Leaching of radionuclides from products of underground nuclear explosions in granite: Experiments with radioactive samples of melted rock from explosion cavities on Semipalatinsk test site

Leaching of ¹³⁷Cs, ⁹⁰Sr, and ^239+240Pu from products of underground nuclear explosions in tunnels 504R, 148/5, and 190 of the Semipalatinsk test site was studied. Samples were taken from central zones of the explosions. Sample characteristics are presented. Leaching experiments were performed in the batch and dynamic modes. The degrees and rates of ¹³⁷Cs, ⁹⁰Sr, and ^239+240Pu leaching with granite aqueous extract were determined. The leach rates determined in batch experiments are in the following ranges: ⁹⁰Sr 2.3 × 10^?6?1.6 × 10^?4, ¹³⁷Cs 5.0 × 10^?6?2.5 × 10^?4, and ^239+240Pu 5.6 × 10^?7?2.1 × 10^?4 g cm^?2 day^?1, and those determined in dynamic experiments are in the following ranges: ⁹⁰Sr 9.2 × 10^?10?10^?8, ¹³⁷Cs 1.0 × 10^?9?1.6 × 10^?7, and ^239+240Pu 1.3 × 10^?11?2.5 × 10^?11 g cm^?2 day^?1. The values obtained are close to those reported in the literature for vitrified radioactive wastes.     

Sorption properties of rocks on the territory of the former Semipalatinsk test site

The ability of granite and diorite, which are the most abundant rocks on the territory of the Semipalatinsk test site, to take up ¹³⁷Cs and ⁹⁰Sr present in tunnel waters of the Degelen mountain massif was studied in a laboratory. Experiments were performed both with simulated solutions and with real radioactive groundwater. The ¹³⁷Cs distribution coefficients were found for samples of typical granites, albitized granite, quartz porphyry, and diorite from the former Semipalatinsk test site. The ¹³⁷Cs distribution coefficient K _D (cm³ g^?1) was found to be n × 10³–n × 10⁴ for albitized granite, 60–80 for quartz porphyry, and 30–50 for granite. The ability of quartz porphyry and granite to take up ⁹⁰Sr from simulated solutions is characterized by the distribution coefficients K _D = 10–20 cm³ g^?1, and that of albitized granite, by K _D = 110?120 cm³ g^?1. The data obtained are required for predicting the migration of long-lived fission products from the nuclear explosion with underground waters from central zones of underground nuclear explosions in the former Semipalatinsk test site and for calculating the mass transfer in specific geologic media.     

Sorption of 60Co, 90Sr, 90Y, and 137Cs from aqueous solutions onto Mg-Ln layered double hydroxides (Ln = Ce, Pr, Sm, Gd)

Sorption of ⁶⁰Co, ⁹⁰Sr, ⁹⁰Y, and ¹³⁷Cs from aqueous solutions onto Mg-Ln layered double hydroxides (LDHs) (Ln = Ce, Pr, Sm, Gd) was studied. All the synthesized LDH-Mg-Ln-CO₃ samples (Ln = Ce, Pr, Sm, Gd) sorb ¹³⁷Cs poorly. At a contact time of the solid and liquid phases of 15 min and V/m = 50 mL g^?1, K _d of ¹³⁷Cs is no higher than 1.0 mL g^?1. Despite similar composition of the LDH-Mg-Ln-CO₃ samples (Ln = Ce, Pr, Sm, Gd), they noticeably differ in the ability to sorb ⁹⁰Sr. At a contact time of the solid and liquid phases of 15 min and V/m = 50 mL g^?1, Kd of ⁹⁰Sr varies from 10 to >5 × 10³ mL g^?1. All the synthesized LDHs efficiently sorb ⁹⁰Y and ⁶⁰Co. At a contact time of the solid and liquid phases of 15 min and V/m = 50 mL g^?1, K _d of ⁹⁰Y and ⁶⁰Co from aqueous solutions with LDH-Mg-Ln-CO₃ (Ln = Ce, Pr, Sm, Gd) exceeds 5 × 10³ mL g^?1.     

