Cesium Leach Research Articles

Preparation of pollucite ceramic matrices as 137Cs ionizing radiation source by spark plasma sintering

The paper proposes an original one-stage method using spark plasma sintering technology (SPS) of manufacturing the ionizing radiation source (IRS) of closed type with a non-dispersible ceramic core based on pollucite obtained using aluminosilicate raw materials saturated (24.3 wt%) with cesium, and doped with 10–40 wt% CsCl. According to the XRD, EDX, AAS and DTA-TG data, optimal conditions for hydrothermal synthesis zeolite NaA were established, the consolidation kinetics and phase transformations into ceramic pollucite under SPS conditions were studied. The ceramics had a relative density of 99.8 % and mechanical strength up to 732 MPa, a low rate of cesium leaching 10−7 g/cm2 day and thermal resistance in air up to 1000 °C. It was confirmed by SEM and EDX that the destruction and deformation of pollucite ceramics in the composition with CsCl and steel at the boundary of their contact in the design of the IRS product was absent and diffusion of cesium outside the ceramics did not occur. The achieved cesium content in the ceramic core of the resulting the ionizing radiation source, which meets high quality and regulatory requirements, is 44.3 wt%. The results of the study may be promising for the manufacture of industrial products.

Solidification of cesium containing magnetic zeolite sorbent by spark plasma sintering

The possibility of effective solidification of spent sorbents containing radioactive cesium by spark plasma sintering (SPS) technology for their safe and long-term disposal was studied. The kinetics of consolidation spent sorbent based on magnetic zeolite after adsorption saturation with cesium (∼23 wt %) under SPS conditions in the range of 700–950 °C was carried out. The structural, elemental and phase composition changes during high-rate (minutes) heating of zeolite powder saturated with Cs+ ions was studied by SEM, EDX and XRD methods. As a result, ceramic samples with mineral-like structure of Fe-substituted pollucite CsFeSi2O6 were successfully obtained. It was determined that the optimal temperature of SPS consolidation was 900 °C, at which the formation of CsFeSi2O6 ceramics with a relative density of 91.2%, compressive strength of 497.6 MPa, Vickers microhardness of 1019 HV, rate of cesium leaching of 2.33 × 10−8 g/cm2 × day was reached. It was found that the mechanism of cesium diffusion from the volume of ceramics corresponded to flushing from the available surface of the sample, the lowest diffusion coefficient was 2.71 × 10−16 cm2/s, as well as the lowest penetration depth of the solution into the volume of ceramics was 1.04 × 10−5 cm. The adsorbents prepared by SPS up to 900 °C retained the magnetization, and at higher temperature the magnetization decreased, which was caused by phase and structural changes of ceramics. The characteristics of ceramic obtained from spent adsorbent complied with the requirements of GOST R 50926–96 and ISO 6961:1982 for solidified Cs-containing radioactive waste.

Vitrification of a Simulator of Vat Residues from Liquid Radioactive Waste

The study on the posibility of the use of the optimal glass compositions for vitrification of an imitator of vat residues of liquid radioactive waste from nuclear power plants with VVER-1000 reactors was carried out. The main process parameters such as vitrification temperature, strength, corrosion resistance, absence of crystalline phases, minimization of glass-forming additives and inclusion the maximum amount of waste were analyzed. It has been established that the melting temperature of lead-borosilicate glass matrices was 1150 °C, which satisfies the requirements for vitrification of low- and medium-level waste. The ultimate compressive strength of the obtained samples of glass matrices was 136.0 MPa. In addition, it has been shown that lead-borosilicate glass matrices are the most resistant to leaching. The cesium leaching rate was 1.5·10-5 g/cm2·day.

