Nuclear Waste Streams Research Articles

Micrometer-sized magnetite synthesis using Fe(OH)2(s) as a precursor for technetium sequestration from liquid nuclear waste streams

• Transformation of Fe(OH) 2 (s) to micrometer-sized magnetite in waste stream at 75 °C. • Solution pH/temperature/oxidation kinetics play key roles in final mineral products. • An increase in pH or Fe(II) amendment improved tc removal from waste streams. • Tc sequestration as TcO 2 •2H 2 O or through Tc(IV)-magnetite incorporation. • More tc incorporation into magnetite at elevated temperatures and pH conditions. Systematic batch experiments under variable adjusted physicochemical conditions were conducted to explore optimization of micrometer-sized magnetite synthesis for Tc sequestration from radionuclide waste streams using Fe(OH) 2 (s) as the precursor. Extensive solid characterizations using X-ray diffraction, microscopy and spectroscopic methods were performed to identify mineral phases, assess changes in particle morphology and size distribution, as well as Tc speciation and incorporation, in the produced mineral phases. The results show that the solution pH, temperature, and oxidation kinetics play key roles in the final mineral products. Micrometer-sized magnetite crystals (0.62–0.96 µm, on average) with well-defined dodecahedral or octahedral structures were synthesized under near neutral (~pH 8) or alkaline (~pH 13) conditions at 75 °C, respectively; whereas goethite dominated the end products at room temperature. An increase in pH at 75 °C improved Tc removal from 27% (near neutral pH) to 42% (alkaline pH), but the removal process remained inhibited by redox-competitive Cr(VI) present in the waste streams. By adding additional Fe(II) to the system, Tc sequestration was dramatically improved up to 87% without observable changes in the solid product. The sequestrated Tc existed as TcO 2 •2H 2 O and/or Tc(IV) incorporated into magnetite, where extended X-ray absorption fine structure (EXAFS) spectroscopy showed that more Tc was incorporated into magnetite at elevated temperatures and pH conditions, with complete Tc(IV) incorporation into magnetite occurring under 75 °C-pH 13 conditions. Our results indicate that optimal micrometer-sized magnetite can be produced for Tc sequestration by reacting Fe(OH) 2 (s) with a waste stream simulant under elevated pH (~13) and temperature (75 °C) conditions. The incorporation of reduced Tc(IV) into stable micrometer-sized magnetite provides a viable supplemental immobilizing technology that may be used to improve nuclear waste treatment and disposal needs.

Management of radioactive waste from application of radioactive materials and small reactors in non-nuclear industries in Canada and the implications for their new application in the future

<abstract> <p>A large number of artificial-origin radionuclides from irradiation in small reactors and/or nuclear reactions in accelerators are currently used in non-nuclear industries such as education, oil and gas, consumer merchandise, research, and medicine. Radioactive wastes from the use of these radionuclides in non-nuclear industries include expired sealed radioactive sources, biological materials, radionuclide-containing chemicals, contaminated equipment, and very small quantities of used nuclear fuel. Although being less challenging and complex than nuclear energy production and research waste streams, these wastes are subject to the common nuclear regulations by the Canadian Nuclear Safety Commission, and are managed following domestic and international standards and guidelines made by the Canadian Standards Association, International Atomic Energy Agency, and International Organization for Standardization. Management practices used in the nuclear industry in Canada are commonly applied to the non-nuclear industry radioactive waste streams, such as waste handling, treatment, packaging, storage, transportation, clearance and exemptions, and disposal. The half-lives of radionuclides in non‑nuclear applications range from hours to thousands of years, and their activities in non-nuclear industrial applications can be as low as their clearance level or as high as the upper limits for intermediate level radioactive waste. Waste containing only short half-life radionuclides is placed in temporary storage to allow decay, and then is cleared and disposed of through non-radioactive waste routes. Non‑clearable waste materials are treated, consolidated, and managed along with radioactive waste generated from the nuclear industries at designated radioactive waste management sites.</p> </abstract>

Exploring competitive metal binding and crystallization of UO22+ and Cu2+ tetrahydrofuran-2,3,4,5-tetracarboxylic acid complexes

