Symposium EN12 : Hierarchical Materials for Nuclear Waste Management

Over the past thirty years, there has been a considerable amount of research on nuclear waste forms of all types: glass, crystalline ceramics, composite ceramics and spent fuel. Most recently, the idea of structural hierarchies has been used to identify new waste form materials. I will briefly survey the history of waste form development and discuss new challenges for the future. One of the very unique aspects of waste form research is the need to develop a means of predicting materials behavior over hundreds of thousands of years. There are a number of approaches: experimental, computational and the use of natural systems that must converge in order to build confidence in these long-term predictions. More recently, there have been discussions of matching the waste form to the characteristics of the waste stream or of selecting waste forms whose durability is compatible with the geochemical and hydrologic conditions in a geologic repository. In both cases, the nuclear waste form adds value to the safety assessment of a geologic repository.

The research and manufacturing associated with the nuclear materials production has resulted in a significant quantity of waste that poses health, environmental, and proliferation hazards. One of the greatest challenges today is the cleanup and remediation of the highly complex, radioactive wastes and contaminated areas surrounding these facilities.
The scientific development of waste forms has evolved through different regulatory histories which, has motivated different approaches to designing, testing, and evaluating each waste form. Consequently, many of the challenges related to vitreous waste forms are well-known whereas, comparatively less fundamental and mechanistic understanding is available for cementitious and other waste forms. These different histories have resulted in significantly more resources expended on developing glass and vitrification technologies compared to other waste forms. Nevertheless, revolutionary advances in glasses and cementitious materials, as well as wholly new storage materials and concepts are needed to ensure the long-term stability and safety of waste storage.
Long-term performance, the criteria on which waste forms are evaluated, requires understanding the fundamental physical and chemical processes that occur in waste forms but, is challenging to predict and correlate across timescales spanning days to millennia. The complexity of waste inventories precludes a single remediation option. New approaches to waste form material synthesis combined with recent advances in computational and experimental approaches, provide the foundation and pathway to address waste remediation challenges. Hierarchical materials are a class of materials that have the potential to effect the aggregate safety, cost, and efficacy of nuclear waste immobilization and sequestration.

At the National Synchrotron Light Source II (NSLS-II, Brookhaven National Laboratory, New York), the X-ray Powder Diffraction (XPD) beamline serves a very broad and diverse user community ranging from physics, chemistry, and materials science to earth science, nuclear science and engineering science. XPD operates with photon beams whose energy is fully tunable between 40 keV and 70 keV, with a nominal beam size of 0.6 × 0.2 mm². Hard X-rays provide the opportunity to probe high-Z or bulk samples, buried interfaces, and through sample cells and devices. The core mission of XPD is to observe materials under conditions that are far from equilibrium, as a function of a process parameter like T, P, reactive gases, or electric field.
XPD is configured for near-field (high Q coverage) and far-field (high peak separation) 2D diffraction using two large-area detectors. They can be quickly interchanged at any time in the course of a reaction or a process. This dual detector arrangement is ideal for total scattering measurements that provide both long-range and short-range atomic ordering information on any material.
Major focus themes of XPD are: 1) in situ synthesis (e.g., high temperature solid state growth, colloidal nanoparticles from solution, flash sintering of ceramics/oxide materials, nucleation & growth of MOFs, and microwave-assisted synthesis) and 2) in situ structure evolution (e.g., catalysis, oxidation reduction reactions using gases, hydrogenation reactions, gas adsorption and separation, and cycling of energy-storage devices).
In the first two years of operation of XPD, a lot of work has been dedicated to irradiated and radioactive materials for nuclear applications. My talk will review the opportunities that high-energy X-ray beamlines at the NSLS-II can offer for multi-length scale structural measurements and development of novel materials for nuclear science.

Safe disposal of nuclear waste is essential to ensure sustainability of nuclear energy and environmental cleanup from nuclear activities. This is especially true for problematic radionuclides such as iodine, cesium, and strontium because of their half-life, volatility, and mobility in most disposal environments. An integrated approach by considering both radionuclide incorporation and release from host materials is essential to improve the performance of waste forms and develop new waste forms. Chemical bonding environment and properties of host material are intimately connected to how radionuclides get incorporated and how they get released to the environment. As an example, incorporation of iodine in apatite is investigated to understand crystal chemistry of iodoapatite using artificial neutral network simulation, which is effective to understand nonlinear relationship between chemical composition and properties of a crystal structure. The result suggests that there are a number of composition combinations of apatite, including Cs, Sr, Ba, Mo, V, and As apatites, that potentially incorporate iodine, greatly extending the compositional space for iodoapatite waste form. Future more, first-principles calculations suggest a number of predicted apatites are energetically stable, and a better route to synthesis is to increase synthesis temperature as the Gibbs free energy formation is calculated to decrease with temperature. The effect of beta decay on the stability such as ¹³⁷Cs → ¹³⁷Ba and ⁹⁰Sr → ⁹⁰Y → ⁹⁰Zr can be mitigated by introducing appropriate electron acceptor at the neighboring sites in the structure. As demonstrated by first-principles calculations, the extra electron from beta decay is localized at a variable valence ion (ferric iron), which changes its oxidation state and becomes ferrous iron, with a significant stability improvement. Due to the nature of ionic bonding of iodine in a covalent bonded apatite lattice, the release of iodine from Pb₁₀(VO₄)₆I₂ apatite is observed to be incongruent. The iodine was initially released at a significantly higher rate than suggested by its stoichiometry with respect to lead and vanadium, gradually decreased over time, caused by an ion-exchange process that is faster than the dissolution rate of the Pb-V-O framework evidenced by the spectroscopic signature of OH groups in leached samples. These studies suggest that apatite-structured materials could be promising nuclear waste forms to incorporate these problematic radionuclides, and are capable to mitigate the beta decay induced instability, and have low iodine release rate in aqueous solutions.

Within the DOE EFRC WastePD (Center for Design and Performance of Nuclear Waste Forms and Containers), the ceramic team targets the incorporation, confinement and transport behaviors of critical radionuclides in bulk crystalline ceramics and across solid-solid and solid-liquid interfaces that can be closely linked with the ceramic waste form degradation and stability under near field conditions. In this talk, the major activities of the ceramic waste form performance and degradation are highlighted with the focus on the development of crystalline host phases to address critical radionuclides of Cs, I and Cl, and their long-term performance. Promising ceramic waste forms are developed based upon apatite and defective perovskite structure types showing extensive cation and anion substitutions and crystal structure flexibility. Chemical dissolution experiments are performed on the designed ceramic waste forms to test their chemical durability and gain mechanistic understanding of the degradation mechanisms. In-situ synchrotron X-ray diffraction and environmental scanning electron microscopy observations are also performed to understand the phase transformation and dissolution behaviors for the ceramic waste forms with water interaction. Surface passivation of the ceramic waste form upon water interaction are also investigated and their impacts on ceramic waste form chemical durability are further discussed.

