Immobilization Of Radionuclides Research Articles

Location of Artinite (Mg2CO3(OH)2·3H2O) within the MgO-CO2-H2O system using ab initio thermodynamics.

The MgO-CO2-H2O system have a variety of important industrial applications including in catalysis, immobilisation of radionuclides and heavy metals, construction, and mineralisation and permanent storage of anthropogenic CO2. Here, we develop a computational approach to generate phase stability plots for the MgO-CO2-H2O system that do not rely on traditional experimental corrections for the solid phases. We compare the predictions made by several dispersion-corrected density-functional theory schemes, and we include the temperature-dependent Gibbs free energy through the quasi-harmonic approximation. We locate the Artinite phase (Mg2CO3(OH)2·3H2O) within the MgO-CO2-H2O phase stability plot, and we demonstrate that this widely-overlooked hydrated and carbonated phase is metastable and can be stabilised by inhibiting the formation of fully-carbonated stable phases. Similar considerations may apply more broadly to other lesser known phases. These findings provide new insight to explain conflicting results from experimental studies, and demonstrate how this phase can potentially be stabilised by optimising the synthesis conditions.

Immobilization of radioactive waste by an aluminum silicate matrix formed from fly ash or bentonite

The decommissioning and recovery of the Fukushima Daiichi Nuclear Power Plant will generate a substantial amount of radioactive waste. Among these wastes, fuel debris is expected to be designated as an intermediate-to-high-level waste that needs to be immobilized within a matrix. The immobilization of radioactive waste in an aluminum silicate matrix formed from fly ash or bentonite was investigated in this study. Three matrix compositions were selected: bentonite 100 wt.%, fly ash 90 wt.% - sodium tetraborate decahydrate (borax) 10 wt.%, and fly ash 80 wt.% - bentonite 20 wt.%. The radioactive waste was simulated using uranium dioxide doped with Sr and Eu. The pellets were formed using 70 wt.% matrix material and 30 wt.% radioactive waste, and were heated at 600°C, 800°C, and 1000°C in a nitrogen atmosphere. This study aims to determine the best matrices and conditions for radionuclide immobilization. All pellets exhibited a high compressive strength of several dozen megapascals, which met the criteria of a minimum of 5 MPa. Matrices that showed the highest resistance to the leaching of U were the fly ash-borax matrix heated at 600°C, 800°C, and 1000°C; fly ash-bentonite matrix heated at 800°C; and bentonite matrix heated at 800°C and 1000°C. Matrices that showed good resistance to both U and Sr leaching were the fly ash-borax matrix heated at 1000°C and the bentonite matrix heated at 1000°C. Eu leached out in very small amounts. U was immobilized by incorporating uranium dioxide particles in the aluminum silicate glassy melt formed during heating. The physical absorption of the waste also contributed to reducing Sr leaching. The fixation of Sr on the surface of fly ash also occurred in matrices containing fly ash.

Medium-Temperature Phosphate Glass Composite Material as a Matrix for the Immobilization of High-Level Waste Containing Volatile Radionuclides

The search for matrices and technological solutions for the reliable immobilization of volatile radionuclides and high-level waste (HLW) components is an actual radiochemical problem. Methods of obtaining of sodium alumino-iron phosphate (NAFP) and iron phosphate (FP) glass composite materials synthesized at temperatures of 450–750 °C, their structure and hydrolytic stability were investigated in this paper. The structure of the samples was studied by XRD and SEM-EDS. It was shown that, in the case of FP materials, the phase composition varies depending on the synthesis temperature, while NAFP materials have a complex multiphase composition at all crystallization temperatures. It has been established that the samples of the obtained glass composite materials have a high hydrolytic stability. At the same time, FP material obtained at 650 °C are the most stable, which makes this medium-temperature method of synthesis promising for the immobilization of volatile HLW components.

