Search

International audience; The scope of the EUFOfusion Work Package Breeding Blanket is to develop a blanket concept for the EU DEMO reactor; this includes the blanket segments inside the Vacuum Vessel and the related Tritium Extraction/Removal Systems. In the Pre-Concept Design (PCD) Phase, two concepts have been selected as candidates; a solid and a liquid breeder blanket cooled with helium and water, respectively. The design of these two blanket systems has been adapted to the DEMO plant design developed in the PCD Phase and performances assessed. A large R&D programme has been implemented with the scope to evaluate different technologies for these blankets; including the development of breeders, tritium extraction and cooling technologies, and the manufacturing of the blanket system. A major milestone in the subsequent Concept Design Phase is the final selection of the blanket concept for DEMO.

The paper reviews the current status of development and qualification of the HCPB TBM’s functional materials, namely, Li-ceramic breeder and Be multiplier materials. The main functional and performance requirements for both functional materials are overviewed. The main results and outcomes of the post-irradiation examinations (PIE) of Li-ceramic breeder and Be/beryllides materials irradiated in HIDOBE and HICU irradiation campaigns are presented aiming at determination of the materials’ performance, properties and characteristics under neutron irradiation, like e.g. morphology and microstructure, tritium retention/release characteristics, mechanical properties. Fabrication of the advanced ceramic breeder material, containing lithium orthosilicate with addition of lithium metatitanate phase in order to improve its mechanical properties, is described. Results of characterization of this advanced ceramic breeder in terms of mechanical properties, porosity, morphology and effective thermal conductivity of a pebble bed are overviewed. In addition, results of characterization of the reference 1 mm Be pebbles produced by Rotation Electrode Process are presented in terms of their microstructure, tritium release characteristics and interaction with air and steam. A necessity to realize in the future a new irradiation experiment for the functional materials, LIBERTI experiment, is discussed in the conclusions.

Uniaxial compression test (UCT) experiments were conducted along with Discrete Element Method (DEM) simulations. The objective was to investigate the effect of the material properties and of the blanket operational parameters (in terms of packing factor, pebble material/size, compressive load and bed height) on the mechanical response of breeder beds subjected to cyclic loading. UCTs were performed with the EU advanced and reference ceramic breeder materials. To investigate the wall effects on the cyclic response of packed beds, a parametric study was performed varying the bed height to pebble size ratio (H/d). To this end monosized commercial zirconia pebbles with different sizes were also used. The numerical experiments were carried out with the KIT-DEM code on pebble assemblies using mixed boundary conditions (periodic and rigid planes). The influence of the bed height, pebble size and pebble material were systematically evaluated to gain an insight about their influence on the macro and micro response of the beds. Thanks to the microscale numerical modelling the macroscale response is presented together with the micro response at the pebble scale. Good agreement was found between experiments and simulations and thus, the KIT-DEM was confirmed to be a reliable predictive tool for the study breeder bed related problems.

The pebble properties of novel two-phase Li 4 SiO 4 pebbles of 1 mm diameter with additions of Li 2 TiO 3 , LiAlO 2 or Li x La y TiO 3 are evaluated in this work as a function of the second phase concentration and the microstructure of the pebbles. The characterization focused on the mechanical strength, microstructure and open as well as closed porosity. Therefore crush load tests, SEM analyses as well as helium pycnometry and optical image analysis were performed, respectively. This work shows that generally additions of a second phase to Li 4 SiO 4 considerably improve the mechanical strength. It also shows that the fabrication processes have to be well-controlled to achieve high mechanical strengths. When Li 2 TiO 3 is added in different concentrations, the determinant for the crush load seems to be the open porosity of the pebbles. The strengthening effect of LiAlO 2 compared to Li 2 TiO 3 is similar, while additions of Li x La y TiO 3 increase the mechanical strength much more. Yet, Li 4 SiO 4 and Li x La y TiO 3 react with each other to a number of different phases upon sintering. In general the pebble properties of all samples are favorable for use within a fusion breeder blanket.

