E. V. Zakharova

Microorganisms in a Disposal Site for Liquid Radioactive Wastes and Their Influence on Radionuclides

Deep subsurface horizons used for the disposal of liquid low- and intermediate-level radioactive wastes of the Siberian Chemical Complex (SCC, Russia) were studied by microbiological, radioisotope, and molecular biological methods. It was shown that a diverse microbial community inhabited the groundwater. The cell numbers of microorganisms of the major metabolic groups and the rates of sulfate reduction, denitrification, and methanogenesis in natural groundwater were low and increased in the zone of wastes dispersion. More than 40 strains belonging to the genera Kocuria, Microbacterium, Pseudomonas, Pantoea, Acinetobacter, Enterobacter, Klebsiella, Stenotrophomonas, Sphingomonas, Staphylococcus, Acidivorax, Shewanella, and Desulfosporosinus were isolated from the disposal sites. Among the isolates, the microorganisms were found that were able to concentrate actinides and other transuranium elements. Aerobic bacteria were able to sorb various radionuclides in laboratory experiments; however, biosorption was low in sample of groundwater and in carbonate solutions containing several radionuclides. Reduction of U(VI) by a sulfate-reducing enrichment culture from the site and reduction of U(VI) and Np(V) by an isolate Shewanella were observed in the presence of various organic substrates. These results show the necessity of further ecosystem characterization based on microbiological and radiochemical studies and modeling of biogeochemical processes at the deep disposal sites for liquid radioactive wastes.

Sorption of Radionuclides by Microorganisms from a Deep Repository of Liquid Low-Level Waste

Sorption of radionuclides on a biomass of aerobic microorganisms isolated from deep repositories of liquid low-level waste was examined. Each strain exhibited substantially different sorption of different radionuclides. In neutral medium, the maximal proportion of the recovered radionuclides was 77, 92, 76, 72, and 33% for Pu, Np, U, Am, and Sr, respectively. None of the isolated strains sorbed Cs. The highest sorption ability was exhibited by Pseudomonas genus bacteria. The results obtained are indicative of different mechanisms of interactions of microorganisms with radionuclides.

Partitioning and speciation of Pu in the sedimentary rocks aquifer from the deep liquid nuclear waste disposal

Abstract Plutonium partitioning and speciation was studied under the conditions of a liquid nuclear waste repository site. The dominating effect of corrosion products (Fe and Cr) oxides on plutonium partitioning was established in batch sorption tests at 150 ℃ and by using sequential extraction. Various techniques including Mössbauer spectrometry, XRD and TEM were used to characterize Fe/Cr oxide phases that form both precipitates and surface coatings which sorb plutonium. The sorption and speciation of plutonium was modeled thermodynamically using Pourbaix diagram that takes into account both surface complexation reaction and precipitation of insoluble PuO2.

Behavior of Cs, Np(V), Pu(IV), and U(VI) in pore water of bentonite

Sorption of Cs, Pu(IV), Np(V), and U(VI) with bentonite from solutions was studied. Physicochemical species of radionuclides in the liquid phase were determined, the sorption mechanisms were established, and the influence of bentonite colloids on the behavior of radionuclides was studied. It was shown that Cs is sorbed by the ion-exchange mechanism, whereas the sorption of actinides at pH > 5 is governed by the reaction with surface hydroxy groups of betonite, and at pH < 5 the competing processes are ion exchange and complex formation. Reduction of Np(V) and U(VI) to Np(IV) and U(IV) in the solution with Fe(II) compounds present in the system was proved by the extraction method. Various methods of separating the solid phase were used in studying the dependence of the distribution coefficients of Np and Pu on the ratio of pore water and bentonite; it was shown that Np and Pu are sorbed on bentonite colloids.

Effect of Redox Conditions on Actinide Speciation and Partitioning with Colloidal Matter

Colloid-facilitated transport of actinides in subsurface environments depends on various geochemical and geological properties among which the redox conditions are very important. In reducing conditions, redox-sensitive actinides are stabilized in low valence states and could potentially form intrinsic colloids while in oxidizing conditions, they form pseudocolloids by sorption or coprecipitation onto natural aquatic colloids. The difference in actinides’ behavior associated with colloids at two sites in Russia under different redox conditions is discussed in this chapter. Successive micro- and ultrafiltrations of groundwater samples were performed by electron microscopic methods, nanoSIMS and sequential extraction for partitioning. Under oxidizing conditions of the PA “Mayak” site (Southern Urals, Russia), U and Np are present in the higher oxidation states while Pu is mostly found in the tetravalent state. Under these conditions, actinides are bound to amorphous hydrous ferric oxide (HFO) and Mn oxides, forming pseudocolloids as demonstrated by nanoSIMS. According to nanoSIMS, at the Tomsk site (Siberian Chemical Combine, Russia) oxidizing waste solutions were mixed with reducing groundwater resulting in reduction of U(VI) and formation of intrinsic U(IV) hydroxocolloids. Most of the U and Pu in a reducing environment are present in a residual fraction of low-mobility that favors slow migration rates.

