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This paper analyzes part of the reactor experiments on the study of tritium and helium release from promising two-phase lithium ceramic (Li2TiO3 and Li4SiO4) of natural lithium enrichment conducted by vacuum extraction.The basis for such an analysis was a more careful study of the time trend of pressure changes of gases in the chamber with the test samples. In a particular case, it was clearly shown that the pressure fluctuations observed during irradiation for gases with mass number M4 (to which both HT and He molecules correspond) are determined only by He, which leaves the intergranular regions of the ceramic through open channels or cracks. The kinetics of changes in the amount of helium that is released during irradiation was traced and both the rate of helium release and the frequency of emissions were determined. It was assumed that the observed emissions correspond to a certain “formation of free paths” from the internal cavities of the irradiated ceramics into the chamber of the facility. The data obtained for the helium emissions were compared with the release of tritium-containing molecules from the ceramics. The quasi-equilibrium levels of the release of tritium-containing molecules and their dependence on the reactor power were estimated. The release of helium and tritium was compared with the calculated values of the tritium generation rate in the test sample.

Advanced lithium orthosilicate (Li4SiO4) pebbles with additions of lithium metatitanate (Li2TiO3) as a secondary phase have attracted international attention as an alternative solid-state candidate for the tritium breeding in future nuclear fusion reactors. In this research, the generation of radiation-induced defects in the Li4SiO4 pebbles with various contents of Li2TiO3 was analysed in-situ by X-ray induced luminescence technique. After irradiation with X-rays, the accumulated radiation-induced defects in the Li4SiO4 pebbles were studied by electron spin resonance, thermally stimulated luminescence and absorption spectrometry. On the basis of the obtained results, it is concluded that the generation mechanism and the structure of primary radiation-induced defects (except Ti3+centres) in the advanced Li4SiO4 pebbles with additions of Li2TiO3 under exposure to X-rays is similar to the single-phase ceramics. In addition, it is expected that the additions of Li2TiO3 can increase the probability for the recombination processes of primary radiation-induced defects in the advanced Li4SiO4 pebbles during irradiation and thereby reduce the formation of thermally stable radiolysis products, such as colloidal lithium particles.

This work presents the preliminary experimental data on the study of gas release from two-phase lithium ceramics Li$_{4}$SiO$_{4}$-Li$_{2}$TiO$_{3}$ under neutron irradiation conditions. Experiments were carried out at the WWR-K research reactor (Almaty, Kazakhstan) for ���4.3 days. The total neutron fluence during the irradiation was ���1.8��10$^{19}$n/cm$^{2}$. In the course of irradiation, two experiments on ceramics heating during irradiation and two experiments with hydrogen isotopes (H$_{2}$ and D$_{2}$) supply into the experimental chamber with the sample were performed at a temperature of 680 ��C and reactor power of 6 MW. During the entire irradiation, the gas composition in the continuously evacuated ampoule device with samples was recorded. The obtained dependences of the release of tritium-containing molecules and helium during the experiment were qualitatively analyzed.

In the development of fusion energy, an important task is the study and improvement of tritium production technologies. In this case, one of the most promising materials for tritium generation is lithium ceramics. Considering the importance of the task, numerous studies are aimed at solving the problem of determining the parameters and mechanisms of tritium release in lithium-containing materials. This paper presents the results of a study of tritium release processes from two-phase lithium ceramics of Li$_{4}$SiO$_{4}$/Li$_{2}$TiO$_{3}$ during reactor irradiation when hydrogen and deuterium are injected into the chamber with irradiated samples. The mechanisms regularities of the tritium yield process in the presence of these isotopes were established. The experiments were carried out in the WWR-K research reactor at a neutron flux density of 5���10$^{13}$n/cm$^{2}$���s and sample temperatures from 650 to 700 ��C.

In a solid breeding blanket, ceramic breeder pebbles are in contact with reduced activation ferritic martensitic (RAFM) steel at high temperatures for years and accordingly form a corrosion layer on the blanket structural steel. Present study focuses on corrosion characteristics of EUROFER97 RAFM steel by an advanced breeder material of Li2TiO3 with excess Li (initial ratio of Li/Ti = 2.2) at 623, 823, and 1073 K under sweep gas (He + 0.1% H2) flow. Formation of a thick oxide double layer was found on the surface of the EUROFER plates heated at 823 and 1073 K, while the corrosion layer formed at 673 K was

The tritium loading of current grades of advanced ceramic breeder pebbles with three different lithium orthosilicate (LOS)/lithium metatitanate (LMT) compositions (20–30 mol% LMT in LOS) and pebbles of EU reference material, was performed in a consistent way. The temperature dependent release of the introduced tritium was subsequently investigated by temperature programmed desorption (TPD) experiments to gain insight into the desorption characteristics. The obtained TPD data was decomposed into individual release mechanisms according to well-established desorption kinetics. The analysis showed that the pebble composition of the tested samples does not severely change the release behaviour. Yet, an increased content of lithium metatitanate leads to additional desorption peaks at medium temperatures. The majority of tritium is released by high temperature release mechanisms of chemisorbed tritium, while the release of physisorbed tritium is marginal in comparison. The results allow valuable projections for the tritium release behaviour in a fusion blanket.