In excess of 100 metric tons of technetium-99 (t_1/2 = 2.13x10⁵ years) has been produced in thermal nuclear reactors as a result of the fissioning of U-235. The isolation and immobilization of technetium from the raffinate of used fuel reprocessing by durable solid waste-forms are a challenge. In the technical process to vitrify high-level radioactive waste effluents, a significant part of the technetium inventory will be oxidized or subjected to disproportionation to consequently volatilize as heptoxide. Oxidation and volatilization of Tc-99 are process-related and a result of slightly oxidizing redox conditions in combination of temperatures targeting 1200°C in the glass melter.
In this research we show that Tc-99 can be successfully immobilized as tetravalent cation in solid state refractory oxides such as pyrochlores and perovskites. Pyrochlores have shown excellent performance in ASTM C1220-10 type corrosion testing and have the ability to structurally bond Tc-99 and therefore avoid the formation of highly-mobile, pertechnetate species under the conditions of a generic repository.
We have fabricated lanthanide technetium oxides using either dry-chemical ceramic processing, or wet-chemical coprecipitation methods. The Tc pyrochlores have shown better Tc retention and corrosion resistance in ASTM C1220-10 based testing compared with Tc-containing LAWE4-type borosilicate glass, combined with 50-times higher waste loading. However, mechanical properties (fracture toughness, compressive strength) of the pyrochlores are lacking and the microstructure shows high open porosity of about 50%. To improve these properties we tested a variety of measures such as hot-pressing or the combination of hot pressing and high-temperature synthesis, but the improvement was minor and Tc and the surrogate metal Ru were partially reduced to the metals. The presence of metallic inclusions has strong impact on Tc retention and release rates are increased tenfold.
We have further developed a wet-chemical coprecipitation synthesis route followed by calcination and a 4-days high-temperature sintering cycle for the model composition Sm₂(Ru_0.5Ti_0.5)₂O₇ where titanium oxide was added as sintering agent. The ceramic surrogate waste-forms showed improved theoretical densities of about 75% combined with sufficient mechanical strength, while maintaining ruthenium in the tetravalent state. To subsequently introduce technetium into the coprecipitation-based process route and to fabricate waste-forms with the stoichiometry Sm₂(Tc_0.25Ru_0.25Ti_0.5)₂O₇ we have successfully tested the performance of reducing agents on pertechnetate. The precipitation of technetium from a mock-up washer solution allows now the incorporation of Tc-99 into the wet-chemical fabrication route. Our aim hereby was to provide a potential solution for continuously depleting the technetium content from a waste treatment facility when vitrifying high and lower-level waste effluents.

Uranyl nanocapsules are poly-peroxo oxometalates with the general formula [UO₂(O₂)_1.5]_n^n- or [UO₂(O₂)(OH)]_n^n-; n has discrete values including 20, 24, 28, and 60. These are closed capsules with square, pentagonal and hexagonal faces. There are also related open capsules (crowns) and derivatives with other bridging polydentate ligands including pyrophosphate and oxalate. All contain alkali counterions inside and outside the capsules; and these dynamically exchange in both aqueous solution and crystalline lattices. To consider employment of these capsules in the many separations, extractions and sequestrations within the nuclear fuel cycle, we need to be able to manipulate them via solvent extraction, dissolution, precipitation, etc. Understanding the behavior of the counterions is key to these processes. In this presentation, I will talk generally about the importance of these counterions in aqueous and non-aqueous systems in defining speciation and solubility. Our major tools of characterization include small-angle X-ray scattering (SAXS) and multi-nuclear solution and solid-state NMR, which respectively provide detailed information and the capsule and the counterions.

We have developed a solvent extraction process that transfers the nanocapsules into organic solvent (i.e. kerosene) based on counterion exchange. For example long chain ammonium surfactants pull the capsules into the non-aqueous phase and the counteranions travel into the aqueous phase. We have demonstrated this entire process starting with simulated spent nuclear fuel, showing complete transfer of the uranium capsules, intact into the kerosene.

Once in the organic phase, the counterions become completely immobilized, different from their behavior in the aqueous phase. This provided opportunity to investigate coordination environments inside the capsule, of Li in particular. Combined computational and NMR studies suggest the surfactant cationic heads surround the capsules and completely block the lithium from exiting, giving rise to unprecedented behavior, including extremely rigid coordination of the alkalis, and rapid motion of the capsule within the inverse micelles.

Acknowledgements: This work was performed by the Materials Science of Actinides, an Energy Frontier Research Center funded by the Department of Energy, under award number DE-SC0001089.

The treatment and recycling of the spent nuclear fuel by the PUREX process in the AREVA-La Hague plant (France) allows to reduce the ultimate nuclear waste volume and radiotoxicity and to save natural resources by recovering valuable materials, such as plutonium and uranium. To do so, the spent fuel is dissolved in nitric acid then uranium and plutonium are co-extracted, separated in two flux and recovered from solution by precipitation into solid phases. Plutonium is precipitated by oxalic acid, then calcined in air in order to obtain PuO₂, used as the main starting material for the Mixed OXide (MOX) fuel fabrication.
In the current industrial process, the plutonium oxalate is precipitated in the form of squared particles of Pu^IV(C₂O₄)₂.6H₂O. Interestingly, this morphology is maintained during the calcination in air which means that the morphology obtained at the precipitation step may influence the textural properties of the final oxide. Researches are currently carried out in collaboration between the CEA-Marcoule and the UCCS-Lille in order to vary the morphology of PuO₂ by modifying the oxalic precipitation conditions. The aim is to be able to control the morphology as soon as the precursor precipitation step occurs.
The crystal shape inherently depends on internal parameters which are related to the symmetry of the crystal structure and to the reticular density. Besides these internal parameters, the size and morphology of crystals can be influenced by various external parameters such as the frequency of nucleation, the rate of crystal growth and the agglomeration of the particles, which are themselves related to the chemical conditions. We herein tempted to manage the morphology of PuO₂ by acting both on internal and external parameters i.e. by modifying the crystal structure of the oxide precursors and by performing modifications of the synthesis conditions.
The experimental work was carried out by precipitating either plutonium (III) or plutonium (IV) oxalates in an acidic medium (HNO₃ ~ 1M) and in presence of thermally labile additives. The chemical conditions were defined on the surrogates Nd₂(C₂O₄)₃(H₂O)₆.4H₂O and Th(C₂O₄)₂.6H₂O prior to the implementation to the plutonium (III) and (IV) systems. The additives effects on the surrogate systems can be listed as follows: modification of the crystal structure and of the particle shape, decrease of the particle size without modification of the crystal structure, modulation of the particles morphology and/or agglomeration without modification of the crystal structure.
The presentation will be dedicated to the multidisciplinary approach combining solid-state chemistry, solution analysis and chemical engineering set up in order to explain the rule of additives on the mechanisms underlying the observed modulations. Another part will be devoted to the origins of the similarities and differences of the thorium (IV) oxalate and the plutonium (IV) oxalate systems towards the additives.

A practical working definition of a hierarchical structure is that of a structural motif contained within a larger structure or framework. Salt Inclusion Materials (SIMs) are a subset of a unique family of crystalline hierarchical structure types that are noteworthy because as a “stuffed” porous material, this type of hierarchical material is of fundamental interest in the development of new waste forms. These materials are of the general formula [A_mB_nX][(UO₂)_p(M_qO_r)_t] where [(UO₂)_p(M_qO_r)_t] is the framework consisting of uranyl, UO₂²⁺, and M_qO_r (M = Si, Ge) units, where B_nX is the salt-inclusion and A are alkali or alkaline earth cations that are not part of the salt-inclusion. The presentation will focus on the synthesis, crystal growth, structures and potential use of a SIMs, including [Cs₃F][(UO₂)(Si₄O₁₀)], [Cs₂Cs₅F][(UO₂)₂(Si₆O₁₇)], [Cs₉Cs₆Cl][(UO₂)₇(Si₆O₁₇)₂(Si₄O₁₂)], [Cs₂Cs₅F][(UO₂)₃(Si₂O₇)₂], and several isostructural germanates, with special emphasis on the overall crystal chemistry of these phases. The synthesis and structural characterization of a series of new phosphates and borates will also be presented.

Uranyl phosphates are important for understanding actinide chemistry for applications in the field of nuclear waste storage, particularly because they are prominent in nature making up 25% of known uranyl minerals and because uranyl phosphates have lower solubilities compared to other uranyl minerals. The molten flux crystal growth method can be used to make a variety of new uranyl phosphate structures including materials featuring new geometrical isomers of the phosphoruranylite topology, cation-cation interactions (CCIs), and three-dimensional frameworks. CCIs are rare in uranyl materials and occur in less than 2% of uranyl structures while CCIs are observed in approximately 50% of neptunyl materials. Three-dimensional structures are also less common due to the tendency of the U(VI) polyhedra to bond through the axial oxygens to form sheet based structures. The presentation will focus on the synthesis, crystal growth, and overall crystal chemistry of new uranyl phosphate materials with unique structural features including: Cs₄(PO₄)₂[(UO₂)₃O₂], Cs₆(PO₄)₄[(UO₂)₇O₄], Cs₂Na₄(PO₄)₂[(UO₂)₅O₅], and K₄(PO₄)₂[(UO₂)₃O₂].