Polymeric matrices promising for immobilization of tritium radioactive wastes

The resistance of polymeric matrices to leaching of tritium water incorporated in oil microcapsules or in chemisorbent particles was studied. The phenol-formaldehyde matrix with tritium and cesium in oil microcapsules ensures highly reliable radionuclide immobilization: The half-decile leaching rate, g cm^?2 day^?1, is 9 × 10^?5 for ³H and 2 × 10^?5 for ¹³⁷Cs. A borate chemisorbent in combination with a polyurethane matrix was suggested for immobilization of highly radioactive tritium water vapor.     

Radioactive Contamination of the Neva River-Lake Ladoga Aquatic System

Many-year monitoring data on the ⁹⁰Sr and ¹³⁷Cs levels in water of the Neva River and Lake Ladoga are presented. In 1970-1990, the ⁹⁰Sr level in the Neva River steadily increased. In 1991, a radiological survey was made of the Kit experimental vessel with radioactive wastes in its holds, which was on berthing in the northwesteren part of Lake Ladoga. ⁹⁰Sr, ¹³⁷Cs, and ^239,240Pu were monitored in water and bottom sediments near the Kit vessel berthing after it was towed outside Lake Ladoga. In 2001, the ⁹⁰Sr concentration in water of Lake Ladoga and the Neva River considerably decreased.     

Sorption-diffusion model of 137Cs uptake by a water body bottom in estimations of water contamination

For a sample including 22 lakes, the ¹³⁷Cs concentrations in water were determined by the sorption-diffusion model. The ¹³⁷Cs levels in waters of lakes of the Kola Peninsula and Karelian Isthmus are satisfactorily described by the suggested model at the distribution coefficient K _d of 4000 L kg^?1 and diffusion coefficient D of 1.0 × 10^?7 cm² s^?1 for 2–4-year exposure. For mid-latitude lakes, the ranges of K _d and D of ¹³⁷Cs were 6000–1200 L kg^?1 and 1.0 × 10^?7–0.2 × 10^?7 cm² s^?1, respectively. The ¹³⁷Cs concentration in water is determined by the major contribution of silts, which are characterized by high K _d of ¹³⁷Cs and by large thickness of sediments on the bottom of lake hollows, to the watter-bottom sorption system. After 2-year exposure of “Chernobyl” ¹³⁷Cs in lakes, its inventory in water relative to the amount that fell out in 1986 did not exceed 14% for deep lakes and 2–3.6% for shallow lakes.     

Association of 90xSr, 137Cs, 239,240Pu, 241Am,and 244Cm with Soil Absorbing Complex in Soils Typical of the Vicinity of the Chernobyl NPP

Association of the main long-lived radionuclides ¹³⁷Cs, ⁹⁰Sr, ^239,240Pu, ²⁴¹Am, and ²⁴⁴Cm with various components of the soil absorbing complexes from soil samples collected along the western, northwestern, and northern tracks of radioactive fallout in the vicinity of the Chernobyl NPP was studied by the sequential leaching. In the samples of the sandy soil collected in the floodplain of the Pripyat river along the northwestern radioactive track, more than 85% of ⁹⁰Sr, 55% of ^239,240Pu, and 75% of ²⁴¹Am and ²⁴⁴Cm are associated with various components of the soil absorbing complex and are potentially mobile species. In the soil samples collected along the narrow western track, 80-85% of ¹³⁷Cs, ⁹⁰Sr, ^239,240Pu, ²⁴¹Am, and ²⁴⁴Cm are incorporated in hot particles. The degree of ¹³⁷Cs, ⁹⁰Sr, ^239,240Pu, ²⁴¹Am, and ²⁴⁴Cm association with different components of the soil absorbing complex is a function of the radionuclide type and physicochemical features of soil.     

Influence of temperature on the sorption properties of rocks from the Nizhnekansky massif

The kinetics of radionuclide sorption onto finely divided samples of rocks from the Nizhnekansky massif, taken from the level of the planned arrangement of deep radioactive waste repository, was studied, and the times in which the sorption equilibrium was attained at 20 and 90°С were determined. The distribution coefficients of actinides (²³³U, ²³⁹Pu, ²⁴¹Am) and fission products (¹³⁷Cs, ⁹⁰Sr) were determined. Increasing the temperature from 20 to 90°С intensifies the sorption of the radionuclides studied, except cesium for which the sorption slightly decreases.     