Chemical compatibility between ceramic breeder (Li 4 SiO 4 + 20 mol% addition of Li 2 TiO 3 ) and EUROFER97 steel was examined in this study. These materials were contacted and heated at 623, 823 and 1073 K under He + 0.1 vol.% H 2 atmosphere for up to 12 weeks. Limited influence was found in the breeder specimens, although losses of the constituent elements appeared near the surface of the breeder pellets heated at 1073 K. For the EUROFER specimens with formation of a corrosion layer, element diffusivity was estimated based on diffusion kinetics. In the temperature range, effective diffusion coefficients of oxygen into EUROFER steel were in the range from 3.5 × 10 −14 to 2.5 × 10 −12 cm 2 /s and found to be faster than that of Li. The coefficients yielded an activation energy of 0.93 eV for oxygen diffusion into EUROFER steel and predicted the possible thickness of the corrosion layer after operational periods.

As the knowledge of the effective thermal conductivity of ceramic breeder pebble beds under fusion relevant conditions is essential for the development of solid breeder blanket concepts, the EU advanced and reference lithium orthosilicate material were investigated with a newly developed experimental setup based on the transient hot wire method. The effective thermal conductivity was investigated in the temperature range RT–700 °C. Experiments were performed in helium and air atmospheres in the pressure range 0.12–0.4 MPa (abs.) under a compressive load up to 6 MPa. Results show a negligible influence of the chemical composition of the solid material on the bed’s effective thermal conductivity. A severe reduction of the effective thermal conductivity was observed in air. In both atmospheres an increase of the effective thermal conductivity with the temperature was detected, while the influence of the compressive load was found to be small. A clear dependence of the effective thermal conductivity on the pressure of the filling gas was observed in helium in contrast to air, where the pressure dependence was drastically reduced.

Reduced activation ferritic/martensitic steel F82H samples with tritium permeation barriers were annealed in contact with ceramic breeder pebbles at 550 °C and 700 °C for up to 32 days under purge gas. Cr2O3-formed F82H rapidly reacted to form three layers at 550 °C, and pure Fe projections grew with annealing time. Diffusion kinetics of O and Li were well fitted with Fick’s first law of diffusion. Er2O3 coatings did not react with the pebbles, but deteriorated by oxygen diffusion through the coatings. The impurity concentrations in the system and the coating thickness affect the coating compatibility in solid breeder blanket environments.

The recycling of tritium breeding materials will be necessary for any future use of nuclear fusion energy due to economical as well as ecological considerations. In the case of the solid breeder blanket concept, the ceramic pebble beds that are intended for the generation of tritium will eventually need to be restored due to depleted lithium levels as well as due to fractured pebbles, which will cause a deterioration of the pebble bed properties. It is proposed that the pebbles, which are fabricated using a melt-based process, are recycled using the same initial process, by replenishing the lithium levels and reforming the pebbles at the same time. To prove this recycling scheme, advanced ceramic pebbles were fabricated and then re-melted multiple times to prove that the reprocessing did not have any negative effect on the pebble properties and secondly, pebbles were produced with a simulated lithium burn-up and subsequently replenished by additions of LiOH to the melt. It was shown that the re-melting and lithium re-enrichment had no effect on the pebble properties, demonstrating that a melt-based process is suitable for recycling used breeder pebbles.

Advanced ceramic breeder pebbles consisting of Li4SiO4 and additions of Li2TiO3 were tested regarding their long-term thermal and to their radiation stability. As-prepared and long-term annealed pebbles were irradiated with accelerated electrons (up to 6 MGy) to investigate the formation of radiation-induced defects and radiolysis products caused by ionizing radiation. By using Raman spectroscopy the formation of significant amounts of radiolysis products can be excluded. Electron spin resonance spectrometry revealed several paramagnetic radiation-induced defects, such as HC2 centres ( SiO 4 3 - and TiO 3 - ), E’ centres ( SiO 3 3 - and TiO 3 3 - ), Ti 3 + centres and peroxide radicals ( Si O O·). Radiation-induced defects forming in the first stage of the radiolysis seem to be more pronounced in long-term annealed samples. Thermally stimulated luminescence showed relatively low temperatures (40–200 °C) for the thermal recovery of the radiation-induced defects by accelerated electrons. Moreover, the main recovery process of defects takes place in the first 8–10 days after the irradiation.

