Breeder Materials Research Articles

Technology readiness assessment of materials for DEMO in-vessel applications

A dedicated procedure was developed to categorize the technology readiness of materials for specific DEMO in-vessel fusion reactor applications. This methodology was employed to assess the technological maturity of materials under development within the EUROfusion materials work package (WPMAT). This covers materials intended for structural, high heat flux, optical and dielectric applications in the European DEMO fusion reactor (breeder materials and barrier coatings are not covered here). The baseline materials have been assigned DEMO Material Technology Readiness Levels (MTRLs) of 4 (EUROFER97), 3 (conventional tungsten) and 4 (Copper-Chromium-Zirconium). In addition, a further 28 candidate materials (and groups of materials) were also assessed. These were generally assigned DEMO MTRLs in the range of 2-3. This process has highlighted the wide range of materials under development within WPMAT. However, it has also brought into focus the many challenges facing DEMO materials development. While the lack of technologically ready materials is clearly a source of risk to DEMO, the introduction of a biennial review of technology readiness within WPMAT is intended to facilitate more effective planning and targeted materials development, in line with the strategic plans of EUROfusion. This paper highlights the methodologies for fusion specific material technology readiness levels, their application for EU-DEMO and the effectiveness of these in strategic materials development.

Integration of LiPb loops for WCLL BB of European DEMO

The eutectic alloy Lithium Lead (LiPb) enriched at 90 % in 6Li is the breeder material for one of the candidate European Breeding Blanket (BB) concepts. Currently under investigation for DEMO reactor, the Water Cooled Lithium-Lead (WCLL), and for the WCLL Test Blanket Module (TBM) that will be qualified in the ITER reactor. The LiPb alloy is used as tritium breeder, neutron multiplier and tritium carrier. The design of the LiPb loops is currently under study and the conceptual design of the main loop components has been completed. For this reason, it becomes mandatory to proceed with the integration of the LiPb loops in the EU DEMO Tokamak building, checking the consistency of the different systems design to be integrated in DEMO reactor building. CAD design and integration of the entire LiPb loops are shown taking into account the building areas assigned, the interfaces with the other systems and the requirement related to the LiPb loop functions. An initial layout of the pipework and the position of the main components have been defined on the basis of the following design requirements: (I) gamma radiation shielding of the components and the pipework; (II) target flow velocity of the LiPb; (III) thermal expansion of the pipes; (IV) possibility to drain the entire loop; (V) redundancy of the loops; (VI) remote maintenance; (VII) position in the building and dimensions of the storage tanks. The 3D model of the entire loops has been provided and integrated in DEMO Tokamak building pointing out the issues related to the interfaces with the other systems and with the building itself.

Numerical investigation of the tritium permeation phenomenon through cooling plates in breeding blankets

Understanding and predicting tritium transport is considered a key issue for the design of safe and self-sufficient fusion power plants. Indeed, tritium generated in the breeder materials is susceptible to permeate to the blanket coolant circuits where it is harder to remove and process. Therefore, the predictive models become very important to evaluate the performance of any blanket design. In this work, 2D finite volume methods are employed to analyze in detail the tritium permeation through a cooling plate considering a set of different parameters that affect the breeding blanket (BB) performance. Different BB configurations are studied varying the geometrical parameters defining the plate: the wall thickness and the cooling channel pitch. Moreover, the analyses cover every permeation regime, from diffusion-limited to surface-limited, including intermediate situations. This way, results are applicable to every BB concept which employs cooling plates with independence of their specific materials, temperatures and tritium concentrations. Results have been compared with those of 1D models in order to define form factors that can be used to increase the precision of system level tritium transport models. A total of 91 simulations have been carried out to find correlations that allows computing the form factors in a wide variety of situations. Results show that the accuracy of 1D models and consequently the accuracy of most system level models strongly depends on the permeation regime and can be poor in some conditions. However, this study demonstrates that the use of form factors can be used for an effective increase of system level exactness.

Current status and future perspectives of EU ceramic breeder development

• EU reference tritium breeding pebbles are produced using a melt-based process. • Process scale-up is underway to ensure the supply for ITER. • Biphasic materials show enhanced mechanical strength and stability. • Design relevant data were summarised in a material property handbook. • Crucial neutron irradiation experiments will complement the material qualification Biphasic ceramic pebbles, consisting of Li 4 SiO 4 and Li 2 TiO 3 , 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.