Wet-chemical fabrication processes are highly adaptable to a wide range of raw materials and are therefore well suited for evaluating new material compositions. Here the established emulsion method was modified to fabricate novel two-phase Li 4 SiO 4 pebbles of 1 mm diameter with additions of Li 2 TiO 3 , LiAlO 2 or Li x La y TiO 3 . As the lithium density of the latter two compounds is relatively low, only moderate contents were added. The Li 2 TiO 3 additions, however, cover the full compositional range. The fabrication process was characterized with regard to its constancy and aptness for the anticipated pebble compositions by optical pebble size measurements. Also the phase content and the elemental composition of the fabricated pebbles were analyzed by XRD and ICP-OES combined with XRF, respectively. This work shows that the emulsion method is an appropriate method to produce pebbles with the anticipated Li 2 TiO 3 and LiAlO 2 concentrations in a Li 4 SiO 4 matrix. However, Li 4 SiO 4 and Li x La y TiO 3 react with each other to a number of different phases. To evaluate the activation properties of the pebbles, FISPACT calculations with a DEMO relevant neutron source are applied as well. The addition of aluminum seems to be unfavorable for a fusion application, but moderate concentrations of lanthanum can be tolerated.

All solid breeder concepts considered to be tested in ITER (International Thermonuclear Experimental Reactor), make use of lithium-based ceramics in the form of pebble-packed beds as tritium breeder. A thorough understanding of the thermal and mechanical properties of the ceramic pebble beds under fusion relevant conditions is essential for the design of the breeder blanket modules of future fusion reactors. In this study, the effect of cyclic loading on the mechanical behaviour of pebble bed assemblies was investigated using a Discrete Element Method (DEM) code. The numerical simulations were compared with the experimental outcomes. The results of numerical simulations show that the pebble size distribution affects noticeably the stress-strain behaviour of the assemblies. A good qualitative agreement between experimental and simulation results was found in terms of difference between residual strains of consecutive cycles. An increase of the oedometric modulus with the compressive load was observed for all investigated compositions in both experimental and DEM simulations. The numerical results show an increase of the oedometric modulus (E) with progressive compaction of the assemblies due to the cycling loading, while no significant influence of the pebbles size distribution was observed.

In advanced designs of DEMO breeding blankets with ceramic breeder, the structural material EUROFER is in direct contact with the pebbles of the ceramic breeder (Li4SiO4 + Li2TiO3) that results in a loss of constituent elements in the ceramic and the formation of a corrosion layer on EUROFER under operation relevant conditions. The impact of this corrosion attack on damage mechanisms which are significantly influenced by microstructural changes at the surface, in particular fatigue, needs to be considered in the design of affected components and is therefore investigated within the work reported here. Fatigue specimens made out of EUROFER in unconstrained contact to ceramic breeder pebbles were exposed to purge gas conditions for different durations. The chemical surface attack observed after the exposure is characterized by round stains, partly even round dimples, consisting of concentric diffusion zones which meet in case of the long exposure time forming an almost closed corrosion multi-layer. The impact of this attack on the fatigue behavior was subsequently investigated by performing strain-controlled low cycle fatigue (LCF) tests at 550 °C. To cover the scatter expected for the fatigue lifetime, a sufficient number of LCF experiments were conducted. The results show significant reduction of the fatigue lifetime, which quantitatively depends on the level of the attack/exposure time as well as the level of the applied load/strain amplitude.

In the breeder units (BUs) of the HCPB TBM, the lithium orthosilicate (OSi) pebbles are contained by cooling plates which are made of Eurofer. The assembly of the TBM box and BUs is based on welding processes which require post welding heat treatment (PWHT). According to the proposed TBM manufacture strategy, the BUs will be packed with the OSi pebbles during the TBM assembly; consequently the pebbles and the Eurofer plates will undergo the PWHT. Therefore the objective of this study is to investigate the effect of the PWHT on both Eurofer and OSi and their interaction. The test unit is a Eurofer box packed with OSi pebbles and Eurofer specimens. This test unit and all Eurofer specimens were heat treated inside a furnace which is capable of providing the required heating and cooling rates under a controlled atmosphere of vacuum or argon. Charpy impact and tensile tests of the Eurofer specimens and crush load tests of the OSi pebbles were performed after the heat treatment to investigate any changes in their mechanical properties. The results of these mechanical tests revealed that both materials did not suffer any significant degradation in their mechanical properties or chemical compatibility problems. The metallurgical analyses of both materials showed minimum chemical interaction.