Phosphorus-containing compounds of pollucite structure and radiochemical problems

New pollucite-like phosphorus-containing compounds with cesium and uni-, bi-, and trivalent cations (Li; Mg, Mn, Co, Ni, Cu, Zn, Cd, Sr, Ba; Al, Cr, Fe), many of which can be present in various combinations in wastes from radiochemical processes, were synthesized. The individual phases obtained were characterized by X-ray phase analysis and IR spectroscopy. The symmetry of the unit cells of the phosphorus-containing compounds decreases relative to the silicon analog (pollucite natural mineral) from Ia3d to I4₁32. The behavior of the compounds on heating to 1000°C, under hydrothermal conditions at 90°C, in aqueous systems (distilled water and seawater) at 25°C, and in a CsCl melt at 680°C was studied. The chemical and phase composition of the samples remained unchanged under all these conditions. The Cs leaching rates varied from 7.1 × 10⁻⁶ to 1.46 × 10⁻⁵ g cm⁻² day⁻¹. The “crystal-chemical modeling” of the new possible compositions of the compounds with the expected pollucite structure was performed.     

Geoceramic Matrices for Immobilization of Nonpartitioned Radioactive Wastes

Immobilization of radioactive wastes (RW) containing large amounts of process alkalis in ceramic matrices based on acid and basic rocks is studied. Immobilizing characteristics of samples with an RW content of 15% are obtained. The best geoceramic matrices are characterized by the forward leaching rates with double-distilled water of 1 × 10^{- 4} and 1 × 10^{- 5} g cm^{- 2} day^{- 1} for Sr and Cs, respectively, and the steady-state leaching rate of about 2 × 10^{- 6} g cm^{- 2} day^{- 1}. Possible fixation mechanisms of the radionuclides in the matrices studied are discussed. Comparative analysis of properties of the geoceramic matrices and borosilicate and some other existing matrices for RW immobilization makes it possible to recommend some kinds of the geoceramics as matrices for immobilization of nonfractionated intermediate- and high-level radioactive wastes.     

Geoconcrete monolith as stable matrix material for immobilization of radioactive wastes

Immobilization of the intermediate-level radioactive waste (RAW) in a new type of binding agents, geoconcrete monoliths, was studied. The experiments were performed using liquid radioactive wastes of various salinities formed in the course of operation of nuclear power plants and various sorbents saturated with radionuclides, in particular, with ¹³⁷Cs. The prolonged operation life of geoconcretes is determined by the chemical and mineralogical composition of the hydrated neogenic compounds, which are analogs of water-resistant rock-forming silicate materials. A procedure for synthesis of a geoconcrete matrix with immobilization of biologically important radionuclides was developed, and the properties of geoconcrete monolith (GCM) were studied. The pilot experiments with real RAW were performed. The strength of the GCM samples exceeds by a factor of 2–4 the level prescribed by the regulations, and the leaching rate of ¹³⁷Cs is lower than the prescribed level by 2–3 orders of magnitude.     

Calculations of gamma ray incineration of 90Sr and 137Cs

The feasibility of the γ-ray incineration method, using the (γ, n) reaction, was examined for the long-lived radioactive waste ⁹⁰Sr and ¹³⁷Cs. The incineration of ⁹⁰Sr and ¹³⁷Cs as well as the buildup of long-lived radioactive by-products were calculated by changing the γ-ray flux as a parameter. The effect of alternative irradiation modes was also studied.     

Sorption of long-lived radionuclides onto main types of rocks of the Novaya Zemlya Archipelago

The ability of black carbonaceous siltstones and silt sandstones, lime sandstones, gray limestones, carbon–silicon carbonate schists with pyrite, and other rocks that most widely occur on the Novaya Zemlya Archipelago to sorb ¹³⁷Cs, ⁹⁰Sr, ^239+240Pu, and ²⁴¹Am was studied. The distribution coefficients K _d (cm³ g^–1) are as follows: for ^239+240Pu, 2.7 × 10³–7.7 × 10³; for 241Am, 2.5 × 10³–1.8 × 10⁴; and for ¹³⁷Cs, 1.1 × 10²–2.0 × 10³. Strontium-85(90) it not noticeably sorbed (within the measurement uncertainty) by any of the rocks studied. ^239+240Pu, ²⁴¹Am, and ¹³⁷Cs are strongly sorbed onto the rocks studied and are not noticeably desorbed from them with distilled water. The data obtained are required for predicting the migration of long-lived radionuclides generated by nuclear explosion with surface waters from test sites on the Novaya Zemlya Archipelago.     