The viability of utilizing salt-inclusion materials (SIMs) as a hierarchical wasteform has resulted in the successful synthetic preparation of a variety of SIMs with uranium-containing frameworks which incorporate oxoanion groups, including silicates and germanates. Considering the pervasive role of the borate anion in current nuclear waste storage processes, it is a logical extension of the currently existing uranium SIMs to investigate the synthetic conditions in which a uranium borate SIM could be prepared. The high temperature flux method has thus far shown to be the prevailing growth technique for SIMs in general, while broader uranium borate crystal chemistry has primarily been dominated by the mild hydrothermal method. This talk will focus on the synthetic techniques employed to synthesize porous uranium borates, in order to ultimately develop a targeted synthesis route to the first uranium borate salt-inclusion material.

Salt-inclusion materials, SIMs, consist of a covalent metal oxide framework containing voids filled by an ionic salt-lattice. Recently, we reported on an enhanced flux growth method for the targeted synthesis of uranyl silicate SIMs. These materials are of interest as nuclear waste storage materials due to their potential to simultaneously immobilize multiple radionuclides. For SIMs to become a viable waste form material, their crystal chemistry must be expanded to allow for the tailoring of the salt channels for specific radioisotopes. For instance, accommodating CsI will require a SIM with larger channels than one designed to accommodate NaF. This can be achieved by expanding the framework to incorporate other building blocks including germanates, molybdates, and lanthanides. The enhanced flux growth method will have to be tuned to these specific building blocks. Initially, this will necessitate gaining an understanding of the conditions which are suitable for the flux growth of oxide compounds with these building blocks. The enhanced flux growth technique can then be modified based on these conditions in order to achieve the synthesis of expanded SIM frameworks.

In nuclear industry, liquid radioactive waste coming from reprocessing plant or Fukushima disaster, has to be treated in order to decontaminate these effluents. One way to decontaminate this waste is to synthesize inorganic monolithic sorbent that are less sensitive to radiolysis phenomena than organic ones. Geopolymer cements are good candidates to fulfill these specifications since intrinsically they are mesoporous with high specific surface area and compatible with specific grafting agents which allow to trap selectively radionucleides of interest (especially the cesium). This work aims to synthesize monolithic geopolymer foams (aluminosilicates binders) with high mechanical resistance which can act both as sorbent for decontamination of liquid radioactive waste and containment matrix. The macroporosity has to be connected in order to facilitate the transport of contaminated fluid without drop pressure, and the control of the chemical parameters of the geopolymers allows to tailor the mesoporous network.
In the present study, a sodium geopolymer is studied and hydrogen peroxide is used as blowing agent. The results show that monolithic materials are obtained with a bubble size distribution controlled according to the nature of surfactants, the concentration of H₂O₂ and chemical composition of the geopolymer. Some X-ray tomography experiments show that the connectivity of the macroporosity (which facilitate the transport of fluid) may be tuned according to the nature of surfactants. In a second part of this work, the precipitation of copper hexacyanoferrate into the porous network has been perform in order to trap selectively the cesium. The results show that monolithic geopolymer is a very good candidate to decontaminate cesium effluent.

Hierarchical waste form materials are a novel approach to nuclear waste sequestration. Their inherent ability to contain various structural motifs within a larger framework or structure make them an interesting candidate to hold various transuranic or fission product elements all within a single entity. Salt-inclusion materials, SIMs, are a class of hierarchical material that consist of a covalent oxide framework containing voids filled by ionic salts potentially of radionuclides of important fission products. The framework allows for structural variability forming uranyl based silicate, germanate, vanadate or borate networks, as well as europium and gadolinium silicates. To widen the class of materials, ion exchange of existing SIM’s can be performed to include targeted isotopic compositions important in nuclear waste. It is therefore of interest to understand the role of the pore sizes created by the salt inclusions and their involvement in ion exchange mechanisms. Moreover, the preparation of these framework materials can take atomic size or charge into consideration during synthesis, though little is known about their thermodynamic stability, including formation enthalpies or Gibbs energies. To date there is no published literature on the thermodynamic properties of SIMS. This work investigates the thermodynamic stability using estimation/correlation techniques such as volume based thermodynamics (VBT) to determine values for the SIM’s, including their separate framework and salt constituents. We use structural information from crystallographic data and build a thermodynamic cycle to calculate entropies, enthalpies and Gibbs energies of formation. Similarly, ion exchange energetics can be predicted by applying hydration enthalpies to VBT. These calculations allow for the determination of relative material stability and the tendency for ion exchange. The results can guide experimental efforts and can be coupled with calorimetric data when available. We aim to provide a library of Gibbs energy values for a set of systems that encompass a multitude of different frameworks and potential salt inclusions to effectively inform the sequestration of radionuclides for waste management. We also investigate the dependence of pore sizes and channel dimensionality within the SIM’s and relate these properties to their framework configuration and propensity for ion exchange.

This work was supported as part of the Center for Hierarchical Waste Form Materials, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DE-SC0016574.

Bimetallic nanoparticles have been identified as candidate systems in which to sequester metallic nuclear waste. Here we analyze some of the basic energetics and kinetics of gold-nickel and gold-copper bimetallic clusters, and of Au/radionuclide clusters using a cluster expansion method. In particular, we characterize the structural stability of interphase boundaries under various relevant stress conditions, and characterize diffusion processes at the interfaces. We also determine structures and diffusion processes in both ordered and random alloys. Implications for sequestration of radionuclides are discussed.

This work was supported by This work was supported by the Center for Hierarchical Wasteform Materials (CHWM), an Energy Frontier Research Center (EFRC) funded by the United States Department of Energy Office of Basic Energy Sciences through Award DE-SC0016574.

Hollandite-type crystalline ceramics are viable waste forms for immobilizing Cs and Ba found in waste generated from nuclear material processing and treatment. Previous research has demonstrated replacing a portion of the Ti in the framework with a tri-valent cation (e.g. Cr, Al, Fe, and Ga) to improve the stability of the hollandite phase while simultaneously incorporating additional waste constituents. Other elements such as Sb, In, Mo, and Tc are also present in typical waste streams and may potentially co-substitute for Ti. The resulting doped hollandite phase is considered a hierarchical waste form material due to the ability to immobilize multiple species in both channel and framework lattice sites of the hollandite phase resulting in enhanced waste storage capacity.

To provide an understanding of the stability of hollandite waste form with different dopants, the Cs₂O-BaO-TiO₂-Sb₂O₃-In₂O₃-TcO₂-MoO₃ system has been thermodynamically assessed according to the CALPHAD methodology encompassing all relevant crystalline phases/solid solutions including hollandite and the liquid phase(s) characterized using the compound energy formalism and two-sublattice partially ionic liquid models, respectively. The resultant thermochemical models and values can allow determination of hollandite phase stability, including limits to substitution/dopant concentrations, temperature behavior, and thermodynamic representations usable for evaluating aqueous solubility and leaching.

This work was supported as part of the Center for Hierarchical Waste Form Materials, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DE-SC0016574.

The fate of a single atom seems inconsequential in the evaluation of risk for long term disposal of radioactive waste. However, consider the incorporation of a single atom (e.g., Cs) in a ceramic (e.g., hollandite) waste form. The release of Cs from the solid waste form may occur prior to matrix dissolution via diffusion along preferential pathways, such as tunnels in the hollandite structure, to grain boundaries. Quantification of the energetic stability of Cs in the crystalline waste form, based on the coordination chemistry of the Cs and the composition-dependent structure of the matrix, is critical to evaluate the risk-mitigation for a specific waste form. Further, atomic scale calculations can identify energetically favorable incorporation mechanisms. For example, divalent cation incorporation into UO₂ (as a reference for spent nuclear fuel) reveals oxidation of the uranium as a dominant incorporation mechanism, which may enhance dissolution of the UO₂ matrix. The position of the incorporated cation at or near a surface of UO₂ will affect the surface energy, and ultimately change the equilibrium morphology and potential for alteration. Non-equilibrium morphologies can be probed using quantum-mechanical calculations of higher energy configurational states, such that compositional controls on radionuclide release may be predicted. This presentation will explore radionuclide incorporation into nuclear waste forms, specifically hollandite and UO₂, and discuss the significance of calculated equilibrium morphologies on the prediction of phase formation and alteration.

