Solid Breeder Materials Research Articles

Surface inventory of tritium on Li 2TiO 3

Though lithium ceramic materials such as Li 2O, LiAlO 2 , Li 2 ZrO 3 , Li 4 SiO 4 and Li 2TiO 3 are considered as breeding materials in the blanket of a D-T fusion reactor, the release behavior of the bred tritium in these solid breeder materials has not been fully understood, yet. 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 breeder material, absorption of water vapor into bulk of the grain, and adsorption of water on surface of the grain, together with two types of isotope exchange reactions for estimation of the tritium inventory in a uniform solid breeder blanket under the steady-state condition. The adsorption capacity and the isotope exchange capacity on the Li 2TiO 3 surface are experimentally obtained in this study. Comparison of the tritium inventory estimated for the Li 2TiO 3 blanket with the inventory for LiAlO 2, Li 2ZrO 3 or Li 4SiO 4 blanket using the model presented by the present authors shows that the Li 2TiO 3 blanket gives the smallest tritium inventory among the candidate ceramic breeding materials.

Compatibility of SiC/SiC Composite Materials with Molten Lithium Metal and Li16-Pb84 Eutectic Alloy

SiC/SiC composite material is one of the candidates of plasma facing and structural materials for a fusion reactor. In an inertial fusion reactor and a magnetic confinement fusion reactor, interactions of solid and liquid breeder materials as well as coolant materials with various structural materials are considered to be very important. Therefore, corrosion tests of SiC/SiC composite materials with molten lithium and Li16-Pb84 alloy were performed. SiC/SiC specimens of five kinds, different in fiber/matrix interface, manufacturers and manufacturing methods, were used. In the case of lithium, the corrosion temperature was 500 K, and it was 573 K in the case of Lil6-Pb84 alloy. Duration for the compatibility tests were about 2.5 Ms (29 days). During the heating of corrosion test, argon gas was made to flow over the liquid metal. The SiC/SiC composite sample, which had a free silicon component within its matrix, was corroded by molten lithium and came to each plane fabrics of fiber. The monolithic specimen that contained a silicon phase was also entirely broken by lithium. While, the attack by Li16-Pb84 alloy was not observed.

Isotope exchange reaction on solid breeder materials

Lithium ceramic materials such as Li 2O, LiAlO 2, Li 2ZrO 3, Li 2TiO 3 and Li 4SiO 4 are considered to be as candidate for the tritium breeding material in a deuterium–tritium (D–T) fusion reactor. In the recent blanket designs, helium gas with hydrogen or deuterium is planned to be used as the blanket purge gas to reduce tritium inventory and promote tritium release from the breeding material. In addition, the rate of isotope exchange reaction between hydrogen isotopes in the purge gas and tritium on the surface of the breeding material is necessary to analyze the tritium release behavior from the breeding materials. However, the rate of isotope exchange reactions between hydrogen isotopes in the purge gas and tritium on the surface of those materials has not been quantified until recently. Recently, the present authors quantified the rate of isotope exchange reaction on Li 2O and Li 2ZrO 3. The overall mass transfer coefficients representing the isotope exchange reaction between H 2 and D 2O on breeding materials or the same between D 2 and H 2O are experimentally obtained in this study. Comparison to isotope exchange reaction rates on various breeding materials is also performed in this study. Discussions about the effects of temperature, concentration of hydrogen in the purge gas or flow rate of the purge gas on the conversion of tritiated water to tritium gas are also performed.

Enhancement of tritium release from ceramic breeders with impregnated catalytic additives

In most current designs of D-T fusion reactor breeding blankets employing Li-based ceramic as breeder materials, the use of a helium sweep gas containing 0.1% of hydrogen is contemplated to extract tritium via isotopic exchange reactions. However, at lower temperatures, the release process of tritium from the breeders is known to be rather slow. For this reason, there is still a need to develop techniques that promote the release of bred tritium. In order to improve recovery of tritium from blankets over a wide range of temperature, the effect of catalytically active metal additives on the heterogeneous isotope exchange reactions at the breeder/sweep gas interface was examined, and main results are summarized in this paper. Platinum and palladium were deposited on solid breeder material pebbles (Li 4SiO 4) by the incipient wet impregnation method. Experiments were performed with hydrogen and predeuterated water using packed bed reactors and gas chromatographic analysis. Out of pile tritium release experiments were also conducted using ceramic breeders irradiated in a research reactor. The experimental results of this work reveal the benefit of the addition of catalytic additive metals, which is very effective to increase the tritium release rate from ceramic breeder materials especially at comparatively lower temperatures.