Optimization of the Solidification Method of High-Level Waste for Increasing the Thermal Stability of the Magnesium Potassium Phosphate Compound

The key task in the solidification of high-level waste (HLW) into a magnesium potassium phosphate (MPP) compound is the immobilization of mobile cesium isotopes, the activity of which provides the main contribution to the total HLW activity. In addition, the obtained compound containing heat-generating radionuclides can be significantly heated, which increases the necessity of its thermal stability. The current work is aimed at assessing the impact of various methodological approaches to HLW solidification on the thermal stability of the MPP compound, which is evaluated by the mechanical strength of the compound and its resistance to cesium leaching. High-salt surrogate HLW solution (S-HLW) used in the investigation was prepared for solidification by adding sorbents of various types binding at least 93% of 137Cs: ferrocyanide K-Ni (FKN), natural zeolite (NZ), synthetic zeolite Na-mordenite (MOR), and silicotungstic acid (STA). Prepared S-HLW was solidified into the MPP compound. Wollastonite (W) and NZ as fillers were added to the compound composition in the case of using FKN and STA, respectively. It was found that heat treatment up to 450 °C of the compound containing FKN and W (MPP-FKN-W) almost did not affect its compressive strength (about 12–19 МPa), and it led to a decrease of high compressive strength (40–50 MPa) of the compounds containing NZ, MOR, and STA (MPP-NZ, MPP-MOR, and MPP-STA-NZ, respectively) by an average of 2–3 times. It was shown that the differential leaching rate of 137Cs on the 28th day from MPP-FKN-W after heating to 250 °C was 5.3 × 10−6 g/(cm2∙day), however, at a higher temperature, it increased by 20 and more times. The differential leaching rate of 137Cs from MPP-NZ, MPP-MOR, and MPP-STA-NZ had values of (2.9–11) × 10−5 g/(cm2∙day), while the dependence on the heat treatment temperature of the compound was negligible.

Nanocomposite SiEA-KNiFe sorbent — Complete solution from synthesis through radiocesium sorption to vitrification using the sol–gel method

This study presents a novel complete solution starting with a synthesis of silica modified with potassium-nickel hexacyanoferrate and ethanolamine (SiEA-KNiFe) sorbent through radiocesium sorption in different process configurations and moving on to the vitrification of the spent sorbent, using the sol–gel method. The experimental data for deionized water solution, as well as seawater solution, correlates strongly with the Langmuir isotherm model. Moreover, the study also presents a method for spent sorbent solidification in the glass matrix. The cesium leaching test confirmed that spent sorbent can be stably bound in the glass matrix after radionuclide removal.

INFLUENCE OF WEATHERING OF BOTTOM ASH ON THE LEACHING BEHAVIOR OF CESIUM

After nuclear accident in 2011, incineration residue contaminated by radioactive cesium had been produced. Government decided in 2011 that incineration residue with less than 8000 Bq/kg of radioactivity could be disposed of into conventional MSW landfill. Since radioactivity of bottom ash was reported to be low compared with fly ash, it had been disposed of MSW landfill based on the government regulation. However, long-term leaching behavior of cesium contained in bottom ash when being disposed of into landfill was not known. Thus, in this study, phase transformation and leaching of cesium from incineration bottom ash were investigated by conducting accelerating weathering experiment. Four kinds of weathering condition were set; namely, blank (under nitrogen gas condition), wetting and drying, freezing and thawing, and exposure to carbon dioxide gas. Leaching of cesium under blank condition didn't change. On the other hand, drastic decrease of leaching of cesium was identified under carbon dioxide exposure. From XRD analysis, formation of calcite was identified. It was deemed that newly formed calcite inhibited the leaching of Cs by covering reactive surface of bottom ash. In both wetting-drying and freezing-thawing condition, leaching of Cs increased initially. But it decreased after several weeks then it became lower than blank finally. This decrease of leaching also seems to be caused by formation of calcite. Besides, in order to confirm the assumption in which the restrain of cesium leaching occurred by calcite formation, SEM-EDS analysis was performed. By the analysis, calcite formation on the surface was clearly identified.

A polyphase geoceramic matrix for joint immobilization of the strontium-cesium and rare earth fractions of high-level waste

Polyphase geoceramic matrices of the (silico)phosphate type for joint immobilization of the strontium-cesium and rare earth fractions of high-level waste with residual actinide content were synthesized. The charge components were iron oxide, technical-grade phosphoric acid, and apatite ore dressing tails. The synthesis method involves fine dispersion of the starting materials and two-step ceramization at 920 and 910°C for 20 min. The total content of the radionuclides in the matrix is on the level of 15 wt %. The cesium leach rate under the conditions of kinetic control (at 90°C) is 5 × 10−7 g cm−2 day−1.