The radioactive decay of incorporated fission products and actinides in nuclear waste forms leads to self-radiation effects and self-heating that may affect the stability, structure and performance of the waste form in a closed system. The principal heat-generating radionuclides in high-level waste are the short-lived fission products, ⁹⁰Sr and ¹³⁷Cs, and if incorporated in waste forms, these radionuclides cause significant self-heating for about the first 600 years. Ionization from gamma rays and beta particles emitted in beta decay of fission products can cause covalent and ionic bond rupture, valence changes, localized electronic excitations and significant changes in ionic mobility. Recent results indicate that ionization effects from beta decay in nuclear waste glasses may negligible due to a threshold in ionization dose rate for observable effects. In the case of waste forms containing actinides or tailored specifically for actinides, self-heating could be important for 1000 to 2000 years. Over the long time periods of deep geologic storage, self-radiation of nuclear waste forms is primarily due to the alpha decay of the actinides. The effects of self-radiation due to alpha decay on the structure and properties of glass and ceramic waste forms have been studied over the past four decades using both short-lived actinides, primarily ²³⁸Pu or ²⁴⁴Cm, and energetic ion beams. A wide range of experimental methods has been employed to characterize the changes in density, stored energy, and local structure as a function of radiation dose and temperature. There are limited studies comparing the response of materials to alpha decay and ion-irradiation damage. Only in a few cases do data from ion beam irradiations correctly predict the behavior in actinide-containing ceramics. In other cases, the dose for amorphization can vary significantly from that predicted by ion beam irradiation, due to ionization-induced recovery either from the ions themselves or from alpha particles emitted in alpha decay, and the temperature dependence of amorphization can shift by several hundred degrees (K), which creates a dilemma in deciding what temperature dependent data can be used, if any, to predict the temperature dependence of actinide-bearing waste forms. Over very long time periods, helium accumulation from alpha decay may lead to the formation of helium bubbles that may cause additional swelling and changes in mechanical properties. The relevance of these results radiation effects in hierarchial nuclear waste forms will be discussed.

This work was supported by the U.S. DOE, BES, MSED.

I will highlight the hierarchical materials challenges associated with Interfacial Dynamics in Radioactive Environments and Materials (IDREAM) Energy Frontier Research Center (EFRC). The mission of this effort is "To master molecular to mesoscale chemical and physical phenomena at interfaces in complex environments characterized by extremes in alkalinity and low water activity, and driven far from equilibrium by ionizing (γ,β) radiation." In doing so, the design and development of robust novel hierarchical materials and associated phenomena is at the heart of the effort. I will present the approach, advances and challenges associated with connecting molecular scale detail to micron particle-particle interaction to macroscopic flow and assembly of materials. Specific phenomena of interest will be speciation, dissolution, crystalization and rheology of Aluminum oxyhydroxides in electrolyte slurries. This is achieved through a diverse team implementing a mix of experimental and theoretical tools, providing new insight to complex, hierarchical phenomena.
This work is supported by the Interfacial Dynamics in Radioactive Environments and Materials, Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences.

A series of glass ceramic and crystalline ceramic waste forms were examined as alternative waste forms for glass. These ceramics and glass ceramics are candidate host materials for immobilizing alkaline/alkaline earth (Cs/Sr-CS) + lanthanide (LN) + transition metal (TM) fission product waste streams from nuclear fuel reprocessing. In this study, glass ceramics were fabricated using a borosilicate glass as matrix in which to incorporate CS/LN/TM combined waste streams. The major phases in these multiphase materials are powellite, oxyaptite, pollucite, celsian, and durable residual glass phases. Al₂O₃ and TiO₂ were combined with these waste components to produce multiphase crystalline ceramics containing hollandite-type phases, perovskites, zirconolite/pyrochlores and other minor metal titanate phases. These alternative waste form materials offer increased solubility of troublesome components in crystalline phases compared to glass. This, in turn, leads to increased waste loading. Also the crystalline network formed in these materials results in higher heat tolerance than glass.
For the radiation stability test, selected glass ceramic and crystalline ceramic samples were exposed to charge particles generated by an ion accelerator, which is used to simulate self-radiation in a waste form. Ion irradiation-induced microstructural modifications, volume swelling and microcracking were examined using X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), and other characterization methods. Our preliminary results reveal very promising radiation tolerance, especially amorphization resistance, in these multiphase nuclear waste forms. However, their stability may be rate dependent which may limit the waste loading that can be employed. To better understand radiation damage effects in nuclear waste forms, several individual crystalline phases in multiphase glass ceramics and crystalline ceramics were fabricated and tested with ex-situ and in-situ ion irradiations. Experimental results show similar radiation damage responses from single crystalline phases and corresponding crystalline phases in multiphase samples. Also, different crystalline phases in these multiphase waste forms exhibit different radiation tolerance under various radiation damage environments.
To investigate radiation damage effects on chemical durability of glass ceramic waste form samples. Static leach testing (28 days/7 days) on non-irradiated and irradiated glass ceramic samples was performed using ASTM C1220 method. Our results suggest that radiation damage definitely affect the chemical durability of glass ceramic samples.

An ideal nuclear waste form will incorporate and immobilize a wide variety of large cations, including actinides, lanthanides, and other fission products. To accommodate this compositional diversity, complex oxides such as the pyrochlore structured A₂B₂O₇ compounds have received a great deal of attention. However, the complex compositions and structures of these materials yield complex, multiscale disordering mechanisms in response to waste disposal conditions, which include self-irradiation. In this work, the disordering of stannate pyrochlores (A₂Sn₂O₇) under irradiation with 2.2 GeV Au ions was characterized. X-ray diffraction characterization demonstrated irradiation-induced transformations of the fluorite-derivative pyrochlore structures to either disordered fluorite or amorphous structures, as has been commonly reported in the literature. In contrast, Raman spectroscopy provided evidence of a more complex reordering of the local structure, with both the disordered fluorite and amorphous irradiation-induced structures exhibiting weberite-type local order. These two phases differ only in that the disordered phase exhibits a long-range, modulated arrangement of weberite-type structural units into an average fluorite structure, while the amorphous phase remains fully aperiodic. Comparison with the behavior of titanate and zirconate pyrochlores showed minimal influence of the high covalency of the Sn-O bond on this phase behavior. An analytical model of damage accumulation was developed to account for simultaneous amorphization and recrystallization of the disordered phase during irradiation.

The fundamental issues of particle-solid interactions and radiation-induced structural changes from a periodic-to-aperiodic state (or metamict state) are an active and important area of research. As different crystalline minerals have been proposed as actinide-bearing crystalline hosts for waste materials, it is important to understand radiation effect on these materials and to evaluate the durability and performance of these actinide-bearing phases. Studying metamicit state is also important for geochemistry, as U-Pd isotope system is used for age dating.
This work reviews early works/arguments on the relation of hydrogen and metamictization and reports new experiment data. Although it has been recognized that alpha-decay damaged minerals turns to have higher hydrogen contents, the relation between water and metamict state has been an unclear issue. Several important questions have been raised regarding the roles of hydrous species in metamictization and recrystallization, e.g., whether hydrous species stabilize the metamict state; why metamict minerals are generally "wet", even for some normally anhydrous minerals; whether hydrogen in radiation-damaged minerals is in the form of H₂O or OH; what is the role of the presence of H₂O in recrystallization? Since structurally incorporated OH or H₂O can be seen as defects in the crystal structures of minerals, and since their sites and bonding are sensitive to the local structure, investigations into the behaviour of hydrogen-related species in radiation-damaged materials may provide insight into local defects cased by radiation damage. Behavior of hydrogen in radiation damaged single crystals of titanite CaTiSiO₅ and zircon ZrSiO₄ (both materials are proposed as nuclear waste forms) were studied by polarized spectroscopy. The results showed that in addition to structural damage, alpha-decay radiation causes an increase in hydrogen content in damaged regions and hydrogen commonly exists in the form of OH. The data show that heating radiation-damaged titanite and zircon to high temperatures leads to the diffusion of hydrogen from damaged amorphous region to crystalline region during recrystallinzation.

