Safe Disposal Of Nuclear Waste Research Articles

Research progress on microbial adsorption of radioactive nuclides in deep geological environments.

Due to the development and utilization of nuclear energy, the safe disposal of nuclear waste needs to be urgently addressed. In recent years, the utilization of microorganisms' adsorption capacity to dispose of radioactive waste has received increasing attention. When compared with conventional disposal methods, microbial adsorption exhibits the characteristics of high efficiency, low cost, and no secondary pollution. In the long term, microbial biomass shows significant promise as specific chemical-binding agents. Optimization of biosorption conditions, identification of rare earth element binding sites, and studies on the sorption capacities of immobilized cells provide compelling reasons to consider biosorption for industrial applications in heavy metal removal from solutions. However, the interaction mechanism between microorganisms and radioactive nuclides is very complex. This mini-review briefly provides an overview of the preparation methods, factors affecting the adsorption capacity, and the mechanisms of microbial adsorbents.

Multiscale modeling of smectite illitization in bentonite buffer of engineered barrier system

With the increasing usage of nuclear energy, how to properly dispose nuclear waste becomes a critical issue. In this study, a multiscale modeling approach combining the experimental findings is presented to address the illitization process, its impact on transport properties, and system behavior of bentonite buffer in engineered barrier systems (EBS). Through the pore-scale modeling, reactive transport properties such as illite generation rate and effective diffusion coefficient of potassium ion as a function of porosity and temperature are quantified by employing the findings of hydrothermal reaction experiments of Bentonil-WRK. The capability of pore-scale modeling has been developed based on the Darcy-Brinkmann-Stokes equation, involving the processes of smectite illitization and clay swelling. Obtained reactive transport properties are utilized as input parameters for the macroscale modeling to predict the long-term behavior of bentonite buffer in EBS. As such, this study involves the whole workflow of quantifying the reaction parameters of smectite illitization through the hydrothermal reaction experiments, and numerically modeling the reactive transport process of smectite illitization in bentonite buffer of EBS from pore-scale to macroscale. The presented multiscale modeling findings are expected to provide reliable solution for safe nuclear waste disposal with EBS.

Matrices for radioactive waste immobilization: a review

Nuclear energy is considered a clean, reliable, and an inexhaustible energy source for power generation. Nuclear power is harnessed from nuclear fission reactions in a dedicated power plant. The by-products (produced in the nuclear power plant) are radioactive and pose a threat to the environment. The safe disposal of nuclear waste is vital to ensure the sustainable use of the nuclear energy. The immobilization of radioactive waste before final disposal is essential for the interim storage and transportation. This review summarizes the recent work on glass, ceramics, and glass–ceramics matrices to immobilize high-level waste. The synthesis methods, leaching behavior, and radiation resistance of matrices are discussed briefly.

Acid response nanochannels of graphene oxide membranes for fast nuclide ions separation

Separation of nuclides from radioactive wastewater is crucial for the safe disposal of nuclear wastes and the sustainable development of resources. However, it still poses great challenges due to the high acidity and high salty of complex composition in radioactive wastewater. Herein, a pH-responsive graphene oxide (GO) membrane, known as a pGO membrane, was developed for fast and efficient selective separation of Sr2+ nuclide ion from highly acidic radioactive wastewater containing multiple mixed salt ions. Different from traditional GO membranes, the interlayer spacings of pGO membranes can be adjusted and increased with increasing acidity in the surrounding solutions due to the presence of grafting molecule polyethylenimine(PEI) within the modified nanochannels of the pGO membrane. The resulting acidic environmentally adaptable membrane has enlarged and positively charged nanochannels in highly acidic solutions. This structure effectively blocks larger and highly charged hydrated Sr2+ nuclide ions while allowing smaller and lowly charged monovalent salt ions (such as Cs+, K+, Na+) to rapidly transport through the membrane. As a result, the pGO membrane exhibits high selectivity for Sr2+/monovalent salt ions with high permeability for monovalent ions. Overall, this work provides a novel sieving method for the separation of radionuclides in highly acidic solutions.