Sorption of 137Cs and 90Sr by potassium production wastes, clay-salt slimes

Sorption of ¹³⁷Cs and ⁹⁰Sr by clay-salt slimes from model aqueous salt solutions (4.0–40.0 g l^?1 NaCl + KCl) was studied under static conditions. The influence of the sorption parameters (time, specific consumption, salt content) on the efficiency of the ¹³⁷Cs and ⁹⁰Sr recovery with finely dispersed sorbents was studied. The activity of the aqueous salt solution decreases by two orders of magnitude owing to the recovery of ¹³⁷Cs.     

Synthetic calcium aluminosilicate as a matrix for radioactive waste immobilization

The dependence of the total leach rate of monoliths with deionized water at 90°? on the keeping time (MCC-1 test) was measured. The monoliths were formed by cold pressing (25°C, 500 MPa, 2 min) of an air-dry powder of synthetic calcium aluminosilicate (SCAS) with added 10, 20, or 30 wt % simulated radioactive waste (SRW) of the composition typical of radioactive and nonradioactive wastes from a closed nuclear cycle. The leach rate of the SCAS-SRW monolith is 6.7 × 10^-7, 7.2 × 10^-7, and 8.3 × 10^-7 g cm^-2 day^-1 at an SRW content of 10, 20, and 30 wt %, respectively.     

Sorption of Cs,Sr, U,and Pu radionuclides on natural and modified clays

The ability of natural and modified montmorillonite clays from Belgorod oblast to sorb Cs, Sr, U, and Pu radionuclides was studied. The clays were modified by treatment with metal (Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Fe²⁺, Zn²⁺) chloride solutions or aqueous HCl. The natural and modified clays studied show high performance in sorption treatment of solutions to remove Cs radionuclides. The natural clay and the Na and Mg forms of clays show the best sorption characteristics with respect to Cs. The distribution coefficient K _d of ¹³⁷Cs in sorption on the above samples from a 0.1 M NaNO₃ solution is (1.1–1.4) × 10⁴ cm³ g⁻¹, which is 4–5 times higher compared to natural clinoptilolite. The Sr, U, and Pu radionuclides are sorbed on the examined clay samples to a considerably lesser extent. The K _d values in sorption of these radionuclides from tap water are lower by 2–3 orders of magnitude than in sorption of Cs. Addition of clay materials in the course of cementation of liquid radioactive wastes, including NPP bottom residues, allows the rate of radiocesium leaching from the hardened cement compounds to be decreased by a factor of 5–16. The most efficient sorption additive in cementation of NPP bottom residues is natural montmorillonite clay.     

Radioactive Fallout and Accumulated Level of Soil Surface Contamination with 90Sr and 137Cs around St. Petersburg in 1954-1999

Systematic data on radioactive atmospheric fallout in 1954-1999 and accumulated levels of contamination of the soil-vegetable cover with ⁹⁰Sr and ¹³⁷Cs around St. Petersburg, estimated on 1998, are presented. Comparative analysis is made of these data and directly obtained data on the vertical distribution of these radionuclides in the surface horizon (0-25 cm) of soddy-podzolic sandy loam soil: the levels of surface contamination with ^90Sr and ¹³⁷Cs were found to be 580±90 and 6500±600 Bq m^-2 on 1998. Before April 28, 1986 the ⁹⁰Sr and ¹³⁷Cs radioactivity in the investigated region corresponded to the global level. After Chernobyl accident the contamination levels with ⁹⁰Sr and ¹³⁷Cs increased, respectively, from 1200 to 1280 (by 6.3%) and from 2400 to 7050 Bq m^-2 (by a factor of 2.9). The ⁹⁰Sr and ¹³⁷Cs vertical distributions throughout the soil profile are considerably different: ¹³⁷Cs is practically totally fixed in the surface horizon, its activity being exponentially declining; whereas the stock of ⁹⁰Sr in horizons 0-10 and 0-25 cm represents only 22 and 59% of the integral accumulated inventory of ⁹⁰Sr in the soil (990 Bq m^{- 2}). The activity ratio of ¹³⁷Cs to ⁹⁰Sr in the atmospheric fallout in 1963-1985 was 1.85±0.42, and in river water in the same period, ca. 0.18, suggesting higher mobility in the soil of ⁹⁰Sr as compared to ¹³⁷Cs, and, therefore, higher rate of self-cleaning of the surface soil horizon with respect to ⁹⁰Sr.     