Characterization of a Li4TiO4 Tritium Breeder by First-Principles Calculation and an Experimental Approach

A tetralithium titanium (Li4TiO4) compound is a tritium breeder material that deserves serious consideration for the fusion reactor blanket concept because of its high lithium density. In the curre...

Characterization of Li-rich Li2TiO3 ceramic pebbles prepared by rolling method sintered in air and vacuum

Li2TiO3 with excess Li was developed as an advanced tritium breeder material for fusion reactors. To improve tritium release performance, the breeder pebbles should have a uniform microstructure with small grains. In this work, Li-rich Li2TiO3 ceramic pebbles were fabricated by an efficient spraying-rolling method and were sintered under air and vacuum atmospheres. Results show that the powder calcination temperature has a significant influence on crush load but negligible effect on grain growth. Excess Li2CO3 in pebbles was not just a lithium source to form the Li-rich phase, but also an effective sintering aid to improve sinterability. Small-grained pebbles with dense and homogeneous structure could be obtained by sintering at a low temperature of 1123 K in air. However, when sintered in vacuum, the pebbles exhibit a porous microstructure with very low crush strength. It is indicated that sintering in air is more suitable for the fabrication of Li-rich Li2TiO3 ceramic pebbles.

An in-pile experimental loop for the irradiation of tritium breeding ceramics in China Mianyang research reactor (CMRR)

• An in-pile irradiation loop has been established in CMRR for in-situ tritium extraction study. • It allows loading or unloading the breeder pellets online by a special designed pneumatic system. • Several typical experiments have been performed to demonstrate its capability. A new in-pile experimental loop has been established in CMRR for ceramic tritium breeder irradiation. The most typical of this loop is the capability of online reloading of irradiated samples. The irradiation temperature in the pellets bed can be controlled within the range of 300～750 °C, with a spatial inhomogeneity of ±10 °C. Up to now, a series of irradiation experiments have been carried out on three different breeder materials, i.e. Li 4 SiO 4 (7.5 % 6 Li), Li 4 SiO 4 (93 % 6 Li) and Li 2 TiO 3 (7.5 % 6 Li) pellets, indicating the stability of the facility.

A global resource for exploring and exploiting genetic variation in sorghum crop wild relatives

AbstractOne response to mitigate the impact of climate change on agricultural systems is to develop new varieties that are tolerant to the new range of biotic and abiotic challenges this change causes. This requires access to novel variants of genes for complex adaptive traits. Crop wild relatives (CWRs) are a potentially valuable source of these genes; however, these materials are often difficult to work with and identifying valuable alleles is difficult without substantial investment in prebreeding. In this study, we describe the development of a nested association mapping population for sorghum [Sorghum bicolor (L.) Moench] using two cultivated grain sorghum reference parents and nine wild and exotic sorghum accessions as donors. The donor parents come from the S. bicolor (L.) Moench subsp. verticilliflorum (Steud.) de Wet ex Wiersema & J. Dahlb., S. bicolor (L.) Moench subsp. drummondii (Steud.) de Wet ex Davidse, and S. bicolor (L.) Moench subsp. bicolor margaritiferum taxa and were sampled from a range of environments across Africa. In total, the resource consists of 13 populations and a total of 1,224 lines. The population has been genotyped with diversity array technology (DArT) markers that produced 42,372 unique single nucleotide polymorphism (SNP) markers covering the genome. We determine the utility of the resource for high resolution mapping of complex traits by demonstrating that the exotics contain unique alleles for some example adaptive trait loci and by using the population for genome‐wide association study (GWAS). The resource should provide useful material for plant breeders attempting to deal with the challenges generated by climate change.

Study on the dynamic behavior of solid breeder materials and neutron multipliers under the perturbation of the magnetic field

The functionality of solid blanket pebble bed in a fusion reactor is inevitably subjected to heat flux and electromagnetic forces during the operation. Since the dynamic behavior under the heat flux and forces has strong influence on the packing factor of the pebble bed, and the packing factor is essential to the higher TBR (tritium breeder rate) of the neutron multiplier pebble bed, it’s necessary to study the impact of the dynamic forces on the neutron multiplier pebble bed. Some researchers have investigated the influences of the heat flux and electromagnetic forces on pebble bed. But, till date the influences of the heat flux and electromagnetic forces on the pebble bed of solid breeder materials in presence of neutron multiplier have not been studied yet. Thus, in this paper, the dynamic behavior of the particle system under the magnetic field are investigated based on the neutron multiplier pebble bed in the solid blanket. The mathematical and physical model of the particle pebble bed under the magnetic field was constructed, and the fundamental rule of the neutron multiplier pebble bed under the disturbance of the magnetic field is established. The presented work will provide basic parameters and data for the CFETR (China fusion testing reactor) pebble bed design.

