Tritium Breeding Materials Research Articles

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.

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.

Characterization of tritium breeding ceramic pebbles prepared by melt spraying

Li4SiO4 and Li2TiO3 have long been recognized as two excellent promising tritium breeding materials. In this paper, two kinds of ceramic pebbles, Li4SiO4 pure phase ceramic pebbles and Li4SiO4-xLi2TiO3 multiphase ceramic pebbles were prepared by a melt spraying method at a superheating temperature of 100 ℃ and then tested for their performance. The proportion of pebbles with a particle diameter of 0.8∼1.2 mm reached the maximum of 24.02 % when the spraying pressure is 0.04 MPa. The surface of the pebbles prepared by the spraying method was smooth, and the surface roughness was reported for the first time to reach 2.039 μm. The sphericity reached 1.027. When the Ti/Si molar ratio was 0.5, the crush load of the pebbles after heat treatment reached 71.6 N and the thermal conductivity of the materials reached its maximum of 3.098 W/(m·K) at 700 ℃.

The effects of irradiation and high temperature on chemical states in Li2TiO3

In the future D-T fusion reactor, tritium will be bred mainly by the reaction of 6Li (n, α) T, as well as 7Li (n, n’ α) T in tritium breeding materials. Solid breeding materials will experience harsh conditions under both irradiations by energetic particles (neutron, tritium, helium and self-particles) and high temperature. The interactions of irradiations and high temperature on lithium ceramics will influence tritium breeding ratio (TBR). The changes of chemical states and its effects on release behavior of hydrogen isotopes in deuterium-irradiated Li2TiO3 and deuterium-exposed Li2TiO3 at high temperature have been investigated. The peak of O-1s shifted to higher binding energy by both irradiation and deuterium exposure, indicating that O-D bonds formed. The amount of O-D bonds enhanced as increase of irradiation fluence and exposure temperature. The main deuterium atoms were trapped by defects for irradiated samples. Annihilation of E-centers was thought to trigger the release of hydrogen isotopes. O-D bonds were the main deuterium trapping sites in deuterium-exposed Li2TiO3. Deuterium recovered by detrapping O-D bonds would require higher temperature. Both deuterium-irradiation and deuterium-exposure at high temperature could result in the change of chemical states in Li2TiO3. The changes in chemical states had effects on deuterium release. It illustrates that tritium breeding materials in fusion reactor will be modified by both irradiation and high temperature and could result in lower tritium recovery.

Mechanical strength stability of tritium breeding materials under high temperature deuterium conditions

The mechanical strength of solid tritium breeding materials is one of the key factors for D-T fusion reactors. The crushing of ceramic breeder pebbles will block both tritium recovery circuit and thermal transmission. The mechanical strength changes of Li4SiO4 and Li2TiO3 pebbles exposed to high temperature and hydrogen isotope atmosphere were investigated. The crush load of Li4SiO4 decreased to 66 % of the original values when the breeders were exposed in deuterium gas at the temperature higher than 750 oC. Nevertheless, the crush load of Li2TiO3 remained stable at all the designated temperatures. Investigations involving phase analysis, defect analysis, grain size analysis, and lithium content analysis were performed to determine the causes of mechanical strength change. No new phase formation and no obvious lithium decrease were demonstrated by X-ray diffraction (XRD) and Inductively Coupled Plasma - Atomic Emission Spectrometer (ICP-AES). Defects arising from reduction reaction didn't play an important role in mechanical strength reduction. The main reason responsible for the decrease of mechanical strength of Li4SiO4 was attributed to grain growth. The grain growth behavior was understood by grain growth kinetics. The kinetic parameters of grain boundary migration for Li4SiO4 pebbles were obtained as n of 4 and activation energy Q of 140 kJ/mol. The mechanism of grain growth was given to demonstrate the mechanical strength change of ceramic breeder pebbles.

