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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.

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

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.

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.

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.

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.

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.

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.

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 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.

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.