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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.

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.

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.

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.

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.

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.

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.

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

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.

Ceramic breeder (CB) pebbles consisting of lithium orthosilicate (Li4SiO4) as the main phase and lithium metasilicate (Li2SiO3) as a secondary phase were analysed with respect to radiation-induced defects and radiolysis products. Therefore, pebbles that were irradiated with neutrons in the so-called HICU experiment ( H igh neutron fluence i rradiation of pebble sta c ks for f u sion) were compared to pebbles irradiated with accelerated electrons and to an unirradiated sample. Fourier transformation infrared spectroscopy was used to investigate changes in the phase composition. Beside an expected increase in the second phase in the neutron-irradiated samples, no further significant changes or additional phases were detected compared to accelerated electron-irradiated samples. Several paramagnetic radiation-induced defects and radiolysis products, such as E’ centres, HC1 and HC2 centres, peroxide radicals and either colloidal lithium particles or F+ centres were detected by electron spin resonance spectrometry. Thermally stimulated luminescence revealed relatively high temperatures (200–450 °C) for the recovery of neutron radiation-induced defects and radiolysis products due to high irradiation temperatures. In general, accelerated electron irradiation can be used to mimic radiation-induced defects and radiolysis products in neutron-irradiated samples.