Sellafield is home to the UK's largest repository of nuclear waste, including reprocessed uranium and plutonium, as well as a backlog of unprocessed used fuel and waste kept in outdated storage facilities; commonly referred to as "legacy waste". For this reason, Sellafield has often been called the most hazardous place in Western Europe and as such, is currently undergoing a multi-billion pound decommissioning and clean-up operation. Each on-site facility has unique challenges associated with it, many of them presenting situations where the radiation chemistry aspects of the material degradation are not well understood. The key factors that can affect water radiolysis processes in the Sellafield challenges are a high pH environment, the presence of magnesium hydroxide, the presence of iron oxide, and the presence of organic materials. This work examines the effect each of these factors has on H2 and H2O2 production in water radiolysis as well as developing a computational model to offer some understanding to the kinetic behaviour of water radiolysis under such conditions. The computational model was able to replicate experimental measurements of radiolytic H2 and H2O2 production in both aerated and deaerated water at neutral pH, and provide a further understanding of the role of dissolved oxygen in water radiolysis. Measurements of H2O2 from solutions containing NaOH have shown that an increase in pH generally results in a higher steady state of H2O2, while measurements of H2 show a similar increase with a maximum production rate at pH ~11. The model was also able to closely replicate these experimental measurements with some over prediction, which highlights a gap in our understanding of high pH radiolysis and also brings into question the validity of the estimated rate constant for the reaction: O- + O2- → 2OH- + O2 k= 6.0×10^8 M^-1 s^-1 which was originally determined from kinetic model calculations designed to describe the decay of ozonide (O3ˉ) during pulse-radiolysis studies of high pH solutions conducted byK. Sehested et al in 1982.The radiolysis of magnesium hydroxide slurry also resulted in an increased yield of hydrogen gas but had little effect on the yield of hydrogen peroxide. The hydrogen yield was 0.52 molecules per 100eV while a NaOH solution of equivalent pH gave a yield of 0.27, however interference from carbonate may be the cause of the increased yield. A surface effect was also estimated to contribute 0.05 molecules per 100 eV to the hydrogen gas yield. Hydrogen gas and hydrogen peroxide was measured from the radiolysis of aqueous methanol. This was modelled with a near agreement, but modifications to the model were necessary; highlighting areas of the model that need improvement, as well as providing a reaction scheme from which a more comprehensive model for aqueous methanol radiolysis could be developed.

The nuclear waste inventory of the UK comprises large quantities of intermediate level wastes (ILW), which will be immobilised by encapsulation within a cementitious grout in stainless steel containers, followed by disposal in a deep engineered geological disposal facility (GDF) within a suitable geological formation. These wastes contain, in addition to radioactive elements, a heterogeneous mix of organic materials, including plastics, cellulose and rubber. Cellulosic items, such as cloth, tissue, filters, paper and wood, are considered particularly problematic, because they are known to be susceptible to degradation under alkaline conditions, forming small chain organic acids with the ability to complex metals and radionuclides. It is predicted that under alkaline conditions isosaccharinic acid (ISA) will form particularly strong complexes with Ni(II), Am(III), Eu(III), Np(IV), Th(IV), and U(IV). As a result, the presence of ISA could affect the migration behaviour of these elements, by increasing their solubility and reducing sorption, thus enhancing their mobility into the near and far field surrounding a GDF. During site operation and then after closure of a GDF, microbial communities have the potential to colonise the steep biogeochemical gradients, running from highly alkaline in the GDF “near field” to circumneutral pH conditions in the surrounding geosphere. Within these steep pH gradients microbial processes can control the fate of organic compounds, such as ISA, and have therefore been considered as an effective self-attenuating mechanism to remove ISA from the groundwater. This thesis aims to deliver a greater understanding of the microbial processes that can potentially use ISA as a carbon source and electron donor, removing it from solution, and thus having a positive impact on radionuclide mobility under GDF-relevant conditions. A microbial enrichment approach was chosen that approaches GDF-relevant conditions to explore the biodegradation of ISA. Cross-disciplinary analyses of water chemistry (pH, Eh, photospectroscopy, IC, ICP), mineralogy (ESEM, XRD, TEM, XAS) and microbiology (light microscopy, next generation sequencing) have demonstrated the ability of bacteria to degrade ISA over a wide range of biogeochemical conditions. Furthermore, key radionuclides (and their non-active analogues), including Ni(II) and U(VI), were precipitated from the groundwater system during ISA biodegradation. Moreover, in the case of uranium, microbial metabolism led to the reduction of U(VI) to U(IV), which is also less soluble. This study highlights the potential for microbial activity to help remove chelating agents from groundwaters surrounding an ILW GDF, and suggests that safety cases that do not include microbial processes may be overly conservative, over-estimating the impact of ISA on radionuclide transport.