Improvement of crushing strength and thermal conductivity by introduction of hetero-element Al into Li4SiO4

Investigations on the doping modification of tritium breeding materials have been becoming one of the interesting research areas in the past few years. Recently, preparation method, mechanical and physical properties and tritium release behavior of Li4+xSi1-xAlxO4 solid solution have been studied because of its potential advantages in the field of tritium breeding materials. In this paper, a facile approach was proposed to prepare Li4+xSi1-xAlxO4 pebbles with a relatively small grain. TG/DSC and XRD were conducted to investigate the formation process of Li4+xSi1-xAlxO4. The results showed the crushing strength and thermal conductivity were obviously improved by introduction of hetero-element Al into Li4SiO4, and the optimal x value for Li4+xSi1-xAlxO4 was 0.2 from the point of high crushing strength and thermal conductivity. This study will be expected to provide references for fabricating other solid solution tritium breeders.

Atomistic Simulations of the Defect Chemistry and Self-Diffusion of Li-ion in LiAlO2

Lithium aluminate, LiAlO2, is a material that is presently being considered as a tritium breeder material in fusion reactors and coating material in Li-conducting electrodes. Here, we employ atomistic simulation techniques to show that the lowest energy intrinsic defect process is the cation anti-site defect (1.10 eV per defect). This was followed closely by the lithium Frenkel defect (1.44 eV per defect), which ensures a high lithium content in the material and inclination for lithium diffusion from formation of vacancies. Li self-diffusion is three dimensional and exhibits a curved pathway with a migration barrier of 0.53 eV. We considered a variety of dopants with charges +1 (Na, K and Rb), +2 (Mg, Ca, Sr and Ba), +3 (Ga, Fe, Co, Ni, Mn, Sc, Y and La) and +4 (Si, Ge, Ti, Zr and Ce) on the Al site. Dopants Mg2+ and Ge4+ can facilitate the formation of Li interstitials and Li vacancies, respectively. Trivalent dopants Fe3+, Ni3+ and Mn3+ prefer to occupy the Al site with exoergic solution energies meaning that they are candidate dopants for the synthesis of Li (Al, M) O2 (M = Fe, Ni and Mn) compounds.

Formation and behaviors of helium bubbles in Li4SiO4: A molecular dynamics simulation

Molecular dynamics simulations were conducted to investigate the formation and behaviors of helium bubbles in Li4SiO4. We found that a single helium atom easily moved to a lithium vacancy with minimal energy, and helium dimers were also observed in Li4SiO4. The temperature directly influenced the formation and diffusion of helium, and helium atoms aggregated and formed helium bubbles at a temperature of 700 K. The release of helium bubbles at several temperatures was simulated to analyze the effects of helium bubble release on the Li4SiO4 surface. The temperature was also important factor influencing the recovery of defects. These results will be useful for establishing a physical model of irradiation-induced microstructure evolution inside tritium breeding material and provide a good reference for understanding the initial stage of helium bubble formation and growth in Li4SiO4.

Low-temperature fabrication of Li2O porous ceramic pebbles by two-stage support decomposition

In this study, Li2O porous ceramic pebbles were synthesized by decomposing the xLi2CO3-yLiNO3 precursor pebbles at low temperature. This efficient decomposition reaction was mainly attributed to the two-stage support decomposition of Li2CO3 and LiNO3. The results indicated that the molar ratio of Li2CO3 and LiNO3 will directly affect the phase composition, pore structure and crush load of sintered pebbles. The high-purity Li2O ceramic pebbles sintered from Li2CO3–LiNO3 precursor pebbles at 730 °C exhibits the open porosity of 24.8 ± 0.7%. The crush load of the ceramic reaches 21.18 ± 1.38 N although the structure was porous. The Li2O porous ceramics fabricated in this study will have potential application in tritium breeder materials.

Fast fabrication of high quality Li2TiO3–Li4SiO4 biphasic ceramic pebbles by microwave sintering: In comparison with conventional sintering

Abstract A comparative study between the microwave and conventional sintering of Li2TiO3–Li4SiO4 biphasic ceramic pebbles was systematically performed. Comparing to samples sintered by conventional method, the microwave sintered samples exhibited homogeneous microstructure with uniform grain size distribution. In addition, the microwave sintered samples showed higher relative density (86.0% T.D.) and crush load (22.7 N). The results indicated that the microwave sintering process is a prospective method for fabricating Li2TiO3–Li4SiO4 biphasic ceramic pebbles with excellent mechanical performance as tritium breeder materials.