Experimental determination of the spent graphite radioactive contamination at plutonium-production reactors of the Siberian Group of Chemical Enterprises (TOMSK-7)

Abstract Spent graphite from decommissioned plutonium-production uranium-graphite reactors is contaminated with radionuclides, and this graphite represents an important fraction of the radioactive wastes accumulated by the Russian nuclear power industry. To select proper ways and dates for the management of graphite, the information regarding the composition and level of the graphite contamination is required. In the paper, results are presented that were obtained in studies carried out at the I-1, EI-2, and ADE-3 reactors of the Siberian Group of Chemical Enterprises (Russia) in 1996-1999. The main feature of the studies is a wide-scale sampling from the graphite piles of the aforementioned reactors followed by complex assays of their radioactive contamination. The analyses performed for the large number of graphite samples made it possible to obtain a detailed picture of the pile contamination, to study radionuclide distributions over the piles, to construct schemes for evaluation of radionuclide stockpiles, and to evaluate stockpiles of several radionuclides including 14C, 3H, 90Sr, 241Am, 244Cm, 238,239,240,241Pu, 137,134Cs, and 60Co.

Experimental Study of the Radioactive Contamination of Graphite Masonry in the Commercial Reactors at the Siberian Chemical Combine

The graphite masonry from decommissioned commercial uranium–graphite reactors is contaminated with radionuclides and is a significant fraction of the radioactive wastes from the nuclear industry. Information about the composition and degree of contamination of the graphite is needed to choose methods and time periods for handling this graphite. The results of investigations performed on I-1, ÉI-2, and ADÉ-3 reactors at the Siberian Chemical Combine in 1996–2001 are presented in this paper. Analysis of a large number of samples made it possible to construct a detailed picture of the contamination of the masonry, study the distribution of the radionuclides of different origin in the masonry, construct schemes for making estimates, and estimate the content of certain radionuclides, including 14C, 3H, 90Sr, 241Am, 244Cm, 238–241Pu, 137,134Cs, and 60Co.

Sorption of 237Np(V), 238U(VI), and 137Cs on clays: Role of surface films of Fe(III) compounds

Mineralogical and sorption properties of sandy-argillaceous rocks proposed as a material of protecting barriers in near-surface repositories of nuclear wastes were studied. Different sorption behavior with respect to Np(V), U(VI), and 137Cs is caused by formation of iron-containing films on the particle surface of coarse (>0.25 mm) and fine (<0.01 mm) rock fractions. The presence of Fe in the compositions of different minerals contained in the rock (chlorite, illite, montmorillonite, and hematite) was determined by scanning electron microscopy with X-ray probe microanalysis (SEM-XPMA), transmitting electron microscopy with electron energy loss spectroscopy (TEM-EELS), and Mössbauer spectroscopy. Hematite is present in the form of both separate particles and films on quartz grains. The Fe fraction in these formations was estimated.

Distribution and activity of microorganisms in the deep repository for liquid radioactive waste at the Siberian Chemical Combine

The physicochemical conditions, composition of microbial communities, and the rates of anaerobic processes in the deep sand horizons used as a repository for liquid radioactive wastes (LRW) at the Siberian Chemical Combine (Seversk, Tomsk oblast), were studied. Formation waters from the observation wells drilled into the horizons used for the radioactive waste disposal were found to be inhabited by microorganisms of different physiological groups, including aerobic organotrophs, anaerobic fermentative, denitrifying, sulfate-reducing, and methanogenic bacteria. The density of microbial population, as determined by cultural methods, was low and usually did not exceed 104 cells/ml. Enrichment cultures of microorganisms producing gases (hydrogen, methane, carbon dioxide, and hydrogen sulfide) and capable of participation in the precipitation of metal sulfides were obtained from the waters of the disposal site. The contemporary processes of sulfate reduction and methanogenesis were assayed; the rates of these terminal processes of organic matter destruction were found to be low. The denitrifying bacteria from the deep repository were capable of reducing the nitrates contained in the wastes, provided sources of energy and biogenic elements were available. Biosorption of radionuclides by the biomass of aerobic bacteria isolated from groundwater was demonstrated. The results obtained give us insight into the functional structure of the microbial community inhabiting the waters of repository horizons. This study indicates that the numbers and activity of microbial cells are low both inside and outside the zone of radioactive waste dispersion, in spite of the long period of waste discharge.

Reduction of neptunium(V) and uranium(VI) with iron(II) in bicarbonate solutions

Reaction of Np(VI) compounds with Fe(II) in bicarbonate solutions was studied. Reaction of Np(V) with Fe(II) in the presence of phthalate ions was briefly considered. Iron(II) compounds reduce Np(V) compounds in solutions saturated with Ar or CO2 at any concentrations of bicarbonate ion. At [Na(K)HCO3] > 0.86 M, Np(V) is reduced during mixing the reactants and recording the spectra. The reaction of Fe(II) with Np(V) in dilute bicarbonate solutions is substantially slower, probably owing to a sharp decrease in the solubility of the Np(V) carbonate complexes. The solubility of the Np(V) compounds increases after saturation of the dilute bicarbonate solutions with CO2. However, in this case reduction remains slow. Uranium(VI) carbonate complexes are reduced with Fe(II) compounds in dilute bicarbonate solutions. The reaction products formed at elevated temperatures are UO2 and FeOOH.