Within the framework of the Collaborative Research Center 499 (SFB 499) of the German Research Foundation (DFG), one task was the development of a zirconia micro turbine as a demonstrator tool to enhance the interaction of the participating workgroups. This case study describes the evolution of the demonstrator and experiences gained with the design and the manufacturing of a micro device. Although it was not the aim of this basic research project to develop a commercial product, the experiences are valuable for improving the performance of industrial product development processes for ceramic micro devices. The various parts of the zirconia micro turbine were prepared by a rapid prototyping process chain (RPPC) that allows for a fast and inexpensive manufacturing of ceramic parts with details down to the micron range. A first design concept was made to mainly demonstrate the shaping feasibility of the process in the micro range. However, some features affected the performance due to their low loading capacity. Thus, a modified design was improved for power output and durability. After optimization of the process chain, dense and homogenous ceramic micro parts could be manufactured. These parts were used for the assembling of a functional micro turbine demonstrator, which was powered by compressed air.

Functional materials should play an important role not only in ITER but also in fusion machines beyond ITER, though it is occasionally claimed that future fusion plants should have much more simple structures and they should be free from auxiliary systems such as diagnostics. Studies on test blanket modules (TBM) clearly show the importance of functional materials there. The paper will review the present status of studies and developments of functional ceramics for nuclear fusion applications, with a perspective on their application in electric-power generating fusion power plant, namely DEMO.

Diamond powders were attempted to be consolidated at elevated temperature using an explosive compaction technique employing a cylindrical configuration. A compacted diamond sample recovered from the center of a cylinder, close to the end plug made of stainless steel showed high hardness and tight interparticle bonding between the diamond powders. The sample was characterized on the basis of X-ray diffraction patterns. No graphitization was observed due to heating or compaction, and the hot compacted sample was highly strained by intensive deformation at elevated temperature.

Most shaping processes for ceramics are based on a powder technological moulding process using a negative mould and subsequent thermal compaction. Especially for prototypes and small-lot series of microcomponents, the outlay for moulds is the major costing factor. Therefore the use of rapid prototyping (RP) processes can decisively reduce the costs and time in product development of ceramic microcomponents. By combining the high resolution of, for example, stereolithography as an inexpensive and fast supply for master models with the high flexibility of low-pressure injection moulding, a rapid prototyping process chain (RPPC) has been established for the fabrication of micropatterned ceramic components as functional models or pre-production lots. This RPPC proved to have a very high moulding precision and accuracy in the submillimetre range, but also enables the fabrication of components with outer dimensions of several centimetres. Different RP techniques were investigated with regard to their suitability to be used as master models in the replication chain. The quality of the master models turned out to be of decisive significance for the quality and reproducibility of the ceramic mouldings.

Cost-intensive and time-consuming manufacturing of new miniaturized or micropatterned ceramic components may profit decisively from the use of rapid prototyping processes. However most known generative processes do not provide a sufficient resolution for the fabrication of microdimensional or micropatterned components or are restricted to polymer materials. In contrast to this, a rapid prototyping process chain (RPPC), which combines e.g., micro stereolithography and a low-pressure shaping method using soft molds, allows the rapid manufacturing of ceramic microcomponents from functional models to preliminary or small lot series.

In the development of the European Helium Cooled Pebble Bed (HCPB) blanket the reference material is slightly overstoichiometric lithium orthosilicate (Li 4 SiO 4 + SiO 2 ) pebbles, fabricated by the melting-spraying method. Slightly overstoichiometric lithium metatitanate (Li 2 TiO 3 + TiO 2 ) and lithium metazirconate (Li 2 ZrO 3 + ZrO 2 ) pebbles, fabricated using a extrusion-spheronization-sintering process, are considered as alternative candidates for the ceramic breeder material. In order to compare the long-term behaviour of the three materials at HCPB-blanket relevant temperature and atmosphere (He + 0.1% H 2 ), a campaign of three long time annealing experiments were carried out at Forschungszentrum Karlsruhe (FZK). Pebbles of each ceramic breeder material (Li 4 SiO 4 + SiO 2 from FZK, Li 2 TiO 3 + TiO 2 and Li 2 ZrO 3 + ZrO 2 from CEA) were annealed for 96 days at 970 °C. During this period the change of the mechanical characteristics, the microstructure, and the chemical composition of the pebbles was analysed. After the end of the annealing experiment, uniaxial compression tests of pebble beds were performed additionally. In the present paper the results from the second experimental campaign are reported and discussed.

