Nuclear Waste Solutions Research Articles

Complexation of novel diglycolamide functionalized calix[4]arenes: Unusual extraction behaviour, transport, and fluorescence studies

Three diglycolamide functionalized calix[4]arenes (calix[4]-nDGA) were synthesized and evaluated for their extraction behaviour towards lanthanide/actinide ions. Exceptionally high D(Am) and D(Pu) values indicate these radiotoxic elements can be selectively removed from nuclear waste solutions. Transport and laser induced fluorescence studies indicated strong complexation of the trivalent metal ions with the calix[4]-4DGA ligand.

Corrosion monitoring of type 304L stainless steel in nuclear near-high level waste by electrochemical noise

Electrochemical noise (EN) monitoring of 304L stainless steel (SS) and sensitized 304 SS in 3 N nitric acid and nuclear near-high level waste (HLW) solution was carried out using a three nominally identical electrode configuration under open circuit conditions. EN signals characteristics of passivation process was obtained for 304L SS in 3 N nitric acid throughout the measurement period, while in near-HLW solution, features of passivation and depassivation were observed. Potentiodynamic polarization of 304L SS in 3 N nitric acid and near-HLW showed spontaneous passivation. Noise resistance was evaluated from EN time record and this parameter was used in the present investigation to qualitatively assess the corrosion behavior during the immersion period. The average noise resistance was found to be lower in near-HLW solution when compared to 3 N nitric acid. The results of the investigation are presented in this article.

The Radiation Chemistry of the Cs-7SB Modifier used in Cs and Sr Solvent Extraction

The compound 1-(2,2,3,3,-tetrafluoropropoxy)-3-(4-sec-butylphenoxy)-2-propanol, also called Cs-7SB, is used as a solvent modifier in formulations containing calixarenes and crown ethers for cesium and strontium extraction from nuclear waste solutions. The compound solvates complexes of both metals and decreases in its concentration result in lowered extraction efficiency for both. The use of Cs-7SB in nuclear-solvent extraction ensures that it will be exposed to high-radiation doses, and thus its radiation-chemical robustness is a matter of interest in the design of extraction systems employing it. The behavior of the compound in irradiated solution, both in the presence and absence of a nitric acid aqueous phase was investigated here using steady state- and pulsed-radiolysis techniques. The rate constants for the aqueous reactions of Cs-7SB with •H, •OH, •NO3, and •NO2 radicals are reported. UPLC-UV-MS results were used to identify major products of the radiolysis of Cs-7SB in contact with nitric acid, and revealed the production of hydroxylated nitro-derivatives. Reaction mechanisms are proposed and it was concluded that the aryl-ether configuration of this molecule makes it especially susceptible to nitration in the presence of radiolytically-produced nitrous acid. Fluoride yields are also given under various conditions.

Substituent effects on BTP's basicity and complexation properties with LnIIIlanthanide ions

Bis(1,2-4 triazin-3-yl)pyridines “BTPs” represent a new class of N-donor extracting agents that separate trivalent actinides and lanthanides from nuclear waste solutions with high An/Ln separation factors. We report here QM calculations on the effect of Rpara–BTP substituents on the protonation and complexation energies of these ligands (1 ∶ 1 and 1 ∶ 3 complexes with LnIII lanthanides) in the gas phase. Both processes follow similar trends and are highly sensitive to the electron donor/acceptor character and polarizability of R. When compared to R–pyr analogues with pyridine, R–BTPs are found to be intrinsically much more basic, by ca. 20 kcal mol−1. In aqueous solution, however (modelled by the continuum PCM model), BTPs and pyridines have a similar basicity, pointing to the importance of solvent environment on their protonation states. In the optimized Ln(R–BTP)33+ complexes with Ln = La, Eu, Yb, complexation energies Ec3 increase with the intrinsic basicity of the ligands, in the order R = NMe2 > NH2 > OMe > C6H5 > tButyl > Me > H > F > Cl. Furthermore, comparison of complexes with different LnIII cations indicates that their stability increases in the order LaIII < EuIII < YbIII, by the same amount with the different R-substituents. The relative contributions of central pyridinyl and lateral triazinyl nitrogens of BTPs are shown to depend on the stoichiometry of the complex and on the LnIII size, possibly contributing to the subtle AnIII/LnIII discrimination by substituted BTPs.