Solvent extractions are used to separate actinide elements from fission products in nuclear waste streams and the successful isolation of specific species utilize subtle differences in metal ligand binding. Metal ligand binding is also important in crystallization of metal organic materials and herein we explore the importance of competitive binding in the tetrahydrofuran-2,3,4,5-tetracarboxylic acid (THFTCA) system. This ligand has been previously evaluated for the selective extraction of uranium from other lanthanides and actinides and in the current study, we evaluate the crystallization of uranyl-THFTCA complexes in the presence of Cu2+, Sr2+, Th4+, and Ce3+. Three major phases were formed in the crystallization experiments and characterized with single crystal X-ray diffraction, powder X-ray diffraction, thermogravimetric analysis, and Raman spectroscopy. Two of the resulting phases were novel (UTHF1 ((C4H10N2)[UO2(C8H8O9)2]∙2H2O) and UTHF2 (Na[(UO2)(C8H5O9)(H2O)]∙ 3.5 H2O)), whereas the third (CuTHF1) was previously reported in the literature. Raman spectroscopy was utilized to evaluate spectral changes in the mother liquor of UTHF1, UTHF2, and CuTHF1 over time to assess the crystallization process. Further, isothermal titration calorimetry was used to determine the binding constants for UO22+ and Cu2+ to the THFTCA ligand in solution and evaluate the role of competitive metal binding in this system. The thermodynamic parameters and the crystallographic data were used to justify the formation of a weaker UO22+-THFTCA complex. Formation of a weaker UO22+-THFTCA complex supports our findings that UTHF1 only forms in homomeric systems, but suggests that the crystallization of UTHF2 and CuTHF1 is reliant on the presence of additional counter ions and ligands to change the amount of available THFTCA ligand.

Energetics of Salt-Bearing Sodalites, Na8Al6Si6O24X2 (X = SO4, ReO4, Cl, I): A Treatment Option for Pertechnetate-Enriched Nuclear Waste Streams

An alternative option for treating anion-enriched reprocessed nuclear waste streams is to immobilize technetium-99 (99Tc, β = 293.7 keV, t1/2 = 2.1 × 105 years) and other anions in micro- and mesop...

Synthesis of N-containing porous aromatic frameworks via Scholl reaction for reversible iodine capture

Abstract The effective capture of radioactive volatile iodine (131I and 129I) from nuclear waste stream is of vital significance. In this work three kinds of porous organic frameworks (POFs) knitting with N-containing building units were constructed via cost-effective and maneuverable Scholl reaction. The obtained PAFs exhibited moderate porosity and satisfying thermal stability. With coexistence of open porous structure, the electron-rich amino group featured with π-π interactions was found to improve the affinity to iodine, and the iodine uptake capacities were 408 wt%, 335 wt% and 268 wt% when the prepared POFs of P-DPDA, P-TPB and P-PC were employed for the adsorption experiments. The results were superior to most of the porous adsorbents including zeolite, metal-organic frameworks (MOFs) and porous organic polymers (POPs). Iodine adsorption capacities of 289.50 mg g−1 (for P-DPDA), 194.63 mg g−1 (for P-TPB) and 259.69 mg g−1 (for P-PC) in organic solution were achieved, and the adsorbed iodine molecules could be released to ethanol. Such promising and cost-effective materials can be envisaged for application in the field of nuclear waste management.

Unprecedented Radiation Resistant Thorium-Binaphthol Metal-Organic Framework.

A thorium-organic framework (TOF-16) containing hexameric secondary building units connected by functionalized binaphthol linkers was synthesized, characterized, and irradiated to probe its radiation resistance. Radiation stability was examined using γ-rays and 5 MeV He2+ ions to simulate α particles. γ-irradiation of TOF-16 to an unprecedented 4 MGy dose resulted in no apparent bulk structural damage visible by X-ray diffraction. To further probe radiation stability, we conducted the first He2+ ion irradiation study of a metal-organic framework (MOF). Diffraction data indicate onset of crystallinity loss upon approximately 15 MGy of irradiation and total loss of crystallinity upon exposure to approximately 25 MGy of He2+ ion irradiation. The high radiation resistance observed suggests MOFs can withstand radiation exposure at doses found in nuclear waste streams and highlights the need for a systematic approach to understand and eventually design frameworks with exceptional radiation resistance.

