A Measurement Platform Scheme and Data Post-processing Method for Thermal Conductivity of Li<Subscript>4</Subscript>SiO<Subscript>4</Subscript> Pebble Bed

The effective thermal conductivity of tritium breeder pebble bed is an important thermal parameter and must be known for the thermo-mechanical design of solid tritium breeder blankets. In order to obtain the parameter, experimental measurement is an effective method. A measurement platform was designed by University of Science and Technology of China for CFETR solid blanket scheme to measure the immediate thermal conductivity data and study the effect of pebble bed temperature, the purge gas pressure and pebble deformation on the thermal conductivity of pebble bed. Measurements were performed based on about 1 mm diameter Li₄SiO₄ pebbles in the temperature range between 100 and 800 °C, with purge gas pressure ranging from 0.1 to 0.3 MPa. This paper described a measurement platform scheme by thermal probe method. On the other hand, for the sake of increasing the precision of thermal conductivity data transformed from temperature data, some improvements for the data post-processing using Monte Carlo inversion method were made in this paper too.     

Reaction Kinetics for the Synthesis of Li<Subscript>4</Subscript>SiO<Subscript>4</Subscript> by Solid State Reaction from Li<Subscript>2</Subscript>SiO<Subscript>3</Subscript> and Li<Subscript>2</Subscript>CO<Subscript>3</Subscript> for Tritium Breeder

Currently, Li₄SiO₄ has been considered one of the best candidates for tritium breeders. Besides, it has also been suggested to be the promising high-temperature carbon dioxide (CO₂) solid sorbent and lithium ion conducting material. As the knowledge of reaction kinetics can offer either a practical or theoretical purpose for predicting and optimizing the reaction process, the kinetics of formation of Li₄SiO₄ from Li₂CO₃ and Li₂SiO₃ were investigated by non-isothermal thermogravimetric technique at four different heating rates. The Malek method results showed that the reaction was the Avarami–Erofeev model, i.e., at about 650 °C the Li₂SiO₃ and Li₂CO₃ with molar ratio of 1:1 formed Li₄SiO₄ crystal nucleus randomly, and then it grew by the diffusion of Li and O through Li₄SiO₄ layer. And the diffusion stage was the rate controlling step. The performances of Li₄SiO₄ pebbles fabricated by a graphite bed process using Li₂SiO₃ and Li₂CO₃ were also evaluated.     

Failure initiation and propagation of Li4SiO4 pebbles in fusion blankets

Lithium orthosilicate (Li₄SiO₄) pebbles are considered to be a candidate as solid tritium breeder in the helium cooled pebble bed (HCPB) blanket. These ceramic pebbles might be crushed during thermomechanical loading in the blanket. In this work, the failure initiation and propagation of pebbles in pebble beds is investigated using the discrete element method (DEM). Pebbles are simplified as mono-sized elastic spheres. Every pebble has a contact strength in terms of critical strain energy, which is derived from a validated strength model and crush test data for pebbles from a specific batch of Li₄SiO₄ pebbles. Pebble beds are compressed uniaxially and triaxially in DEM simulations. When the strain energy absorbed by a pebble exceeds its critical energy it fails. The failure initiation is defined as a given small fraction of pebbles crushed. It is found that the load level for failure initiation can be very low. For example, if failure initiation is defined as soon as 0.02% of the pebbles have been crushed, the pressure required for uniaxial loading is about 2.5 MPa. Therefore, it is essential to study the influence of failure propagation on the macroscopic response of pebble beds. Thus a reduction ratio defined as the size ratio of a pebble before and after its failure is introduced. The macroscopic stress–strain relation is investigated with different reduction ratios. A typical stress plateau is found for a small reduction ratio.     