Adsorption of cesium (I) from aqueous solution using oxidized multiwall carbon nanotubes

Multiwall carbon nanotubes (MWCNTs) were modified by nitric acid solution and then used to study the adsorption of cesium from aqueous solution using a batch technique under ambient conditions. As produced and oxidized MWCNTs were characterized by nitrogen adsorption/desorption, Boehm’s titration method and Fourier transform infrared spectroscopy. The physical properties of MWCNTs such as functional groups, total number of acid sites and specific surface area were greatly improved after oxidation, and these were responsible for more sorption of cesium from aqueous solution and made them more dispersible in water. The adsorption of cesium ions as a function of contact time, initial concentration of cesium, pH, ionic strength and oxidized MWCNT concentrations was also investigated. The results showed that cesium adsorption percentage strongly depended on the pH value, oxidized MWCNT content and on the solution ionic strength. Kinetic data indicated that the adsorption process achieved equilibrium within 80 min. Equilibrium data for as produced and oxidized MWCNTs was well described by both Freundlich and Langmuir isotherms. The dominant mechanism of cesium adsorption on oxidized MWCNTs may be mainly attributed to ion exchange. This study suggests that oxidized MWCNTs can be a promising candidate for the removal of cesium from nuclear waste solution.

Lithium, rubidium and cesium ion removal using potassium iron(III) hexacyanoferrate(II) supported on polymethylmethacrylate

Potassium iron(III) hexacyanoferrate(II) supported on poly methyl methacrylate, has been developed and investigated for the removal of lithium, rubidium and cesium ions. The material is capable of sorbing maximum quantities of these ions from 5.0, 2.5 and 4.5 M HNO3 solutions respectively. Sorption studies, conducted individually for each metal ion, under optimized conditions, demonstrated that it was predominantly physisorption in the case of lithium ion while shifting to chemisorption with increasing ionic size. Distribution coefficient (K d) values followed the order Cs+ > Rb+ > Li+ at low concentrations of metal ions. Following these findings Cs+ can preferably be removed from 1.5 to 5 M HNO3 nuclear waste solutions.

Technetium‐99 MAS NMR Spectroscopy of a Cationic Framework Material that Traps TcO4− Ions

No chance for technetium: 99Tc magic-angle spinning (MAS) NMR spectra show that TcO4− ions, which are generated by nuclear fission and can contaminate the environment, can be trapped within the channels and cavities of a cationic framework material (see picture). These spectra are among the first 99Tc MAS NMR spectra reported to date, and show that the TcO4− ions can be efficiently removed from simulated nuclear waste solutions.

Uptake behavior of titanium molybdophosphate for cesium and strontium

This study investigates uptake of cesium and strontium from aqueous solution similar to nuclear waste on three samples of titanium molybdophosphate (TMP) synthesized under various conditions. Effects of concentration of sodium nitrate, pH and contact time on the uptake of cesium and strontium have been studied by bath method. The results showed that TMP has high affinity toward cesium and strontium at pH > 2 and relatively low concentration of sodium nitrate. Kinetic data indicated that cesium uptake process to achieve equilibrium was faster than strontium. Cesium and strontium breakthrough curves were examined at 25 °C using column packed with H3O+ form of TMP and breakthrough curves showed symmetrical S-shaped profiles. At the same time, the calculated breakthrough capacity for cesium was higher than strontium. The results of desorption studies showed that over 99% of cesium and strontium was washed out of column by using 4 M NH4Cl solution. This study suggests that TMP can have great potential applications for the removal of strontium and specially cesium from nuclear waste solution.

Method for the prediction of nuclear waste solution decontamination by coprecipitation of strontium ions with barium sulphate using the experimental data obtained in non-radioactive environment

The theoretical and experimental prediction of the decontamination factor during the treatment of nuclear waste solutions is generally a cumbersome problem due to their complex composition and extremely dangerous environment. Here, a methodology is proposed and experimentally validated allowing the prediction of the decontamination factor by carrying out experiments in small pilot installations in non-radioactive environment. These experiments provide the population density and the crystal growth data of the coprecipitating agent which serve for calculations according to the general relation presented in this paper.