Cette thèse s’inscrit dans le cadre des études sur le stockage des déchets radioactifs HA/MAVL en couches géologiques profondes. L’objectif est de tester la capacité des codes couplés chimie-transport à prendre en compte l’impact des variations de porosité sur le transport des solutés, et ce, en concevant des expériences de diffusion colmatage/dissolution les plus simples possibles, pour limiter au maximum le nombre de degrés de liberté du système.Pour ce faire, des montages expérimentaux de type « diffusion traversante » ont été adaptés pour l’étude de différents milieux poreux de complexité croissante : du sable compacté, des frittés de verre, de la craie, du grès jusqu’aux matériaux proches de ceux envisagés dans le cadre d’un stockage, les argilites de Tournemire. En même temps qu’une expérience de diffusion de traceur inerte, des réactions de précipitation (oxalate de calcium, gypse ou barytine) ou de dissolution (attaque acide de la craie) ont été réalisées, de manière à évaluer leur influence sur le flux diffusif de ces traceurs. En fin d’expérience, les milieux poreux et les précipités ont été caractérisés par MEB-EDS. Enfin, ce large jeu de données a permis de tester un des codes de chimie-transport utilisé dans cette thèse, le code Crunch. Les résultats obtenus à partir des premières expériences de diffusion/colmatage réalisées au travers des sables et des frittés de verre tendent à démontrer que le premier montage expérimental utilisé n’était pas adapté à ces milieux poreux très perméables. Les courbes de flux sont en effet très bruitées, et les observations des solides au MEB ne révèlent pas l’existence d’un front de précipité, comme attendu, mais seulement de nouvelles phases éparpillées au sein des milieux poreux. Ceci est à mettre en lien avec de possibles écoulements parasites de solution lors des prélèvements.En revanche, les expériences réalisées au travers de la craie, qui est plus imperméable, n’ont pas été entachées par ces artefacts. Dans ce cas, il a été clairement observé au MEB un front de précipitation au sein de la craie, dont l’effet sur les flux diffusifs de traceurs de l’eau (HTO ou HDO), est plus ou moins important suivant le type de précipité. Ainsi, d’un côté, la cellule avec précipitation de barytine est impactée dès le départ par le colmatage, avec une baisse continue du flux de HDO, qui peut être jusqu’à 40 fois plus faible que le flux mesuré dans le milieu poreux sain. En revanche, la cellule avec précipitation de gypse est impactée beaucoup plus tardivement par le colmatage (70 jours après le lancement de l’expérience), et de manière moins marquée, avec des valeurs de flux baissant d’un facteur 3 par rapport à celles mesurées dans le milieu sain. Tout ceci tend à suggérer que l’efficacité du colmatage est plus liée à la nature du minéral qu’à la quantité précipitée, la barytine étant probablement plus dense et moins poreuse que le gypse. En plus, après plus de deux mois d’expérience, le flux de HTO de la cellule contenant l’échantillon d’argilite de Tournemire ne montre aucun impact en lien avec l’éventuelle précipitation de gypse, ce qui est cohérent avec les très faibles coefficients de diffusion mesurés dans cette roche. En outre, pour les frittes de verre, on a vu un léger effet de la précipitation de la barytine sur la diffusion. En ce qui concerne la simulation, les premiers essais réalisés avec Crunch n’ont pas permis de reproduire les résultats