GeoLaB – Geothermal Laboratory in the crystalline Basement: synergies with research for a nuclear waste repository

Abstract. Challenges for geosciences with societal relevance include the use of crystalline geothermal reservoirs for the energy transition as well as final disposal. Crystalline rocks, along with clay and salt, are being considered as host rocks in the ongoing search for a suitable site for a nuclear waste repository in Germany. Currently, there is no existing experience in terms of excavating a repository within crystalline rocks in Germany. In the case of geothermal energy, the greatest potential – as yet untapped – lies in the crystalline basement. New targeted, science-based strategies are the key to harness this energy under safe, sustainable, predictable, and efficient conditions. The research infrastructure GeoLaB (“Geothermal Laboratory in the crystalline Basement”) is the first underground laboratory to be built in Germany for many years and offers unique opportunities for the geoscientific community. In the following, GeoLaB will be described, and the synergies it offers for geothermal research and also for nuclear waste disposal will be elaborated. GeoLaB aims to scientifically address and overcome the key challenges of deep geothermal exploitation of fractured crystalline reservoirs. To reach these goals of (1) sustainable reservoir technology, (2) wellbore safety, and (3) productivity enhancement in deep crystalline reservoirs, GeoLaB is designed as an underground laboratory consisting of a main gallery starting at the surface, providing access to several subsurface openings. These are typically small caverns in which or from which the planned scientific experiments will be conducted and observed. The first phase of the project began in January 2023 and will primarily consist of an exploration campaign to identify and characterize a suitable site. The Black Forest and Odenwald are considered suitable locations for the planned underground laboratory related to specific scientific and technical criteria. Here, the rocks of the crystalline basement, which are an important geothermal reservoir in the Upper Rhine Graben, have been lifted up and are thus accessible for science. In the Upper Rhine Graben and Black Forest–Odenwald complex, a good database and a high level of expertise has been built-up over decades by science, the petroleum industry, and geothermal activities. The technical measures in the development phase until 2029 include intensive monitoring as well as underground mining development and the construction of a control and visitor center above ground. A digital twin of the GeoLaB, named “Virtual GeoLaB”, will accompany the development of the research infrastructure already during the construction phase and support the scientific investigations, including planning, analysis, and documentation, in the operation phase. It is used for visual analysis and data storage and will play an important role in communication with stakeholders, decision-makers, and the public. In its dialog and participation concept, GeoLaB focuses on transparency and offers citizens opportunities for active participation. It takes action in communication and knowledge transfer as well as education. GeoLaB will be developed as a blueprint for environmentally sound technology developments in the underground, and it will incorporate multiple research perspectives. With BGE (Bundesgesellschaft für Endlagerung) joining GeoLaB, the exploration and building phases, in particular, have become a mutual effort. From the perspective of safe nuclear waste disposal, several topics of the GeoLaB project are of interest even though the target rock characteristics differ to some extent. The collaboration starts with the planning phase for the underground laboratory. The geophysical exploration together with the required drilling, logging, testing, and monitoring of exploration wells is comparable – to a large extent – regardless of the purpose of the mine. Several topics with relevance to Germany's current nuclear waste disposal site selection process are part of the scientific work within GeoLaB, for instance the characterization and quantification of the subsurface stress regime. Furthermore, regulatory topics and outreach intersect greatly. Finally, the excavation of the mine is of utmost interest from the perspective of nuclear waste disposal. Recent experience of mining in Germany is rare, especially with respect to crystalline rocks.

Confronting the Uncertainties Associated with Long-Time Scales: Analysis of the Modes of Preservation of Memory of Radioactive Waste Burial Sites

The burial of radioactive waste in deep geological repositories has gradually imposed itself since the 1980s in various countries. Considered more stable, safe, and responsible than storage above ground, this solution is seen as a way of keeping the waste out of human reach, and of freeing oneself of the obligation to monitor repositories. However, it soon became clear that the idea of relying on the relative stability of deep geological repositories for the safe disposal of nuclear waste to remove it from the ‘all too human’ risks associated with history’s turbulences, has produced new uncertainties, and raised new questions, given the multi-millennial time scales involved. This article presents a study of the strategies adopted by the actors in charge of radioactive waste management in the face of the temporal constraints imposed by the length of their radioactive life. More specifically, it is intended to question the representations of temporality that allow the designers of these projects to assume their responsibilities by transmitting information and warnings towards to an extremely distant future, to recipients who are not easily representable and for whom they already have a responsibility even though they do not yet exist.