Surfactant-modified siliceous zeolite Y for pertechnetate remediation

Separation of pertechnetate (99TcO4−) from contaminated groundwater and nuclear waste streams remains a technical challenge for the long-term management of 99TcO4− released to the environment. Herein, a siliceous zeolite Y phases(s) were modified with cationic surfactant and subsequently used to sequester TcO4− from acidic aqueous solution containing competing ions. Pristine and modified zeolite Y phase(s) were characterized with X-ray diffraction, X-ray absorption spectroscopy (XAS), sorption isotherms, zeta potential, and BET surface area measurements. Measured TcO4− sorption capacity of the modified zeolite Y (MSAV) was 337 mg/g, which was in good agreement with modeled estimates, 357 mg/g. The Langmuir and the pseudo-second-order models adequately described the sorption of TcO4−on the modified zeolite Y phases. Kinetic data suggest that liquid film diffusion and intraparticle diffusion were the rate-limiting steps governing TcO4− sorption. Analyses of the XAS spectra data revealed that ReO4−, a chemical analogue for TcO4−, was associated with the modified zeolite Y phase(s) as an inner-sphere complex of ReO4− possibly incorporated into a structural cavity, and there was no evidence of the reduced form of Re(IV). The modified zeolite Y exhibited fast sorption kinetics, significant sorption capacity, and robust uptake performance in acidic conditions (pH 3–5) and in the presence of high concentrations of competing anions. An important attribute of this remediation technology is that it sequesters TcO4− (Tc oxidized form) that exists naturally in most surface environments and waste streams, and it provides an effective alternative to the existing reductive precipitation technologies that have limited long-term environmental applications.

Insight into volatile iodine uptake properties of covalent organic frameworks with different conjugated structures

The effective capture and storage of volatile radionuclide iodine from the nuclear waste stream is of great significance for environment remediation. Here we synthetize two types of two-dimensional (2D) covalent organic frameworks (COFs) for iodine vapor uptake, including COF-LZU1 with a whole π-π conjugated structure and TpPa-1 with a combination of π-π and p-π conjugated structure. The capacities for iodine vapor uptake are acquired for COF-LZU1 (530 wt%) and TpPa-1 (245 wt%). Thorough studies reveal that the large and intact π-π conjugated system formed by phenyl rings and imine (−CN−) linkers in COF-LZU1 provides materials with ultrahigh iodine vapor uptake by inducing the charge-transfer interactions to transform adsorbed iodine molecules into polyiodide anions. However, the combined π-conjugated structure in TpPa-1 shows no obvious effect on iodine adsorption enhancement. Hence, this study can provide a guidance for the design and construction of ultrahigh-capacity iodine adsorbents.

Comparative analysis of ZVI materials for reductive separation of 99Tc(VII) from aqueous waste streams.

Technetium-99 (Tc) is a long-lived radioactive contaminant present in legacy nuclear waste streams and contaminated plumes of the nuclear waste storage sites worldwide that poses risks for human health and the environment. Pertechnetate (TcO4-), the most common chemical form of Tc under oxidative conditions, is of particular concern due to its high aqueous solubility and mobility in the subsurface. One approach to treatment and remediation of TcO4- is reduction of Tc7+ to less soluble and mobile Tc4+ and its removal from the contaminated streams such as liquid secondary waste generated during vitrification of the Hanford low activity tank waste. Zero valent iron (ZVI) is a common reactive agent for reductive treatment of environmental contaminants, including reducible heavy metal ions, which can offer a potential solution to this challenge. Here, we present a comparative study of eleven commercial ZVI materials manufactured by different methods that were evaluated for the reductive removal of TcO4- from an aqueous 80 mM NaCl solution at near neutral pH representing low activity waste off-gas condensate. Performance of ZVI materials was analyzed in relation to time-dependent Fe2+ dissolution as well as pH and ORP profiles of the contact solution. Large variability in the efficiency and kinetics of Tc7+ reduction by different ZVI materials was contingent on their origin. ZVI materials manufactured by electrolytic method exhibited superior performance, and the kinetics of the Tc7+ reduction correlated to particle size. ZVI materials manufactured by iron pentacarbonyl reduction with hydrogen were ineffective for Tc7+ reduction. In general, our results highlight the need for thorough performance analysis of commercial ZVI materials for any contaminant of interest.