Composite metal phosphates for selective adsorption and immobilization of cesium, strontium, and cobalt radionuclides in ceramic matrices

Zr–Ca–Mg composite phosphates of various chemical compositions were prepared by facile heterogeneous synthesis and their adsorption properties towards 137Cs, 85,90Sr, 60Co radionuclides were studied. The obtained composites showed a high efficiency for 137Cs, 85,90Sr, and 60Co adsorption (Kd > 104 mL/g) from model 0.1M NaNO3 aqueous solution. A sharp decrease in adsorption properties was found against 0.01M CaCl2 and seawater background, due to a decrease in the adsorbents selectivity. Zr–Ca–Mg phosphates exhibited high efficiency of radionuclides immobilization during metal ions leaching in water (desorption efficiency ∼10%). At the same time, the desorption rate of radionuclides in seawater solutions increased by 4–8 times. Calcination of saturated adsorbents at 1000 °C led significantly to increase in the efficiency of radionuclides immobilization. Thus, the activity of 60Со and 85Sr radionuclides in seawater solutions after exposure for 28 days did not exceed the lower detection limit, and the desorption efficiency of 137Cs was less than 4%. This was due to the formation of mixed Cо0.5Zr0.5(PO4), SrZr4(PO4)6 phosphates, characterized by high efficiency of radionuclides immobilization. The obtained Zr–Ca–Mg composite phosphates are promising materials for the adsorption and immobilization of 137Cs, 85,90Sr, and 60Co radionuclides.

Synthesis and spark plasma sintering of solid-state matrices based on calcium silicate for 60Co immobilization

An effective sorption material for the immobilization of cobalt radionuclides into highly safe and reliable solid-state matrices is proposed. The resulting silicate sorbent СaSiO 3 had an amorphous mesoporous structure (A BET 53 m 2 /g) and a sorption capacity Co ions of 3.32 mmol/g. The physico-chemical characteristics of the СaCoSi 2 O 6 sample obtained after Co 2+ ions sorption were studied using XRD, N 2 and Ar adsorption-desorption, SEM-EDX and TG/DTA methods. Solid-state silicate matrices characterized by high density values (2.86–3.16 g/cm 3 ), compressive strength (150–637 MPa) and Vickers microhardness (1.80–5.25 GPa) were obtained by spark plasma sintering (SPS). The sample obtained at 1000 °C had the lowest values of Co 2+ ions leaching (R Co ~10 −7 g/(cm 2 ×day)) and diffusion coefficient (De 1.73 ×10 −17 cm 2 /s) from silicate matrices. Thus, the obtained СaCoSi 2 O 6 silicate matrices saturated with Co ions comply with the regulatory requirements of GOST R 50926–96 and ANSI/ANS 16.1 for 60 Co immobilization. • Amorphous CaSiO 3 with 3.32 mmol/g Co ions sorption capacity was prepared. • SPS method allowed to obtain the CaCoSi 2 O 6 matrices at 1000 °C for 2 min. • CaCoSi 2 O 6 solid-state matrices exclude Co 60 ions leaching and diffusion. • The characteristics of CaCoSi 2 O 6 matrices exceeded the GOST and ANSI/ANS 16.1 requirements.

Reaction synthesis of SrTiO3 mineral-like ceramics for strontium-90 immobilization via additional in-situ synchrotron studies

The paper presents spark plasma sintering-reaction synthesis (SPS-RS) of SrTiO3-based mineral-like ceramics with a perovskite structure, which is promising for immobilization of Sr-90 radionuclides. Detailed time-resolved study of phase transformations taking place in the reactive mixture (SrCO3 and TiO2) within 20–1000 °C temperature range was conducted using both in situ heating synchrotron XRD and TGA. Structure and composition dependence on consolidation temperature was revealed by the means of SEM and EDX. We determined optimal temperature conditions for rapid formation of SrTiO3 ceramics with density – 4.49 g cm−3, Vickers hardness – up to 6.2 GPa, compressive strength – 279 MPa, and strontium leaching rate of 10−5–10−6 g cm−2·day. These results clearly show strong applied potential of the presented material for radioactive waste management and isotope production fields.