R&D status on Water Cooled Ceramic Breeder Blanket Technology

Japan Atomic Energy Agency (JAEA) is performing the development of a Water Cooled Ceramic Breeder (WCCB) Test Blanket Module (TBM) as one of the most important steps toward DEMO blanket. Regarding the blanket module fabrication technology development using F82H, the fabrication of a real scale mockup of the back wall of TBM was completed. In the design activity of the TBM, electromagnetic analysis under plasma disruption events and thermo-mechanical analysis under steady state and transient state of tokamak operation have been performed and showed bright prospect toward design justification. Regarding the development of advanced breeder and multiplier pebbles for DEMO blanket, fabrication technology development of Li rich Li₂TiO₃ pebble and BeTi pebble was performed. Regarding the research activity on the evaluation of tritium generation performance, the evaluation of tritium production and recovery test using D-T neutron in the Fusion Neutronics Source (FNS) facility has been performed. This paper overviews the recent achievements of the development of the WCCB Blanket in JAEA.     

Fabrication of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/FeAl Coating as Tritium Permeation Barrier on Tritium Operating Component on Quasi-CFETR Scale

Limited CFETR-scale experience of engineering preparation techniques of tritium permeation barrier (TPB) exists up to date. Aimed at processing some real components that are usually tubular components sealed in one end, in the tritium cycling systems of China Fusion Engineering Test Reactor (CFETR), an Al₂O₃/FeAl coatings as TPB was prepared on tubular components of 321 type stainless steel components with a length of 400 mm and an external diameter of 150 mm, by Al-electroplating followed by heat treating and selective oxidation. The ability to construct TPB coated components on quasi-CFETR scale was demonstrated, with fabricating a TPB of Al₂O₃/FeAl coating with a double-layered structure, consisted of an outer γ-Al₂O₃ layer with a thickness of 0.3 µm and an inner (Fe,Cr,Ni)Al/(Fe,Cr,Ni)₃Al layer of 40 µm in thickness. The tritium permeation reduction factors of the Al₂O₃/FeAl TPB on component were 229 and 96 at 500 and 600 °C respectively. Finally, signatures and gaps of TPB mass process on CFETR-scale were discussed.     

Study of ceramic pebble beds in Post Irradiation Examination of the Pebble Bed Assemblies irradiation experiment

The irradiation experiment Pebble Bed Assemblies (PBA) consists of four mock-up representations (test elements) of the EU Helium Cooled Pebble Bed (HCPB) concept. The four test elements contain a ceramic breeder pebble bed sandwiched between two beryllium pebble beds and are regarded as one of the first DEMO representative HCPB blanket irradiation tests, with respect to temperatures and power densities. The design value of the PBA were to irradiate pebble beds at a power density of 20–26 W/cc in the ceramic breeder, to a maximum temperature of 800 °C.Two test elements contain lithium orthosilicate pebbles (Li₄SiO₄; FZK/KIT) and were irradiated with target temperatures of 600 and 800 °C, respectively. The other test elements have lithium metatitanate (Li₂TiO₃; CEA) with different grain sizes and were both irradiated with a target temperature of 800 °C. The PBA have been irradiated for 294 Full Power Days (12 cycles) in the High Flux Reactor (HFR) in Petten to a total neutron dose of 2–3 dpa in Eurofer, and an estimated (total) lithium burnup of 2–3% in the ceramic pebbles.This work presents results of Post Irradiation Examinations (PIE) on the four HCPB test elements. Using e.g. SEM, the evolution of compressed pebble beds and pebble interactions like swelling, creep, sintering, etc., under irradiation and thermal loads are studied for the candidate pebble materials Li₂TiO₃ and Li₄SiO₄. (Chemical) interactions between ceramic pebbles and Eurofer (e.g. chrome diffusion) are observed. Looking at different sections of the pebble beds, correlations between temperatures and thermal–mechanical behaviour are clearly observed.     

