Breeder Materials Research Articles

Tritium Solubility in Li2TiO3 from First-Principles Simulations

An understanding of the fundamental processes governing the release of tritium from a ceramic breeder material is a requirement for the development of breeder blanket technologies for future fusion reactors. Here, density functional theory is used to study tritium accommodating defects in one of the leading breeder blanket candidate materials, Li2TiO3. Defect formation energies are combined with simple thermodynamics to predict the mechanisms of tritium solubility across a range of conditions relevant to a fusion reactor. The simulations predict very different modes of tritium accommodation depending on the stoichiometry of the crystal and the oxygen partial pressure. In addition, the simulations show how the incorporation of a significant tritium concentration can modify the defect chemistry of the host matrix.

Numerical-experimental analyses by Hot-Wire method of an alumina cylinder for future studies on thermal conductivity of the fusion breeder materials

The determination of the thermal conductivity of breeder materials is one of the main goal in order to find the best candidate material for the fusion reactor technology.Experimental tests have been and will be carried out with a dedicated experimental devices, built at the Department of Civil and Industrial Engineering of the University of Pisa. The methodological approach used in doing that is characterized by two main phases strictly interrelated each other: the first one focused on the experimental evaluation of thermal conductivity of a ceramic material, by means of hot wire method, to be subsequently used in the second phase, based on the test rig method, to determine the thermal conductivity of pebble bed material. To the purpose, two different experimental devices have been designed and built. This paper deals with the first phase of the methodology.In this framework, the equipment set up and built to perform Hot wire tests, the ceramic material (a cylinder of alumina), the experimental procedure and the measured results obtained varying the temperature, are presented and discussed. The experimental campaign has been lead from 50°C up to 400°C.The thermal conductivity of the ceramic material at different bulk temperatures has been obtained in stationary conditions (detected on the basis of the temperature values measured during the experiment). Numerical analyses have been also performed by means of FEM code Ansys©. The numerical results were in quite good agreement with the experimental one, confirming also the reliability of code in reproducing heat transfer phenomena.

Fabrication of Li4SiO4 pebbles by wet method with modified powders synthesized via sol–gel process

Li4SiO4 pebbles have been recognized as attractive tritium breeder materials in the fusion reactor blanket of international thermonuclear experimental reactor (ITER). In this work, we present a facile method to prepare Li4SiO4 pebbles of high density and sphericity by using a directive wet method with the Li4SiO4 powders synthesized via sol–gel process. The Li4SiO4 powders were prepared with two-step calcinating method, followed by a ball-milling process. Thermal and phase analysis, morphologies and sintering behaviors observations of the pebbles were carried out systematically. Experimental results show that the pure phase powders with white color that prepared by using two-step calcinating method is different from the powders prepared by the traditional direct calcinating method. The subsequent ball milling process proves to be effective to improve the relative density of the sintered body. When sintered at the temperature as low as 850°C for 4h, the favorable Li4SiO4 pebbles with uniform size (∼1mm), good sphericity (1.02), and high density (above 90% T.D.) were fabricated by using a directive wet method. The as-fabricated pebbles hold good potential as tritium breeding materials for blankets.

Solution based synthesis of mixed-phase materials in the Li2TiO3–Li4SiO4 system

As candidate tritium breeder materials for use in the ITER helium cooled pebble bed, ceramic multiphasic compounds lying in the region of the quasi-binary lithium metatitanate–lithium orthosilicate system may exhibit mechanical and physical advantages relative to single phase materials. Here we present an organometallic solution-based synthesis procedure for the low-temperature fabrication of compounds in the Li2TiO3–Li4SiO4 region and investigate phase stability and transformations through temperature varied X-ray diffraction and scanning calorimetry. Results demonstrate that the metatitanate and metasilicate phases Li2TiO3 and Li2SiO3 readily crystallise in nanocrystalline form at temperatures below 180°C. Lithium deficiency in the region of 5% results from Li sublimation from Li4SiO4 and/or from excess Li incorporation in the metatitanate phase and brings about a stoichiometry shift, with product compounds exhibiting mixed lithium orthosilicate/metasilicate content towards the Si rich region and predominantly Li2TiO3 content towards the Ti rich region. Above 1150°C the transformation of monoclinic to cubic γ-Li2TiO3 disordered solid-solution occurs while the melting of silicate phases indicates a likely monotectic type system with a solidus line in the region 1050–1100°C. Synthesis procedures involving a lithium chloride precursor are not likely to be a viable option for breeder pebble synthesis as this route was found to yield materials with a more significant Li-deficiency exhibiting the crystallisation of the Li2TiSiO5 phase at intermediate compositions.