Ceramic microreactors can be used for applications that cannot be covered by metal or polymer systems, because special material properties, such as high thermal and chemical resistance are required. However, application of ceramic microcomponents often fails due to the time-consuming and costly manufacturing of components with patterning details in the micrometer range. A promising solution to this problem is a rapid prototyping process chain. It offers a fast and precise fabrication of ceramic components down to the micrometer range by combining stereolithography and low-pressure ceramic injection molding. Its fast and flexible tooling allows rapid product development and manufacturing of ceramic components as functional models or in small series. For use in chemical microreaction technology, a modular ceramic microreactor with inner dimensions in the sub-millimeter range has now been developed by means of this process chain.

Time-intensive and cost-consuming manufacturing of new ceramic components may be improved significantly by the use of rapid prototyping processes especially in the development of miniaturized or micropatterned components. Their molding is generally very expensive and finishing is difficult to the point of impossibility. Most known generative ceramic molding processes do not provide a sufficient resolution for the fabrication of microstructured components. In contrast to this, a rapid prototyping process chain that combines micro-stereolithography and low-pressure injection molding, for example, allows the rapid manufacturing of ceramic microcomponents from functional models to preliminary or small-lot series, microcomponents from functional models to preliminary or small-lot series.

Among the international fusion solid breeder blanket community, there exists steady progress on the experimental, phenomenological, and numerical characterizations of the pebble bed effective thermo physical and mechanical properties, and of thermomechanic state of the bed under prototypical operating conditions. This paper summarizes recent achievements in pebble bed thermomechanics that were carried out by members of the IEA Fusion Nuclear Technology Subtask I Solid Breeding Blanket. A major goal is on developing predictive capability while identifying a pre-conditioned equilibrium stress state that would warrant pebble bed integrity during operations. The paper reviews and synthesizes existing computational modeling approaches for pebble bed thermomechanics prediction, and differentiating points of convergence/divergence among existing approaches. The progress toward modeling benchmark is also discussed. These advancements have led to a framework to help navigate future research.

Preparation, crystal structure and some basic properties of a new fullerene-containing molecular solid, C60S8CS2, comprising C60 fullerene molecules, S8 rings and linear CS2 groups are described. The orthorhombic structure ( a = 9.973(1) A , b = 13.356(2) A , c = 28.654(4) A , space group Pnma, z = 4), as determined from single crystal X-ray data collected at room temperature, has been refined to a resolution of about 1.8 A. The structure is built up of six-membered zig-zag rings of C60 molecules in the b-c plane which share common edges and are stacked closely upon each other along a. In this way they form a 3D framework with large 1D channels along a containing the S8 and CS2 molecules. Within the resolution of the experiment the fullerene molecules in C60S8CS2 appear to be orientationally disordered while the S8 and the CS2 molecules are well ordered. The compound is an insulator with a reddish-brown colour and almost isotropic light absorption.

The authors greatly acknowledge the technical and experimen-tal support of O. Valtenbergs and L. Avotina (Institute of Chemical Physics, University of Latvia). The work is performed in the frames of the University of Latvia financed project No. Y9-B044-ZF-N-300, “Nano, Quantum Technologies, and Innovative Materials for Eco-nomics”., Advanced lithium orthosilicate (Li4SiO4) pebbles with additions of lithium metatitanate (Li2TiO3) as a secondary phase are suggested as a potential source for tritium breeding in future nuclear fusion reactors. The advanced Li4SiO4 pebbles with different contents of Li2TiO3 were examined before and after simultaneous action of 5 MeV accelerated electron beam (dose rate: up to 10 MGy h−1) and high temperature (up to 1120 K) in a dry argon atmosphere. The accumulated radiation-induced defects (RD) and radiolysis products (RP) were studied by electron spin resonance (ESR) spectrometry and thermally stimulated luminescence (TSL) technique. The phase transitions were studied with powder X-ray diffraction (p-XRD). The microstructure and mechanical strength of the pebbles, before and after irradiation, were investigated by scanning electron microscopy (SEM) and comprehensive crush load tests. The obtained results revealed that the irradiation temperature has a significant impact on the accumulation of RD and RP in the advanced Li4SiO4 pebbles, and with an increasing content of Li2TiO3, the concentration of accumulated paramagnetic RD and RP decreases. Major changes in the mechanical strength, microstructure and phase composition of the advanced pebbles were not detected after irradiation., University of Latvia Y9-B044-ZF-N-300; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²