Preparation and characterization of lithium-titanate pebbles by solid-state reaction extrusion and spherodization techniques for fusion reactor

For the development of TBM for fusion reactors, lithium containing ceramics as against the metal are preferred as tritium breeding material. Lithium titanate (Li₂TiO₃) is one such chosen ceramic tritium breeder. Li₂TiO₃ pebbles are conventionally prepared by sol-gel process and wet process. Solid state reaction of lithium carbonate with titanium dioxide is preferred route for the bulk production of Li₂TiO_3. Thermo-gravimetric and differential thermal analysis (TG-DTA) techniques have been used in the present study to understand the solid state reaction of intimate mixture of lithium carbonate and titanium dioxide. It was found out that single phase lithium titanate (Li₂TiO₃) is produced at 750 °C and the reaction is completed in 6 h. Fine powders of lithium titanate obtained after milling and classification were mixed with aqueous solution of PVA to prepare green pebbles of desired size and shape. The pebbles were subsequently sintered at 900 °C and the effect of sintering time on the properties of sintered pebbles was studied. The reaction mechanisms and the product qualities obtained by the solid state reaction, extrusion and spherodization techniques are discussed in this paper.     

New progress on design and R&D for solid breeder test blanket module in China

ITER will be used to test tritium breeding module concepts, which will lead to the design of DEMO fusion reactor demonstrating tritium self-sufficiency and the extraction of high grade heat for electricity production. China plans to test the HCCB TBM modules during different operation phases. Related design and R&D activities for each TBM module with the auxiliary system are introduced.The helium-cooled ceramic breeder (HCCB) test blanket module (TBM) is the primary option of the Chinese TBM program. The preliminary conceptual design of CN HCCB TBM has been completed. A modified design to reduce the RAFM material mass to 1.3 ton has been carried out based on the ITER technical requirement. Basic characteristics and main design parameters of CN HCCB TBM are introduced briefly. The mock-up fabrication and component tests for Chinese test blanket module are being developed. Recent status of the components of CN HCCB TBM and fabrication technology development are also reported. The neutron multiplier Be pebbles, tritium breeder Li₄SiO₄ pebbles, and structure material CLF-1 of ton-class are being prepared in laboratory scale. The fabrication of pebble bed container and experiment of tritium breeder pebble bed will be started soon. The fabrication technology development is proceeding as the large-scale mock-up fabrication enters into the R&D stage and demonstration tests toward TBM testing on ITER test port are being done as scheduled.     

Tritium permeation in EUROFER in EXOTIC and LIBRETTO irradiation experiments

The reduced activation martensitic steel (RAFM) EUROFER is foreseen as a structural material in test breeder module (TBM) in ITER and breeder blanket in DEMO design. In a number of irradiation experiments conducted in high flux reactor (HFR) in Petten EUROFER was used as a containment wall of the breeder material, through which tritium permeation was monitored on line. Thus in EXOTIC-9/1 (EXtraction Of Tritium In Ceramics) experiment where Li₂TiO₃ pebbles were the breeder material, EUROFER was irradiated up to 1.3 dpa at 340–580 °C. In LIBRETTO experiments (LIBRETTO-4/1, -4/2 and -5) the breeder material was lead lithium eutectic which was in direct contact with the EUROFER containment wall. The neutron damage in steel achieved in the LIBRETTO experiments varied from 2 to 3.5 dpa. The irradiation temperature was 350 °C (LIBRETTO-4/1), 550 °C (LIBRETTO-4/2), and 300–500 °C (LIBRETTO-5).Tritium permeability was studied by varying the irradiation temperature and hydrogen concentration in the purge gas. From the analysis of the temperature transients performed in all four experiments yielded the tritium diffusion coefficients were derived, which appear to be factor ten lower than the literature data obtained in the gas driven permeation experiments.     