Influence of surface morphology and surface area on release behavior of hydrogen isotopes in LiNbO3

Surface processes have profound influence on tritium release behavior in ceramic breeder materials. In this paper, the release behavior of hydrogen isotopes in LiNbO3 is studied by thermal desorption spectroscopy (TDS) with focusing on the influence of the surface morphology and surface area. It is found that the amount of surface hydroxyl groups is proportional to the specific surface area and can be decreased by smoothing the surface roughness through heating pretreatment at high temperatures. The isotope exchange reaction between the surface hydroxyl groups and water molecules residue in the system is discussed and turns out to proceed fast. The release behavior of hydrogen isotopes in LiNbO3 is compared with that in Li2TiO3 studied in our previous work. It reveals that LiNbO3 and Li2TiO3 have similar surface environment and similar concentration of surface hydroxyl groups with the level of 1020m−2. The formation mechanism of hydroxyl groups on the surface is discussed and a model to explain the experimental observations is proposed.

Thermal Discrete Element Analysis of EU Solid Breeder Blanket Subjected to Neutron Irradiation

Due to neutron irradiation, solid breeder blankets are subjected to complex thermo-mechanical conditions. Within one breeder unit, the ceramic breeder bed is composed of spherical-shaped lithium orthosilicate pebbles, and as a type of granular material, it exhibits strong coupling between temperature and stress fields. In this paper, we study these thermo-mechanical problems by developing a thermal discrete element method (Thermal-DEM). This proposed simulation tool models each individual ceramic pebble as one element and considers grain-scale thermo-mechanical interactions between elements. A small section of solid breeder pebble bed in a helium-cooled pebble bed (HCPB) is modelled using thousands of individual pebbles and subjected to volumetric heating profiles calculated from neutronics under ITER-relevant conditions. We consider heat transfer at the grain scale between pebbles through both solid-to-solid contacts and the interstitial gas phase, and we calculate stresses arising from thermal expansion of pebbles. The overall effective conductivity of the bed depends on the resulting compressive stress state during the neutronic heating. The Thermal-DEM method proposed in this study provides access to the grain-scale information, which is beneficial for HCPB design and breeder material optimization, and a better understanding of overall thermo-mechanical responses of the breeder units under fusion-relevant conditions.

Size-Dependent Crush Analysis of Lithium Orthosilicate Pebbles

The crushing strength of the breeder material [lithium orthosilicate (Li4SiO4 or OSi)] in the form of pebbles to be used for EU solid breeder concept is investigated. The pebbles are fabricated using a melt-spray method, and hence, a size variation in the pebbles produced is expected. Knowledge of the mechanical integrity (crush strength) of the pebbles is important for a successful design of a breeder blanket. In this paper, we present the experimental results of the crush (failure) loads for spherical OSi pebbles of different diameters ranging from 250um to 800um. The ultimate failure load for each size shows a Weibull distribution. Furthermore, the mean crush load increases with increase in pebble diameter. It is also observed that the level of opacity of the pebble influences the crush load significantly. The experimental data presented in this paper and the associated analysis could possibly help us to develop a framework for simulating a crushable polydisperse pebble assembly using discrete element method.

Synthesis of Li2TiO3 by sol–gel combustion method and its gel-casting formation

Lithium metatitanate (Li2TiO3) has been recognized as a candidate of ceramic tritium breeder materials for its excellent performance. In this paper, Li2TiO3 powder was synthetized by a modified sol–gel combustion process, and the ratio of fuel to oxidizer was optimized. DTA–TG analysis indicated that an intense exothermic reaction occurred at 539 K, which led to the formation of a voluminous white powder. No additional annealing process was required, as pure crystalline β-Li2TiO3 was directly obtained from the combustion reaction. The powder had nano-scale structure and large specific surface area of 9.6m2/g. Li2TiO3 pebbles with desired size and shape were prepared by gel-casting technique using the as-prepared powder. The pebbles reached a high density (81–86% T.D) after sintered at a relatively low temperature (⩽1373K) for 2h. The sintering pebbles had a smooth surface, homogeneous microstructure and small grain size.