Multiple tritium breeder recycling is an attractive option to make a future fusion power reactor ecologically and economically sustainable. This research project aims at exploring the feasibility of multiple breeder ceramics recycling by remelting with regard to the activation behavior. The average activation level of multiple reused breeder pebbles is calculated for a DEMO reactor with HCPB (Helium Cooled Pebble Bed) blankets on the assumption that all spent pebbles are mixed and reprocessed by remelting. The calculation loop verifies the feasibility of a multiple reprocessing scheme with an acceptable waiting period of approximately 22 y for the remote handling dose rate level—even after 15 times recycling. This paper also describes the limitation of the current approach for the average activation property and provides some recommendations for future improvements.

In the framework of the EUROfusion’s Power Plant Physics and Technology, the working package breeding blanket (BB) aims at investigating four different BB concepts for an EU demonstration fusion reactor (DEMO). One of these concepts is the helium-cooled pebble bed (HCPB) BB, which is based on the use of pebble beds of lithiated ternary compounds and Be or beryllides as tritium breeder and multiplier materials, respectively, EUROFER97 as structural steel and He as coolant. This paper aims at giving an overview of the EU HCPB BB Research and Development (R&D) being developed at KIT, in collaboration with Wigner-RCP, BUTE-INT, and CIEMAT. The paper gives an outline of the HCPB BB design evolution, state-of-the-art basic functionalities, requirements and performances, and the associated R&D activities in the areas of design, functional materials, manufacturing, and testing. In addition, attention is given also to the activities dedicated to the development of heat transfer augmentation techniques for the first wall and the corresponding testing. Due to their nature as design drivers, a brief overview in the R&D of key HCPB interfacing areas is given as well, namely, the tritium extraction and recovery system, the primary heat transfer and power conversion systems, and safety topics, as well as some specific activities regarding the integration of in-vessel systems through the BB. As concluding remarks, an outline of the standing challenges and future R&D plans is summarized.

The effect of the operating pressure on the droplet formation dynamics of a molten laminar jet was studied using a high-speed camera. A melt was formed in a platinum alloy crucible at 1380 °C, after which a pressure was applied to the system to form a jet through a 300 μm nozzle. The molten jet breaks up into droplets as described by the Plateau-Rayleigh theory, which are then solidified using liquid nitrogen to form small ceramic spheres. A high-speed camera was positioned directly below the nozzle to obtain footage for a full range of operating pressures at 3500 frames per second, which was then analysed using image processing methods. An optimum operating pressure was found at which the desired instabilities grow the fastest and the variance between the droplet spacing is at a minimum. This resulted in an increased yield and a narrower size distribution of the final product.

The development of silicon microcomponents and microsystems, being an outgrowth of the processing techniques commonly used for the fabrication of integrated circuits, was constrained to mostly planar dimensions. This was because optical lithography and most photoresists were developed for this specific purpose. In many applications, however, the components require that the vertical dimensions be larger than those obtained by traditional optical lithography methods. This led to the development of high aspect ratio processing in which the vertical dimension can be large compared to the lateral dimensions. While there is no apparent agreed-upon definition of high aspect ratio, the processes are generally characterized by x-ray lithography and/or micromechanical machining methods, with postprocessing and replication methods suitable for the dimensions involved. While there have been many accomplishments toward making these processes as economically viable as integrated circuit processes, much work remains to be done. The following sections describe the high aspect ratio processes and replication techniques. The work is a compilation of efforts worldwide and is meant to provide an overview of the processes and techniques.

Lithium orthosilicate pebbles with 2.5 wt% excess of silica are the reference tritium breeding material for the European solid breeder test blanket modules. On the surface of the pebbles chemisorption products of carbon dioxide and water vapour (lithium carbonate and hydroxide) may accumulate during the fabrication process. In this study the influence of the chemisorption products on radiolysis of the pebbles was investigated. Using nanosized lithium orthosilicate powders, factors, which can influence the formation and radiolysis of the chemisorption products, were determined and described as well. The formation of radiation-induced defects and radiolysis products was studied with electron spin resonance and the method of chemical scavengers. It was found that the radiolysis of the chemisorption products on the surface of the pebbles can increase the concentration of radiation-induced defects and so could affect the tritium diffusion, retention and the released species.

Low-pressure injection moulding is a very efficient process for net shape manufacturing of ceramic micro parts. In order to obtain sintered ceramic specimens without shape distortion or damages, density gradients in the green bodies have to be avoided. Especially feedstocks with a solid loading near the critical powder volume content often tend to segregate the binder while flowing. However, the value of critical powder content can be significantly influenced by particle size, particle size distribution and particle morphology. This paper compares two powder mixtures of identical chemical compositions with different specific surfaces and morphology and evaluates their workability for low-pressure injection moulding. The aim of this paper is to identify the influence of morphology on feedstock rheology as well as on accuracy, mechanical properties and microstructure of net shape manufactured reaction-bonded zircon ceramics.