Transformation of inherent microorganisms in Wyoming-type bentonite and their effects on structural iron

Bentonite is one of the materials used to construct engineered barriers in high-level radioactive nuclear waste geological disposal with its many advantageous features such as low hydraulic conductivity, self-sealing ability, durability, adsorption and immobilization of metals and radionuclides and reduction of microbial activity. Many of these properties are linked with the bentonite swelling capability. Transformations of indigenous microorganism communities from Wyoming-type bentonite and the Finnish repository site groundwater and their effects on the bentonite structural iron over five years were studied in repository relevant anoxic and oxic slurry conditions. Active sulfate reduction (0.06 nmol mL−1 day−1) was detected in the anoxic microcosm waters after a year, however after two years sulfate reduction was not active anymore. Microbial numbers determined by quantitative PCR in the bentonite slurry of both experiment types supported the finding of decrease of overall microbial activity after a year of incubation that was not maintained anymore by the dissolving organic carbon from the bentonite. Regular electron donor additions (final concentration of 2 mM for formate and acetate each, three times per year) activated the microbiome resulting in increasing numbers of bacterial 16S rRNA gene copies and sulfate reducers (dsrB gene copies) as well as detection of sulfide in the water phase of both experiment types. After 4.9 years the structural iron in the fine portion of the montmorillonite had become completely reduced in all microbial microcosms and minor smectite illitization was detected especially in anoxic microcosms. Dominating bacterial groups at the end of the experiment were mainly known sulfur/sulfate and iron reducers. Archaea and fungi constituted a minor part of the microbiomes. In originally oxic microcosms, the bacterial 16S RNA and dsrB gene copy numbers were lower than in the anoxic experiment but started to significantly increase after the electron donor additions. Microorganisms originating from the repository environment could reduce the bentonite structural iron in a few years to an extent likely to affect the bentonite swelling ability if sufficient amounts of suitable electron donors are available in localized areas where bentonite is not at high density and pressure in the geological disposal.

Adsorption behaviour of simulant radionuclide cations and anions in metakaolin-based geopolymer

Geopolymers are a class of alkaline-activated materials that have been considered as promising materials for radioactive waste disposal. Currently, metakaolin-based geopolymers (MK-GPs) are attracting interest for the immobilisation of radionuclides in contaminated water from the Fukushima Daiichi Nuclear Power Station. However, the associated chemical interaction mechanisms and the theoretical prediction of the adsorption behaviour of MK-GP in response to cationic radionuclides have not been thoroughly studied or fully understood. In addition, there is a lack of studies on the adsorption capacity of MK-GP for anionic radionuclides. In this study, two types of metakaolin-based (Metastar501 and Sobueclay) geopolymers were synthesised at a K2O:SiO2:H2O ratio of 1:1:13. The binding capacity and interaction mechanism of MK-GP with Cs+, Sr2+, Co2+, I-, IO3-, SeO32-, and SeO42- were evaluated based on the zeta potential, radionuclide binding, and alkali leaching. The results showed that MK-GP does not have the ability to incorporate anionic radionuclides irrespective of the metakaolin source used, but both types of geopolymers have a high capacity to immobilise cationic radionuclides. The uptake of Cs+ was observed as a one-to-one exchange between Cs+ and K+ whereas both one–two and one–one ion exchanges are possible in the case of Sr2+ and Co2+ with K+. The formation of cobalt blue (CoAl2O4) also contributed to the binding of Co2+. Thermodynamic modelling was conducted according to the ion exchange mechanism which predicts the binding of Cs+ and Sr2+ at low concentrations.

Simultaneous immobilization of radionuclides Sr and Cs by sodium zirconium phosphate type ceramics and its chemical durability