Feasibility of Li4SiO4 pebbles by drip casting and preliminary characterization

Abstract Open issues for fusion reactor technology are related to the breeder material to be used for the nuclear fusion reaction. Despite in the last decades several studies were carried out, the selection, characterization and reliability of breeder material are still open issues. In this study, the attention is focused on the production of lithium orthosilicate (Li4SiO4) by means of drip casting method. The feasibility of this method was verified using an experimental rig designed and constructed purposely at the Pisa University, Department of Civil and Industrial Engineering (DICI). The green pebbles were produced starting from a suspension of Li4SiO4 precursors (i.e. LiOH and Si(OCH2CH3)4) prepared by sol-gel method. These pebbles were dried at 40 °C for 4 h, and then sintered in air at 900 °C for 10 h. The morphological and physical properties of the obtained pebbles were analyzed by scanning electron microscope (SEM), X-ray and compression tests. The X-ray results showed the main characteristic peaks of Li4SiO4 crystalline phase and small peaks attributable to calcium metasilicate, while the SEM analysis showed a homogeneous and uniform structure of the pebbles. The obtained preliminary results showed the feasibility of dripping at room temperature and provided further useful information for the optimization of the process.

Highly efficient preparation of Li2O breeder materials with core‐shell structure by oil‐based granulation route

AbstractThis paper gives an oil‐based granulation approach to realize the efficient preparation of Li2O breeder materials, which solve the problems of long preparation cycle, high manufacturing cost and difficult to be homogenization and densification in the past. Li2O precursor spheres with narrow size distribution and uniform spherical shape can be obtained using paraffin oil as binders. The Li2O feed powder with bimodal size distribution will self‐organize to form a core‐shell structured sphere during granulation, in which the core and shell are composed of small particles and large particles, respectively. Notably, the Li2O ceramic spheres sintered at 900°C can achieve a crush load of 40.69 ± 4.01 N and a theoretical density of 79.66% TD, which was not achievable by the methods reported in the previous literature. The strategy proposed in this work not only can be easily extended to other material systems but also opens up a new avenue for the design and preparation of tritium breeder materials with uniform size distribution and satisfactory mechanical properties.

Developments on the tritium extraction and recovery system for HCPB

Abstract The tritium release from the ceramic materials of the DEMO Breading Blanket (BB) is a complex mechanism due mainly to the chemistry of the interface between the breeder material and the purge gas. The tritium atomic form produced inside the ceramics structure is converted into tritiated water at the surface of the breeder material due to interaction with residual moisture / water layer on the ceramics and free oxygen atoms after Li burn-up. Therefore, in contrast to the liquid BB where tritium is produced as Q2 solely, the Tritium Extraction and Recovery (TER) from any solid BB must handle both the molecular tritium and the tritiated water. Most of the processes considered for T extraction from the purge gas foresee to handle Q2O and Q2 separately and consecutively, making necessary a two-stage process for full tritium removal.

Effects of sintering conditions on the microstructure of Li2TiO3 tritium breeding materials

Lithium meta-titanate (Li2TiO3) have been proposed as candidate breeder materials for fusion breeding blankets. For the efficient tritium release, the breeder materials are recommended to have homogeneous microstructure with small grain sizes which can be strongly affected by the sintering conditions such as temperature, heating rate, atmosphere and so on. The Li2TiO3 pebbles were fabricated by the slurry droplet wetting method with nano-powder. In this study, the effects of sintering conditions on the microstructure of sintered pebbles were investigated. Phase analysis and morphological observation were carried out by X-ray diffraction and electron microscopy, respectively. The grain growth is strongly affected by the increasing temperature and heating rate. The sintering temperature of 900 °C and 10 °C/min were optimal condition for small grain size.

Plant genetic resources in India: management and utilization.