Effect of moisture in sweep gas on chemical compatibility between ceramic breeder (Li 4 SiO 4 with 20 mol% of Li 2 TiO 3 ) and EUROFER97 was examined in this study. These materials were contacted and heated at 623, 823, and 1073 K for up to 12 weeks where the H 2 O concentrations of the outlet gas were 0.13-0.24 vol.%. In the two-phase breeder specimen, Li 4 SiO 4 was preferably decomposed into Li 2 SiO 3 with a possible formation of Li 2 O or LiOH depending on environmental condition. Formation of corrosion layers on the EUROFER specimen was enhanced in the wet condition, especially at the elevated temperatures of 823 and 1073 K. Though these results showed parabolic growth rates, an extrapolation from data of the elevated temperatures to 623 K was not feasible in the wet atmosphere, which could derive from the difference of the chemical form in the breeder specimen. Effective diffusion coefficient of oxygen into EUROFER at 623 K was given to be 4.5 × 10 −14 cm 2 /s and a possible thickness of the corrosion layer after a 2-year use was predicted.

The mechanical properties of ceramic parts are strongly influenced by the quality of the surface. In the present work it is demonstrated that for the surface quality, material specific properties (sintering activity, grain morphology) as well as process-related factors (debinding and sintering) are vitally important. For the exemplarily investigated materials zirconium dioxide (ZrO2) and silicon nitride (Si3N4) the surface morphology of micro specimens turned out to be the limiting factor for high bending strength. Due to its fine and homogenous microstructure ZrO2 allows the formation of very smooth surfaces with correspondingly high characteristic strength up to 3,000 MPa. Si3N4 is characterized by a highly anisotropic microstructure, pronounced grain growth, and thereby high surface roughness. Although both materials reveal virtually the same strength level in macro dimensions, in micro dimensions Si3N4 seems to be the inferior choice with a maximum strength of 1,200 MPa.

Lithium orthosilicate and lithium aluminate are the candidates of the breeder material of the tritium-breeding blanket. To enhance the rate of tritium release from these breeder materials, 0.2 wt.% of palladium was impregnated into the breeder materials by the incipient wet impregnation method. The lithium orthosilicate and the lithium aluminate pebbles with or without Pd were irradiated in the Kyoto university research reactor, and out-of-pile annealing experiments were performed. When the breeder materials were deposited with a catalytic additive, such as Pd the ratio of the tritium discharged in the molecular form of tritium (HT) increased. It was also found that the amount of the tritium discharged from the lithium orthosilicate pebbles with a catalytic additive at the low temperatures is greater compared with lithium aluminate.

In order to study the significance of as-sintered surfaces for the strength of Si3N4 micro components, micro bending specimens with dimensions of 240 μm × 240 μm × 1400 μm were prepared from sintered reaction-bonded silicon nitride. The processing parameters sintering temperature and dwell time were varied, and two different types of powder bed were applied during sintering. The characteristic strength σ0 – determined by micro-3-point-bending tests – varied from about 500 to 1200 MPa. Strength values of about 1000 MPa and higher were observed when powder beds were applied which were newly prepared and doped with the sintering additives Y2O3, Al2O3, and MgO. In that case, the mass loss during sintering was negligible or even a small mass gain occurred. Samples sintered in used and undoped powder beds by contrast showed mass loss and an enrichment of the secondary phase YN-melilite (Y2Si3O3N4) at the surface; for these samples σ0 was found to be restricted to about 500–700 MPa. Contrary to expectations, it was observed that the residual porosity – ranging from less than 1% up to 11% – does not act as a predominant strength-determining factor. Instead, the more or less pronounced formation of surface-near defects, due to the decomposition and volatilization of Si3N4 during sintering, is considered to be decisive for the achievable strength. Mass loss and YN-melilite formation are indicators for defect creating reactions. Due to the increased surface-to-volume ratio of micro components, the affected surface zone in particular determines the overall mechanical properties.

The solid breeder blanket concept proposed by the European Union features the tritium breeding ceramics as pebble beds inside several breeder units. The pebble bed will not take part in the structural rigidity of a breeder unit. Still the pebbles need to withstand forces originating from thermal expansion mismatches and neutron irradiation. A designated breeder ceramic for this concept is lithium orthosilicate with a slight surplus of silica. It has been demonstrated that lithium orthosilicate can be processed by melt spraying, which is favorable to other processes in terms of recycling. This work summarizes changes to the process and investigations of the current limitations that have to be overcome to enhance not only the mechanical properties of the pebbles, but also the yield of the process. A key feature of the improvements is to control the forming of the single pebbles more precisely than in the past. Thereby, not only the generation of pores should be avoided, but also the size distribution of the pebbles should be more accurately controlled.