Sodium zirconium phosphate (NaZr2(PO4)3, NZP) type phosphate compounds have been considered as a candidate material for the immobilization of radionuclides. In this work, the highly densified NZP-type ceramic waste forms for immobilizing simulated radionuclides Sr and Cs, which were designed as the formula of Cs1-2xSrxZr2(PO4)3 (x = 0, 1/12, 2/12, 3/12, 4/12, 5/12, and 6/12), were prepared by microwave-assisted solid-state sintering method. The effects of Sr and Cs incorporation on the phase composition, microstructure, densification, and chemical durability of Cs1-2xSrxZr2(PO4)3 ceramic waste forms were systematically discussed. It was shown that the single CsZr2(PO4)3 (CsZP) phase was generated in the samples when x ≤ 2/12, while two phases of CsZP and Sr0.5Zr2(PO4)3 (SrZP) were formed when 3/12 ≤ x ≤ 5/12. The Rietveld refinement results revealed that Sr/Cs could be incorporated in the NZP crystal structure. The as-prepared samples all presented a well dense microstructure, whose relative density reached up to approximately 98% with Sr incorporation. In addition, the Product Consistency Test (PCT) leaching results demonstrated that the ceramics waste forms simultaneously immobilizing Sr and Cs exhibited superior leaching resistance, and the leaching rates of Sr and Cs elements were in the order of 10−3-10−4 g m−2 d−1. The increase of Sr incorporation brought about the decreased leaching rates of ceramic samples.

Effect of alkali activation on coal fly ash and its role in microwave-sintered ceramic for radionuclide immobilization

Considerable amounts of coal fly ash (CFA) are produced annually without adequate resource utilization. Given the abundance of silica and alumina in CFA, as well as its established reactivity when subjected to an alkali medium, alkali-activated CFA was introduced to prepare ceramics for treating neodymium-contaminated soil. The effect of alkali activation on the phase, microstructure, and elemental distribution of the CFA particles and microwave-sintered ceramics was systematically investigated, and crude CFA was used for comparison. The results indicated that the alkali activation of CFA led to a desilication reaction, and new phases rich in sodium were generated. Additionally, the alkali-activation of CFA was conducive to enhancing the mechanical properties of the ceramics. The alkali-activated CFA bearing 30 wt% Nd2O3, which was microwave sintered at 1200 °C, exhibited the highest bulk density (2.79 g/cm3), while that of untreated CFA was only 2.57 g/cm3. Moreover, the Nd ions were homogeneously distributed in the Ca2Nd8(SiO4)6O2 matrix.

Preparation of ceramic matrices based on calcium silicate by spark plasma sintering for immobilization of cobalt-60

Calcium silicates are a special class of inorganic materials, promising for sorption and immobilization of long-lived radionuclides. A number of domestic and foreign studies pay attention to the sorption properties of amorphous and crystalline calcium silicates nCaO∙mSiO2.

Evaluation of sorption test of iodide on carbon nanotubes to support anionic radionuclide immobilisation method

Evaluation of sorption test of iodide on carbon nanotubes to support anionic radionuclide immobilisation method

Advances in immobilization of radionuclide wastes by alkali activated cement and related materials

Nuclear power is seen a significant part of new energy industry because of the low carbon emission feature and high efficiency. Safe and reliable immobilization of radionuclide wastes from nuclear power station is important for its operation. When the Portland cement-based solidifying materials encounter unsatisfying acid resistance, alkali activated cements (AACs) emerge as more durable, low-cost and environmentally friendly options. This paper provides an overview of recent advances of research on immobilization of radionuclide wastes by AAC and related materials. It discusses mechanisms and efficiency of different AACs on the immobilization of low- and intermedium-level radionuclide wastes, representatively the simulating radionuclides strontium and cesium. Immobilization of other radionuclides, such as 60Co, 113Cd and 129I, by AACs is also reviewed. It is agreed that the immobilization efficiency of radioactive nuclides in AACs is generally better than that in Portland cement-based materials. Technical challenges and future research and development are proposed.