Ultrasonic Quality Assurance at Magnesia Shotcrete Sealing Structures.

Engineered barriers are a key element to enable safe nuclear waste disposal. One method currently under research for their construction is magnesia concrete applied in a shotcrete procedure. In this study, the ultrasonic echo method is evaluated as a means for quality assurance. Imaging of internal structures (backwall, boreholes) and defects, such as delamination, has successfully been achieved in the shotcrete. Additionally, detailed information about the potential cause of selected reflectors are obtained by phase analysis. In several test blocks of various sizes, no consistent concrete section boundaries have been found by ultrasonic imaging, which was verified by subsequent drilling and complementary tests. An experiment with artificial defects imitating cracks, air-filled voids, and material with lower density has been challenging and shows the limitations of the current methods. Although significant defects, such as a large delamination, are reliably identified, several smaller defects are not identified. Generally, ultrasonic imaging provides a suitable base as a mean for quality assurance during and after the construction of sealing structures. However, further developments are required to enhance the reliability of the method and a full validation is still pending. Still, the method has potential to increase the safety of nuclear waste repositories.

Lessons from archaeology and heritage studies for the long-term preservation of records, knowledge and memory concerning deep geological disposal sites for nuclear waste

Abstract. Safe disposal of nuclear waste in deep geological repositories requires secure knowledge transfer or knowledge recovery in time spans of many tens of thousands of years. Never before has any detailed record, knowledge or memory been reliably preserved or recovered over comparable time periods. This challenge has been extensively addressed since the late 1980s, initially during the SANDIA workshops in the USA and more recently in the Nuclear Energy Agency/Organisation for Economic Co-operation and Development (NEA/OECD) project on Preservation of Records, Knowledge and Memory Across Generations (Schröder, 2019). Experts from many disciplines including engineering, the natural sciences, information technology, social studies of science and technology, semiotics, public management, and design as well as artists have contributed to these discussions. Some scholars from the humanities have been involved in working on these issues, especially in recent decades. At the same time, much of the existing work has drawn on assumptions about human history, archaeological monuments and cultural heritage that have been scrutinized and deemed deeply flawed by Joyce (2020). The authors of the present paper are archaeologists and cultural heritage experts. For the past decade, they have been working with the challenge of preserving records, knowledge and memory concerning deep geological disposal sites for nuclear waste (Holtorf and Högberg, 2021). From the perspective of the human sciences, in particular archaeology and heritage studies, the unique task at hand involves not only the previously recognized challenges that require consideration of long-term material durability, linguistic intelligibility, and appropriate sense-making of any communicated information but also two challenges not previously addressed: Human action as informed by cultural and social processes. In designing of various long-term mechanisms, we risk overlooking that what people will do is not going to be governed by mechanics. How human beings learn, reason, value, decide, and act is informed by specific cultural and social processes creating context and meaning. We must avoid ignoring these complexities governing human thinking and agency. This challenge requires more work on understanding how sentient and intelligent beings like humans act in variable contexts across time and space. Our anticipatory assumptions. A proverb states that “nothing ages faster than the future”. In making assumptions about future generations' understandings, meanings, and significances of our nuclear waste we risk “colonizing” the future, fail to embrace variability over time, and miss realizing multiple futures and emerging conditions. We must therefore not foreclose uncertain futures but instead create circumstances favorable for change and transformation of relevant knowledge and memory. This challenge requires more work with processes of translation between generations. The challenges of assessing our anticipatory assumptions and understanding how humans act will also need to be addressed in transmitting records, knowledge and memory for the benefit of future generations.