Immobilization of cesium by zirconium phosphate

Mixtures of cesium nitrate and ammonium-ion type zirconium phosphate, NH4Zr2(PO4)3, with varying molar ratios were thermally treated at 600 or 750 °C, yielding partial cesium-ion type zirconium phosphates, CsxH1−xZr2(PO4)3 (x = 0.4, 0.6, 0.8). Immobilization of cesium on the CsxH1−xZr2(PO4)3 was examined. Leaching tests at 90 °C using pseudo-seawater as leachant revealed that the cesium leach rates of the materials with x = 0.4 and 0.6 were in the order of 10−2 g m−2 day−1, smaller than those reported for borosilicate glasses, the generally accepted first-generation waste form. This indicated the materials examined were better cesium immobilizers than borosilicate glasses.

Polymer-inorganic coatings containing nanosized sorbents selective to radionuclides. 1. Latex/cobalt hexacyanoferrate(II) composites for cesium fixation.

Here we present a new approach to improve fixation of radionuclides on contaminated surfaces and eliminate their migration after nuclear accidents. The approach consists in fabrication of latex composite coatings, which combine properties of polymeric dust-suppressors preventing radionuclides migration with aerosols and selective inorganic sorbents blocking radionuclides leaching under contact with ground waters and atmospheric precipitates. Latex/cobalt hexacyanoferrate(II) (CoHCF) composites selective to cesium radionuclides were synthesized via "in situ" growth of CoHCF crystal on the surface of carboxylic or amino latexes using surface functional groups as ion-exchange centers for binding precursor ions Co(2+) and [Fe(CN)6](4-). Casting such composite dispersions with variable content of CoHCF on (137)Cs-contaminated sand has yielded protective coatings, which reduced cesium leaching to 0.4% compared to 70% leaching through original latex coatings. (137)Cs migration from the sand surface was efficiently minimized when the volume fraction of CoHCF in the composite film was as low as 0.46-1.7%.

Synthesis and leachability study of a new cesium immobilized langbeinite phosphate: KCsFeZrP3O12

A potassium cesium iron zirconium phosphate, KCsFeZrP3O12, was synthesized by simple solution method for the first time. Powder X-ray diffraction analysis confirmed that the compound crystallizes in cubic langbeinite structure (space group: P213) with the cell parameters of a=10.103 (2)Å and V=1031Å3. Fourier transform infrared spectrum showed the stretching and bending vibrational bands of phosphate tetrahedra. Energy dispersive X-ray analysis proved the presence of elements. Leachability of a powder specimen was studied by MCC-5 test for a period of one month. Inductive coupled plasma and atomic absorption spectroscopic results revealed the negligible leaching of cesium. The normalized mass loss of potassium, iron, zirconium and phosphorous were found to be in the order of 10−4–10−3g/m2, 10−6–10−4g/m2, 10−5–10−3g/m2and 10−4–10−3g/m2 respectively. For comparison, a cesium zirconium phosphate (CsZr2P3O12) with the sodium zirconium phosphate structure was synthesized, and its leach rates of zirconium and phosphorous were comparable to those of the langbeinite phase. The cesium leach rate was negligible for both compounds.

Iron pyrophosphate matrix as a form for immobilization of cesium radionuclides

Several modifications of CsFeP2O7 were prepared by fine dispersion procedure: glass, glass ceramic, and finely crystalline composite. The cesium leach rates from these CsFe pyrophosphate forms at 90°C were determined. The pyrophosphate glass shows promise as a form for isolation of fission products in a deep underground repository.