Fabrication of Li2TiO3 pebbles by a freeze drying process

Li₂TiO₃ pebbles were successfully fabricated by using a freeze drying process. The Li₂TiO₃ slurry was prepared using a commercial powder of particle size 0.5–1.5 μm and the pebble pre-form was prepared by dropping the slurry into liquid nitrogen through a syringe needle. The droplets were rapidly frozen, changing their morphology to spherical pebbles. The frozen pebbles were dried at ?10 °C in vacuum. To make crack-free pebbles, some glycerin was employed in the slurry, and long drying time and a low vacuum condition were applied in the freeze drying process. In the process, the solid content in the slurry influenced the spheroidicity of the pebble green body. The dried pebbles were sintered at 1200 °C in an air atmosphere. The sintered pebbles showed almost 40% shrinkage. The sintered pebbles revealed a porous microstructure with a uniform pore distribution and the sintered pebbles were crushed under an average load of 50 N in a compressive strength test. In the present study, a freeze drying process for fabrication of spherical Li₂TiO₃ pebbles is introduced. The processing parameters, such as solid content in the slurry and the conditions of freeze drying and sintering, are also examined.     

Fabrication and characterization of Li4SiO4 pebbles by melt spraying method

The solid breeder blanket concept proposed by the China features the tritium breeding ceramic as pebble beds in several submodules. The lithium orthosilicate (Li₄SiO₄) is considered as first candidate ceramic breeder materials fabricated by the melt spraying method, which is favorable to other processes in terms of density and recycling. The production process involves rapid quenching of the liquid droplets from the melt to room temperature which cause internal stresses and leads in some cases to formation of microcracks and the dispersion of mechanical properties. Molar ratio (Li/Si) of the pebbles was evaluated by ICP–OES. It is shown that the Li/Si ratio of the pebbles is slightly varying from batch to batch because of evaporation of lithium at high temperatures. The crush tests on single pebbles show that a mean value of 7.0 N was obtained in crush load experiments of 40 pebbles with a diameter of 1.0 mm. It results that heat treatment of pebbles improves the density and mechanical stability. The activation characteristics for the current composition of Li₄SiO₄ pebbles were assessed. The calculations were used to identify critical amounts of impurities and were compared to the results of pure material without impurities.     

Out-of-pile tritium release study on Li4SiO4 pebbles from TRINPC-I experiments

Out-of-pile tritium release examinations of irradiated Li₄SiO₄ pebbles were performed in TRINPC-I experiments for evaluating material performance and verifying the system design. To generate tritium the specimens were irradiated with neutrons. Li₄SiO₄ pebbles were made by a freeze-drying method. In the experiments, concentrations of tritium in the form of tritium gas (HT + T₂) and tritiated water (HTO + T₂O) in the outlet streams of a reactor tube were measured separately with an ionization chamber and a liquid scintillation radiometer. The results show that the percentage of tritium gas (HT + T₂) and tritiated water trapped by the breeder pebbles were about 72% and 19% of totally released tritium, respectively. Thus, more tritium was released in the form of tritium gas in this work. In addition to tritium trapped by the breeder pebbles, the amount of free tritium was also measured by breaking on-line a quartz capsule containing Li₄SiO₄ pebbles, the percentage of which was 9% of totally released tritium. The temperature peaks of tritium gas mainly appeared at about 477 °C and 654 °C, while the temperature peak of tritiated water appeared at about 402 °C, under which most of tritiated water released.     

Synthesis & fabrication of lithium-titanate pebbles for ITER breeding blanket by solid state reaction & spherodization

⁶Li produces tritium by (n, α) nuclear reaction, ⁶Li + ¹n → ⁴He + ³H. Lithium titanate (Li₂TiO₃) enriched with ⁶Li, is the most promising candidate for solid test blanket module (TBM) material for fusion reactors. Various processes are reported in literature for the fabrication of Li₂TiO₃ pebbles for its use as TBM material. A process has been developed based on the solid state reaction of lithium-carbonate and titanium-dioxide for the synthesis of lithium titanate and pebble fabrication by extrusion, spherodization and sintering. This paper discusses the sequence of steps followed in this process and the properties obtained. Analytical grade titanium-dioxide and lithium-carbonate were taken in stoichiometric ratio and were milled to ensure thorough intimate mixing and obtain fine particles less than 45 μm before its calcination at 900 °C. Following calcination, the agglomerated product was again milled to fine particles of size less than 45 μm. Aqueous solution of ploy-vinyl-alcohol was added as binder prior to its feeding in the extruder. The extruded strips were spherodized and spherical pebbles were dried and sintered at 900 °C for different duration. Pebbles of desired density and porosity were obtained by suitable combination of sintering temperature and duration of sintering. Properties of the prepared pebbles were also characterized for sphericity, pore size distribution, grain size, crushing load strength, etc. The values were found to be conforming to the desired properties for use as solid breeder. The attractive feature of this process is almost no waste generation.     