Plant genetic resources (PGR) are the foundation of agriculture as well as food and nutritional security. The ICAR-NBPGR is the nodal institution at national level for management of PGR in India under the umbrella of Indian Council of Agricultural Research (ICAR), New Delhi. India being one of the gene-rich countries faces a unique challenge of protecting its natural heritage while evolving mutually beneficial strategies for germplasm exchange with other countries. The Bureaus activities include PGR exploration, collection, exchange, characterization, evaluation, conservation and documentation. It also has the responsibility to carry out quarantine of all imported PGR including transgenics meant for research purposes. The multifarious activities are carried out from ICAR-NBPGR headquarters and its 10 regional stations located in different agro-climatic zones of India. It has linkages with international organizations of the Consultative Group on International Agricultural Research (CGIAR) and national crop-based institutes to accomplish its mandated activities. NBPGR collects and acquires germplasm from various sources, conserves it in the Genebank, characterizes and evaluates it for different traits and provides ready material for breeders to develop varieties for farmers. ICAR-NBPGR encompasses the National Genebank Network and at present, the National Genebank conserves more than 0.40 million accessions. NBPGR works in service-mode for effective utilization of PGR in crop improvement programmes which depends mainly on its systematic characterization and evaluation, and identification of potentially useful germplasm. NBPGR is responsible for identifying trait-specific pre-adapted climate resilient genotypes, promising material with disease resistance and quality traits which the breeders use for various crop improvement programmes. The system has contributed immensely towards safeguarding the indigenous and introducing useful exotic PGR for enhancing the agricultural production. Presently, our focus is on characterization of ex situ conserved germplasm and detailed evaluation of prioritized crops for enhanced utilization; assessment of impact of on-farm conservation practices on genetic diversity; genome-wide association mapping for identification of novel genes and alleles for enhanced utilization of PGR; identification and deployment of germplasm/landraces using climate analog data; validation of trait-specific introduced germplasm for enhanced utilization. Key words: plant genetic resources; gene banks; wild relatives; biotic and abiotic stresses; marker-assisted selection.

Numerical investigation of purge gas flow through binary-sized pebble beds using discrete element method and computational fluid dynamics

Abstract The main function of the breeder materials, such as Li2TiO3, Li4SiO4, and Li2ZrO3, in a solid type breeding blanket is to produce tritium, which is transferred to the fuel cycle system by helium purge gas and is used as a fuel for nuclear fusion reactions. Since the configuration of the breeder should be designed considering the purge gas flow, the form of pebble beds is mainly adopted, thereby leading to the smooth flow inside a blanket. The flow characteristics can be important design drivers because they also affect the design of the fuel cycle system. In order to increase a tritium production rate, a large number of breeders should be placed in the same space and various sizes of pebbles instead of one size of pebbles can be thus applied to increase the packing fraction of pebble beds. In this case, however, the flow resistance may increase and hence the relationship between the pebble bed configuration and the purge gas flow should be carefully investigated. In this study, a computational framework for one-way coupling analysis of the discrete element method and computational fluid dynamics is established to simulate the purge gas flow through binary-sized pebble beds. The configurations of pebble beds are obtained using the discrete element method and their geometries are modified by smoothing the sharp circular edges at the contact points of pebbles to construct proper meshes for computational fluid dynamics simulations. The flow resistance of binary-sized pebble beds is then investigated in terms of the pressure drop in the breeding zone.

Experimental study on corrosion-erosion behavior of ITER blanket structure materials in flowing Pb-17Li

Abstract A rotating corrosion test device has been developed to study the compatibility between liquid tritium breeder material and structure materials for fusion facility. The corrosion behaviors of three samples made of reduced activation ferritic/martensitic steel (RAFMS), 316 L stainless steel and oxide dispersion strengthened steel (ODS), are studied in liquid Pb-17Li at 500℃ for an exposure of 1000 h. The weight changes of the specimens are obtained before and after test. The analyses of surface morphology and component are characterized by SEM, EDS and XRD methods. Results show that weight-loss rate of RAFMS is lower than that of 316 L and ODS in the experiment. It indicates that the corrosion mechanism is governed by some or all dissolution of alloy elements (such as Cr, Ni) from the sample surface into liquid Pb-17Li and the exfoliation from the corroded surface by the flowing Pb-17Li. Specifically, the dispersed Al2O3 particles in ODS is easy to be corroded in liquid Pb-17Li and then provide corrosion points to accelerate dissolution of Ni-Fe austenite. The compatibility of RAFMS with liquid Pb-17Li is slightly better than that of 316 L and ODS in the current experimental period.