External force estimation for robotic manipulator base on particle swarm optimization

The safe disposal of nuclear waste in radioactive environment urgently needs cost-effective approaches. Toward this goal, this article developed a method to external force estimation based on the identified model without force sensors. Firstly, the mathematical model including joint friction was obtained and transformed into the linear combination of unknown parameter to be estimated. Secondly, the unknown parameters were identified based on the improved particle swarm optimization algorithm, the identification procedure was implemented by optimizing the excitation trajectories to excite joint motion and sampling relevant data. Identified results were compared with the biogeography-based optimization algorithm and the cuckoo search algorithm. Then, the identified dynamic parameter was applied to external force estimation. Finally, the verification of external force estimation has been carried out using the Kinova Jaco2 robot manipulator, and the experimental results showed that the external forces by the proposed method could be estimated with an root mean square error of 0.7 N.

Phase distribution of uranium in matrices obtained by co-melting basalt and uranium-containing aluminum oxide

Safe disposal of nuclear waste is key to sustainable use of nuclear energy. Exploring safe and reliable disposal methods for nuclear waste has become a research hotspot. The phase distribution of uranium in matrices obtained by co-melting basalt and uranium-containing Al2O3 was examined for the development of a potential material for radioactive waste immobilization. The co-melting of basalt with UO3-containing Al2O3 produced matrices containing glass, plagioclase, and spinel as major phases. The highest uranium content (97 mass % UO3) was observed on the non-dissolved fragment of the Al2O3 grains. The titanium-containing complex oxide phase, supposedly, ilmenite, contained up to 62 mass % UO3. The glass phase exhibited an uneven distribution of uranium, with its content ranging ~ 5 to ~ 10 mass % UO3. Data on the leaching of uranium into H2O from the co-melt of basalt and UO3-containing Al2O3 were obtained.

An Experimental Study on the Mechanical Properties of High-Temperature Granite under Natural Cooling and Water Cooling

With the further development of deep rock mechanics engineering, such as the exploitation and utilization of geothermal resources, the exploitation of deep mineral resources, and the safe disposal of nuclear waste, the study of mechanical properties of deep high-temperature rock is gaining the attention of the researchers. However, not only the high temperature but also the cooling condition/method that will be used in the construction such as drilling cooling will also greatly affect the mechanical properties of the rock. In this paper, the mechanical behaviour and the evolution of the mechanical properties of the high-temperature (600°C–1,000°C) granite under different cooling methods are studied. The following conclusions can be obtained: (1) The peak stress of the granite decreases with the heating temperature. Compared with natural cooling, water cooling has a more significant effect on strength degradation. (2) The increase of the heating temperature increases the maximum axial strain of the granite. The water cooling method more greatly induces the maximum axial strain of granite than the natural cooling. The maximum axial strain of the specimen under the water cooling reaches 117.3% of that under natural cooling (800°C). (3) The elastic modulus of the granite decreases with the heating temperature. Water cooling will have a stronger effect on the reduction of the elastic modulus than natural cooling. The maximum difference value (2.02 GPa) of the elastic modulus under the different cooling methods occurs at the temperature of 800°C. (4) Poisson’s ratio of the granite increases with heating temperature, and the cooling method does not have an evident effect on it. The relationship between Poisson’s ratio and the heating temperature under different cooling methods can be described using the linear model. (5) According to the influence of the temperature on the peak stress, the elastic modulus, and Poisson’s ratio, the heating temperature domain can be divided into the unapparent zone, the significant zone, and the mitigation zone. (6) The thermal stress due to the nonuniform temperature field and the different thermal expansion coefficients is incompatible. Such incompatibility stresses the essences of the degradation of the mechanical properties of the granite.