Reactive spark plasma sintering of Cs-exchanged chabazite: characterisation and durability assessment for Fukushima Daiichi NPP clean-up

ABSTRACT Ion-specific media (ISM) have played an integral role in the clean-up and remediation efforts at the Fukushima Dai-ichi disaster site, through the processing of contaminated wastewaters. The use of these materials generates a secondary nuclear waste stream, presenting its own series of engineering problems arising from stringent handling and long-term storage requirements. A reactive spark plasma sintering (SPS) method was investigated for conditioning of the spent cesium exchanged zeolite, chabazite. A natural form of the zeolite was used as an analogue to the engineered ISM used at the Fukushima NPP site. Simulant wasteforms were sintered using different temperature and pressure parameters followed by analysis of phase assemblage, density, and durability (using the product consistency test (PCT)). The results indicated that zeolite structure had collapsed completely, with the exchanged cesium partitioned primarily into a durable feldspar to assure stability of the sintered material for passively safe storage or geological disposal.

Porous iron material for TcO4- and ReO4- sequestration from groundwater under ambient oxic conditions.

Technetium-99 (99Tc) is a major contaminant at nuclear power plants and several US Department of Energy sites. Its most common aqueous species, pertechnetate (TcO4-), is very mobile in the environment, and currently there are no effective technologies for its sequestration. In this work, a porous iron (pFe) material was investigated for TcO4- and perrhenate (ReO4-) sequestration from artificial groundwater. The pFe was significantly more effective than granular iron for both TcO4- and ReO4- sequestration under oxic conditions. The Tc removal capacity was 27.5 mg Tc/g pFe at pH ˜6.8, while the Re removal capacity was 23.9 mg Re/g pFe at pH ˜10.6. Tc K-edge XANES and EXAFS analyses indicated that the removed Tc species was 70-80% Tc(IV) that was likely incorporated into Fe corrosion products (i.e., Fe(OOH), Fe3O4) and 20-30% unreduced TcO4-. In contrast, the removed Re species was ReO4- only, without detectable Re(IV). In addition, the sequestered ReO4- was not extracted (<3%) by 0.1 M Na2SO4 and 1 M KI solution, which indicated that ReO4- and by chemical analogy, unreduced TcO4-, was likely incorporated into Fe corrosion products. This inexpensive pFe material may be applied to the sequestration and stabilization of 99TcO4- from contaminated environments and nuclear waste streams.

Iodine speciation in a silver-amended cementitious system

Silver-impregnated zeolite (AgIZ) has been used for removing radioiodine from contaminated groundwater and nuclear waste streams and the worldwide inventory of such secondary waste is rapidly increasing. The objective of this study was to 1) quantify the effectiveness of two grout waste forms for disposing of the used AgIZ, and 2) determine the I speciation leached from AgIZ encapsulated in grout. A 60-day kinetics batch experiment demonstrated that AgIZ encapsulated in slag-free grout was extremely effective at immobilizing I and Ag, a potential non-radioactive carcinogen. However, AgIZ encapsulated in slag-containing grout, the most common type of grout used for low-level radioactive waste disposal, was entirely ineffective at immobilizing I. While the slag-free grout with AgIZ released only 3.3 μg/L Itotal into the contact solution, the slag-containing grout released 19,269 μg/L Itotal. Based on thermodynamic calculations, the strongly reducing conditions of the slag-containing system (Eh was −392 mV) promoted the reductive dissolution of the AgI, forming Ag0(aq) and releasing iodide (I−) into the aqueous phase. The slag-free grout system was maintained under more oxidizing conditions (Eh was 439 mV) and a minimal amount of I was released from the grout. In both grout systems, the aqueous I, originally added to the AgZ as iodide, was composed primarily of iodide and org-I, and essentially no iodate was detected. More organo-I was detected in the slag-free than the slag-containing grout system because the high redox potential of the former system was more conducive to the formation of oxidized I species, such as I2, which may be intermediates in the covalent bonding of I with organic C in grout. Iodine K-edge XANES analysis indicated that I existed exclusively as silver iodide in both AgIZ-grout samples. Together, these results indicate that subsurface grout disposal of AgIZ waste should be done under oxidizing conditions and that radioiodide released from AgIZ can undergo speciation transformations that have important implications on subsequent mobility and estimated risk.