Isotope Exchange Capacity on Li4SiO4 and Comparison of Tritium Inventory in Various Solid Breeder Blankets

It has been pointed out by the present authors that it is essential to understand such mass transfer steps as diffusion of tritium in the grain of a breeder material, absorption of water vapor into bulk of the grain, adsorption of water on surface of the grain, and exchange capacity of tritium to be trapped to surface of the grain together with two types of isotope exchange reactions for evaluation of the tritium inventory in a solid breeder blanket under various conditions. The isotope exchange capacity on the Li₄SiO₄ surface is experimentally obtained in this study. Most of the properties required for evaluation of the tritium inventory for various blanket materials have been already quantified by the present authors. Then it has become possible to compare the tritium inventory in solid breeder blankets packed with either Li₂O, LiAlO₂, Li₂ZrO₃, Li₂TiO₃ or Li₄SiO₄ using the calculation model previously presented by the present authors.     

Fabrication of the Li2TiO3 tritium breeder pebbles by a capillary-based microfluidic wet process

Li₂TiO₃ is one of the most promising candidates among tritium breeders. In this work, Li₂TiO₃ pebbles with a narrow size distribution, high density, small grain size, and good sphericity are synthesized using the commercial Li₂TiO₃ powder as the raw material by a co-flow capillary-based microfluidic wet process. Highly uniform slurry droplets containing Li₂TiO₃ powder and polyvinyl alcohol were formed in the microfluidic device and then solidified with the cross-linking reaction between polyvinyl alcohol and borax. Li₂TiO₃ pebbles were finally obtained after sintering the green bodies. The size of sintered Li₂TiO₃ pebbles with a good sphericity (D_max/D_min < 1.05) can be controlled precisely in a size range of 400–1000 µm by adjusting the flow rate of disperse and continuous phases. The effects of the calcination method, sintering condition, solid content of slurry, and the particle size of Li₂TiO₃ powder on the relative density of Li₂TiO₃ pebbles prepared were studied in detail, and 97.16% T.D. (Theoretical Density) maximum relative density of the pebbles with 5 µm grain size were fabricated.     

Fabrication and improvement of the density of Li2TiO3 pebbles by the optimization of a sol-gel method

Li₂TiO₃ is regarded as a promising candidate breeder in solid blanket concepts. Pebble configuration has been a preferred option due to its potential advantages in blanket design. Li₂TiO₃ pebbles were successfully fabricated by a water-based sol-gel method previously. However, the sintered density of the pebbles was very low (less than 70% theoretical density). The process parameters were optimized and the sintered density of the pebbles was improved significantly in this work. Li₂TiO₃ pebbles with density as high as 85% were obtained at a relatively lower sintering temperature (1100 °C) and shorter sintering time (4 h). The experimental results showed that the viscosity of the sol was influential to the sphericity of the gel-spheres and thus the sintered pebbles. The variety of lithium source, the pH value of the solution and the sintering conditions demonstrated significant influences on the microstructure and density of the sintered Li₂TiO₃ pebbles.     

Investigation of the packing structure of pebble beds by DEM for CFETR WCCB

Li₂TiO₃/Be₁₂Ti mixed pebble beds with multi-sized particles are one of the potential candidates for the WCCB (water-cooled ceramic breeder blanket) of the CFETR (China Fusion Engineering Test Reactor). To meet the neutronics requirements of a WCCB, a study of the packing structure of the concerned pebble bed is necessary. In this paper, the discrete element method (DEM) is applied to produce a prototypical blanket pebble bed by directly simulating the contact state of each individual particle using basic interaction laws. According to the current simulation, the packing factor of a mono-sized pebble bed is 0.62–0.64, while the value will become more than 0.75 for Li₂TiO₃/Be₁₂Ti mixed breeding pebble bed with a diameter ratio of not less than 5 as well as an appropriate mixed volume ratio, and thus can meet the neutronics requirements.     