Theoretical Calculation and Numerical Investigation for the Effective Thermal Conductivity of Packed Li<sub>2</sub>TiO<sub>3</sub> Pebble Bed

Lithium titanate (Li2TiO3) in form of packed pebble bed is promising concept for breeder material in the fusion reactor blanket, and worldwide efforts have been dedicated to its R&D in the last two decade. Li2TiO3 pebble beds with the efforts of tritium breeding and thermal removing play a major role in the thermal performance of the fusion reactor blanket. Specifically, the effective thermal conductivity of Li2TiO3 is an important parameter for the design and thermal mechanical of fusion reactor blanket. The theoretical calculation and numerical simulation for the effective thermal conductivity of Li2TiO3 pebble bed are performed in this paper. The theoretical equations and modeling result for the thermal conductivity of Li2TiO3 pebble bed are given and compared with the experimental data. The results show that the effective thermal conductivity of Li2TiO3 pebble bed can be preliminarily obtained by numerical simulation and theoretical calculation.

Preparation and Characterization of the Lithium Metatitanate Ceramics by Solution-Combustion Method for Indian LLCB TBM

In fusion DEMO reactors, the blanket requires lithium-containing ceramics as the tritium breeder material. Lithium metatitanate (Li2TiO3) is being considered as a promising tritium breeding material for thermonuclear fusion reactors because of its reasonable lithium atom density, prominent tritium release rate at low temperatures, low activation characteristics, low thermal expansion coefficient, high thermal conductivity, etc. Li2TiO3 will be used in the Indian Lead-Lithium–Cooled Ceramic Breeder concept to be tested in ITER. Li2TiO3 powder has been synthesized by the solution-combustion technique using a less expensive precursor of titanium, i.e., titanium dioxide (TiO2), at Institute for Plasma Research. Titanium oxynitrate [TiO(NO3)2] and lithium carbonate (Li2CO3) with citric acid fuel are used as the raw materials. The combustion reaction was carried out at citrate-to-metal ratios of 0.8 to 1.5, as well as for various pH values ranging from 1 to 5. Citric acid was used as a fuel material for the reaction. Calcination of the powder was carried out at 600°C. The powders were characterized for phase purity, grain size, and surface area using X-ray diffraction, scanning electron microscopy, and a Brunauer-Emmett-Teller surface area analyzer. Finally, Li2TiO3 pebbles were prepared by extrusion followed by spheronization with a diameter range from 1 to 1.5 mm. The details of the powder systemization, pebble formation, and their various characterizations are discussed in this paper.

On ion implantation and damage effect in Li2TiO3 as a fusion breeder blanket: A technological approach for degradation testing

The present work is a technological approach to establish testing techniques able to evaluate the release of the two Li-transmutation products and characterize the degradation of Li2TiO3 as breeder material. In an attempt to find the outgassing/release temperature of light gases and to study its bulk diffusion, nuclear reaction analysis (NRA) was used for profiling the 3He and D depth distributions of the as-implanted and damaged samples as a function of the annealing temperature. The defects generated by γ-rays and heavy-ion (Ti4+) irradiation were characterized by microRaman spectroscopy and confocal microscopy luminescence (CML) techniques. Considering the relevance of the microstructure (porosity and grain size), the study was supported by transmission electron microscope (TEM) observations and Hg-porosimetry measurements. Concerning D-implanted samples, the results suggest the complete release at temperatures higher than 200°C. On the other hand, the implanted 3He desorbs continuously with temperature, a 7–8% still being measured at 700°C.