Production behavior of irradiation defects in lithium silicates and silica under ion beam irradiation

For the performance assessment of fusion reactor solid breeder materials, the production behavior of irradiation defects in the ternary lithium ceramics of Li 2SiO 3 and Li 4SiO 4 was studied by an in situ luminescence measurement technique under ion beam irradiation of H + and He +. For comparison the measurement was also performed with vitreous SiO 2. The temperature dependence of the luminescence intensity and the transient behavior of the luminescence intensity on temperature changes were measured for kinetic aspects of the involved reactions, and the production mechanisms and kinetics of the irradiation defects in these lithium ceramics were discussed.

Modeling and analysis of time-dependent tritium transport in lithium oxide

Tritium behavior in the solid breeder blanket is one of the key factors in determining tritium self-sufficiency, as well as safety, of fusion reactors. Therefore, it is important to understand the tritium transport mechanisms and processes, in order to accurately determine the tritium release and inventory in the blanket. A model has been developed at UCLA to describe the tritium behavior in solid breeder materials, together with a computer code which can predict the tritium release and inventory. However, the model was limited in some cases: for example, it did not include the capability to account for surface to bulk fluxes and for trapping inside the grain which can have major effects on the tritium transport. The model capabilities have been extended to enable its application over a wide range of experimental conditions. Improvements include: (1) implementing an initial model to account for the general effect of trapping in the bulk, (2) addition of a dissolution flux which would enable a more accurate modeling of solubility driven by hydrogen partial pressure in the solid breeder porosity, (3) inclusion of more hydrogen isotopes species in the pore, and, (4) modification of the existing computer code to allow for broader application, such as the possibility of accounting for both first order and second order adsorption/desorption. The code was used to analyze experimental data on tritium behavior in Li 2O from the BEATRIX-II experiment.

Irradiation effects in ceramic breeder materials

For understanding of the irradiation effects in solid breeder materials, extensive studies have been performed with lithium-containing ceramics such as Li 2O, LiAlO 2, Li 4SiO 4 and Li 2ZrO 3. In parallel with the studies of ordinary irradiation performance testing, a number of the studies have been performed focussed on the fundamental aspects of irradiation-induced defects and their effects in lithium ceramics. In the present paper, the fundamental studies are reviewed and discussed to improve our knowledge on the irradiation effects in lithium ceramics. Their production behaviors, reaction kinetics, and effects on the tritium release performance and microstructural changes of lithium ceramics are described.

Production behavior of irradiation defects in ternary lithium ceramics under ion beam irradiation

For the performance assessment of solid breeder materials for a fusion reactor, the production behavior of irradiation defects in some candidate materials of Li 2TiO 3, Li 2ZrO 3 and Li 2SnO 3 were studied by an in situ luminescence measurement technique under ion beam irradiation. The transient behavior of the luminescence intensity on temperature changes was measured in kinetic aspects of the involved reactions, and the production mechanisms and kinetics of the irradiation defects in these lithium ceramics were determined.

Tritium inventory estimation in solid blanket system

Although lithium ceramic materials such as Li 2O, LiAlO 2, Li 2ZrO 3 and Li 4SiO 4 are considered as breeding materials in the blanket of a D-T fusion reactor, the release behavior of the bred tritium in these solid breeder materials is not yet fully understood. Most results of in situ tritium release experiments are analyzed assuming that the overall release process of tritium is mainly controlled with tritium diffusion in the crystal grain of a solid breeder material. However, the diffusivities by various authors for solid breeder materials do not agree with each other. An insufficient estimation of the surface reactions, the effect due to irradiation defects or the system effect may be the reason. Effects of diffusion of tritium in the grain, absorption of water in the bulk of grain, and adsorption of water on the surface of grain, together with two types of isotope exchange reaction on the tritium inventory in the steady-state condition, are discussed in this study. Comparisons of tritium inventory estimated in this study with data obtained in several in-situ experiments are also performed, and good agreement is obtained when the existence of some water vapor is assumed in the purge gas.

Isotope exchange reaction in Li 2ZrO 3 packed bed

To understand the release behavior of bred tritium in a solid breeder blanker, the tritium transfer rate and tritium inventory for various mass transfer processes should be investigated. The contribution of the surface reactions (adsorption, desorption and two kinds of isotope exchange reactions) to the release process cannot be ignored. It is believed that two kinds of isotope exchange reactions (gaseous hydrogen-tritiated water and water vapor-tritiated water) occur on the surface of the solid breeder materials when hydrogen is added to the sweep gas to enhance the tritium release rate. The isotope exchange reaction study in H–D systems was carried out using a Li 2ZrO 3 packed bed. The exchange reaction between gaseous hydrogen and water was the rate controlling step among the two kinds of exchange reactions. The reaction rate constants were quantified, and experimental equations were proposed. The equilibrium constant of the isotope exchange reaction in the H–D system was obtained from experimental data and was found to be 1.17.