Aminosilane-grafted bismuth-alumina adsorbents: Role of amine loading and bismuth content in iodine immobilization from aqueous solutions

Development of adsorbent materials that can efficiently remove radioactive species, in particular, iodine from aqueous waste streams is of great importance to prevent the discharge of these highly toxic chemicals into waterways, while providing a means for a safe disposal of nuclear wastes. In this study, we present a detailed investigation of the performance of a series of bismuth-loaded alumina adsorbents (Bi/Al) grafted with three different aminosilanes including 3-Aminopropyl-triethoxysilane (APS), N-methylaminopropyl-trimethoxysilane (MAPS), and 3-N,N-dimethylaminopropyl-trimeth-oxysilane (DMAPS), in the removal of iodine from aqueous solutions. Specifically, we studied the effects of bismuth loading, amine type and content, as well as iodine concentration and capture temperature to determine the best performing material and removal conditions for iodine immobilization from caustic scrubber solutions. Our results indicated that amine grafting can enhance the iodine removal performance of Bi-Al, with DMAPS giving rise to highest capacity. It was also found that iodine capture over amine-grafted Bi/Al changes dramatically with amine content. Among the materials investigated, Bi5/Al-DMAPS with an amine content of 60 wt% exhibited the highest iodine capture capacity of 215.7 mg/g at 50 °C. Moreover, amine grafting improved the retention of adsorbed iodine species within the adsorbent over time with no degree of leaching after 24 h, unlike Bi/Al2O3 analogue.

Activated carbon additives for technetium immobilization in bentonite-based engineered barriers for radioactive waste repositories

Safe disposal of nuclear waste in a geologic repository will rely on natural geologic features and engineered barriers to greatly retard the movement of radionuclides from the repository. Clay minerals including bentonite are effective in retarding the migration of many radionuclides, but are ineffective for anionic radionuclides, of which pertechnetate is of particular concern owing to its relatively long half-life and the lack of natural isotopes that dilute it. Activated carbon is proposed as an additive material for reducing pertechnetate mobility in the nearfield. Activated carbon materials of different origins quantitatively sorb pertechnetate from aqueous solution under oxidizing conditions during the first day of contact, and sequential extraction showed that 73 % of this technetium is in the strongly bound fraction. X-ray photoelectron spectra (XPS) and extended X-ray absorption fine structure (EXAFS) spectra both demonstrated that no reduction of technetium occurred in the studied systems. The interaction of technetium with a composite material consisting of bentonite and activated carbon was studied at the first time. Effective technetium sorption was shown, with distribution coefficients (Kd) up to 740 cm3. g−1.

The new K, Pb-bearing uranyl-oxide mineral kroupaite: Crystal-chemical implications for the structures of uranyl-oxide hydroxy-hydrates

AbstractKroupaite (IMA 2017-031), ideally KPb0.5[(UO2)8O4(OH)10]·10H2O, is a new uranyl-oxide hydroxylhydrate mineral found underground in the Svornost mine, Jáchymov, Czechia. Electron-probe micro-analysis (WDS) provided the empirical formula (K1.28Na0.07)Σ1.35(Pb0.23Cu0.14Ca0.05Bi0.03Co0.02Al0.01)Σ0.48 [(UO2)7.90(SO4)0.04O4.04(OH)10.00]·10H2O, on the basis of 40 O atoms apfu. Sheets in the crystal structure of kroupaite adopt the fourmarierite anion topology, and therefore kroupaite belongs to the schoepite-family of minerals with related structures differing in the interlayer composition and arrangement, and charge of the sheets. Uptake of dangerous radionuclides (90Sr or 135Cs) into the structure of kroupaite and other uranyl-oxide hydroxy-hydrate is evaluated based on crystal-chemical considerations and Voronoi-Dirichlet polyhedra measures. These calculations show the importance of these phases for the safe disposal of nuclear waste.

Ligand architectural effect on coordination, bonding, interaction, and selectivity of Am(iii) and Ln(iii) ions with bitopic ligands: synthesis, solvent extraction, and DFT studies.