Comparison of coating processes in the development of aluminum-based barriers for blanket applications

Reduced activation ferritic-martensitic steels (RAFM), e.g. Eurofer 97, are envisaged in future fusion technology as structural material, which will be in direct contact with a flowing liquid lead–lithium melt serving as breeder material. Aluminum-based barrier layers had proven their ability to protect the structural material from corrosion attack in flowing Pb–15.7Li and to reduce tritium permeation into the coolant.Coming from scales produced by hot dipping aluminization (HDA), the development of processes based on electrochemical methods to produce defined aluminum-based scales on RAFM steels gained attention in research during the last years. Two different electrochemical processes are proposed: The first one, referred to as ECA process, is based on the electrodeposition of aluminum from volatile, metal-organic electrolytes. The other process called ECX is based on ionic liquids.All three processes exhibit specific characteristics, for example in the field of processability, control of coating thicknesses (low activation criteria) and heat treatment behavior. The aim of this article is to compare these different coating processes critically, whereby the focus is on the comparison of ECA and ECX processes. New results for ECX-process will be presented and occurring development needs for the future will be discussed.

Point Defects and Non-stoichiometry in Li2TiO3

The intermediate-temperature, monoclinic β-phase of Li2TiO3 shows a stoichiometry range from 47 to 51.5 mol % TiO2. This broad stoichiometric range may be exploited for industrial applications, such as breeder material in a fusion reactor or a microwave dielectric. Here, density functional theory is employed to calculate formation energies for the intrinsic defect species, allowing the identification of the mechanisms responsible for accommodating both excess Li2O and TiO2 across a wide range of temperatures and oxygen partial pressures. The results predict that while the exact mode of accommodating non-stoichiometry depends on factors such as the temperature and oxygen partial pressure, cation disorder plays a major role in the incorporation of non-stoichiometry and that oxygen defects are of relatively minor importance.

Genotype - Environment Interaction for Plant Development and Some Yield Components in Common Bean (Phaseolus vulgaris L.) during the 2012 Wet Season

A multi- location field trial was conducted at two location to estimate genotype by environment interaction for growth, yield and yield characters in nine accession lines of common bean Phaseolus vulgaris L. A randomised complete block design with three replication was use for the field trials. A highly significant differences among the tested genotypes in the two environments were detected for Leaf count, Days after Sowing to first flowering, Days after Sowing to 50% flowering, Plant height at 50% flowering, Pod count per plant, Pod length per plant, Seed diameter per pod, Seed count per pod, Pod yield per plot, Seed yield per plot and Dry matter weight among the testes accessions. The genotype and environment interaction exhibited significance for some characters suggests that genotypes interacted considerably with the environments in the expression of characters, and will behave differently under different environment. A wide mean range of variation was detected which reflects the extent of phenotypic variability in respect of the characters and a good scope for selection of suitable basic material for plant breeders for further crop improvement. Further multi-location trial for seasons should be carried out for the proper selection of the desired genotype.

리튬용액 침투방법에 의한 Li2TiO3페블 제조

To fabricate spherical lithium titanate (Li₂TiO₃) pebbles which are used for a breeder material in fusion reactor, titanium oxide (TiO₂) granules were used as a starting material. The granules were pre-sintered, and then aqueous lithium nitrate solution infiltrated into the granules at vacuum condition. The granules were crystallized to Li₂TiO₃ after sintering under the control of process parameters. In this study, the concentration of lithium in the solution, as well as the number of penetration times and sintering temperature affected the final crystallite phase and the microstructure of the pebbles. In particular, the sphericity and size of the pebbles were effectively controlled by a technical rolling process. The useful spherical Li₂TiO₃ pebbles which have 10 ~ 20% porosity and 60 ~ 120 N compressive strength were obtained through the sintering at 1000 ~ 1100℃ in the multi-times infiltration process with 50 wt% solution. The physical properties of pebbles such as density, porosity and strength, can be controlled by a selection of TiO₂ powders and control of processing parameters. It can be thought that the lithium penetration method is a useful method for the fabrication of mass product of spherical Li₂TiO₃ pebbles.

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 1.3dpa 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 3.5dpa. 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.