Low-pressure injection moulded (LPIM) zirconia (3Y-TZP) micro bending bars with bending strength up to 2808 MPa were fabricated. It was found that an increase of the surfactant concentration of the processed feedstocks results in an improvement of surface finish and strength of sintered micro bending bars. During thermal debinding, the formation of a binder film on the surface of the micro parts could be observed, which is responsible for the healing of surface defects. Due to high surface-to-volume ratio surface defects determine the fracture behaviour. Thus, the mechanical properties are improved, if the number and size of surface defects and the surface roughness of the micro bending bars can be reduced. However, an improvement of the surface quality is usually associated with rounding of edges and, in the worst case, the loss of geometrical integrity. Several material and process-related factors were found to influence shape retention and, as a result, surface quality and strength of LPIM ceramic micro parts during the binder removal. To ensure a reproducible fabrication it is necessary to control all material and process-related factors. In this study an interesting opportunity for an in-process improvement of the strength of ceramic micro parts is presented.

The dependence of the rheological properties of zirconia–paraffin feedstocks for low-pressure injection moulding (LPIM) on binder composition, storage time and temperature was investigated. Feedstocks with varying amounts of dispersant were fabricated to work out the required quantity for a monolayer of dispersant molecules on zirconia particles. Experimental results revealed that a shear rate dependent characteristic of the viscosity against the amount of dispersant exists. The observations were compared with calculated values according to an adsorption model, which overestimated the required quantity of dispersant to form a monolayer. Feedstocks stored at elevated temperature for several days exhibited a time-dependent decrease of the yield stress and viscosity, which is supposed to be caused by physical or chemical interactions among zirconia and the dispersant. Increasing working temperature and decreasing solids loading were found to significantly decrease the yield stress as well as the viscosity. Flow activation energies and flow indices were calculated and compared with literature. This study shows that the dispersant used in this investigation has a remarkable influence especially on the time-dependent flow behaviour of zirconia–paraffin feedstocks that affects further processing and reproducibility.

This second part of the report deals with, how the sintering additives Y 2 O 3 , Al 2 O 3 , and MgO influence the sintering behaviour of SRBSN. Paraffin-based feedstocks with varying sintering aid compositions and silicon grain size were used for moulding macro- and micro-scale samples. It was observed that compositions with smaller Si grain size (with correspondingly high SiO 2 content) and containing Al 2 O 3 as sintering additive exhibit higher shrinkage and lower residual porosity when sintered at 1700–1800 °C after nitridation. The mechanical properties determined for micro-scale samples were obtained by three-point bending tests, with the resulting characteristic strength values σ 0 ranging from 500 MPa up to 1200 MPa. Surprisingly residual porosity did not play the role of a strength limiting factor; rather it was observed that the presence of crystalline secondary phases – mainly Y 2 Si 3 O 3 N 4 – was responsible for reducing the micro-bending strength. As micro-samples exhibit a large surface-to-volume ratio they are in particular affected by decomposition of Si 3 N 4 and volatilization of SiO 2 which is considered to be responsible for the occurrence of secondary phases preferred at the sample surface. The powder bed condition was also found to play a prominent role in the development of the secondary phases during liquid phase sintering.

For the manufacturing of reaction-bonded ceramic microparts by low-pressure injection moulding, feedstocks with an optimized rheological behaviour are required. In order to evaluate the main influences on the rheological properties of feedstocks and their possible interactions, various compositions were systematically tested in the frame of design of experiments (DoE). For this purpose, ZrSi 2 , ZrO 2 , Al 2 O 3 , and MgO, and two different paraffins were used as starting materials. The influences of powder volume content, Zr/Si ratio, binder composition and processing temperature on the flowability of the feedstocks were observed in this line of experiments. A four-factorial fully fractional CCC-model was used. Finally the reliability of the computed statistical model was experimentally verified by means of two compositions, whose rheological behaviour has been predicted by the software.

The low-pressure injection moulding process (LPIM) allows near-net shape manufacturing of ceramic microcomponents. Feedstocks used in this process have to fulfil different requirements. They have to show good flowability for the machining but they also need to have high solids content to reduce shrinkage during sintering. This work focuses on the flow behaviour of alumina feedstocks with high solids content, based on two different binder systems and two different alumina powders. By varying particle size, organic binder system, and solids content, the rheological properties of the feedstocks were modified significantly. The material parameters such as viscosity, the derived relative viscosity, and yield stress were investigated systematically. The Herschel–Bulkley model turned out to be suitable for the description of the rheological behaviour of these ceramic feedstocks. The dependency of the relative viscosity on solids content could be well described by models like Krieger–Dougherty, the Eilers, and the Quemada models, while the best results for the maximum packing factors were obtained with the Janardhana Reddy et al. model.

