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Advanced ceramic breeder (ACB) pebbles consisting of 65 mol% lithium orthosilicate (Li4SiO4) and 35 mol% lithium metatitanate (Li2TiO3) are developed for tritium breeding in the European Union’s (EU) helium cooled pebble bed (HCPB) breeder blanket concept of the demonstration fusion power plant (DEMO). Electron-nuclear double resonance (ENDOR) spectroscopy is a powerful and widely used magnetic resonance technique that combines aspects of electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR). In the present work, ENDOR spectroscopy was applied to investigate hyperfine (HF) interactions of paramagnetic radiation-induced defect centres (containing unpaired electrons) in the biphasic ACB pebbles after irradiation with 10 MeV accelerated electrons. To separate individual EPR signals of the formed and accumulated paramagnetic centres, isochronal annealing of the irradiated ACB pebbles followed by EPR spectra measurements at X- and Q-microwave frequency bands at room and low temperature were performed. Afterwards, X-band ENDOR spectra at low temperature were recorded for selected EPR signals to reveal HF interactions between the unpaired electron spins with the spins of magnetic 7Li and 1H isotope nuclei. The g-factor and HF coupling values determined from EPR and ENDOR spectra simulations provide novel insights into the local structure of paramagnetic hole- and electron-type centres in the irradiated ACB pebbles.

Advanced ceramic breeder (ACB) pebbles consisting of the two phases Li4SiO4 and Li2TiO3 serve as the EU reference tritium breeding material. In the present study, the phase stability with regard to a lithium-burn-up is investigated. The loss of lithium due to its transmutation will lead to a compositional change of the breeder material. Li4SiO4 and Li2TiO3 will partly be transformed to the lithium-poorer phases Li2SiO3 and Li4Ti5O12, respectively. However, from the phase diagram it is anticipated that only Li2SiO3 will occur as a third phase after a lithium burn-up as the generated Li4Ti5O12 is expected to further react with Li4SiO4 to form again Li2TiO3 and more Li2SiO3. Therefore, the reaction between Li4SiO4 and Li4Ti5O12 and its temperature dependence were investigated in detail and its reaction kinetics were determined. It was found that this diffusion-controlled reaction with an activation energy of 75 kJ/mol (0.78 eV) will slow down at low irradiation temperatures and may not take place at very low temperatures.

This paper presents the results of 4 reactor campaigns on the irradiation of biphasic lithium ceramics containing different ratios of lithium orthosilicate (LOS) and lithium metatitanate (LMT) components (25 and 35 mol% LMT in LOS). The size distribution of pebbles in pebble beds was 250–1250 μm and 500–710 μm, respectively. The studies were carried out sequentially with each type of ceramics. In experiments carried out using the vacuum extraction method, the composition of gases released from lithium ceramic samples was registered in in-situ mode. The absence of purge gas during the experiments minimized the possibility of T2O and HTO formation, significantly simplifying processing of the results and providing more opportunities for results analysis. The main goal of the present paper was to identify the general patterns of the release of gases with mass numbers M2 (H2), M4 (He + HT), M6 (T2) and M18 (H2O) from ceramic samples throughout the entire irradiation experiment in 4 campaigns. Release trends of main gases with mass numbers M2, M4, M6 and M18 for all four campaigns are presented and their comparative analysis was performed. The average partial pressure of tritium release in the form of T2 and HT molecules for all campaigns was determined as 5.8 × 10-7 Torr. The dependences of formation rates of helium release peaks on the irradiation time were plotted. The nature of peak emissions does not have a monotonic relationship; upon irradiation, both an increase in the frequency of peaks and a decrease in it are observed. During irradiation, the process of peak helium release does not stop.The simulation was carried out by the finite element method, assuming that tritium release from the sample is determined by diffusion and desorption processes from the sample surface. The experimental curves are satisfactorily described by a number of sets of desorption and diffusion parameters. One way or another they lie in the range of specified values. The Arrhenius dependences of the effective diffusion coefficient and desorption coefficient obtained for lithium ceramics 35 LMT are equal to:D=5,2×10-11m2sexp-21kJmoleRT,K=1.21×10-4m2sexp-64kJmoleRT.The values of the effective diffusion coefficient and tritium desorption coefficient in 25 LMT ceramics were 15 and 20% lower than in 35 LMT ceramics.