Aluminum oxyhydroxide (boehmite, AlOOH) and aluminum hydroxide (gibbsite, Al(OH)₃) are prominent components in high-level nuclear waste stored in large quantities at the Hanford Site, Washington, U.S.A., and at the Savannah River Site, South Carolina, U.S.A, with future processing plans dependent on developing a predictive understanding of the growth and dissolution behavior of these two materials in highly alkaline solution. However, mechanisms of crystal growth, dissolution, and transformation of these minerals still remain poorly understood, particularly in the complex environment of concentrated sodium hydroxide at low water activity. In this work, magic angle spinning nuclear magnetic resonance (MAS-NMR), high resolution atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS), high resolution powder X-ray diffraction (XRD), and X-ray Pair Distribution Function (PDF) techniques were conducted to investigate the crystallization of gibbsite/boehmite from amorphous aluminum hydroxide gel precursors, the dissolution of gibbsite/boehmite, and the transformation of gibbsite to boehmite. By focusing on understanding the role of aluminum coordination change dynamics from tetrahedral in solution to octahedral in solids and vice versa, and the intermediate pentacoordinate state, some unifying principles governing these transformation emerge, which are of importance for developing reliable techniques to manage the aluminum based minerals in nuclear waste.

In many application areas there is often a need for fit-for-purpose and efficient separation strategies. This is even more important when looking for nuclear waste management solutions where the materials challenges are just one aspect. Holistic assessments including; materials creation, through to application, through to waste disposal, all play a role in informing practical solutions for the extreme environments encountered.
Our research has focused upon the synthesis of novel hierarchical bead materials optimized for selective separations. In line with holistic nuclear waste treatment, the sorbent materials are fabricated with base media that could also potentially be used, post-adsorption, for waste immobilization or as transmutation matrices. To impart selectivity into the sorbents the work has been based upon incorporating extractants, currently used in solvent extraction processes, into solid-phase materials.
Materials for separations in a nuclear context must also be assessed for their chemical stability, including both hydrolytic and radiolytic stability. Group IV metal oxide and phosphonate based materials can impart superior hydrolytic and radiolytic stability and as such are the foundation of much of this work.
Finally, using hierarchical structured beads of separation materials provide significant advantages when implemented in practice. A hierarchical bead structure is desirable as it allows ease of handling and close packing in chromatographic columns. There is also a volume and waste by-product advantage of solid-liquid separation when compared to the more conventional liquid-liquid separation processes. It has also been demonstrated that hierarchical bead morphologies provide greater availability of functional surface with the same selectivity, higher extraction loading, and the advantage of improved kinetics.

Hierarchical materials containing multiscale porosity are promising candidates for improving the performance of radioactive waste containment matrices. Understanding the effect of multiscale porous structures and chemistry on diffusion, extraction kinetics, and capacity for radioactive species is important in designing advanced waste form materials. In this work, we will present a 3D microstructural-dependent diffusion model for investigating the effect of porous structures, and thermodynamic and kinetic properties on uptake kinetics during ion exchange. To demonstrate the model’s capability we simulated Sr²⁺ uptake kinetics in porous Na-LTA zeolites. The thermodynamic and kinetic properties of the ion exchange system were assessed from experimental data. The effect of zeolite particle size, chemical potential and heterogeneous diffusivity on the spatial and temporal evolution of Sr²⁺ and Na⁺, and Sr²⁺ uptake kinetics in a spherical zeolite particle and an aggregation of zeolite particles were simulated and compared with experimental results. It is found that the uptake is kinetically limited by two diffusion phenomena in agreement with experimental observations. Predicted microstructure dependent uptake kinetics is also consistent with experimental data. The model can also be extended to study the effect of bonding phase in the beads of Na- zeolite, and the anisotropic thermodynamic and kinetic properties in salt-inclusion compounds on ion exchange kinetics.
This work was supported as part of the Center for Hierarchical Waste Form Materials, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DE-SC0016574.

Nuclear activities (nuclear power plants, reprocessing plants, dismantling activities…) produce important volume of radioactive effluents which must be treated to minimize their impact on environment. Also, large volumes of contaminated groundwater and seawater have been generated during the Fukushima-Daiichi nuclear power plant disaster. Among the fission products present in these effluents, ⁹⁰Sr is one of the most abundant and hazardous radionuclides for human health. Processes commonly used for ⁹⁰Sr sorption generally involve an ion exchange mechanism on specific sorbents such as sodium titanates and zeolites ¹. However, ⁹⁰Sr removal efficiency by these ion exchange minerals is significantly reduced by the presence of competitive cations (Ca, Mg). On the other hand, in the radioactive liquid effluent treatment stations, barium sulfate is used to extract selectively radioactive strontium from effluents by a coprecipitation process ². This process is selective towards Ca but leads to large volumes of radioactive sludge that have to be managed. To address these concerns, our approach consists in coupling ion exchange and coprecipitation processes. A barium zeolite material has proved to be an effective and selective sorbent for the extraction of strontium from effluents of high salinity like seawater ³. This material demonstrates considerably high capacity and selectivity for strontium with distribution coefficient K_d of 18200 mL.g^-1, obtained from batch sorption tests with seawater spiked with ⁹⁰Sr (59355 Bq.L^-1, m/V = 5 g.L^-1). As evidenced by the SEM image in Figure 1, and XRD analysis, the precipitation of BaSO₄ occurs at the surface of the zeolite grains. The evidenced mechanisms of Sr extraction with this material are quite complex, involving both ion exchange with the Ba ions from the zeolite and the coprecipitation in insoluble barium sulfate formed at the surface of the zeolite grains upon contact with the effluent containing sulfate ions. We propose to expose in more details this material’s characteristics and related extraction mechanisms at the conference.
1) TEPCO Decommissioning plan of Fukushima Daiichi nuclear power - Contaminated water treatment. http://www.tepco.co.jp/en/decommision/planaction/alps/index-e.html (accessed 20/08/2015).
(2) IAEA Radioactive Waste Management Profiles: A compilation of data from the waste management database; Vienna, 2000.
(3) Kim, K. W.; Lee, K.Y.; Lee, E. H.; Baek, Y.; Chung, D. Y.; Moon, J.K. Nuclear Technology, 2016, 193, 318-329.

Radiocesium is one of the most radiotoxic waste water contaminants for human health produced routinely in the nuclear reactors and in accidents such as Fukushima Daiichi. One of the popular methods for removing radiocesium is using Iron ferrocyanide based compounds - Prussian Blue Analogous (PBA) as sorbents for Cesium (Cs) [1-2]. Although Cs sorption properties of various PBAs have been studied, sorption mechanisms involved remain unclear. This can be attributed to the fact that Cs sorption experiments have not been performed under consistent conditions and there is lack of detailed characterization studies using a systematic test matrix.

Preliminary studies indicate that depending on the transition metal in the PBA structure [1], and the ratio K/Cs, the Cs sorption properties and selectivity of the PBA are different. In our current approach we utilize synchrotron based X-ray absorption spectroscopic methods to study the local and chemical structure of the PBAs containing different transition metal ions (Ni²⁺, Co²⁺ and Cu²⁺). Extended X-ray Absorption Fine Structure (EXAFS) and X-Ray Absorption Near Edge structure (XANES) are used to investigate the local coordination environment and chemical structure of PBAs. In addition, the effect of exposure to an acidic environment and changes in local and chemical structure after Cs adsorption is reported. Characterizing the local coordination environment and chemical structure of different PBAs will help better understand the structural stability and Cs sorption capacity of PBAs. This knowledge gained will further provide fundamental understanding of sorption mechanisms involved in Cs extraction as well as aid the design of the next generation PBA sorbents for Cs removal from nuclear waste.