Assessing the mechanism of the synergistic action of calixarenes and Co-dicarbollides in lanthanide extractions

The extraction pathway of lanthanide ions from simulated nuclear waste solutions and the synergistic effects of a calixarene and a cobalt dicarbollide anion (CD) have been investigated on a model system by the combined use of electrospray ionization MS, FTMS ICR, together with 1H and 11B high field NMR spectroscopy techniques. Moreover, the extracting capabilities of the calixarene/BrCD mixture were determined by liquid–liquid extraction tests. An enhanced extraction efficiency of the calixarene/BrCD mixture with respect to the calixarene alone was observed. MS analysis allowed the assessment of a supramolecular ternary association, while the NMR data gave information on the calixarene binding sites for lanthanide ions. Herein, we report the experimental evidence of the formation of a ternary lanthanide/calixarene/Co-dicarbollide aggregate, which allowed us for the first time to assess the mechanism of the synergistic action of calixarenes and CD anions in lanthanide ion extraction.

Finland's nuclear waste solution

This article speaks about the nuclear waste solution in Finland. Posiva, the Finnish company building an underground repository here, says it knows how to imprison nuclear waste for 100 000 years. These multimillennial thinkers are confident that copper canisters of Scandinavian design, tucked into that bedrock, will isolate the waste in an underground cavern impervious to whatever the future brings: sinking permafrost, rising water, earthquakes, copper-eating microbes, or oblivious land developers in the year 25 000. If the Finnish government agrees-a decision is expected by 2012. This site will become the world's first deep, permanent repository for spent fuel.

Preparation and characterization of zirconium (IV) molybdo tungsto vanado silicate as a novel inorganic ion exchanger in sorption of radionuclides

A new mixed metal heteropoly anion-based cation exchanger Zr(IV) molybdo tungsto vanado silicate (ZMTVS) was prepared under varying conditions. The material was characterized by FTIR, X-ray diffraction, TGA-DTA and SEM techniques. Its ion exchange capacity (IEC) for K + was found to be 0.86 meq g −1. Distribution coefficients ( K d) values for 10 metal ions and three radioisotopes were determined. On the basis of K d values, two important and analytically difficult quantitative binary separations viz. Ni(II)–Co(II) and Ni(II)–Pb(II) were achieved on its column. Decontamination of aqueous nuclear waste solution was also studied.

Evaluation of polymer inclusion membranes containing crown ethers for selective cesium separation from nuclear waste solution

Transport behaviour of 137Cs from nitric acid feed was investigated using cellulose triacetate plasticized polymer inclusion membrane (PIM) containing several crown ether carriers viz. di-benzo-18-crown-6 (DB18C6), di-benzo-21-crown-7 (DB21C7) and di- tert-butylbenzo-18-crown-6 (DTBB18C6). The PIM was prepared from cellulose triacetate (CTA) with various crown ethers and plasticizers. DTBB18C6 and tri- n-butyl phosphate (TBP) were found to give higher transport rate for 137Cs as compared to other carriers and plasticizers. Effect of crown ether concentration, nitric acid concentration, plasticizer and CTA concentration on the transport rate of Cs was also studied. The Cs selectivity with respect to various fission products obtained from an irradiated natural uranium target was found to be heavily dependent on the nature of the plasticizer. The present work shows that by choosing a proper plasticizer, one can get either good transport efficiency or selectivity. Though TBP plasticized membranes showed good transport efficiency, it displayed poor selectivities. On the other hand, an entirely opposite separation behaviour was observed with 2-nitrophenyloctylether (NPOE) plasticized membranes suggesting the possible application of the later membranes for the removal of bulk 137Cs from the nuclear waste. The stability of the membrane was tested by carrying out transport runs for nearly 25 days.