An exploration of benchtop X‐ray emission spectroscopy for precise characterization of the sulfur redox state in cementitious materials

The evolution of sulfur chemistry in cements is best known in the bailiwick of failure mechanisms via sulfate attack, but is equally important for its contributions to the reduction capacity of cementitious materials often used for immobilizing nuclear waste streams destined for long‐term storage, for example, cementitious waste forms (CWF). The total reduction capacity of CWFs, encompassing contributions from both S and Fe reductants, and its implications toward radionuclide immobilization is most often studied by destructive wet chemistry methods requiring acid digestion in the presence of Ce(IV) and subsequent titration and colorimetric interpretation. Here, we investigate a similarly analytical but nondestructive alternative, benchtop high resolution wavelength‐dispersive X‐ray fluorescence spectroscopy, most commonly known as X‐ray emission spectroscopy (XES), for probing the bulk sulfur oxidation state distribution. We present here an initial investigation of S XES, including an improved experimental protocol for lab XES of inhomogeneous samples, both as a complement to the Ce(IV) test and for new scientific opportunities that it enables for observing changes in sulfur chemistry. We discuss future improvements and opportunities, including: (1) the practical challenges associated with coordinating XES and Ce(IV) liquid extraction for a more comprehensive perspective on reduction capacity and for a high‐precision evaluation of uncertainties in the Ce(IV) test; and (2) new opportunities, due to the nondestructive nature of XES, for controlled evolution studies aimed at elucidating specific chemical responses of CWFs exposed to invasive gas or liquid species or to accelerated aging by radiative dose or thermal treatment.

Graphene oxide-mediated the reduction of U(VI), Re(VII), Se(VI) and Se(IV) by Fe(II) in aqueous solutions investigated via combined batch, DFT calculation and spectroscopic approaches

Although thermodynamically feasible, reduction of U(VI), Re(VII), Se(VI) and Se(IV) in homogeneous Fe(II) solution could be hardly observed. Whereas, surface-mediated reduction by Fe(II) has been generally regarded as a major pathway for the immobilization of these radionuclides. In this paper, graphene oxide (GO) was firstly revealed to mediate the reduction of U(VI), Re(VII), Se(VI) and Se(IV) by aqueous Fe(II) via a combined batch, DFT calculation and spectroscopic investigation. The kinetics for all adsorption systems could be fitted by the pseudo-second-order model, which was indicative of a chemical interaction. The isotherms for all reaction systems could be described by the Freundlich model better than that by the Langmuir model. Spectroscopic studies indicated that the enhancement effects of GO were attributable to the facilitated electron transfer by the graphitic surfaces and particularly to the decreased Fe(III)-Fe(II) redox potential by surface adsorption of Fe(II) with O-bearing functional groups on GO. Additionally, the presence of coexisting dissolved fulvic acid (FA) could significantly promoted GO-mediated reduction, by enhancing the electron shuttle ability of GO. Therefore, owning to the combined strong mediation efficiency and excellent adsorption affinity, GO exhibited an important role in mediating the reductive immobilization of radionuclides in a wide range of redox-stratified environments.

Mechanisms and thermodynamic modelling of iodide sorption on AFm phases

Both, experimental and modelling evidence is presented in this study showing that interlayer anion exchange is the dominant sorption mechanism for iodide (I-) on AFm phases. AFm phases are Ca-Al(Fe) based layered double hydroxides (LDH) known for their large potential for the immobilization of anionic radionuclides, such as dose-relevant iodine-129, emanating from low- and intermediate-level radioactive waste (L/ILW) repositories. Monosulfate, sulfide-AFm, hemicarbonate and monocarbonate are safety-relevant AFm phases, expected to be present in the cementitious near-field of such repositories. Their ability to bind I- was investigated in a series of sorption and co-precipitation experiments. The sorption of I- on different AFm phases was found to depend on the type of the interlayer anion. Sorption Rd values are very similar for monosulfate, sulfide-AFm and hemicarbonate. A slightly higher uptake occurs by AFm phases with a singly charged anion in the interlayer (HS-AFm) as compared to AFm with divalent ions (monosulfate), whereas uptake by hemicarbonate is intermediate. No significant sorption occurs onto monocarbonate. Our derived thermodynamic solid solution models reproduce the experimentally obtained sorption isotherms on HS-AFm, hemicarbonate and monosulfate, indicating that anion exchange in the interlayer is the dominant mechanism and that the contribution of I- electrostatic surface sorption to the overall uptake is negligible.