References:

Agne`s Grandjean, Carole Delchet, Jeremy Causse, Yves Barre, Yannick Guari, Joulia Larionova, The Journal of Radioanalytical and Nuclear Chemistry, 2016, 307, 427–436
Merwen Aouadi, Giulia Fornasieri, Valerie Briois, Pierrick Durand, and Anne Bleuzen, Chemistry – A European Journal, 2012, 18, 2617 – 2623

Designing porous materials that can uptake target ions from contaminated effluents in a continuous flow process is a desired way to efficiently contain the ions. Unfortunately, the typical ion-exchange resins suffer from mechanical instability and swelling under flow. Silica and aluminosilicate supports, such as zeolites, are promising because they do not swell and allow for binding specific radionuclides by functionalizing with selective organic chelating complexes. Ultimate confinement can be achieved using a multi-scale silica-based support containing sequestered elements within the pores, potentially forming a precursor for a containment matrix. In fact, mesoporous silica-based materials exhibit properties very similar to high-activity waste glass containment systems.

An aluminosilicate based system of interest is LTA zeolite. We performed density functional theory (DFT) calculations of the binding energies of different ions being incorporated at the 3 distinct doping sites in LTA zeolite. In our study we are used pure silica LTA (ps-LTA), and we introduced 3 different ions, Na⁺, Sr²⁺ and Ba²⁺, in the cavity of the ps-LTA. We considered the system to be in vacuum and in a solvent (water), simulating a more realistic case of the LTA being submerged in a water solution. We show that each of the ions will preferably occupy the same doping site in the ps-LTA, with the binding energy increasing from Ba²⁺ to Sr²⁺ to Na⁺. In addition, we observe that the Na⁺ ion becomes the stable dopant at a high electron chemical potential (Fermi level), < 4.5 eV.

This work was supported as part of the Center for Hierarchical Waste Form Materials, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DE-SC0016574.

Nuclear industry produces high amount of radioactive wastewater from various origins requiring specific and highly selective treatment and confinement matrices. Cesium-137 is found in most of the effluents and is the subject of various studies with a view to improving the uptake and producing the lowest level of final waste. Thus, the development of innovative porous materials is of the highest importance for the treatment of radioactive contaminated wastewater such as column decontamination processes. Designing the most efficient architecture requires to synthesize hierarchically porous materials having a strong affinity for cesium ions, but also to understand pore-dependent reactive transport processes. Previous studies have shown the capacity and selectivity of cyanoferrates nanoparticles (Prussian Blue Analogues) to ion-exchange K⁺ with Cs⁺ even in complex aqueous matrices, but they failed in developing a suitable shape. The use of silica monoliths prevents the constraints of dispersibility that are problematic in the context of final waste management. In addition, monoliths are remarkable supports that improve efficiency and productivity of column processes thanks to an interconnected hierarchically porous network.

Synthetic glass-based silica monoliths were functionalized with potassium/copper hexacyanoferrates nanoparticles inserted into the mesoporosity of the silica support. Extraction experiments of cesium were performed in fresh water using a 3 cm long monolithic column with a diameter of 0.8 cm and a weight of 0.5 g. With a contact time lasting less than one minute, it showed a steep breakthrough curve. The effects of the hierarchical pore structure on hydrodynamics were highlighted for both macroporosity and mesoporosity. The modeling of experimental data took into account the full chemistry of competitive ion sorption as well as advective transport in the interskeleton macropores and diffusion within the mesoporous skeleton. On the other hand, the mechanisms of Cs sorption of both bulk and silica supported hexacyanoferrates were investigated by XRD and EXAFS analyses at the synchrotron facility NSLS-II in order to optimize the crystalline structure of this ion exchanger for cesium capture. This work constitutes the first mechanistic study of hierarchically porous monoliths applied to Cs decontamination processes. The acquired results on silica monoliths will help to design porous glass-based monoliths with the optimized pore size distribution for Cs decontamination. The last step is the thermal collapse of the monolith to immobilize Cs in a dense waste form.

In the nuclear field, functionalized porous silica are envisaged for the decontamination of radioactive effluents [1]. Indeed, molecules having specific head functions can enhance the adsorption capacity and the selectivity of the sorbent material. More particularly, phosphonate group is of great interest because of their selectivity regarding actinides. However, classical grafting of alkoxysilanes requires the use of organic solvents, which presents many drawbacks. Supercritical carbon dioxide (SC CO₂) is a green option and has already given good results on the grafting of Self-Assembly Monolayers (SAM) of different alkoxysilanes. Moreover, SC CO₂ is a green solvent with physical properties facilitating the access of soluble molecules to low dimension gap such as nanopores [2]. Here for the first time, we report an experimental method using supercritical CO₂ grafting process to functionalize planar and concave silica surfaces (Si/SiO₂ wafer, SiO₂ nanochannels consisting in two parallel and planar SiO₂ surfaces spaced of few nm and mesoporous silica) with a phosphonate head group. The method consists on the SC CO₂ grafting of 3-(iodopropyl)trimethoxysilane (IPTMS), followed by a post-functionnalization with a triethylphosphite. This method avoids the possible reactions of the phosphonate head group with surface silanols and allows a homogeneous grafting all over the surface [3].

First, in order to find the optimal process parameters allowing the obtention of a monolayer, the IPTMS SC CO₂ grafting is performed on silica planar surfaces, and characterized by X-Ray Reflectivity, X-ray photoelectron spectroscopy, contact angle and Atom Force Microscopy. Then, to determine the well adapted pore size to the grafting process, silica nanochannels having gap comprise between 3 and 5 nm were also functionalized using the optimal process and characterized by Hard X-Ray Reflectivity at 27 keV (BM32 – ESRF) [4]. A confinement size of 5 nm being the well adapted size to our functionalization method, ordered mesoporous silica (SBA-15 type having a 6 nm pore size) was prepared [5] and characterized using are Small Angle X-Ray Scattering (SAXS), N₂ adsorption-desorption, FTIR-ATR and NMR to validate the presence of phosphonate groups at the pore surface.

Finally, the stability of the grafted silica regarding aqueous solutions at various pH was determined using in-situ SAXS measurements and infrared spectroscopy [6].

References:
[1] Makowski, P. et al., 2012. New Journal of Chemistry, 36, 531-541.
[2] Sanli, D. & Erkey, C., 2015. Journal of Materials Science, 50, 7159–7181.
[3] Corriu, R. et al., 2010. New Journal of Chemistry, 31, 911–915.
[4] Baum, M. et al., 2017.Procedia Earth and Planetary Science, 17, 682–685.
[5] Zhao, D.Y. et al., 1998. Advanced Materials, 10, 1380–1385.
[6] Gouze, B. et al., 2014. Microporous and Mesoporous Materials, 183, 168–176.

Porous aromatic frameworks (PAFs) incorporating a high concentration of acid functional groups possess characteristics that are promising for use in separating lanthanide and actinide metal ions, as required in the treatment of radioactive waste. These materials have been shown to be indefinitely stable to concentrated acids and bases, potentially allowing for multiple adsorption/stripping cycles. Additionally, the PAFs combine exceptional features from metal organic frameworks (MOFs) and inorganic/ activated carbons giving rise to tunable pore surfaces and maximum chemical stability. The adsorption of selected metal ions, Sr2+, Fe3+, Nd3+, and Am3+, from aqueous solutions employing a carbon-based PAF, BPP-7 (Berkeley Porous Polymer-7) has been investigated. This material displays high metal loading capacities together with excellent adsorption selectivity for neodymium over strontium. X-ray absorption spectroscopy studies show that the stronger adsorption of neodymium is attributed to multiple metal ion and binding site interactions resulting from the densely functionalized and highly interpenetrated structure of BPP-7. Recyclability and combustibility experiments demonstrate that multiple adsorption/stripping cycles can be completed with minimal degradation of the polymer adsorption capacity.