The isolation of Am(iii) ion from Ln(iii) ions is very crucial for the safe disposal of nuclear wastes and thus, studies are being continuously pursued to accomplish this goal. In view of this, herein, a new conformationally rigid bitopic ligand, N,N,N',N'-tetra(2-ethylhexyl)piperazine-di-methylenecarboxamide (PIPDA) has been synthesized and studied for the separation of Am(iii) from Ln(iii) ions. The effect of structural rigidification on the selectivity of Am(iii) over Ln(iii) was compared with an open chain flexible compound, namely, N,N,N',N'-tetra(2-ethylhexyl)-3,6-(N'',N'''-dibutyl)diaza-octane-1,8-diamide (DADA). Two oxygen atoms of the diamide moiety seem to be responsible for controlling the metal ion extraction ability of PIPDA, whereas two nitrogen atoms of the piperazine moiety most probably dictate the separation factor between the Am(iii) and Eu(iii) ions in PIPDA. In addition, scalar relativistic density functional theory (DFT) in conjunction with Born-Haber thermodynamics was used herein to compliment the experimental selectivity. The experimentally observed preferential selectivity of PIPDA for Am(iii) ion over the Ln(iii) ion was corroborated by the computed extraction free energy, ΔGext. The covalent nature of bonding between the metal ions and the ligand was confirmed by analyzing the Mayer bond order and bond character analysis using the atom in molecule concept. Though the conformational rigidity of PIPDA gives stronger interaction than DADA, it does not offer a significant advantage over DADA in terms of the separation factor. The marginal increase in the separation factor for PIPDA over DADA might be attributed to the piperazine nitrogen and to the ligand architecture during complex formation.

Effect of solution chemistry on the iodine release from iodoapatite in aqueous environments

To ensure the safe disposal of nuclear waste, understanding the release process of radionuclides retained in the nuclear waste forms is of vital importance. Iodoapatite Pb9.85(VO4)6I1.7, a potential waste form for iodine-129, was selected as a model system for ceramic waste forms in this study to understand the effect of aqueous species on iodine release. Semi-dynamic leaching tests were conducted on bulk samples in cap-sealed Teflon vessels with 0.1 mol/L NaCl, Na2CO3, Na3PO4, and Na2SO4 solutions under 90 °C, fixed sample surface area to solution volume ratio of 5/m, and periodic replacement of leaching solutions. The reacted solutions were then analyzed by Inductively Coupled Plasma-Mass Spectrometry and Inductively Coupled Plasma-Optical Emission Spectrometry; the leached surfaces were characterized by X-ray diffraction, scanning electron microscopy, and infrared spectroscopy. The result shows that, compared to deionized water, the ion-rich solutions enhanced the iodine release as a result of the increased ionic strength, reduced activity coefficient of dissolved species, and increased solution pH. Surface reactions can lead to the formations of secondary phases by ion-exchange and precipitation. These findings suggest that an ion-rich environment in the geological repository can be detrimental to the disposal safety of the nuclear waste form.

Investigation on the mechanism of the immobilization of CeO2 by using cullet-based glass (CBG)

The safe disposal of nuclear waste is a key factor restricting the sustainable use of nuclear energy. Exploring safe and reliable disposal methods for nuclear waste has become a research hotspot. In this paper, cullet-based glass (CBG) was chosen as a new immobilization agent to host the plutonium (Pu). The immobilization mechanism of Pu in the CBG was investigated by using cerium (Ce) as a surrogate for Pu, and the interaction was characterized by using XRD, XPS, FTIR and SEM-EDS analyses. The results indicate that there are two synergistic immobilization mechanisms by which Ce is hosted in the vitrified products. One mechanism is doping of the Ce into the crystal lattice of the glassy matrix to form CeOSi bonds, which increases the proportions of non-bridging oxygen (NBO) bonds in the glass structure. The other mechanism is Ce co-existing in the glass structure gaps as individual crystals of CeO2. The synergistic immobilization makes Ce hard to release. The PCT results show that the normalized leach rate of Ce decreases with the Ce partitioning ratio in the CeO2 crystalline phase. These data show that CBG can be used as a good immobilization agent to vitrify the Pu presented in high-level radioactive wastes.

Ultratrace Determination of 99Tc in Small Natural Water Samples by Accelerator Mass Spectrometry with the Gas-Filled Analyzing Magnet System.