expérimentaux, particulièrement ceux issus de l’expérience de diffusion/précipitation de barytine. Ceci peut s’expliquer par la façon dont sont prises en compte les lois cinétiques dans le code, mais plus probablement par la formulation de facto trop empirique de la loi d’Archie, utilisée pour relier l’évolution du coefficient de diffusion à celle de la porosité / This phD is in the frame of nuclear waste storage . The purpose is to test the ability of coupled chemical transport code to study the impact of porosity changes on solute transport, by designing clogging/dissolution experiments .To do this, experimental setups "through-diffusion" have been adapted to study the various porous media with increasing complexity: the compacted sand, glass frit , the chalk, sandstone close up to the materials used in the context of a storage, argillites Tournemire. Together with an inert tracer, diffusion experiment, precipitation reactions (calcium oxalate, gypsum or barite) or dissolution (acid attack chalk) were performed to assess their influence on the diffusive flux of these tracers. At the end of experience, the porous media and precipitates were characterized by MEB-EDS. Finally, this large data set was used to test one of the coupled chemical transport codes used in this phD, the Crunch code. The results from the first diffusion / clogging experiments performed through sand and glass frits show that the first experimental setup was not suited to such highly permeable porous media. The flux Curves are very noisy, and SEM observations do not reveal the existence of a precipitate front, as expected, but only new scattered phases in porous media. This is due to possible parasitic flow solution during sampling. In contrast, experiments through the chalk, have not been touched by these artifacts. In this case, it was clearly observed by SEM a precipitated front within the chalk, the effect on diffusive flux of water tracers (HTO and HDO) is more or less important depending on the type of precipitate. Thus, on the one hand, the cell with barite precipitation is impacted from the start by clogging , with a continued decrease of the HDO flux , which can be up to 40 times lower than the flux measured in the porous media without clogging. However, the cell with gypsum precipitation is affected much later by the clogging (70 days after the start of the experiment), and less markely, with flux values lowering a factor of 3 compared to those measured in healthy environment. All this tends to suggest that the efficiency of the clogging is more related to the nature of mineral than the precipitated amount, barite is probably denser and less porous than gypsum. In addition, after more than two months of experience, the HTO flux of the cell containing the Argilite Tournemire sample shows no impact related to the possible gypsum precipitation, which is consistent with the very low diffusion coefficients measured in this rock. In addition, for glass frits, there has been a slight effect of the barite precipitation on the flux. With regard to the simulation, the tests carried out with Crunch did not permit to reproduce the experimental results, particularly those from the diffusion / precipitation barite experiment. This can be explained by the way are considered kinetic laws in the code, but more probably to empirical formulation of the Archie law, used to link the evolution of diffusion coefficient with the porosity .