Lithium metatitanate (Li2TiO3) pebbles were irradiated with neutrons within the HICU (High neutron fluence Irradiation of pebble staCks for fUsion) experiment to investigate their material properties and tritium release behaviour in a post-irradiation examination (PIE). The irradiation temperature is the most significant influence on the material. Besides a higher irradiation temperature, a higher initial Li–6 content tends to lead to an increased tritium generation resulting in the formation of the secondary lithium depleted phase Li4Ti5O12. A pre-compaction of the pebble bed does not have an apparent influence on the material properties. Tritium is released as semi-tritiated water and semi-tritiated hydrogen (HTO and HT) and its release is enhanced at elevated irradiation temperatures.

Two-phase ceramics Li2TiO3-Li4SiO4 are one of the potentially promising materials for creating a ceramic blanket for DEMO reactor. However, until now, a limited number of studies have been carried out on the release of tritium from this material under the influence of neutron irradiation, which poses fundamental problems for its application.In this work, the pebbles of two-phase lithium ceramics consisting of 25 mol% Li2TiO3 and 75 mol% Li4SiO4 were studied using thermal desorption spectroscopy (TDS) after their irradiation with neutrons at the WWR-K research reactor. Under the conditions of the irradiation experiment, which lasted 21.5 days at a reactor power of 6 MW, the samples were exposed to thermal neutrons with a flux density of 2·1013n/(cm2 × s). The thermal neutron fluence accumulated as a result of irradiation was 3.7·1019 cm−2.It has been established that tritium comes out mainly in the form of HT molecules and has three visible TDS peaks, which can be described by three rates of the first-order desorption reaction with an activation energy of 90 kJ/mol. Experiments have shown that the main amount of tritium is evenly distributed throughout the pebble’s pores and voids. Its yield is determined by desorption and transfer from inner voids to the boundaries that communicate with the outer surface of the sample. The kinetics of tritium release depends on the number, size and depth of the exit paths of such areas inside the ceramic.

Biphasic lithium ceramics based on lithium orthosilicate Li4SiO4 and lithium metatitanate Li2TiO3 is one of the most promising materials for breeder blankets of future fusion reactors. One of the important issues of biphasic lithium ceramics application in the fusion reactor blanket is to determine the parameters and mechanisms of tritium transfer within and from the ceramics.This paper continues the analysis of irradiation experiments carried out at the WWR-K reactor (Almaty, Kazakhstan) with a sample of biphasic lithium ceramics Li4SiO4-Li2TiO3 (pebbles of lithium orthosilicate with 35 mol% lithium metatitanate with diameter 250––1250 μm). The section of the experiment in which the reactor was temporarily shutdown for 1.5 h was investigated in detail. During this period of time the sample temperature rapidly decreased from 665 °C to 100 °C, generation of tritium and helium in the lithium ceramic sample ceased, but the desorption of previously generated gases from the ceramic surface continued. The experiments were carried out by the vacuum extraction method.The nature of tritium-containing molecules and helium release for that specified time interval was analyzed. The kinetics of tritium release from ceramics in the experiment during reactor shutdown was simulated and the expression for the effective diffusion coefficient D = 5e-11(m2/s)∙exp(-20(kJ/mole)/RT) was determined. It was suggested that one the most realistic mechanisms for tritium release is the mechanism associated with both diffusion and desorption of tritium from the pebbles surface and release from the open pores of the pebble.This mode of the experiment made it possible to estimate the parameters of tritium release immediately after irradiation, which imitates the conditions of breeding blanket operation in the fusion reactor.