Synthesis of nanostructured silica via sol–gel process with incorporation of cesium compound

Cesium was incorporated into silica matrix by direct mixture of cesium chloride into the casting solution formed by water and tetramethylorthosilicate. The casting solutions were prepared with cesium content ranging from 0.035 to 4.9 wt.%. For structural characterization the techniques of nitrogen sorption, mercury pycnometry, thermogravimetric analysis (TGA), differential thermal analysis (DTA) and X-ray diffraction were employed. Specific surface area, pore-size distribution and bulk density of the gels were obtained by nitrogen sorption (BET) and mercury pycnometry. The pore morphology was inferred through curves of nitrogen-sorption hysteresis. After drying and heat treatment, the gels obtained with hydrofluoric (HF) as catalyst remained unbroken. The specific surface area of the silica gels decreases in the presence of cesium. The shape of curves of nitrogen-sorption hysteresis did not change with the cesium content studied. The presence of cesium makes the pore-size distribution narrower. Gels obtained with 1.1 wt.% Cs (content in casting solution) and treated at two dwell temperature (500+800°C) show the indication of a lower cesium leaching.

Radiocesium removal from high level liquid waste and immobilisation in sodium silicotitanate for geological disposal

The isolation of the fission product cesium from high-level alkaline liquid waste is studied with inorganic ion-exchange materials. High separation and concentration factors can be obtained using sodium silicotitanate (CST). The leaching behaviour of cesium from CST was studied in clay porewater and Quinary brine at 90 °C. It is shown that cesium leaching in Quinary brine can be drastically reduced by thermal treatment of cesium loaded CST above 1000 °C. To this end, neutralisation of the ion exchange material before heating is required in order to reduce cesium volatility. It is concluded that CST is a good candidate material for geological disposal of the fission product cesium.

Improvement of Cs leaching resistance of solidified radwastes with copper ferrocyanide (CFC)-vermiculite

Cesium removal from de-ionized water, seawater, and limewater using copper ferrocyanide (CFC) and porous media including silica gel, bentonite, vermiculite, and zeolite as adsorbents were investigated; CFC was incorporated with vermiculite to prepare a compound adsorbent for improving the Cs-leaching resistance of solidified borate radwastes. It was shown that the Cs-removal efficiency by CFC, defined as the percentage of cesium removed or adsorpted from solution, was largely affected by pHs of the solutions. Good removal efficiency occurred at pHs ranging from 3 to 12 with the best from 7 to 10. Vermiculite and zeolite were shown to have better removal power than silica gel and bentonite, and vermiculite was chosen to incorporate with CFC to make compound adsorbents because of its good compatibility with CFC floc. Compound adsorbents with different CFC contents were used as additives in the solidification of radioactive borate wastes for improving the cesium leaching resistance of the solidified products. Experimental results showed that the cesium leachability index measured following the method described in ANSI/ANS 16.1 increased from 7.96 to 9.76 by adding 0.25% of a compound adsorbent containing 20% CFC and 80% vermiculite. It indicated that the compound adsorbent is very useful for improving cesium-leaching resistance of the solidified borate wastes.

Overview of the French Program on Leaching Various Waste Forms

Performing leach tests provides quality assurance and verification that radioactive waste packages are acceptable for final shallow land burial, in accordance with environmental policy. An overview assessment was completed on the leaching tests and on the leachability of various waste forms, regarding both the industrial and the research and development aspects of the leachability and illustrated by several examples. The most important aspects extracted from this overview are as follows: (1) Leaching tests must be performed on all types of package candidates for an ultimate near-surface disposal whatever the waste form. (2) Representativeness of leached specimens remains to be fully proved, especially when they are small samples. (3) Quality of the waste packages whose type has been accepted should be periodically audited to establish deviations from the quality agreed on. (4) An exhaustive material balance of the released activity in leachates and fixed on surfaces must be established when performing leaching tests, especially for alpha emitters that can become attached to surfaces in the leaching loops. (5) Influence of temperature and pressure on the leaching results have been identified as important factors for cesium leaching of cemented wastes. (6) More generally, there has been good progress in leaching knowledge, butmore » the influence of many parameters, such as durability effect on leaching, remains incompletely defined.« less