Sorption of cesium, strontium, and yttrium radionuclides from the aqueous phase on layered double hydroxides

Sorption of ¹³⁷Cs, ^85,90Sr, and ⁹⁰Y from aqueous solutions on the solid phase of layered double hydroxides (LDHs) and layered double oxides (LDOs) of various compositions was studied. On the solid phase of Mg-Al and Cu-Al LDHs and Mg-Al LDO, the Sr and Cs radionuclides are very weakly sorbed from aqueous solutions containing 10^?5 M of the corresponding element (Cs⁺ or Sr²⁺). Introduction of EDTA ions into Mg-Al LDH increases the distribution coefficients K _d of Sr by a factor of more than 40. After 96-h contact of the solid and liquid phases, K _d of radioactive Sr in sorption from aqueous solution on Mg-Al LDH and Mg-Al LDH-EDTA is 2.4 and 100 ml g^?1, respectively. The Sr and Y radionuclides are efficiently sorbed from aqueous solutions containing 10^?5 M Sr²⁺ and Y³⁺ on the Mg-Nd LDH solid phase. After 5-min contact of the solid and liquid phases, K _d of Sr exceeds 10⁵ ml g^?1. For Y, the distribution coefficients equal to 700–800 ml g^?1 are attained after 30-min contact of the solid and liquid phases. Aging of the Mg-Nd LDH precipitate does not affect its sorption properties toward Sr and Y radionuclides. With an increase in the Sr(NO₃)₂ concentration in the solution from 10^?5 to 10^?1 M, the distribution coefficients K _dz of Sr drastically decrease (virtually to zero) and those of Y change insignificantly.     

Development and trials of an improved process for decontamination of liquid radioactive wastes from Radon Moscow Research and Production Association

Speciation of radionuclides in low-salinity liquid radioactive wastes (LRW) formed in the course of reprocessing and storage of radioactive wastes at the Radon Moscow Research and Production Association was studied. ¹³⁷Cs is present in LRW mainly in the ionic form; α-emitting radionuclides, in the colloidal form; and ⁹⁰Sr, in both colloidal and ionic forms. Coagulation methods with FeCl₃ and methods of peroxide and permanganate oxidation were tested for removing organic substances from LRW. A two-step reverse-osmosis apparatus was tested for removing the major fraction of radionuclides and salts from the LRW. Laboratory experiments on sorption refinement of the permeate from the first step of reverse osmosis were performed with ionexchange resins and an NZhS type ferrocyanide sorbent. A new flowsheet for a water-treatment installation (WTI), allowing considerable reduction of the volume of secondary radioactive wastes, was suggested. The WTI was put into operation in 2010.     

Phosphorus-containing cesium compounds of pollucite structure. Preparation of high-density ceramic and its radiation tests

Multicomponent oxides of pollucite structure, containing Cs and Ba, were synthesized as powders and ceramics. Their chemical compositions, Cs[MgAl_0.5P_1.5O₆] and Cs_0.875Ba_0.125[Li_0.125Zn_0.875Al_0.5P_1.5O₆], were modeled on the basis of the known structural features, taking into account the principles of iso- and heterovalent isomorphism of cations. From powdered samples synthesized using sol-gel process, a ceramic was prepared by spark plasma sintering (SPS). The sintering time was 3–4 min in the temperature interval 600–850°C. The relative densities were 97 and 99%. To evaluate the radiation resistance of the ceramics, the samples were irradiated with ¹³²Xe²⁶⁺ ions (E = 167 MeV) in the fluence interval from 6 × 10¹⁰ to 1 × 10¹³ cm^?2 (ion flux density ～10⁹ s^?1 cm^?2). The amorphization took place at fluences of (1.2–1.3) × 10¹² cm^?2. This fact suggests the decisive role of the ion energy loss for ionization in the generation of radiation defects. Conditions were found for the transition of the metamict form into the crystalline form on heating.