<p>Reflection seismic and seismic tomography methods can be used to image the upper kilometer of hard bedrock and the loose unconsolidated sediments covering it. Developments of these two methods and their application, as well as identifying issues concerning their usage, are the main focus of the thesis. Data used for this development were acquired at three different sites in Sweden, in Forsmark 140 km north of Stockholm, in the Oskarshamn area in southern Sweden, and in the northern part of the Siljan Ring impact crater area.</p><p>The reflection seismic data were acquired with long source-receiver offsets relative to some of the targeted depths to be imaged. In the initial processing standard steps were applied, but the uppermost part of the sections were not always clear. The longer offsets imply that pre-stack migration is necessary in order to image the uppermost bedrock as clearly as possible. Careful choice of filters and velocity functions improve the pre-stack migrated image, allowing better correlation with near-surface geological information.</p><p>The seismic tomography method has been enhanced to calculate, simultaneously with the velocity inversion, optimal corrections to the picked first break travel times in order to compensate for the delays due to the seismic waves passing through the loose sediments covering the bedrock.</p><p>The reflection seismic processing used in this thesis has produced high-quality images of the upper kilometers, and in one example from the Forsmark site, the image of the uppermost 250 meters of the bedrock has been improved. The three-dimensional orientation of reflections has been determined at the Oskarshamn site. Correlation with borehole data shows that many of these reflections originate from fracture zones. </p><p>The developed seismic tomography method produces high-detail velocity models for the site in the Siljan impact area and for the Forsmark site. In Forsmark, detailed estimates of the bedrock topography were calculated with the use of the developed tomography method.</p>

<p>Diss. Stockholm : Handelshögsk.</p>

Welds in materials intended for service at temperatures above the creep activation temperature often develop damage before the base metal. The weld is a discontinuity in the material and stresses and strains often accumulate in the weld. Knowledge of the properties of the weld is essential to the safe operation of the component containing the weld. The work in this thesis has been aimed at the study of welds in service at high temperatures: The work is divided into two main chapters. The first chapter deals with welds in stainlesssteels and dissimilar metal welds and includes three papers, and the second chapter dealswith welds in copper intended for nuclear waste disposal, also including three papers. Common to both parts is that the temperature is high enough for most of the damage in the welds to result from creep. In the first part the role of the weld microstructure on the creep crack propagation properties has been studied. Experiments using compact tension specimens have been performed on service exposed, low alloyed heat resistant steels. The results show good correlation with the crack tip parameter, C*, during steady state creep crack growth. The test methodology has also been reviewed and sensitive test parameters have been identified. The results from the creep crack propagation tests on service exposed material has been modeled using uniaxial creep data on both new and ex-service material. The development of the weld microstructure in a dissimilar metal weld between two heat resistant steels has also been investigated. A weld was made between one ferritic and one martensitic steel and the development of the microstructure during welding and post-weldheat treatments has been studied. The results show that the carbon depleted zone that develops near the weld metal in the lower alloyed steel depends on the formation and dissolution of the M23C6-carbide. Variations of the weld parameters and the post-weld heat treatment affect the size and shape of this zone. The process has been successfully modeled by computer simulation. The second part focuses on oxygen free copper intended for nuclear waste disposal containers. The containers are made with an inner core of cast nodular iron and an outer core of copper for corrosion protection. The copper shell has to be welded and two weld methods has been tested, electron beam welding and friction stir welding. Creep specimens taken from both weld types have been tested as have base metal specimens. The technical specifications of the waste canisters demand that the creep ductility of both the copper shell and the welds has to be as high as possible. The creep test results show that base material doped with at least 30 ppm phosphorus has high creep ductility, and friction stir welds made from this material has almost as high creep strength and creep ductility. Copper without phosphorus does not exhibit the same ductility. The creep properties evaluated from testing has been modeled and extrapolated for the intended purpose / QC20100719

This dissertation focuses on key intergroup and intragroup separations found in the back end of the nuclear fuel cycle, specifically americium from lanthanides and americium from other actinides, most importantly americium from curium. Our goal is to implement a liquid-solid separation process to reduce waste and risk of contamination by the development of metal(IV) phosphate phosphonate inorganic organic hybrid ion exchange materials with the ideal formula of M(O6P2C6H4)0.5(O3POA) * nH2O, where M = Zr or Sn, A = H or Na. These materials have previously shown to have high affinity for Ln, this work will expand on the previous studies and provide methods for the above target separation, exploiting oxidation state and ion charge to drive the separation process. The optimum hydrothermal reaction conditions were determined by adjusting parameters such as reaction temperature and time, as well as the phosphonate to phosphate (pillarto-spacer) ligands ratio. Following these results four bulk syntheses were performed and their ion exchange properties were thoroughly examined. Techniques such as inductively coupled mass spectrometry and liquid scintillation counting were used to determine the affinity of the materials towards Na+, Cs+, Ca2+, Sr2+, Ni2+, Nd3+, Sm3+, Ho3+, Yb3+, NpO2+, Pu4+, PuO22+, Am3+, AmO2+, and Cm3+. Separation factors in the thousands have been observed for intergroup separations of the Ln from the alkali, alkaline earth, and low valent transition metals. A new method for Am oxidation was developed, which employed Na2S2O8 as the oxidizing agent and Ca(OCl)2 as the stabilizing agent for AmO2+ synthesis. Separation factors of 30-60 for Nd3+ and Eu3+ from AmO2+, as well as 20 for Cm3+ from AmO2+ were observed at pH 2. The work herein shows that a liquid-solid separation can be carried out for these difficult separations by means of oxidation and ion exchange.