The Structure and Properties of Two New Silicotitanate Zeolites

ABSTRACTTwo new zeolitic crystalline phases with stoichiometry, CSTiSi2O6.5 and CS2TiSi6O15, have been discovered. CSTiSi2O6.5 has a crystal structure isomorphous to the mineral pollucite, CsAlSi2O6, with Ti+4 replacing Al+3. This replacement requires a mechanism for charge compensation. A combination of techniques including neutron diffraction, single crystal x-ray diffraction and x-ray absorption spectroscopy have revealed that eight extra oxygens are present per unit cell Cs2TiSi2O6.5 as compared to pollucite. As a result of the extra oxygen, the titanium coordination geometry is five-fold. Pentacoordinate titanium and tetrahedral silicon form a network structure with Cs residing in cages formed by the network. The crystal structure of Cs2TiSi6O15 is unique, with titanium octahedra and silicon tetrahedra forming an open framework structure with the Cs residing in large cavities. The largest covalently bonded ring opening to the Cs cavities in both compounds are smaller than a Cs ion, revealing that the Cs ion has minimal mobility in the structure. Cesium leach rates for both compounds are lower than or comparable to borosilicate glass.

α‐Decay Damage Effects in Curium‐Doped Titanate Ceramic Containing Sodium‐Free High‐Level Nuclear Waste

A polyphase titanate ceramic incorporating sodium‐free simulated high‐level nuclear waste was doped with 0.91 wt% of 244Cm to accelerate the effects of long‐term self‐irradiation arising from α decays. The ceramic included three main constituent minerals: hollandite, perovskite, and zirconolite, with some minor phases. Although hollandite showed the broadening of its X‐ray diffraction lines and small lattice parameter changes during damage ingrowth, the unit cell was substantially unaltered. Perovskite and zirconolite, which are the primary hosts of curium, showed 2.7% and 2.6% expansions, respectively, of their unit cell volumes after a dose of 12 × 1017α decays.g‐1 Volume swelling due to damage ingrowth caused an exponential (almost linear) decrease in density, which reached 1.7% after a dose of 12.4 × 1017α decays.g‐1. Leach tests on samples that had incurred doses of 2.0 × 1017 and 4.5 × 1017 a decays g‐1 showed that the rates of dissolution of cesium and barium were similar to analogous leach rates from the equivalent cold ceramic, while strontium and calcium leach rates were 2–15 times higher. Although the curium, molybdenum, strontium, and calcium leach fates in the present material were similar to those in the curium‐doped sodium‐bearing titanate ceramic reported previously, the cesium leach rate was 3–8 times lower.

Continuous-Flow Leaching Studies of Crushed and Cored SYNROC

Both crushed (150- to 300- ..mu..m) and cored (1,8- mm-diam) samples of SYNROC have been leached with single-pass continuous-flow leaching equipment. Crushed samples of cesium-hollandite were also leached in a similar experiment. Temperatures used were 25/sup 0/ and 75/sup 0/C and leachates were 0.03 N NaHCO/sub 3/ and distilled water. Leaching rates from SYNROC-C were ranked cesium > strontium greater than or equal to calcium > barium > zirconium. A comparison of leaching rates is made between crushed SYNROC, cored SYNROC, and Pacific Northwest Laboratory 76-68 glass beads. This comparison depends on how the surface areas are determined for each sample. Based on geometric surface areas for SYNROC cores and glass beads, cesium leach rates from SYNROC compare well with both sodium and neptunium leached from the glass. The other elements leached from SYNROC are lower than sodium and neptunium leached from glass. They also vary for each element, while glass shows nearly the same leach rate for both sodium and neptunium.

An Experimental Comparison of Alternative Solid Forms for Savannah River High-Level Wastes

ABSTRACTSamples of borosilicate glass, high-silica glass, tailored ceramic, and SYNROC, incorporating simulated Savannah River high-level defense waste sludges, were leached by the MCC-1 procedure for times up to 28 days. Cesium, uranium, and cerium leach rates are reported for waste forms containing a composite sludge, at 40°C in deionized water, and at 90°C in deionized water, silicate water, and brine. The ordering of the waste forms from best to worst differs for each element leached, and none of the forms show a clear advantage for all the key radwaste elements. Some cesium leach rates for forms containing high-aluminum or high-iron sludges also are presented. So far, only small effects of sludge type have been observed, with one exception. This study is one of several inputs for selection of an alternative waste form for Savannah River waste.