Constructing "breathing" dynamic skeletons with extra π-conjugated adsorption sites for iodine capture.

Radioiodine (129I and 131I) emission from the nuclear waste stream has aroused enormous apprehension because of its quick diffusion and radiological contamination. Conventional porous adsorbents such as zeolites and carbon with rigid skeletons and constant pore volumes reveal a limited performance for reliable storage. Here, a series of soft porous aromatic frameworks (PAFs) with additional π-conjugated fragments is disclosed to serve as physicochemical stable media. Due to the flexibility of the tertiary amine center, the PAF products provide sufficient space for the binding sites, and thus exhibit a considerable capability for iodine capture from both gaseous and soluble environments. The obtained capacity of PAFs is ca. 1.6 times higher than that of PAF-1 which possesses similar aromatic constituents featuring an ultra-large specific surface area (BET = 5600 m2 g−1). The novel paradigm of dynamic frameworks is of fundamental importance for designing adsorbents to treat environmental pollution issues.

Corrosion of 1050, 1085, and 5083 grade aluminium in the pore water of a GGBS/CEM I grout up to 777 d

Aluminium forms a significant part of a nuclear waste stream that will be encapsulated in CEM I (Portland cement)/GGBS (ground granulated blast furnace slag) grout. The aluminium is in the form of irradiated fuel cladding or containers used for transporting waste. The rapid rate of corrosion of aluminium in the cement pore water and the associated hydrogen generation during the initial curing of cement systems is well known. Longer-term corrosion data however, are relatively sparse, hence this work investigates corrosion up to 777 d measured from the rate of hydrogen generation.After an initial rapid corrosion stage, it has been shown that the corrosion rate decreases in a predictable linear manner depending on the inverse of the depth corroded. Eventually the corrosion rate decreases to such a low value that the corrosion effectively ceases.When the corrosion that has taken place is compared with the voidage generated by the hydrogen generation whilst the grout is in the plastic state, then it is apparent that insufficent corrosion is likely to take place for the expansive corrosion product generated to fill these voids. Hence the formation of these corrosion products does not significantly affect the performance of the wasteform and container.

The use of columns of the zeolite clinoptilolite in the remediation of aqueous nuclear waste streams

Mud Hills clinoptilolite has been used in an effluent treatment plant (SIXEP) at the Sellafield nuclear reprocessing site. This material has been used to remove 134/137Cs and 90Sr successfully from effluents for 3 decades. Samples of the zeolite have been tested in column experiments to determine their ability to remove radioactive Cs+ and Sr2+ ions under increasing concentrations of competing ions, Ca2+, Mg2+, Na+ and K+. These ions caused increased elution of Cs+ and Sr2+. Ca2+, Mg2+ and K+ were more effective competitors than Na+. For Na+, it was found that if concentration was reduced, then column performance recovered rapidly.

Azo-Bridged Calix[4]resorcinarene-Based Porous Organic Frameworks with Highly Efficient Enrichment of Volatile Iodine

The effective capture and storage of volatile radionuclide iodine from the nuclear waste stream is of paramount importance for environment remediation. In this work, we report the first examples of azo-bridged calix[4]resorcinarene-based porous organic frameworks (CalPOFs), synthesized by diazocoupling reaction of 4,4′-biphenyldiamine and C-alkylcalix[4]resorcinarenes (RsCns; n stands for the associated alkyl chain length) under mild conditions. The resulting CalPOFs are permanently porous, and their porous properties could be adjusted by varying the alkyl chain lengths of RsCns. With the alkyl chain length increasing from methyl, ethyl to propyl, the Brunauer–Emmett–Teller surface areas are decreasing from 303, 154 to 91 m2 g–1 for CalPOF-1, CalPOF-2 and CalPOF-3, respectively. The presence of a great many of effective sorption sites including azo (−N═N−) groups, macrocyclic π-rich cavities and phenolic units in the skeleton as well as permanent porous structures provides these materials with ultrahigh i...

Complexation of U(VI) with Cucurbit[5]uril: Thermodynamic and Structural investigation in aqueous medium.