En el siglo pasado se ha desarrollado una nueva energía: la nuclear. Su aplicación comprende tanto el campo militar como el campo civil. Unos de los aspectos más interesantes es el uso de esta energía para la producción de electricidad de uso civil. El combustible nuclear es de larga duración: unas barras de uranio pueden llegar a producir energía eléctrica durante una década aproximadamente. Sin embargo, a mediados del siglo pasado se planteó el problema de los residuos nucleares. Este problema es muy complejo por que, aunque la duración del combustible sea de una década, el periodo de decaimiento es del orden de siglos. Esta tesis quiere aportar su granito de arena a esta nueva rama de la ciencia nuclear, estudiando algunos aspectos del comportamiento geotécnico de los agregados de sal, para su uso como material de relleno en los depósitos definitivos de los residuos nucleares de mayor peligrosidad.La regla fundamental de diseño de dichos depósitos es que su función debe desarrollarse sin la ayuda del hombre o de máquinas y que la misma naturaleza tiene que ser quien proteja al hombre del riesgo de radiaciones durante el transcurso de los siglos. Parece que al día de hoy la solución mundialmente aceptada es la de un depósito profundo en formaciones rocosas, a veces utilizando antiguas minas, incluso de sal. Se prevé que entre esta década y la próxima entrarán en funcionamiento varios de estos depósitos en los países en que la investigación en este área está más avanzada.En Europa se está haciendo un esfuerzo común, bajo supervisión de la Unión Europea y por medio de proyectos de investigación, para estudiar el funcionamiento del almacenamiento y la difusión de los resultados de esos estudios entre los miembros de la Unión Europea, a fin de dar el mayor provecho a las diferentes investigaciones que en la actualidad se están desarrollando en todos los centros de investigación de Europa.Esta tesis trata sobre el comportamiento termo hidráulico de los agregados de sal debido a las altas temperaturas que el decaimiento de los residuos nucleares produce. Este fenómeno crea unos efectos en los materiales de relleno y en la formación de base de alrededor del deposito.La tesis tiene un desarrollo básicamente experimental. En el laboratorio se realizaron fundamentalmente dos ensayos; el primero permitió determinar la curva de retención de los agregados de sal altamente compactados. Para ello se utilizó la técnica de traslación de ejes para aplicar la succión matricial y se construyó una placa de succión apta a los materiales salinos para la realización de estos ensayos.Este ensayo permitió determinar el comportamiento de una propiedad básica de los materiales salinos no saturados, sus resultados pudieron aplicarse al resto de los trabajos que se realizaron con posterioridad en esta tesis.El objeto del segundo ensayo de laboratorio fue reproducir los fenómenos de variación de porosidad debidos a la presencia de una diferencia de temperatura en las extremidades de la muestra. Dicho ensayo tiene una importancia fundamental para los almacenamientos profundos, porque los residuos radiactivos serán una fuente de calor durante varios siglos y la presencia de elevadas temperaturas induce fenómenos de flujo y transporte en el contacto con los contenedores de carburante nuclear agotado. Además, en el caso de los agregados de sal, a estos fenómenos cabe añadir el intercambio de sal entre la fase sólida y la líquida (precipitación /disolución).Para desarrollar estas pruebas de laboratorio, se dedicó una parte importante de la investigación al desarrollo y comprobación del equipo de ensayo, siendo éste poco convencional y completamente novedoso. Este último ensayo, en su segunda fase, fue convalidado mediante un modelo numérico (CODE BRIGHT). Los resultados del modelo numérico han permitido confirmar todo cuanto se determinó en la fase experimental y la importancia de la succión matricial en estos fenómenos.