Low-cost fabrication of Li2TiO3 tritium breeding ceramic pebbles via low-temperature solid-state precursor method

Abstract Lithium metatitanate (Li2TiO3) ceramic pebbles were fabricated from the powder synthesised via low-temperature solid-state precursor method. Solid H2TiO3 and LiOH·H2O react chemically during ball milling process to form a nano-sized precursor powder. Pure β-Li2TiO3 powder can be obtained by calcining the precursor powder at 500 °C, which is half the temperature of conventional solid-state method. The synthesis process is simple and low-cost, which would be more available to achieve batch production among all feasible techniques. The low-temperature calcination will effectively avoid hard particle aggregates and poor sinterability caused by high-temperature heat treatment, which is conducive to prepare ceramics with good properties. The results show that the powder exhibits high sinterability with small particle size of 19 nm. The Li2TiO3 ceramic pebbles sintered at 800 °C have small grain size (470 nm), high relative density (83%) and good crush load (45 N), which has great potential as tritium breeding materials for fusion reactors.

Measurement of thermal conductivity of Li2TiO3 pebble bed by laser flash method

In order to verify the performance and safety of the solid-type breeding blanket concept, the thermal conductivity of the pebble bed is required as an essential parameter for design and analysis of the blanket. The hot wire, hot disk, guarded hot plate, and laser flash method are considered as measurement technique for the thermal conductivity of pebble bed. This study aims at the provision of the thermal conductivity of Li2TiO3 pebble bed, which is one of the candidates for tritium breeder material in the solid-type breeding blanket. The laser flash method was adopted due to its advantages compared with other methods, such as high accuracy and repeatability, easy sample preparation, absolute measurement technique, and so on. In this method, the thermal diffusivity is directly measured by the laser flash system, and then the thermal conductivity is obtained by multiplying density and specific heat to the measured thermal diffusivity. The thermal conductivity of the Li2TiO3 pebble bed with packing factor of 57% increased with increasing temperature up to 500 °C and was saturated to 1.3 W/m/K at more than 500 °C.

Preparation of fine-grained Li4SiO4 pebbles by a combined sol–gel and hydrothermal method and their thermal cycling behavior

Li4SiO4 has been regarded as the vigorous competitors for tritium breeding materials due to its distinct advantages. In this work, Fine-grained Li4SiO4 pebbles were prepared by a combined sol–gel and hydrothermal method (SG–H method). According to our researches, hydrothermal treatment was helpful to gain the ultrafine powders, and two-step sintering process showed obvious advantages in the preparation of fine-grained Li4SiO4 pebbles. The related physical parameters of the samples were measured. Afterwards, thermal cycling tests were carried out to study the microstructure evolution and the variation of crushing load. SEM analyses and compression tests results indicated the Li4SiO4 pebbles obtained by SG–H method exhibited favorable stability under thermal cycling. It suggested that the obtained Li4SiO4 pebbles might meet the requirement of high service temperature in fusion reactor blanket.

Preparation of High Sphericity Li<sub>2</sub>TiO<sub>3</sub> Tritium Breeder by Polymer Assisted Sedimentation Method

The tritium breeder materials used in solid tritium breeding blanket of fusion reactor are lithium-based ceramics, like lithium titanate (Li2TiO3), lithium orthosilicate (Li4SiO4), lithium aluminate（LiA1O2) and lithium zirconate (Li8ZrO6). Among them, Li2TiO3 has the advantages of stable chemical property, high mechanical strength and high lithium content, so it has become one of the preferred materials. Based on the design of solid blanket and tritium extraction process, breeder materials need to be made into a certain shape of pebbles. B Spherical breeder is widely used in the design of fusion reactor for the larger specific surface area, more channels between the pebbles to diffuse and release tritium, and easier loading or unloading. However, due to the high requirements of the breeder pebbles, the production control are not complete. In this paper, a new preparation method of ceramic pellets with high sphericity, polymer assisted sedimentation method, was used to prepare ceramic pebbles. The slurry consists of micro-scaled ceramic powder and organic monomers. Spherical droplets of ceramic slurry were obtained by sedimentation method in viscous liquids. The organic monomer contained in the droplets was heated to polymerize into small pellet preforms during the sedimentation process. By optimizing the sintering process, the fabricated pebbles could perform high compressive load (>18 N), spherical shape (0.95~1.02), uniform particle size (0.8~1.2mm) and porosity (15~20 %). The main advantages of this method is the mild preparation conditions (50~110 °C), the simplified transfer and purification process and convenient industrialization.. The obtained product has high spherical shape, uniform particle size, high compactness and mechanical strength.