Covalent organic frameworks (COFs) are prominent in gas storage/separation, catalysis, and energy-related applications. The crystalline nature of COFs with defined pore sizes allows for a precise structural design to sequester environmental pollutants such as radioactive wastes. The chemical stability of COFs, in general, limits their practical application for adsorbing ions/molecules that are environmentally impactful. Compared with COFs, highly crystalline porous molecular materials, such as hydrogen bonded organic frameworks¹ (HOFs) and porous organic molecules^2,3 that rely on weak interactions to stabilize their frameworks, are often too labile for their wide adoption under environmental settings. Developing hydrogen-bonded crosslinked organic frameworks⁴ (H_COFs), will leverage the advantages of both COFs and HOFs, thus affording high chemical stability for selectively adsorbing environmentally impactful guests. We report the design and synthesis of H_COF-1 through a single crystal to single crystal (SCSC) transformation from molecular precursor via photo-irradiated thiol-yne reactions. H_COF-1 adsorbs I₂ rapidly in an aqueous environment with high uptake capacity and efficiency, associated with an increase in the density of the material that simplifies its isolation. Interestingly, the adsorbed I₂ can interrupt the crystallinity of H_COF-1, which expands its void space to accommodate more I₂ beyond its theoretical capacity. The crystallinity of H_COF-1 can be recovered by releasing the enriched I₂ after solvent evacuation, demonstrating the elastic and recyclable properties of H_COF-1 and its potential in practical application for the active enrichment and removal of radioactive iodine isotopes (¹²⁹I and ¹³¹I) that are liberated during nuclear fuel treatment and nuclear accidents such as the Fukushima nuclear disaster.
References:
(1) He, Y., Xiang, S., Chen, B. J. Am. Chem. Soc., 2011, 133, 14570-14573.
(2) Zhang, G., Presly, O., White, F., Oppel, I. M., Mastalerz, M. A Angew. Chem. Int. Ed, 2014, 53, 1516-1520.
(3) Hasell, T., Cooper, A. I. Nat. Rev. Mater., 2016, 1, 16053.
(4) Lin, Y., Jiang, X., Kim, S. T., Alahakoon, S. B., Hou, X., Zhang, Z., Thompson, C. M., Smaldone, R. A., Ke, C. J. Am. Chem. Soc., 2017, 139, 7172–7175.

Development of novel constituents and architectures is essential for a fundamental understanding of the mechanisms involved in actinide integration inside extended structures necessary to optimize nuclear waste administration. Hybrid materials, such as metal-organic frameworks (MOFs), can be utilized as a foundation for the engineering of actinide-containing materials. The unprecedented modularity of MOFs can address current challenges for efficient storage, separation and selective sequestration of nuclear waste.
In this work, we applied a sequential multi-step approach towards actinide immobilization through utilization of the modularity and versatility of MOFs, which cannot be replicated in any other type of materials. The first examples of actinide-based MOFs with “unsaturated” metal nodes necessary for the further building of hierarchical complexity of actinide-containing materials were prepared. As a result, actinide-bimetallic MOFs were prepared through metal node extension and transmetallation for the first time. Through a combination of solid-state metathesis, guest incorporation, and capping linker installation, we were able to achieve the highest Th wt% in MOFs with minimal structural density. Overall, the role of framework modularity towards stepwise actinide incorporation inside extended structures was demonstrated, which is essential for more efficient nuclear waste management.

Radiotoxic elements constitute a source of pollution from the nuclear industry, for which it is important to anticipate effective processes. Among the elements of high radiotoxicity, ¹³⁷Cs constitutes a priority since it possesses a long half-life of more than 30 years and it decays to ^137mBa presenting a high gamma emission. Liquid solid extraction is a seducing process able to transfer large amounts of Cs from liquid wastes to solid matrices than can be further processed more easily than liquids. However, solid matrices have to present excellent properties in terms of mechanical and chemical resistance in order to be able to withstand hydrodynamic stresses, irradiation damages and aggressive environments.
In this communication, we present in a first part the elaboration, careful characterization and Cs sorption tests of carbon nanotubes papers functionalized with hexaferrocyanate nanoparticles. Two distinct sorption sites were identified on functionalized carbon nanotubes, among which one is highly Cs selective. , A high maximum sorption capacity of about 230 mg.g^-1 is determined, including 80 mg.g^-1 possessing a Cs selective character¹.
In a second part, we present the elaboration and characterization of porous ceramic matrices made of silicon carbide, or silicon oxicarbide. A so-called soft templating approach, involving polymeric templates and polycarbosilane precursors is followed in order to control the porosity in the resulting materials. For example, mesoporous SiC presenting a specific surface area of up 400 m².g^-1 has been obtained.² These materials can be further functionalized in order to be used as Cs sorption matrices.

¹ H. Draouil et al., New Journal of Chemistry, 41(15), 7705-7713 (2017)
² T. Nardin et al., Materials Letters 185, 424-427 (2016)

During the past years, concerns over nuclear plant securities increased following several major crises. Decontamination of radionuclides is one of the main countermeasures to be applied during the initial stage of the response to a severe nuclear emergency. They are mainly based on sorption processes using specific materials. In the present study, complete thermodynamic approach, combining sorption isotherms and calorimetric measurements have been developed on nanostructured solids such as zeolites, mesoporous zeolites, layered materials, or innovative hybrid silicas.
Synthetic zeolites such as Linde-Type A (LTA) 4A are particularly efficient for cesium and strontium decontamination. The determination of the exchange capacity and the displacement enthalpy measured with Isotherm Titration Calorimetry (ITC) has shown that sorption is endothermic for Sr whereas it is exothermic for Cs. The structure evolution of materials during cation adsorption has been studied to determine the various cation sites and their occupation. The enthalpy could be linked to cation diffusion and hydration shell.
In the case of mesoporous LTA zeolite prepared with various amount of structuring agent, sorption capacity and kinetics are not strictly ascribed to the degree of mesoporosity. Nevertheless, the heat effect is first exothermic for the first exchanged Strontium, and becomes endothermic when the surface coverage increases, illustrating the changes in the various contributions of the heat effects (adsorption of strontium, desorption of sodium, and dehydration-hydration properties of these ions). Moreover, the curves are shifted towards lower surface coverage when the amount of structuring agent increases, demonstrating the influence of the mesoporosity on the exchange and dehydration mechanisms.
Nanoflower-like manganate nanostructures were synthesized. The lamellar structure of such materials is considered to strongly influence the retention performance. These materials exhibit high sorption capacity of Strontium from ultrapure water or multi-component aqueous solutions. Nevertheless, in the presence of calcium, competition between the various species is observed, which was correlated with the displacement enthalpy of the various species. In some cases, molecular simulations were performed to rationalize the retention process.
Concerning anions, the sorption of iodide has been studied for hybrid ionosilicas prepared with tetrasylilated ammonium precursor and CTAB as structuring agent. Thanks to ITC measurements, it has been possible to evidence the high radiolytic stability of these original and innovative materials. No morphological, textural and chemical modifications of the material were detected upon electron irradiation, and their performances are similar.
In all cases, complementary information was obtained from the combination of original materials and detailed thermodynamic study of the sorption processes, and especially from ITC measurements.

For the past 30 years, the development of durable materials for radionuclide immobilization has been central to efforts to dispose of wastes generated by the nuclear fuel cycle. There still exist, however, large gaps in the understanding of fundamental modes of waste form degradation under repository conditions. Comprehensive evaluation of waste form performance, including resistance to corrosion, requires detailed knowledge of the atomic-scale effects of long-term self-irradiation. We have recently shown that pair distribution function (PDF) analysis of neutron total scattering measurements can be applied to uniquely characterize radiation effects in a wide range of waste form materials, including fluorite-derivative pyrochlores, spinels, actinide oxides, and glasses. These measurements allow a detailed analysis of both cation and anion defect behavior, and short range order and disorder, which is particularly important for the investigation of aperiodic glasses. Recent results for several simple oxides (e.g., yttria-stabilized zirconia) and complex oxides (e.g., pyrochlore) demonstrate that disordering is much more complex than previously thought with distinct processes occurring over different length scales. Interestingly, local structural units maintain a certain level of atomic order and exist in configurations that appear to be different as expected from measurements at longer length scales.

It has been shown in various metallic systems that chemical stability and nanostructure can be tuned by alloying with a chemically more inert noble metal. For example, Au-Cu alloy and intermetallic nanoparticles with Cu atomic fractions below the parting limit are resistive to dealloying in the presence of a chemical etchant, while those with Cu content above the parting limit tend to form spongy nanoframes. This tuneability thus makes noble metal systems interesting as potential hierarchical waste forms for volatile species such as Mo, Tc, or Rh
Detailed information on atomic scale ordering and nanostructure in order-disorder (intermetallic-solid solution) phase transitions is not easily accessible. This is a result of the so called “nanostructure problem” within the field of classical crystallography. The depth and inverse nature of this problem requires complex modelling to fit observed data, so that structure – processing - property relationships can be leveraged to tailor material performance. Here we probe the correlated nature of disorder in Au-Cu systems, which show prototypical order-disorder (o-d) transformations. We achieve this through a novel approach, by tuning an evolutionary algorithm for global optimization of large ensembles to fit observed pair distribution functions (PDFs). These models are further refined using density functional theory (DFT) to elucidate the subtle bond-length fluctuations associated with correlated disorder.