Study on tritium release from solid breeding materials using the research reactor “YAYOI”

Tritium release behavior from solid breeding materials was studied in order to clarify the mechanism of migration process. In-situ tritium release experiments (TTTEx) and in-situ luminescence measurements from radiation defects using the research reactor YAYOI at the University of Tokyo played important roles in these studies combining with off-pile experiments and modeling studies. In in-situ tritium release experiments, intra-grain and grain boundary diffusivities were separately evaluated. The chemical from of the released tritium was found to be determined by the oxygen potential in the system. Surface reaction, which often becomes the rate determining step in tritium release, was intensively studied. The surface reaction rate was found to be determined by the oxygen potential in the system and swamping reaction with H 2 or H 2O. It was also suggested that at low oxygen potential bonding strength of surface -OT differs from that at high oxygen potential. This was also supported by off-pile infrared absorption study of surface -OH. Tritium surface reaction model (DAD) was constructed based on these results. The tritium residence time agreed well with experimental results. In order to study the behavior of radiation defects and its interaction with tritium migration behavior, in-situ luminescence measurement and tritium release experiments under reactor irradiation at high temperature and under controlled atmosphere were conducted. As a first step, defects behavior under irradiation was studied.

Isotope exchange capacity of solid breeder materials

Though lithium ceramic materials such as Li 2O, LiAlO 2, Li 2ZrO 3 and Li 4SiO 4 are considered as the candidates of tritium breeding materials in the blanket of a D-T fusion reactor, the release behavior of the bred tritium in these solid breeder materials is not yet fully understood. It is observed in this study that LiAlO 2 or Li 4SiO 4 has an additional capacity for tritium uptake on the surface through isotope exchange reactions, the isotope exchange capacity, other than the adsorption capacity. It is considered that some -OH bases attributed to the strongly bound chemically adsorbed water or the crystal water formed on the grain surface act as tritium trap, when the isotope exchange reaction are active. It is also observed in this study that Li 2ZrO 3 has no isotope exchange capacity. Effects of the isotope exchange capacity on the tritium inventory in the breeding blanket is discussed based on the data observed in this study where effects of diffusion of tritium in the grain, absorption of water in the bulk of grain and adsorption of water on the surface of grain together with isotope exchange reactions are considered.

In situ luminescence measurement of irradiation defects in ternary lithium ceramics under ion beam irradiation

For the performance assessment of fusion reactor solid breeder materials, the production behavior of irradiation defects in some candidate materials of Li 2TiO 3, Li 2ZrO 3 and Li 2SnO 3 was studied by an in situ luminescence measurement technique under ion beam irradiation. The luminescence was observed to be composed of multiple luminescence bands and the temperature dependence of the luminescence intensity was measured under He + or H + ion beam irradiation. The production mechanism of irradiation defects was discussed by comparing the present results with those previously obtained for Li 2O. The effects of irradiation defects on tritium recovery kinetics and material stability were pointed out.

Tritium inventory in a LiAlO 2 blanket

Although lithium ceramic materials such as Li 2O, LiA1O 2, Li 2ZrO 3 and Li 4SiO 4 are considered as breeding materials in the blanket of a D-T fusion reactor, the release behavior of the bred tritium in these solid breeder materials has not been fully understood. Most of the results of in situ tritium release experiments are analyzed assuming that the overall release process of tritium is mainly controlled with the process of tritium diffusion in the crystal grain of a solid breeder material. However, it has been pointed out by many authors that contribution of the surface reaction cannot be ignored. Effects of diffusion of tritium in the grain, absorption of water in the bulk of grain and adsorption of water on the surface of grain together with isotope exchange reactions on the tritium inventory under the steady state condition are discussed in this study. Comparison of the estimated tritium inventory using the way of this study with the data obtained in several in situ experiments shows that good agreements are obtained when the effective diffusivity obtained from the data by Kwast et al. is used. The better agreements are obtained when existence of some water vapor is assumed in the purge gas.