Inorganic Waste Forms for Efficient Immobilization of Radionuclides

Inorganic Waste Forms for Efficient Immobilization of Radionuclides

Microwave Vitrification of Uranium Tailings: Microstructure and Mechanical Property

In this work, the dense glass matrix of uranium tailings was successfully fabricated via microwave sintering process with Na2CO3 as a sintering aid. The effects of Na2CO3 additive and sintering temperature on the microstructure and mechanical properties of as-prepared solids were systematically investigated. XRD results confirmed the vitrified forms can be achieved at 1200°C within 30 min with 20 wt.% Na2CO3 addition. Importantly, the Na2CO3 additive significantly reduced the firing temperature from 1500°C to 1200°C and promoted densification. FT-IR analysis demonstrated that the main characteristic peaks of the sintered samples were attributed to the vibration of Si-O-Si. Microstructural studies presented the homogeneous distribution of glass phases. The results of mechanical properties of the sintered forms show that bulk density and Vickers hardness increased with increasing Na2CO3 content as well as sintering temperature, and the highest bulk density (2.45 ± 0.01 g/cm3) and Vickers hardness (823 ± 25 HV) were obtained at the temperature of 1300°C with 20 wt.% Na2CO3 addition, the heating rate of 20°C/min, and the soaking time of 30 min. It implied that the combination of microwave sintering with the appropriate addition of Na2CO3 would provide an efficient method for the immobilization of radionuclides in uranium tailings.

Electrode and Electrolyte Materials From Atomistic Simulations: Properties of LixFEPO4 Electrode and Zircon-Based Ionic Conductors

LixFePO4 orthophosphates and fluorite- and pyrochlore-type zirconate materials are widely considered as functional compounds in energy storage devices, either as electrode or solid state electrolyte. These ceramic materials show enhanced cation exchange and anion conductivity properties that makes them attractive for various energy applications. In this contribution we discuss thermodynamic properties of LixFePO4 and yttria-stabilized zirconia compounds, including formation enthalpies, stability, and solubility limits. We found that at ambient conditions LixFePO4 has a large miscibility gap, which is consistent with existing experimental evidence. We show that cubic zirconia becomes stabilized with Y content of ~8%, which is in line with experimental observations. The computed activation energy of 0.92eV and ionic conductivity for oxygen diffusion in yttria-stabilized zirconia are also in line with the measured data, which shows that atomistic modeling can be applied for accurate prediction of key materials properties. We discuss these results with the existing simulation-based data on these materials produced by our group over the last decade. Last, but not least, we discuss similarities of the considered compounds in considering them as materials for energy storage and radiation damage resistant matrices for immobilization of radionuclides.

Biogeochemical Modelling of Uranium Immobilization and Aquifer Remediation Strategies Near NCCP Sludge Storage Facilities

Nitrate is a substance which influences the prevailing redox conditions in groundwater, and in turn the behaviour of U. The study of groundwater in an area with low-level radioactive sludge storage facilities has shown their contamination with sulphate and nitrate anions, uranium, and some associated metals. The uranyl ion content in the most contaminated NO3–Cl–SO4–Na borehole is 2000 times higher (1.58 mg/L) than that in the background water. At the same time, assessment of the main physiological groups of microorganisms showed a maximum number of denitrifying and sulphate-reducing bacteria (e.g., Sulfurimonas) in the water from the same borehole. Biogenic factors of radionuclide immobilization on sandy rocks of upper aquifers have been experimentally investigated. Different reduction rates of NO3−, SO42−, Fe(III) and U(VI) with stimulated microbial activity were dependent on the pollution degree. Moreover, 16S rRNA gene analysis of the microbial community after whey addition revealed a significant decrease in microbial diversity and the activation of nonspecific nitrate-reducing bacteria (genera Rhodococcus and Rhodobacter). The second influential factor can be identified as the formation of microbial biofilms on the sandy loam samples, which has a positive effect on U sorption (an increase in Kd value is up to 35%). As PHREEQC physicochemical modelling numerically confirmed, the third most influential factor that drives U mobility is the biogenic-mediated formation of a sulphide redox buffer. This study brings important information, which helps to assess the long-term stability of U in the environment of radioactive sludge storage facilities.