A major obstacle to the disposal of high-level nuclear waste is the fabrication of durable materials that can safely immobilize radionuclides. Ceramic structures proposed as waste forms for underground repositories must demonstrate stability against self-irradiation of the incorporated actinides as well as chemical durability such that radioisotopes are not leached into the environment. One proposed ceramic waste form, the isometric spinel structure (AB₂O₄), is adopted by many chemical compositions and exhibits excellent radiation tolerance due to its ability to accommodate atomic-scale disordering. However, it is currently not yet fully understood how this disordering mechanism proceeds over a range of length scales and how it influences particle transport and phase stability under operational conditions, which entail high temperatures and self-irradiation. We have shown with neutron total scattering experiments that the short-range structure of disordered Mg_1-xNi_xAl₂O₄ spinel is much more complex than previously thought with highly local cation-ordered distortions affecting the long-range lattice. Pair distribution function analysis suggests that this short-range ordering influences the response of spinel to ion irradiation. This new insight provides a framework by which the behavior of spinel can be more accurately modeled under the extreme environments important for the immobilization of nuclear wastes.

Spent nuclear fuel contains valuable raw materials, such as plutonium and uranium, which can be separated from other elements in order to be recycled. The industrial process implemented at AREVA-La Hague includes a first stage in which the spent fuel is dissolved in hot nitric acid. During this operation, adverse precipitation involving fission products such as molybdenum and zirconium may occur. Understanding of the precipitation mechanisms and appropriate management of the precipitate require a detailed knowledge of the solid phases formed. This work was aimed at deepening the structural study of the mixed zirconium and plutonium (or cerium) molybdates in order to take into account the possible influence of tetravalent elements on the composition of the precipitates.
The study was first conducted by precipitation in the Mo/ Zr-Ce^IV surrogate system where cerium can be considered as a chemical analogue of plutonium. The solid composition strongly depends on the tetravalent element initially contained in solution. The results show the predominance of three phases that are ZrMo₂O₇(OH)₂(H₂O)₂ (1), Ce₃Mo₆O₂₄(H₂O)₄ (2) and Ce₂Mo₃O₁₂(NO₃)₂(H₂O)₂.H₂O (3). Each of these compounds was proved being a solid solution within which the Zr^IV and Ce^IV atoms can substitute each other. Single–crystals of (2) were obtained by hydrothermal synthesis. The crystal-structure was solved in the no-centrosymmetric space group, Cc, which allows to find a structural arrangement in good agreement with the one previously reported, without disorder. Phase (3) was unknown and not reported in the structural database at the beginning of this study. As no single-crystal could be obtained, the crystal structure was solved ab-initio by combining X-ray powder diffraction and X-ray absorption spectroscopy (beamline MARS, synchroton SOLEIL). The structure of Ce₂Mo₃O₁₂(NO₃)₂(H₂O)₂.H₂O derives from Scheelite, ABO₄, within which the Mo atoms are in a tetrahedral environment.
The transposition of the study to the Mo/Zr-Pu^IV system shows that the phases precipitated in the presence of plutonium are isotypes to those formed in the presence of cerium. As such, the mixed zirconium and tetravalent elements molybdates can be seen as a reference system for comparing the behavior of the 4f and 5f elements, even if the precipitation domains differ according to the tetravalent element; the precipitation rate in presence of plutonium appears slower than in presence of cerium.

Stability of materials in radiation harsh environment is an important issue because damage build-up within a material in such an environment can exhibit complex behavior. Of special interest is the response of materials to fission-fragments that can produce extended damage known as fission tracks. To address this, the response of a wide range of crystalline ceramics to swift heavy ion (i.e. fission fragment-like) irradiation has been systematically investigated for nuclear waste applications [1-3].
Dense electronic excitation in the wake of the swift heavy ion can lead to nanoscale material damage along its trajectory called ion (i.e. fission) track. The thermal spike scenario describes this process as a transfer of the deposited swift heavy ion energy from the electronic subsystem into the phonon subsystem via electron-phonon coupling. If the density of the deposited energy is sufficient to induce melting, then an ion track can be formed during rapid quenching of the melt. Otherwise, the deposited energy simply dissipates away without producing damage. This is the origin of the ion track formation threshold and its high valueis desirable for any material used in nuclear energy applications. Lowering of threshold value can have significant, detrimental effects.
We present results of RBS/c, TEM and AFM investigations of ion track formation thresholds in swift heavy ion irradiation resistant materials MgO, Al₂O₃, MgAl₂O₄ and GaN. In case of oxides, we compare thresholds for ion track formation in the bulk [3], [4] and on the surface [5]. Based on the AFM measurements, we present evidence that ion tracks can be easily formed on the oxide surfaces after grazing-incidence swift heavy ion irradiation. Similar to our previous studies on ion tracks on GaN, SrTiO₃ and TiO₂ surfaces [6-8], we show how the threshold for ion track formation can be significantly reduced by applying grazing incidence irradiation geometry.
Another way how this threshold can be reduced is via introduction of defects, usually by means of low energy ion irradiation. Recently, synergistic effects of nuclear and electronic energy loss came into research focus, for example defects in SrTiO₃ can promote ion track formation [9], [10]. As a follow-up of our previous study on GaN [6], we investigated the role of defects in this material with respect to the ion track formation and report additive effects similar to the case of SrTiO₃.

[1] W. J. Weber et al., J. Mat. Res. 13 (1998) 1434
[2] W. J. Weber et al., MRS Bulletin 34 (2009) 46
[3] G. Szenes, J. Nucl. Mater. 36 (2005) 81
[4] S.J. Zinkle et al., Nucl. Instr. Meth. B 191 (2002) 758
[5] V.A. Skuratov et al., Nucl. Instr. Meth. B 250 (2006) 245
[6] M. Karlušić et al., J. Phys. D: Appl. Phys. 48 (2015) 325304
[7] M. Karlušić et al., J. Phys. D: Appl. Phys. 50 (2017) 205302
[8] M. Karlušić et al., J. Appl. Cryst. 49 (2016) 1704
[9] W.J. Weber et al., Sci. Rep. 5 (2015) 7726
[10] H. Xue et al., Acta Materialia 127 (2017) 400

To retain iodine-129 with waste forms in geological settings is challenging due to its extremely long half-life and high volatility in natural environments. To evaluate the long-term performance of nuclear waste forms, it is imperative to characterize the release mechanism of radionuclides in the host materials. This study investigated the release mechanism of iodine retained in apatite structure waste form Pb_9.85(VO₄)₆I_1.75 to determine whether and how diffusion and dissolution control the chemical durability of apatite waste form in aqueous solutions. A series of standard semi-dynamic leaching tests were conducted in different solutions: deionized water, organic pH buffers, and 1 mol/L of NaCl, Na₂CO₃, Na₃PO₄, and Na₂SO₄ solutions. During the leaching experiment, a sample pellet is periodically exposed to fresh leachant solution in a cap-sealed Teflon vessel under constant temperature 90±0.5°C. The leachant solution is replaced every 24 hours. The leachate solutions were analyzed and the leached surfaces were characterized. The results show that the release of iodine is subjected to both diffusion and dissolution processes and is highly sensitive to the solution conditions: 1) in pH neutral solutions, iodine release is controlled by short-term diffusion through ion-exchange and by long-term dissolution of structure matrix; 2) in acidic and basic solutions, dissolution is enhanced and dominates the iodine release from apatite. In addition, new phases are observed on leached surfaces, such as chervetite at pH 4 and hydroxylvanadinite at pH 10. The finding of this study provides two implications for long-term disposal safety of iodine-apatite waste form: i) avoid ion-rich storage environment, especially chloride; ii) create and maintain neutral pH conditions surrounding waste forms.