Interaction of Eu3+ with N,N,N′,N′-tetraoctyl diglycolamide: A time resolved luminescence spectroscopy study

N,N,N′,N′-tetraoctyl diglycolamide (TODGA) has been identified as one of the promising extractants for the partitioning of minor actinides from high-level nuclear waste solutions. Solvent extraction studies have shown that stoichiometry of the extracted species of Eu3+ with TODGA depend on the nature of diluent. Time resolved luminescence spectroscopy (TRLS) has been employed to investigate the complexation of Eu3+ with TODGA under different experimental conditions. The effects of different experimental parameters such as aqueous phase acidity, nature of diluent, and TODGA concentration on the luminescence lifetime of Eu3+ ions have been investigated. The lifetime measurements of the complexed fraction of Eu3+ with TODGA suggested the absence of water molecules in the inner coordination sphere of the metal ion in different solvents. In ethanol–water (60/40%) mixture, the complexation of Eu3+ with TODGA under varying ligand-to-metal ratios suggested the formation of 1:1, 1:2, and 1:3 species, viz., Eu(TODGA)3+, Eu(TODGA)23+, and Eu(TODGA)33+, respectively. The conditional stability constants logβ1, logβ2, and logβ3 were calculated as 6.1±0.5, 10.8±0.7, and 14.3±0.6, respectively. The nature of diluent did not influence the luminescence spectra of Eu3+ in the presence of TODGA.

An Inorganic Microsphere Composite for the Selective Removal of 137Cesium from Acidic Nuclear Waste Solutions. 1: Equilibrium Capacity and Kinetic Properties of the Sorbent

A new inorganic ion exchange composite consisting of ammonium molybdophosphate, (NH4)3P(Mo3O10)4·3H2O (AMP), synthesized within hollow aluminosilicate microspheres (AMP‐C) has been developed. Two different batches of the sorbent were produced resulting in 20% and 25% AMP loading for two and three loading cycles, respectively. The selective cesium exchange capacity of this inorganic composite was evaluated using simulated sodium bearing waste solution as a surrogate for the acidic tank waste currently stored at the Idaho National Laboratory (INL). Equilibrium isotherms obtained from these experiments were very favorable for cesium uptake and indicated maximum cesium loading of approximately 9% by weight of dry AMP. Batch kinetic experiments were also performed to obtain the necessary data to estimate the effective diffusion coefficient for cesium in the sorbent particle. These experiments resulted in effective intraparticle cesium diffusivity coefficients of 4.99 × 10−8 cm2/min and 4.72 × 10−8 cm2/min for the 20% and 25% AMP‐C material, respectively.

An Inorganic Microsphere Composite for the Selective Removal of 137Cesium from Acidic Nuclear Waste Solutions. 2: Bench‐Scale Column Experiments, Modeling, and Preliminary Process Design

A new inorganic ion exchange composite for removing radioactive cesium from acidic waste streams has been developed. The new material consists of ammonium molybdophosphate, (NH4)3P(Mo3O10)4·3H2O (AMP), synthesized within hollow aluminosilicate microspheres (AMP‐C), which are produced as a by‐product from coal combustion. The selective cesium exchange capacity of this inorganic composite was evaluated in bench‐scale column tests using simulated sodium bearing waste solution as a surrogate for the acidic tank waste currently stored at the Idaho National Laboratory (INL). Total cesium loading on the columns at saturation agreed very well with equilibrium values predicted from isotherm experiments performed previously. A numerical algorithm for solving the governing partial differential equations (PDE) for cesium uptake was developed using the intraparticle mass transfer coefficient obtained from previous batch kinetic experiments. Solutions to the governing equations were generated to obtain the cesium concentration at the column effluent as a function of throughput volume using the same conditions as those used for the actual column experiments. The numerical solutions of the PDE fit the column break through data quite well for all the experimental conditions in the study. The model should therefore provide a reliable prediction of column performance at larger scales.

Application of Laser-Induced Photoacoustic Spectroscopy for Determination of Plutonium Concentration in Nuclear Waste Solutions

Laser-induced photoacoustic spectroscopy was used in a quantitative analysis of Pu in HNO3 medium. Plutonium was quantitatively oxidized to Pu(VI) using Ce(IV). The photoacoustic measurement of Pu(VI) with maximum absorption at 830.5 nm was subsequently performed to determine the concentration. The photoacoustic signal was linearly proportional to the Pu(VI) ion concentration. The detection limit of Pu(VI) was estimated to be 0.5 microg mL(-1) (3sigma) in 3 M HNO3. By the proposed method, Pu concentration was successfully determined in a nuclear waste solution for use in nuclear materials management.