Advanced ceramic breeder (ACB) pebbles consisting of lithium orthosilicate (Li4SiO4) and the strengthening phase lithium metatitanate (Li2TiO3) are accepted as the European Union’s reference solid-state material for tritium breeding in thermonuclear fusion reactors. Previously, the influence of various radiation types on Li4SiO4-based ceramic breeder materials has already been investigated and described by several groups of researchers. Electron paramagnetic resonance (EPR) spectroscopy is one of the most frequently used analytical and non-destructive techniques that can be selectively applied for qualitative and quantitative research of radiation-induced defect centres with paramagnetic properties (electron spin S ≠ 0). In the present work, individual signals of paramagnetic radiation-induced defect centres in the ACB pebbles after irradiation with X-rays were separated by EPR spectra simulations for the first time. Multiple signals of spin S = ½ systems with distinctive symmetries and g-factor (g) values were identified and characterised. Individual signal decay at room temperature was monitored and isochronal annealing of the irradiated ACB pebbles was performed in order to evaluate the stability of accumulated radiation-induced defect centres. Based on the obtained results, it was determined that radiation-induced defect centres with EPR signals in the g > 2.00 region are similar to irradiated single-phase Li4SiO4 ceramic materials, while additions of Li2TiO3 as the second phase in the ACB pebbles also stimulates the formation of several titanium-related electron centres with signals in the g

This work presents the preliminary experimental data on the study of gas release from two-phase lithium ceramics Li4SiO4-Li2TiO3 under neutron irradiation conditions. Experiments were carried out at the WWR-K research reactor (Almaty, Kazakhstan) for ∼4.3 days. The total neutron fluence during the irradiation was ∼1.8·1019n/cm2. In the course of irradiation, two experiments on ceramics heating during irradiation and two experiments with hydrogen isotopes (H2 and D2) supply into the experimental chamber with the sample were performed at a temperature of 680 °C and reactor power of 6 MW. During the entire irradiation, the gas composition in the continuously evacuated ampoule device with samples was recorded. The obtained dependences of the release of tritium-containing molecules and helium during the experiment were qualitatively analyzed.

In the development of fusion energy, an important task is the study and improvement of tritium production technologies. In this case, one of the most promising materials for tritium generation is lithium ceramics. Considering the importance of the task, numerous studies are aimed at solving the problem of determining the parameters and mechanisms of tritium release in lithium-containing materials. This paper presents the results of a study of tritium release processes from two-phase lithium ceramics of Li4SiO4/Li2TiO3 during reactor irradiation when hydrogen and deuterium are injected into the chamber with irradiated samples. The mechanisms regularities of the tritium yield process in the presence of these isotopes were established. The experiments were carried out in the WWR-K research reactor at a neutron flux density of 5∙1013n/cm2∙s and sample temperatures from 650 to 700 °C.

This paper analyzes part of the reactor experiments on the study of tritium and helium release from promising two-phase lithium ceramic (Li2TiO3 and Li4SiO4) of natural lithium enrichment conducted by vacuum extraction.The basis for such an analysis was a more careful study of the time trend of pressure changes of gases in the chamber with the test samples. In a particular case, it was clearly shown that the pressure fluctuations observed during irradiation for gases with mass number M4 (to which both HT and He molecules correspond) are determined only by He, which leaves the intergranular regions of the ceramic through open channels or cracks. The kinetics of changes in the amount of helium that is released during irradiation was traced and both the rate of helium release and the frequency of emissions were determined. It was assumed that the observed emissions correspond to a certain “formation of free paths” from the internal cavities of the irradiated ceramics into the chamber of the facility. The data obtained for the helium emissions were compared with the release of tritium-containing molecules from the ceramics. The quasi-equilibrium levels of the release of tritium-containing molecules and their dependence on the reactor power were estimated. The release of helium and tritium was compared with the calculated values of the tritium generation rate in the test sample.

In a solid breeding blanket, ceramic breeder pebbles are in contact with reduced activation ferritic martensitic (RAFM) steel at high temperatures for years and accordingly form a corrosion layer on the blanket structural steel. Present study focuses on corrosion characteristics of EUROFER97 RAFM steel by an advanced breeder material of Li2TiO3 with excess Li (initial ratio of Li/Ti = 2.2) at 623, 823, and 1073 K under sweep gas (He + 0.1% H2) flow. Formation of a thick oxide double layer was found on the surface of the EUROFER plates heated at 823 and 1073 K, while the corrosion layer formed at 673 K was