Development of fabrication technologies for advanced breeding functional materials For DEMO reactors

Demonstration power reactors (DEMOs) require advanced tritium breeders and neutron multipliers that have high stability at high temperatures. Lithium titanate (Li₂TiO₃) is one of the most promising candidates among tritium breeders because of its tritium release characteristics. Li₂TiO₃ with additional Li (Li_2+xTiO_3+y) has increased stability in a reducing atmosphere at high temperatures. In this work, Li_2+xTiO_3+y pebbles were fabricated using the emulsion method, which is a sol–gel method. The raw material for the fabrication of Li_2+xTiO_3+y pebbles was synthesized from a mixture of LiOH·H₂O and H₂TiO₃ at specific ratios. The average diameter and the sphericity of the pebbles fabricated by the emulsion method were 1.40 mm and 1.02, respectively. In addition, beryllium (Be) intermetallic compounds (beryllides) are promising materials for advanced neutron multipliers. The results of the trial fabrications in this work showed that beryllides of Be–Ti and Be–V intermetallics could be synthesized using the plasma sintering method.     

Conceptual design of a water cooled breeder blanket for CFETR

China Fusion Engineering Test Reactor (CFETR) is an ITER-like superconducting tokamak reactor. Its major radius is 5.7 m, minor radius is 1.6 m and elongation ratio is 1.8. Its mission is to achieve 50–200 MW of fusion power, 30–50% of duty time factor, and tritium breeding ratio not less than 1.2 to ensure the self-sufficiency. As one of the breeding blanket candidates for CFETR, a water cooled breeder blanket with superheated steam is proposed and its conceptual design is being carried out. In this design, sub-cooling water at 265 °C under the pressure of 7 MPa is fed into cooling plates in breeding zone and is heated up to 285 °C with saturated steam generated, and then this steam is pre-superheated up to 310 °C in first wall (FW), final, the pre-superheated steam coming from several blankets is fed into the other one blanket to superheat again up to 517 °C. Due to low density of superheated steam, it has negligible impact on neutron absorption by coolant in FW so that the high energy neutrons entering into breeder zone moderated by water in cooling plate help enhance tritium breeding by ⁶Li(n,α)T reaction. Li₂TiO₃ pebbles and Be₁₂Ti pebbles are chosen as tritium breeder and neutron multiplier respectively, because Li₂TiO₃ and Be₁₂Ti are expected to have better chemical stability and compatibility with water in high temperature. However, Be₁₂Ti may lead to a reduction in tritium breeding ratio (TBR). Furthermore, a spot of sintered Be plate is used to improve neutron multiplying capacity in a multi-layer structure. As one alternative option, in spite of lower TBR, Pb is taken into account to replace Be plate in viewpoint of safety. In this contribution, study on neutronics and thermal design for a water cooled breeder blanket with superheated steam is reported.     

Progress on design and R&D for helium-cooled ceramic breeder TBM in China

The helium-cooled ceramic breeder (HCCB) test blanket module (TBM) is the primary option of the Chinese TBM program. Current progress on the design and R&D for Chinese helium-cooled ceramic breeder TBM (CN HCCB TBM) in China is presented. The main updated design and related R&D of CN HCCB TBM are introduced briefly. The mock-up fabrication and component tests for Chinese test blanket module are being carried out. Recent status of the components and fabrication technology development is also reported. The neutron multiplier Be pebbles, tritium breeder Li₄SiO₄ pebbles, and structure material CFL-1 are being prepared in the laboratory scale. The fabrication of 1/3 sized mock-up and construction of a He test loop are being carried out. The key technology development is proceeding to the large scale mock-up fabrication and demonstration tests toward on ITER testing.