In the frame of studies on the safe disposal of nuclear waste, there is a great interest for understanding the migration behavior of 99Tc. 99Tc originating from nuclear energy production and global fallout shows environmental levels down to 107 atoms/g of soil (∼2 fg/g). Extremely low concentrations are also expected in groundwater after diffusion of 99Tc through the bentonite constituting the technical barrier for nuclear waste disposal. The main limitation to the sensitivity of the mass spectrometric analysis of 99Tc is the background of its stable isobar 99Ru. For ultratrace analysis, the Accelerator Mass Spectrometry (AMS) setup of the Technical University of Munich using a Gas-Filled Analyzing Magnet System (GAMS) and a 14 MV Tandem accelerator is greatly effective in suppressing this interference. In the present study, the GAMS setup is used for the analysis of 99Tc in samples of the seawater reference material IAEA-443, a peat bog lake, and groundwater from an experiment of in situ diffusion through bentonite in the controlled zone of the Grimsel Test Site (GTS) within the Colloid Formation and Migration (CFM) project. With an adapted chemical preparation procedure, measurements of 99Tc concentrations at the fg/g levels with a sensitivity down to 0.5 fg are accomplished in notably small natural water samples. The access to these low concentration levels allows for the long-term monitoring of in situ tracer tests over several years and for the determination of environmental levels of 99Tc in small samples.

Additives in Plasticised Polyvinyl Chloride Fuel Microbial Nitrate Reduction at High pH: Implications for Nuclear Waste Disposal

The safe disposal of nuclear waste necessitates a thorough understanding of the role microorganisms play on the fate of the waste materials over time. Despite significant volumetric contributions to nuclear waste inventories, little is known about the fate of plastics under the high pH and irradiating conditions of a geological repository. We conducted microbial enrichment experiments to assess the potential for additives in plasticised polyvinyl chloride (PVC) sheet, commonly used in the nuclear industry, to fuel nitrate reduction by a microbial community adapted to grow at pH 10. These additives include plasticisers and flame retardants, amongst others. PVC powder (no additives) served as an additive-free control, and samples of both materials were gamma irradiated (1 MGy) under hyperalkaline conditions, representative of conditioned nuclear waste, to assess the effect of irradiation on bioavailability. Plasticised PVC supported near-complete nitrate reduction, whether irradiated or not, although irradiated PVC sheet supported less nitrate reduction. No nitrate reduction was observed with non-irradiated PVC powder, although irradiated powder supported minor nitrate reduction. These results highlight the bioavailability of volumetrically significant plasticised PVC under conditions relevant to its geological disposal in cementitious intermediate level nuclear waste and highlight the critical need to constrain downstream effects on biogeochemical processes ultimately impacting on the safety case for disposal.

Nollmotzite, Mg[UV(UVIO2)2O4F3]·4H2O, the first natural uranium oxide containing fluorine.

Nollmotzite (IMA2017-100), Mg[UV(UVIO2)2F3O4](H2O)4, is a new uranium oxide fluoride mineral found in the Clara mine, Black Forest Mountains, Germany. Electron microprobe analysis provided the empirical formula (Mg1.06Cu0.02)Σ1.08[UV(UVIO2)2O3.85F3.15][(H2O)3.69(OH)0.31]Σ4.00 based on three U and 15 O + F atoms per formula unit. Nollmotzite is monoclinic, space group Cm, with a = 7.1015 (12) Å, b = 11.7489 (17) Å, c = 8.1954 (14) Å, β = 98.087 (14)°, V = 676.98 (19) Å3 and Z = 2. The crystal structure [twinned by reticular merohedry; refined to R = 0.0369 with GoF = 1.09 for 1527 unique observed reflections, I > 3σ(I)] is based upon [UV(UVIO2)2F3O4]2- sheets of β-U3O8 topology and contains an interlayer with MgF2(H2O)4 octahedra. Adjacent sheets are linked through F-Mg-F bonds, as well as via hydrogen bonds. The presence of fluorine and pentavalent uranium in the structure of nollmotzite has potentially important implications for the safe disposal of nuclear waste.