The assessment of cucurbituril (CBn) for selective removal of actinides from nuclear waste streams requires comprehensive understanding of binding parameters and coordination of these complexes. The present work is the first experimental report on complexation of actinide ion with Cucurbit[5]uril (CB5) in solution. The thermodynamic parameters (ΔG, ΔH and ΔS) for complexation of CB5 with U(VI) in formic acid water medium were determined using microcalorimetry and UV-Vis spectroscopy. The enthalpy and entropy of complexation revealed the partial binding of U(VI) to CB5 portal. The partial binding was confirmed by spectroscopic techniques viz. extended X absorption fine structure spectroscopy (EXAFS), 1H and 13C NMR. The EXAFS χ(r) versus r spectra for U-CB5 complex has been fitted from 1.4 to 3.5 Å with two oxygen shells and a carbon shell. The presence of three carbon atom in secondary shell shows the involvement of only three carbonyl oxygens directly bonding to U(VI) which is in contrast to that calculated from gas phase DFT calculation of unhydrated system. The combined effect of hydration and formic acid encapsulation led to the enhanced stability of partially bound U(VI) to CB5. In the present work the binding of formic acid has also been studied by fluorescence spectroscopy. ESI-MS data shows the unusual stabilization of U(VI) by CB5 in gas phase.

Technetium Stabilization in Low-Solubility Sulfide Phases: A Review

Technetium contamination remains a major environmental problem at nuclear reprocessing sites, e.g., the Hanford Site, Washington, USA. At these sites, Tc is present in liquid waste destined for immobilization in a waste form or has been released into the subsurface environment. The high environmental risk associated with Tc is due to its long half-life (214 000 years) and the mobility of the oxidized anionic species Tc(VII)O4–. Under reducing conditions, TcO4– is readily reduced to Tc(IV), which commonly exists as a relatively insoluble and therefore immobile, hydrous Tc-oxide (TcO2·nH2O). The stability of Tc(IV) sequestered as solid phases depends on the solubility of the solid and susceptibility to reoxidation to TcO4–, which in turn depend on the (biogeo)chemical conditions of the environment and/or nuclear waste streams. Unfortunately, the solubility of crystalline TcO2 or amorphous TcO2·H2O is still above the maximum contaminant level (MCL) established by the U.S. EPA (900 pCi/L), and the kinetics of...

Silver-functionalized silica aerogels and their application in the removal of iodine from aqueous environments.

One of the key challenges for radioactive waste management is the efficient capture and immobilization of radioiodine, because of its radiotoxicity, high mobility in the environment, and long half-life (t1/2 = 1.57 × 107 years). Silver-functionalized silica aerogel (AgAero) represents a strong candidate for safe sequestration of radioiodine from various nuclear waste streams and subsurface environments. Batch sorption experiments up to 10 days long were carried out in oxic and anoxic conditions in both deionized water (DIW) and various Hanford Site Waste Treatment Plant (WTP) off-gas condensate simulants containing from 5 to 10 ppm of iodide (I-) or iodate (IO3-). Also tested was the selectivity of AgAero towards I- in the presence of other halide anions. AgAero exhibited fast and complete removal of I- from DIW, slower but complete removal of I- from WTP off-gas simulants, preferred removal of I- over Br- and Cl-, and it demonstrated ability to remove IO3- through reduction to I-.

Iodine Adsorption in Metal Organic Frameworks in the Presence of Humidity.

Used nuclear fuel reprocessing represents a unique challenge when dealing with radionuclides such as isotopes of 85Kr and 129I2 due to their volatility and long half-life. Efficient capture of 129I2 ( t1/2 = 15.7 × 106 years) from the nuclear waste stream can help reduce the risk of releasing I2 radionuclide into the environment and/or potential incorporation into the human thyroid. Metal organic frameworks have the reported potential to be I2 adsorbents but the effect of water vapor, generally present in the reprocessing off-gas stream, is rarely taken into account. Moisture-stable porous metal organic frameworks that can selectively adsorb I2 in the presence of water vapor are thus of great interest. Herein, we report on the I2 adsorption capacity of two microporous metal organic frameworks at both dry and humid conditions. Single-crystal X-ray diffraction and Raman spectroscopy reveal distinct sorption sites of molecular I2 within the pores in proximity to the phenyl- and phenol-based linkers stabilized by the I···π and I···O interactions, which allow selective uptake of iodine.