Carbonation is a naturally-occurring process whereby Ca-containing cement phases lose their hydration water and are converted to carbonate minerals by reaction with atmospheric CO&#8322;. As these secondary minerals develop in the microstructure of hydrated cement, porosity, pore-size distribution and permeability are decreased. These are all considered desirable properties in a wasteform. The objective of this study was to examine the effect of carbonation and different pozzolans on the leach performance and mechanical strength of ordinary Portland cement (OPC) wasteforms. Two methods of accelerated cement carbonation were used: <ol> <li>A vacuum carbonation method, where wasteforms are placed in an evacuated, sealed cell and subjected to small additions of CO&#8322; over several days at near vacuum conditions; and <li>A one-step carbonation method, where CO&#8322; gas is added to the wasteform paste as it is being mixed. </ol> Thirteen elemental constituents of interest to the safety assessments of long-term management of Ontario Power Generation's radioactive waste (Cl, N, S, Se, 13C, Th, Pb, Co, Ni, Cu, Sr, Ba and Cs) were stabilised/solidified via cement mix water. Wasteforms were produced with only OPC, OPC and fly ash, or OPC and silica fume. Most wasteforms were carbonated using one of the carbonation methods. Some wasteforms were not carbonated and served as controls. Wasteforms were subjected to either standard leach tests or compressive strength tests. The extent of carbonation was found to be about 20% for vacuum carbonation method, substantially higher than that for one-step treatment (up to about 10%). For vacuum carbonated wasteforms, carbonation occurred at the outer selvages of the wasteforms, whereas one-step treatment resulted in homogenous carbonation. Generally, compared to uncarbonated OPC wasteforms, vacuum carbonation increased leaching of elements that are anionic in cementitious conditions (Cl, N, S, Se, 13C, Th), decreased leaching of large metal cations (Sr, Ba, Cs, Pb) and had negligible effect on the leaching of the elements that form hydroxyl complexes (Co, Ni, Cu). 13C was the only anionic element whose leachability was reduced by vacuum carbonation, as it may be precipitated in the form CO32- in the large quantity of secondary carbonate minerals produced during the vacuum carbonation process. One-step carbonation did not result in substantial reductions in leachability, compared to uncarbonated OPC wasteforms. However, it had an interesting inverse effect on large metal cation leachability from fly ash- and silica fume-containing wasteforms. A model is presented that proposes that porewater pH changes can have an effect on waste element leachability because 1) the C-S-H Ca/Si ratio is dependent on the equilibrating porewater pH and 2) the degree of ion sorption on C-S-H is dependent on the C-S-H Ca/Si ratio. This model should be tested experimentally as it has important implications on wasteform design. Because of this inverse behaviour, overall neither pozzolan outperformed the other with respect to leachability. Generally, for uncarbonated wasteforms, OPC retained the elements more effectively than OPC with pozzolans. For pozzolans, the leachability of these elements from OPC with fly ash was lower than that of OPC with silica fume. Leaching of Cs was anomalously low from uncarbonated OPC wasteforms, but follow-up experimentation did not corroborate this anomaly. Further testing of these wasteforms to determine how the mineralogical fate of Cs can differ between wasteforms is recommended. All wasteforms tested were of acceptable strength (<0. 689 MPa). Fly ash, and, to a greater degree, silica fume, improved wasteform strength when compared to OPC wasteforms. Carbonation treatments had little effect on wasteform strength. This study has provided much information about the leaching characteristics of a representative set of waste elements from several cement-based wasteform treatments. Although it has not indicated a wasteform design that is ideal for all elements studied, it does suggest that some treatments may be effective for certain groups of elements. Most notably, vacuum carbonation shows promise in improving the immobilisation of isotopes of large metal cations such as Sr, Ba, Cs and Pb as well as 14C (as suggested by 13C here) in cement-based wasteforms.