Fundamental analysis for electrochemical removal and monitoring of oxide impurities in lead lithium using LiCl-KCl melt

• The liquid LiPb alloy is expected to be used in the blanket system. • The oxide impurities in the LiPb is forcused in this research. • LiCl-KCl is useful for the transfer of oxide impurities. • The oxide impurities in the molten salt can be extracted by applying voltage. The electrochemical treatment using chloride molten salt was focused as a method for removal and monitoring of oxide impurities in the liquid breeder materials. The anodic reactions on a glassy carbon electrode in a LiCl-KCl eutectic melt contacting with Li-Pb eutectic melt without and with Li 2 O addition were investigated. The transfer of oxide impurities to the molten salt from liquid metal was proved by the change of the concentration of oxide ions in the molten salt estimated from the cyclic voltammetry. The content of oxide impurities in the Li-Pb can be estimated by calculating backward from the transfer ratio and this technique may also be applied to monitoring. The visual observation and the weight loss of the anode indicated that the oxide impurities transferred to the molten salt could be removed as gases such as CO 2 by applying a voltage between an anodic glassy carbon rod in the molten salt and the Li-Pb used as a cathode.

The HICU PIE results of EU ceramic breeder pebbles: General characterization

The HICU (High neutron fluence Irradiation of pebble staCks for fUsion) experiment was performed in the High Flux Reactor (HFR) in Petten, NL, in order to irradiate candidate tritium breeder materials in a fusion relevant environment. The presented work focuses on the post-irradiation examination of the irradiated lithium orthosilicate based breeder pebbles. The pebble samples showed three different contents of Li-6 and were irradiated at two different temperatures and in mechanically constrained and unconstrained state. In this particular publication, the influences of the irradiation conditions on the pebble morphology, microstructure, porosity, and mechanical strength are addressed.The results indicate that in general a high irradiation temperature seems to be advantageous for maintaining the mechanical strength of the irradiated pebbles. A higher mechanical strength and a significantly lower closer porosity is observed for samples that were irradiated at high temperatures in comparison to pebbles that were irradiated at low temperatures. The effects on the pebble properties with respect to the Li-6 content are small in contrast to effects of the irradiation temperature. With an increased Li-6 content, no deterioration of the material properties was observed, especially for samples irradiated at high temperatures.

Tritium diffusion in a Li2TiO3 crystal terminated with the (001) surface from first-principles calculations.

The tritium release behavior of the Li2TiO3 crystal has become an important index to evaluate its comprehensive performance as a solid breeder material in nuclear fusion reactors. The tritium diffusion on the surface (surface diffusion) and diffusion from the inside to the surface (hopping diffusion) in Li2TiO3 crystals with a 1/3-Li(001) surface are systematically investigated by the first-principles method. Possible adsorption sites, diffusion pathways and energy barriers of surface diffusion and hopping diffusion have been calculated and analyzed, respectively. Tritium atoms are found to diffuse preferentially along the [100] direction on the surface and two equivalent pathways across the surface were identified. The obtained activation energies are about 0.50 eV for surface diffusion and 1.56 eV for hopping diffusion. The local density of states and Bader charge for typical surface diffusion and hopping diffusion pathways are calculated and analyzed. The results reveal that the tritium (T) atom bonds with neighboring oxygen (O) atoms during the surface diffusion, while the T-O interaction is significantly weakened in the hopping diffusion which results in the higher activation energy than that of surface diffusion. In combination with our previous work, a complete tritium diffusion model for the Li2TiO3 crystal is proposed and the corresponding tritium diffusion coefficients are obtained. Our obtained activation energies are in the same range as previous experimental data and could provide theoretical support for the future related experiments.

First-Principles Study of Tritium Diffusion in the Li3TaO4 Crystal.

Li3TaO4 with a high melting point, good thermal stability, and higher lithium content has become a possible choice for breeder materials, which have potential applications in future fusion reactors. Perfect and defect crystal models of Li3TaO4 are set up, and all of the tritium-diffusion pathways have been studied by the first-principles method. The activation energy barriers of different diffusion pathways are calculated and analyzed considering the pathway length and tritium–oxygen interactions. The obtained minimum energy barrier for tritium diffusing in the perfect Li3TaO4 crystal is only 0.34 eV. The minimum energy barrier is less than 0.72 eV when tritium diffuses in the defect Li3TaO4 crystal in the presence of a lithium vacancy. Finally, the diffusion coefficients of tritium in the Li3TaO4 crystal are calculated, which further confirm that it is easy for tritium to escape from the trap of the lithium vacancy and diffuse in the crystal. Such a tritium-diffusion behavior is in favor of the tritium-release process of the Li3TaO4 crystal and could provide theoretical guidance for the future applications of Li3TaO4 materials.