Slightly overstoichiometric lithium orthosilicate pebbles are fabricated from lithium hydroxide and silica by a melting and spraying method in a semi-industrial scale facility. The authors performed out-of-pile annealing tests using the lithium orthosilicate pebbles irradiated in a research reactor. Moreover, the effect of the deposition of palladium in the lithium orthosilicate pebbles on the behavior of tritium release was investigated. The lithium orthosilicate pebbles were irradiated in a research reactor. In the out-of-pile annealing experiments, the temperature of lithium orthosilicate pebbles was raised from ambient temperature to 1173 K at a constant rate of 5 K/min under the stream of 0.1% hydrogen/nitrogen sweep gas. The experimental results indicate that almost all tritium was released as tritiated water vapor from the virgin lithium orthosilicate pebbles. It was also found that a lager amount of tritium was released as the molecular form (HT) from the lithium orthosilicate pebbles deposited with palladium.

Out-of-pile tritium release experiments were performed using slightly overstoichiometric lithium orthosilicate pebbles irradiated in a research reactor. In the experiments, various sweep gases such as pure nitrogen gas, hydrogen/nitrogen gas and water vapor/nitrogen gas are used to investigate the effect of difference in sweep gas composition on tritium release form the breeder material. The experimental results indicate that almost all tritium was released as tritiated water vapor regardless of difference in gas composition of the sweep gas. Most of the chemical form of tritium released was tritiated water vapor even when the hydrogen/nitrogen gas was used for the sweep gas. It was also found that a considerably smaller amount of tritium was released when a pure nitrogen gas was used as the sweep gas. Comparison of the experimental results reveals that the tritium release rate is substantially enhanced if the sweep gas contains water vapor.

Lithium orthosilicate (Li4SiO4) pebbles with 2.5 wt.% excess of silicon dioxide (SiO2) are the European Union's designated reference tritium breeding ceramics for the Helium Cooled Pebble Bed (HCPB) Test Blanket Module (TBM). However, the latest irradiation experiments showed that the reference Li4SiO4 pebbles may crack and form fragments under operation conditions as expected in the HCPB TBM. Therefore, it has been suggested to change the chemical composition of the reference Li4SiO4 pebbles and to add titanium dioxide (TiO2), to obtain lithium metatitanate (Li2TiO3) as a second phase. The aim of this research was to investigate the formation and accumulation of radiation-induced defects (RD) and radiolysis products (RP) in the modified Li4SiO4 pebbles with different contents of TiO2 for the first time, in order to estimate and compare radiation stability. The reference and the modified Li4SiO4 pebbles were irradiated with accelerated electrons (E = 5 MeV) up to 5000 MGy absorbed dose at 300–990 K in a dry argon atmosphere. By using electron spin resonance (ESR) spectroscopy it was determined that in the modified Li4SiO4 pebbles, several paramagnetic RD and RP are formed and accumulated, like, E' centres (SiO33−/TiO33−), HC2 centres (SiO43−/TiO3−) etc. On the basis of the obtained results, it is concluded that the modified Li4SiO4 pebbles with TiO2 additions have comparable radiation stability with the reference pebbles.

Slightly overstoichiometric lithium orthosilicate (Li4SiO4) pebbles are fabricated by a melt-spraying process, where the droplets are rapidly quenched to room temperature. Depending on their size, the solidified pebbles exhibit different microstructures. While the crystallised pebbles always display some pores and cracks, the amorphous ones are mostly free of any defects. The crystallisation process of amorphous pebbles was therefore investigated to decrease the amount of cracks and pores with the aim of improving the mechanical properties of the final product. The development of phases during crystallisation was studied by thermoanalysis, X-ray diffraction and electron microscopy. The crystallisation takes place in three steps. At about 300–350 °C an unidentified, intermediate phase is nucleated. In a second step the formation of Li4SiO4 takes place at about 500 °C, while lithium metasilicate (Li2SiO3) is the last phase to appear at temperatures above 600 °C.

For the reuse of ceramic breeder materials in fusion technology, the possibility to recycle these materials is a precondition and an important issue due to the high costs of 6Li-enriched materials and waste considerations. For lithium orthosilicate pebbles, fabricated by a melt-spraying process, the remelting was investigated, as it represents a facile reprocessing process without an additional recycling step. For this purpose, Li4SiO4 pebbles with a lithium content according to expected DEMO end-of-life burn-ups were fabricated and then remelted with an addition of lithium hydroxide hydrate to gain the composition of the usual reference material with a surplus of 2.5 wt% SiO2. It could be demonstrated, that the remelting process does not deteriorate the properties of the pebbles.