Tritium self-sufficiency of HCPB blanket modules for DEMO considering time-varying neutron flux spectra and material compositions

Significant transmutation of solid-type breeding blanket materials affects the time and spatial variation of neutron energy within such materials. This has an impact on simulation assumptions required to accurately assess tritium surplus quantities for conceptual power plant devices. This paper details an investigation, via simulation, of the consequences for the tritium breeding ratio and the tritium self-sufficiency of a DEMO concept with homogeneous Helium-Cooled Pebble Bed blanket modules containing Li4SiO4 ceramic breeder material. For this purpose, a code was developed to couple MCNP5 and FISPACT to supply material compositions from activation calculations to the neutron transport calculation in an iterative loop covering several time steps. Simulation results are presented for a simple 1D spherical device model and a DEMO tokamak model.

Mass loss of Li2TiO3 pebbles and Li4SiO4 pebbles

It has been known that water vapor is released from ceramic breeder materials into the purge gas due to desorption of adsorbed water under dry atmosphere and due to the water formation reaction under hydrogen atmosphere. However, an effect of water vapor in the purge gas to Li mass loss has not been understood. In this study, mass loss of Li2TiO3 (NFI) and Li4SiO4 (FzK) under hydrogen atmosphere (1000Pa H2/Ar), and mass loss of Li2TiO3 (NFI) and Li2TiO3 with additional Li which is in a developmental stage (JAEA) under water vapor atmosphere (50Pa H2O/Ar) were compared, respectively. It was found that under hydrogen atmosphere Li mass loss of Li4SiO4 and Li2TiO3 is same degree although the amount of water vapor released from Li4SiO4 is larger than that from Li2TiO3. It was clarified with regard to Li2TiO3 that Li mass loss in water vapor atmosphere is larger than that in hydrogen atmosphere. Mass loss of Li2TiO3 with additional Li (JAEA) was larger than that of Li2TiO3 (NFI). It was observed by X-ray analysis that Li deposits formed on the inner wall of the quartz tube contain Li2SiO3.

Sorption and desorption behavior of tritiated water on lithium titanate with additional Li

Tritium sorption capacity is an important parameter to evaluate tritium behavior on lithium ceramic breeder materials. In the present study, sorption and desorption behavior of tritiated water on Li2TiO3 with additional Li, which is in a developmental stage in Japan Atomic Energy Agency as an advanced tritium breeder materials, was observed at 20, 300, 600, and 900°C. Tritium sorption capacity on Li2TiO3 with additional Li is larger than that on Li2TiO3. At 600 and 900°C, the sorption capacity approximately agrees with the sum of physical adsorption capacity and chemical adsorption capacity, but at 20 and 300°C it is smaller than that. The overall mass transfer coefficient for tritium sorption increases with temperature in the range from 20 to 600°C but it decreases considerably at 900°C. The sorption capacity and the mass transfer coefficient at 600°C for the sample once used in sorption and desorption experiment at 900°C are smaller than that for original ones.

Measurement of Hg-203 activity induced in LiPb breeder material in D-T like neutron field

Hg-203 (T1/2 46.59 days) produced from Pb isotopes by fast neutrons is one of the most important radionuclide dominating in Pb in fusion power plant a few months after irradiation. Pb in Li15.7Pb breeder material will be irradiated in the HCLL TBM model by high fluxes of 14MeV D-T fusion neutrons during test in ITER. The objective of the present work was to validate available computational tools and nuclear data used for calculation of Hg-203 activity produced in Pb and LiPb material irradiated in 14MeV D-T like neutron fields. The samples of Pb and LiPb were irradiated in several day long experiment performed at FNG Laboratory in Frascati. Hg-203 activity in samples was measured after complete decay of Pb-203 interfering radioisotope with the use of HPGe spectrometers. The MCNP model of the experiment was developed and neutron fluxes in each sample were calculated with the use of FENDL-2.1 transport library. Hg-203 activities in samples were calculated using two methods: FISPACT-2007 code with EAF-2010 library and based on MCNP calculated reaction rates (Pb-206(n, α) and Pb-207(n, n′α)) with the use of JEFF-3.1, JENDL-4.0 and FENDL-2.1 libraries. Finally, the experimental and simulated results were compared and C/E values with assessed uncertainties calculated. The C/E value for JEFF-3.1 and EAF-2010 is 0.85 and for JENDL-4.0 is 1.16. The results with the use of FENDL-2.1 data are nearly one order of magnitude overestimated against experiment.