Adsorptive Separation of Palladium from a Simulated Nuclear Waste Solution with Activated Carbon Fibers

Adsorptive separation of palladium from a nitric acid solution was performed by using a pitch‐based activated carbon fiber modified with NaOH (NaOH‐ACF). The NaOH‐ACF showed a markedly improved adsorption performance for the palladium ions as compared with the untreated ACF. It also showed a much higher preferential adsorption for the palladium ion over the rhodium and rhenium ions. The equilibrium capacity of the NaOH‐ACF for palladium was 1.25 meq · g−1. The breakthrough behavior obtained by using the bed packed with the NaOH‐ACF showed a complete separation of palladium from the coexisting ions of rhodium and rhenium. The spent bed was eluted effectively with nitric acid and a palladium nitrate with a purity of higher than 99% could be recovered.

Influence of hydroxyl-water interaction on structural transformations during Cs + exchange in microporous H-CST

Crystalline silicotitanate (CST) molecular sieve is being tested as an ion exchanger for targeted removal of Cs from high level radioactive nuclear waste solutions. CST is resistant to high radiation fields and highly alkaline/acidic solutions which make it an ideal sequestration candidate. The mechanisms and dynamics of the crystallographic structural changes involved are the fundamental properties of selectivity in this material, and have not been studied previously. This study investigates the molecular processes of ion exchange in real time using neutron and X-ray diffraction as well molecular dynamics (MD) simulations. These techniques in unison have provided the most complete description of the Cs exchange process in hydrogen form of CST (H-CST). In the H-CST structure, the elliptical channels cannot accommodate Cs due to unacceptable short Cs–O bond distances. From time-resolved X-ray diffraction studies, the elliptical channels in H-CST transform to circular geometry after 8 wt.% Cs exchange. Although structural transformations were observed and the symmetry changed from space group P42/mbc to P42/mcm, the mechanisms were still unknown. Identifying all atom types and positions in the CST structure will ultimately explain the mechanisms of selectivity for Cs in H-CST. Time-resolved and static neutron diffraction studies added the proton positions on the water molecules and hydroxyl units to the model derived from X-ray scattering. The results show that as Cs diffuses through the structure, water molecules must change orientations and positions to hydrate Cs and maintain the hydrogen bond network. The new configuration of the proton on the water sites are closer to the framework and force close proton–proton repulsion with the framework hydroxyl. This repulsion bends the hydroxyl and drives the transformation of the CST structure. Hydroxyl bending and water dynamics have been confirmed using MD simulations. The short timescales of MD simulations is ideal to study these fast molecular motions and emphasizes the important role hydrogen has in CST toward Cs selectivity.

Development and Characterization of Rare Earth-Rich Glassy Matrices Envisaged for the Immobilization of Concentrated Nuclear Waste Solutions

New nuclear highly durable glass compositions, able to immobilize a higher concentration of high-level nuclear wastes than current borosilicate nuclear glasses, are being studied. Investigations are performed on rare earth (RE)-rich glasses, known as durable matrices. After a preliminary study on complex and simplified compositions, a basic glass composition was selected and studied (wt%): 51.0 SiO2–8.5 B2O3–12.2 Na2O–4.3 Al2O3–4.8 CaO–3.2 ZrO2–16.0 RE2O3. Chemical durability, physical properties (viscosity, transformation temperature), and crystallization tendency of glasses containing either a mixture of RE (La + Ce + Pr + Nd) or only one RE were studied and compared. The local environment of RE (for RE = Nd) in the glass and its evolution according to Nd2O3 concentration (from 1.3 to 30 wt%) was also studied by coupling characterization methods such as extended X-ray absorption fine structure spectroscopy at the neodymium LIII-edge and optical absorption spectroscopy. 11B, 27Al magic angle spinning-nuclear magnetic resonance, and Raman spectroscopy were also used to study glass structure.