the recovery and sustainable socio-economic development of the affected areas [12, 25].
Within the framework of the ISTC project #3189, a draft technical specification on the content of the OMA was developed. According to the technology implementation plan, a new project for production of a pilot lot of OMAs and their field testing was prepared .
The analysis of existing methods and production technology of organomineral mixtures in the project has shown that it is possible to produce a kind of powdered or granulated fertilizer by mechanical mixing of sapropel, industrial waste with additives of microcells and/or peat and brown coal. The production process consists of following operations: preparation of organic and/or mineral raw materials (clearing, sifting), batch in bunkers; dosed supply and forced mixture of components, and packaging.
Organomineral mixtures of various structure and appointment on the basis of natural raw materials (peat and silica sapropel) are produced according to TS RB 800009966.001-2003 on the industrial base of Open Joint-stock Company ‘Zhitkovichichemservice’ in Belarus, which could be used to produce OMAs.
The method of amendments insertion in agricultural soils contaminated by Cs-137 and Sr-90 are carried out in the same way as application of traditional organic and/or mineral fertilizers, with one exception. They require a good mixing of soil. The dose of OMA application in contaminated soils depends on their type and physicochemical properties, and on the physicochemical properties of the OMAs themselves. But as an example, on a Belarussian soddy-podzolic sandy soil with specific density of 1300 kg/m3, the rate of OMA application would be 50-100 t/ha.
Currently, there are three basic gaps in knowledge about applying OMAs to rehabilitate radioactively contaminated soils in Belarus:
- quantitative relationship between type/physicochemical property of soil/field conditions and dose rate
- R&D on specification of OMA based on specific contaminated soil physicochemical properties
- absence of industrial technology for OMA production.
International organizations including the European Community and the United Nations have worked actively on economic activity revival in the Chernobyl-contaminated zone for a long time [10-12]. The importance of rehabilitating the rural areas of Belarus, the Russian Federation and Ukraine contaminated by the accident was underlined in a recent (2010) UN resolution relating to the need to mitigate and minimize the consequences of the Chernobyl accident .
More recently, European Union experts are preparing a strategy of social and economic development of the Chernobyl-restricted zone in Ukraine. One of the prospective directions of such development is the cultivation of rapeseed on radioactively-contaminated soils [26, 27].
A project (#B-1905, ISTC, Moscow) has recently been proposed by the Joint Institute for Power and Nuclear Research – Sosny, National Academy of Sciences of Belarus (Minsk) to develop a theoretical basis for application of clay-salt slimes as an effective and low-cost sorbent of radionuclides, and development of the technology of aluminosilicate extraction (illite, feldspar) from clay-salt slimes to produce a matrix for immobilization of Cs-137.
The proposed project was approved by the ISTC governing board, but without financing. At present, we are looking for partners who are ready to support the practical implementation of the project, and foreign collaborators who are interested in cooperation with our team on the topic of rehabilitation of the radioactive contaminated soils by using OMA.
Implementation of this project is of special significance because it will produce real decontamination data based on applying series-produced soil additives to real contaminated soils, at a pilot level. It will also enable the establishment of a Belarussian production base using locally-sourced natural materials and industrial wastes.
The given approach could also be developed for use in Japan and other countries for rehabilitation of soils contaminated as a result of the nuclear accident at Fukushima, and for minimization of the consequences of a possible radiation accident.
Table 1: Scale of Cs-137 soil contamination in the Russian Federation, Belarus and Ukraine, in km^2
Table 2: Traditional decontamination methods in Russia, Belarus and Ukraine, and their effectiveness
Table 3: Parameters of selective sorption of Sr-90 by OMA
Table 4: Parameters of selective sorption of Cs-137 by OMA
LN Maskalchuk (firstname.lastname@example.org), Joint Institute for Power and Nuclear Research – “Sosny”, NAS of Belarus, Remediation of Techno Polluted Territories Laboratory, Head, 99, academic A.K. Krasin str., 220109, Minsk, Belarus, Tel/Fax: (+375) 17 2994768.
This article was originally published in the February 2012 issue of Nuclear Engineering International magazine.
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