The formation of crystalline Li 4 SiO 4 and Li 2 SiO 3 from lithium orthosilicate glasses has been studied by means of in situ high-temperature X-ray diffraction. The first phase that crystallizes from the glass could not be identified, but was followed by the transformation to crystalline orthosilicate and metasilicate. Orthosilicate formation was tracked at temperatures between 600° and 650°C, whereas at higher temperatures, the formation of a small amount of crystalline metasilicate occurs. The kinetics of the initial phase of both crystallization processes are described by the Avrami-Erofeev equation, resulting in activation energies of 90 kJ/mol for the formation of the unidentified phase, and 68 kJ/mol for the formation of Li 4 SiO 4 . The rate constant for the crystallization of the unidentified phase is 0.014 s -1 at 510°C, equaling that of the orthosilicate formation at 630°C. After isothermal heat treatment for 100-800 s, depending on temperature, 80%-95% of the sample is crystallized and further crystallization is controlled by diffusion in both cases.

This article presents the development of a ceramic micro heat exchanger. Starting from its geometrical design, results obtained from a simplified simulation of the heat exchanger efficiency will be given. The alumina microcomponents were manufactured by a rapid prototyping process chain. Here, stereolithography was combined with a low-pressure injection molding process for the rapid and precise manufacturing of ceramic components down to the micrometer range. A micro heat exchanger, joined from sintered components using a glass solder, was applied as a prototype and checked for its performance at a system pressure of 8 bar.

Based on a KIT–CIEMAT collaboration on the radiation damage effects of light ions sorption/desorption in ceramic breeder materials, candidate materials for the ITER EU TBM were tested for their outgassing behavior as a function of temperature and radiation. Lithium orthosilicate based pebbles with different metatitanate contents and pellets of the individual oxide components were exposed to a deuterium atmosphere at room temperature. Then the thermally induced release of deuterium gas was registered up to 800 °C. This as-received behavior was studied in comparison with that after exposing the deuterium-treated samples to 4 MGy total dose of gamma radiation. The thermal desorption spectra reveal differences in deuterium sorption/desorption behavior depending on the composition and the induced ionizing damage. In these breeder candidates, strong desorption rate at approx. 300 °C takes place, which slightly increases with increasing amount of the titanate second phase. For all studied materials, ionizing radiation induces electronic changes disabling a number of trapping centers for D 2 adsorption.

Conventional shaping processes for ceramics are mostly based on a powder‐technological molding process using a negative mold and subsequent thermal compaction. Especially for prototypes and small lot series of microcomponents the outlay for molds are the major costing factor. Consequently the use of rapid prototyping (RP) processes can decisively reduce the costs and time in product development of ceramic microcomponents. In spite of the fact, that a large number of freeform fabrication techniques for different materials were developed in recent years, most generative techniques of ceramics still have different drawbacks for the fabrication of prototypes and often exhibit limited resolution compared to those of polymers. The combination of RP techniques such as micro stereolithography and ceramic injection molding in a RP process chain can fill in the gap between the limited applicability of solid freeform fabrication of ceramics and the restricted material properties of polymers.

The fabrication of lithium orthosilicate pebbles by melt-spraying enables a facile reprocessing of irradiated material by direct remelting. However, the necessary waiting period for the reprocessing is determined by the long-term activation of the material under irradiation that is dominated by the impurities. The activation characteristics for the current composition of lithium orthosilicate pebbles were assessed on the basis of three-dimensional activation calculations for a fusion power reactor. The calculations were used to identify critical amounts of impurities and were compared to the results of a hypothetical, pure material without impurities, as well as to a calculated Li-6 enriched OSi composition.

The design of solid breeder blankets is strongly affected by the low values of thermal conductivity and density of ceramic breeder pebble beds. A significant rise of both quantities would enhance the thermal performance and lead to an increased tritium breeding ratio. In order to improve these quantities pretreated lithium orthosilicate pebble material was dry pressed and subsequently sintered. The thermal conductivity of cylindrical pellets was determined by the heat pulse method using a laser flash device. A pebble bed characteristic sample was also investigated in order to check the measurement accuracy in comparison with previous results. Furthermore, two samples of low density cellular ceramics were also prepared by infiltration of polymer foams with a ceramic slurry. The thermal conductivity results show that the values are affected both by the particle size and the sample density. Thermal conductivity values of higher than 2 W/m K were obtained using large particles and sintering at 1000 °C.