Fabrication of Li 2 O ceramic pebbles by freeze drying process

Lithium oxide (Li2O) is one of the best tritium breeding materials. A new wet process, freeze drying method, was used to prepare Li2O pebbles by Li2CO3 powder. Preliminary fabrication tests including freezing, calcination and decomposition were carried out to investigate the fabrication technique parameter of Li2O pebbles. It is suggested that Li2O pebbles can be fabricated by the decomposition of Li2CO3 pebbles at 720 ℃, which were prepared by freeze drying method and calcinated at 650 ℃. Li2O pebbles sintered at 900 ℃ have a diameter of 0.8 mm, grain size of 9 m, density of 69.5% theoretical density and porous structure.

Corrosion characteristics of copper in static liquid lithium under high vacuum

Copper (Cu) materials are extensively used as heat sinks and sealing gaskets in fusion devices because they have the properties of good thermal and electrical conductivity and high plasticity. Meanwhile, liquid lithium (Li) is considered as a potential blanket coolant and tritium breeder and/or plasma facing material in fusion devices. Studying the corrosion characteristics of Cu materials by liquid Li under extreme fusion conditions is important because the corrosion of Cu by liquid Li may affect simultaneous application of these materials in fusion devices. The corrosion behavior of Cu in static liquid Li at 620 K for 15 h under high vacuum was investigated. After exposure to liquid Li, the weight loss rate of Cu in liquid Li is 466.1 g m−2 h−1, which is equivalent to 458.7 mm⋅a−1 for the average corrosion depth rate. The entire surface of each specimen was seriously damaged. Visible grain boundary corrosion was observed on the surface of the specimens. Also, Cu debris entered the liquid Li from the corroded surface, resulting in considerable Cu loss from the specimen. These results demonstrate a corrosion protection grade of Cu in liquid Li of 10, Cu cannot withstand the corrosion of liquid Li under the given conditions. Additionally, the corrosion process of Cu in liquid Li at 620 K under high vacuum was studied using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS). The results from these simulations indicate that the corrosion of Cu in liquid Li is induced via physical dissolution and intergranular corrosion, where intergranular corrosion is the dominant mechanism.

First principles review of options for tritium breeder and neutron multiplier materials for breeding blankets in fusion reactors

The current breeding blankets proposed in the different conceptual fusion power plants are based mainly on the use of Li4SiO4 and/or Li2TiO3 as tritium breeder and Be/Be12Ti as neutron multiplier or an eutectic Li17Pb83 for as a hybrid tritium and neutron multiplier. While these materials offer some tritium breeding capabilities, some recent studies show that the tritium self-sufficiency may not be ensured with these materials due to the strong reduction of blanket coverage after the integration of other in-vessel reactor systems (heating and current drive, limiters, large or double-null divertor systems, etc.). Also, some materials like Be raises several key feasibility concerns. The goal of this paper is to perform an update of the screening for tritium breeder and neutron multiplier materials and to assess the tritium breeding performance of the selected compounds in order to reveal new options. As for the neutron multiplier materials, a new subdivision between solid and liquid multipliers is proposed. For the selected compounds, detailed 3D heterogeneous neutronic analyses have been performed with MCNP5-1.60 assuming the architecture of the current EU DEMO Helium Cooled Pebble Bed (HCPB) as a benchmark breeding blanket. From the point of view of ceramic breeders, Li8ZrO6 has been found to outperform Li4SiO4 by more than 4% in terms of tritium breeding, having 6% higher melting point. From the point of view of solid neutron multipliers, Be12Cr, Be12V, Be13Zr and Be13Y show a similar performance as Be12Ti, while LaPb3, Zr5Pb4 and YPb2 offer a solution for a Be-free blanket. As for liquid multipliers, Pb in combination with a ceramic breeder shows a very promising option. Moreover, Pb compounds like Pb90Mn10 and Pb95Ba5 offer similar performance as Pb with a lower melting point (290 °C). Due to the significant advantages of molten Pb as neutron multiplier, future work will be conducted to define a design of a helium cooled Molten Lead Ceramic Breeder blanket, as simple, cost effective blanket concept.