Carbonation is a naturally-occurring process whereby Ca-containing cement phases lose their hydration water and are converted to carbonate minerals by reaction with atmospheric CO&#8322;. As these secondary minerals develop in the microstructure of hydrated cement, porosity, pore-size distribution and permeability are decreased. These are all considered desirable properties in a wasteform. The objective of this study was to examine the effect of carbonation and different pozzolans on the leach performance and mechanical strength of ordinary Portland cement (OPC) wasteforms. Two methods of accelerated cement carbonation were used: <ol> <li>A vacuum carbonation method, where wasteforms are placed in an evacuated, sealed cell and subjected to small additions of CO&#8322; over several days at near vacuum conditions; and <li>A one-step carbonation method, where CO&#8322; gas is added to the wasteform paste as it is being mixed. </ol> Thirteen elemental constituents of interest to the safety assessments of long-term management of Ontario Power Generation's radioactive waste (Cl, N, S, Se, 13C, Th, Pb, Co, Ni, Cu, Sr, Ba and Cs) were stabilised/solidified via cement mix water. Wasteforms were produced with only OPC, OPC and fly ash, or OPC and silica fume. Most wasteforms were carbonated using one of the carbonation methods. Some wasteforms were not carbonated and served as controls. Wasteforms were subjected to either standard leach tests or compressive strength tests. The extent of carbonation was found to be about 20% for vacuum carbonation method, substantially higher than that for one-step treatment (up to about 10%). For vacuum carbonated wasteforms, carbonation occurred at the outer selvages of the wasteforms, whereas one-step treatment resulted in homogenous carbonation. Generally, compared to uncarbonated OPC wasteforms, vacuum carbonation increased leaching of elements that are anionic in cementitious conditions (Cl, N, S, Se, 13C, Th), decreased leaching of large metal cations (Sr, Ba, Cs, Pb) and had negligible effect on the leaching of the elements that form hydroxyl complexes (Co, Ni, Cu). 13C was the only anionic element whose leachability was reduced by vacuum carbonation, as it may be precipitated in the form CO32- in the large quantity of secondary carbonate minerals produced during the vacuum carbonation process. One-step carbonation did not result in substantial reductions in leachability, compared to uncarbonated OPC wasteforms. However, it had an interesting inverse effect on large metal cation leachability from fly ash- and silica fume-containing wasteforms. A model is presented that proposes that porewater pH changes can have an effect on waste element leachability because 1) the C-S-H Ca/Si ratio is dependent on the equilibrating porewater pH and 2) the degree of ion sorption on C-S-H is dependent on the C-S-H Ca/Si ratio. This model should be tested experimentally as it has important implications on wasteform design. Because of this inverse behaviour, overall neither pozzolan outperformed the other with respect to leachability. Generally, for uncarbonated wasteforms, OPC retained the elements more effectively than OPC with pozzolans. For pozzolans, the leachability of these elements from OPC with fly ash was lower than that of OPC with silica fume. Leaching of Cs was anomalously low from uncarbonated OPC wasteforms, but follow-up experimentation did not corroborate this anomaly. Further testing of these wasteforms to determine how the mineralogical fate of Cs can differ between wasteforms is recommended. All wasteforms tested were of acceptable strength (<0. 689 MPa). Fly ash, and, to a greater degree, silica fume, improved wasteform strength when compared to OPC wasteforms. Carbonation treatments had little effect on wasteform strength. This study has provided much information about the leaching characteristics of a representative set of waste elements from several cement-based wasteform treatments. Although it has not indicated a wasteform design that is ideal for all elements studied, it does suggest that some treatments may be effective for certain groups of elements. Most notably, vacuum carbonation shows promise in improving the immobilisation of isotopes of large metal cations such as Sr, Ba, Cs and Pb as well as 14C (as suggested by 13C here) in cement-based wasteforms.