Tritium recovery from liquid metals

Abstract Liquid breeder materials have many inherent advantages over solid breeder materials, and the liquid metals pure lithium and Pb-17Li alloy are expected to be potential candidate materials. This paper reviews recent work on tritium recovery from the two candidate materials, and discusses the implications of the work for successful future development. The physical and chemical properties of these two materials are also discussed in order to make the similarities and differences clear.

Characteristics of Li 2O pebbles fabricated by the melting granulation method

Lithium ceramics have been considered as candidates for solid breeder materials for fusion reactors. In lithium ceramics, lithium oxide (Li 2O) is one of the best tritium breeders from the standpoint of high lithium density and high thermal conductivity. Recently, several studies have been carried out on the fabrication of small, spherical Li 2O forms to reduce the induced thermal stress in the breeder. A mass-production process for making small pebbles was developed using the melting granulation method. In the present work, the characteristics of Li 2O pebbles fabricated by this method, and mass transfer properties within a Li 2O pebble bed, are discussed.

In situ observation of surface -OH and -OD on lithium oxide

To elucidate the tritium release mechanism from the surface of solid breeding materials, surface -OH and -OD on Li 2O were studied at high temperature up to 400°C and under controlled H 2O (D 2O) vapor pressure or H 2(D 2) partial pressure by using a diffuse reflectance method of FTIR (Fourier transform infrared absorption spectroscopy). During the drying process of Li 2O, a large peak of precipitated LiOH was observed at 3662 cm −1. When D 2O vapor was added to a concentration higher than the precipitation limit, the O-D peak from LiOD was observed at 2696 cm −1. Below the limit D 2O vapor pressure, absorption peaks of isolated -OD on the surface were found at least at two positions: 2748 cm −1 and 2717 cm −1. This experimental fact shows that the surface is not homogeneous for D 2O adsorption. The peaks shifted to the lower wave numbers with the precipitation of LiOD at lower temperature. When D 2 was added to He + D 2O sweep gas, the intensity of the peak from isolated -OD became higher, indicating D 2 is dissociatively adsorbed on the Li 2O surface to produce -OD.

In-pile tritium release rate limiting process study

Diffusion and desorption are normally taken into consideration as the main rate limiting processes for tritium release from solid breeder materials in thermo-nuclear fusion devices. The importance of the correct identification of the process, in order to determine correct process parameters, is universally recognized. Because the two processes are temperature dependent, generally following an Arrhenius law, it is very important to determine, for a certain material microstructure, the temperature ranges in which they are effective, or the existence of a transition zone. The process identification in in-pile experiments is normally very difficult and generally requires several measurements on the same materials with different grain sizes. Even so it is almost impossible to assure that materials, different in grain sizes, have identical all the other microstructural parameters. This again induces data interpretation problems. In this paper the theoretical possibility to discriminate between diffusion and desorption, especially for a particular but interesting in-pile case, is pointed out. For the particular case of big stepwise temperature decrease, the process, at the final temperature, can be identified by a single step. The identification is possible, in full analogy to the out-of-pile case, by observing the slope of the early tritium release, at small times compared with the residence time, in a log-log plot.

BEATRIX-II: A multinational solid breeder materials experiment

BEATRIX-II is an in situ tritium recovery experiment in the fast flux test facility (FFTF) reactor designed to characterize the feasibility of utilizing solid breeder materials at extended burnups in a fast neutron flux. Although not yet complete, the BEATRIX-II experiments have already substantiated that the solid breeder selected for ITER, Li2O, has good irradiation stability and tritium recovery. Temperature stability, lithium transport, dimensional stability and tritium recovery issues of Li2O up to 5% Li burnup were addressed in this experiment, Temperature gradients far more severe than in the ITER design, 400 to 1000°C, were found to be essentially unchanged by burnup and produced no observable instability, either from swelling or lithium vapor transport. Temperature change experiments illustrated that lithium inventories do not appear to increase as a result of irradiation to burnups of 5%.

Fabrication, properties, and tritium recovery from solid breeder materials

The breeding blanket is a key component of the fusion reactor because it directly involves tritium breeding and energy extraction, both of which are critical to development of fusion power. The lithium ceramics continue to show promise as candidate breeder materials. This promise was recognized by the International Thermonuclear Experimental Reactor (ITER) design team in its selection of ceramics as the first option for the ITER breeder material. Blanket design studies have indicated properties in the candidate materials data base that need further investigation. Current studies are focusing on tritium release behavior at high burn-up, changes in thermophysical properties with burn-up, compatibility between the ceramic breeder and beryllium multiplier, and phase changes with burn-up. Laboratory and in-reactor tests, some as part of an international collaboration for development of ceramic breeder materials, are underway.