Comparison of the microwave and conventional sintering of Li2TiO3 ceramic pebbles

Microwave sintering was employed in the fabrication of Li2TiO3 ceramic pebbles using the powders synthesized via hydrothermal method. The as-prepared Li2TiO3 powders exhibited high reactivity with an average particle size as small as 40 nm. A comparative study between the microwave and conventional sintering behavior of Li2TiO3 pebbles was systematically investigated. The microstructure and density analyses showed that the presence of microwaves accelerated the densification and grain growth, thus decreasing the sintering temperature. Besides, an accelerated phase transformation from α-Li2TiO3 to β-Li2TiO3 was observed in microwave processing. The Li2TiO3 ceramic pebbles obtained by microwave sintering exhibited high density, good mechanical property and uniform microstructure, which might hold good potential as tritium breeding materials for blankets. The results showed that the microwave sintering was a promising process for the fabrication of Li2TiO3 pebbles.

(Invited) Lithium Recovery and Lithium-6 Enrichment Using Innovative Electrodialysis with Lithium Ionic Conductor Membrane

As a means of addressing global warming, the world is increasingly turning to the use of Li-ion batteries in electric vehicles and as storage batteries in the home; therefore, there is a growing need for Li. Furthermore, as a fuel for fusion reactors, tritium is produced by the reaction of lithium with neutrons in a tritium-breeding material. Li is primarily recovered from salt lakes in South America but is also present in seawater. Thus, I propose a method for recovering Li from seawater by using innovative dialysis, wherein Li only permeates from the negative electrode side to the positive electrode side through a Li ionic superconductor functioning as a Li separation membrane (LISM) (Fig.1). Measurements of the Li ion concentration at the positive electrode side as a function of dialysis duration showed that the Li recovery ratio increased to approximately 7% after 72 h with no applied electric voltage, and an electrical power of 0.04 V and 0.1 mA was generated. Moreover, other ions in the seawater did not permeate the LISM. With both ends of the LISM bound with a negative and positive electrode, hydrated Li ion was transformed to Li ion only because Li ion can permeate through the LISM [1]. Furthermore, tritium needed as a fuel for fusion reactors is produced via neutron capture by lithium-6 (6Li). However, natural Li contains only about 7.8% 6Li, and enrichment of 6Li up to 90% is required for adequate tritium breeding in fusion reactors. Therefore, new Li isotope separation technique using a Li ionic rconductor functioning as a LISM have been developed. Because the mobility of 6Li ions is higher than that of 7Li ions, 6Li can be enriched on the cathode side of a cell. Using Li0.29La0.57TiO3 (LLTO) as the Li ionic superconductor was prepared, and the relationship between the 6Li separation coefficient and the electrodialysis time was investigated. After electrodialysis, we obtained a maximum of 1.05 for the 6Li isotope separation coefficient. This result showed that the 6Li isotope separation coefficient of this method is the same as that of the amalgamation process using mercury (1.06). Thus, this method has the potential to be a superior 6Li enrichment method to produce 90% enriched tritium breeder for fusion reactors. [1] T. Hoshino, Desalination, 359, 59-63 (2015). Figure 1

Microstructure and phase evolution of Li4TiO4 ceramics pebbles prepared from a nanostructured precursor powder synthesized by hydrothermal method

Li4TiO4 ceramics pebbles have been proposed to be a promising tritium breeder materials for the fusion reactor blanket of the International Thermonuclear Experimental Reactor (ITER). In the present study, the hydrothermal method was first used to synthesize the nanostructured precursor powders for fabricating the Li4TiO4 ceramic pebbles. The precursor powders were composed of LiOH, Li2CO3 and Li2TiO3 with a grain size of 30–80 nm. Moreover, the microstructure and phase evolution of Li4TiO4 ceramics pebbles were investigated. The results indicated that the formation temperature of Li4TiO4 by hydrothermal method was lower than that of other methods. The pure phase Li4TiO4 ceramic pebbles with small grain size (average value 0.50 μm), satisfactory crush load (average value 41 N) and reasonable pore structure was sintered at 750 °C for 4 h in vacuum. This study provides a new method for preparing a high quality Li4TiO4 ceramic pebble, which has potential application as a tritium breeding material.