International audience; As candidate tritium breeder materials for use in the ITER helium cooled pebble bed, ceramic multiphasic compounds lying in the region of the quasi-binary lithium metatitanate-lithium orthosilicate system may exhibit mechanical and physical advantages relative to single phase materials. Here we present an organometallic solution-based synthesis procedure for the low-temperature fabrication of compounds in the Li 2 TiO 3-Li 4 SiO 4 region and investigate phase stability and transformations through temperature varied X-ray diffraction and scanning calorimetry. Results demonstrate that the metatitanate and metasilicate phases Li 2 TiO 3 and Li 2 SiO 3 readily crystallise in nanocrystalline form at temperatures below 180°C. Lithium deficiency in the region of 5% results from Li sublimation from Li 4 SiO 4 and/or from excess Li incorporation in the metatitanate phase and brings about a stoichiometry shift and product compounds with mixed lithium orthosilicate/ metasilicate content towards the Si rich region and predominantly Li 2 TiO 3 content towards the Ti rich region. Above 1150°C the transformation of monoclinic to cubic γ-Li 2 TiO 3 disordered solid-solution occurs while the melting of silicate phases indicates a likely monotectic type system with a solidus line in the region 1050°-1100°C. Synthesis procedures involving a lithium chloride precursor are not likely to be a viable option for breeder pebble synthesis as this route was found to yield materials with a more significant Li-deficiency exhibiting the crystallisation of the Li 2 TiSiO 5 phase at intermediate compositions.https://doi.org/10.1016/j.jnucmat.2014.09.028; En tant que matériaux sélectionneurs de tritium candidats à utiliser dans le lit de galets refroidi à l'hélium ITER, les composés céramiques multiphasiques situés dans la région du système métasitanate de lithium-orthosilicate de lithium quasi-binaire peuvent présenter des avantages mécaniques et physiques par rapport aux matériaux monophasés. Nous présentons ici une procédure de synthèse à base de solutions organométalliques pour la fabrication à basse température de composés de la région Li 2 TiO 3-Li 4 SiO 4 et étudions la stabilité de phase et les transformations par diffraction des rayons X à température variée et calorimétrie à balayage. Les résultats démontrent que les phases de métatitanate et de métasilicate, Li 2 TiO 3 et Li 2 SiO 3, cristallisent facilement sous forme nanocristalline à des températures inférieures à 180 ° C. Une déficience en lithium dans la région de 5% résulte de la sublimation de Li de Li 4 SiO 4 et / ou de l'incorporation excessive de Li dans la phase de métatitanate et entraîne un déplacement de la stoechiométrie et des composés du produit avec une teneur mixte en orthosilicate de lithium / métasilicate vers la région riche en Si teneur en Li 2 TiO 3 principalement vers la région riche en Ti. Au-dessus de 1150 ° C, la transformation de la solution solide désordonnée en γ-Li 2 TiO 3 monoclinique en cubique se produit, tandis que la fusion des phases silicatées indique un système de type monotectique probable avec une ligne solide située dans la région allant de 1050 à 1100 ° C. Les procédures de synthèse faisant appel à un précurseur de chlorure de lithium ne constitueront probablement pas une option viable pour la synthèse de galets reproducteurs, car il a été constaté que cette voie conduisait à des matériaux présentant un déficit en Li plus important, présentant la cristallisation de la phase Li 2 TiSiO 5 dans des compositions intermédiaireshttps://doi.org/10.1016/j.jnucmat.2014.09.028

For high-temperature applications of a microreactor system the precise temperature control inside the reactor is important to optimize the yield of chemical reactions. If a microreactor system is operated inside a tube furnace, at best a homogeneous temperature distribution will be achieved inside the reactor. Heating elements that are integrated in the reactor have the potential to realize localized heating. Moreover, integrated heaters offer an efficient heating and a more convenient handling. Ohmic resistance heating for a modular ceramic microreactor system was investigated by direct connection and induction heating. The ceramic reactor system was modeled with FEM methods. Heat transfer and stress were calculated and compared with measurements taken.

Biphasic ceramic pebbles, consisting of Li4SiO4 and Li2TiO3, are being developed as the EU reference tritium breeding material for ITER and DEMO. A modified melt-based process for the pebble fabrication was established and optimised with regard to the process stability and yield. In parallel, the properties of pebbles and pebble beds were evaluated in dedicated experiments. These include investigations into e.g. the long-term thermal stability of pebbles, the thermomechanical properties of pebble beds, as well as the compatibility between the ceramics and the structural material. The available data was recently summarised and issued as the Material Property Handbook on Advanced Ceramic Breeders. During the EU DEMO conceptual phase, two main issues are proposed to be pursued in view of ITER. Irradiation campaigns are planned to ensure that the advanced ceramic breeder material is adequately tested and an extensive process scale-up with an increased production rate is foreseen to secure the supply for the ITER test blanket modules.