PU Yiming ¹, FENG Guanlin ¹, OYA Yasuhisa ², ZHOU Qilai ¹

(1. School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, Hubei, China;

2. Faculty of Science, Shizuoka University, Shizuoka 422-8529, Japan)

Extended abstract:

[Background and purposes] Fusion energy is regarded as a clean, efficient and sustainable alternative energy source. The development of reliable solid-state tritium breeding materials is a key requirement for the application of fusion energy. Lithium titanate (Li₂TiO₃) ceramic is a promising tritium breeding material, owing to its high chemical inertness and structural stability, as compared with Li₂O. Lead (Pb) has a low affinity for hydrogen isotopes, which is expected to reduce tritium retention and promote tritium release at lower temperatures. Pb is also a neutron multiplier, which can offer additional neutrons based on the (n, 2n) reaction. Therefore, the introduction of Pb into the Li₂TiO₃ ceramic body would improve the tritium release properties, e.g., decrease of tritium desorption temperature and increase of tritium desorption rate. In addition, the porosity of ceramic materials also affects the diffusion and release of tritium. This study was aimed to study the effects of Pb content and porosity on tritium release behavior of Li₂TiO₃ ceramics, providing an experimental basis for the design of tritium breeding lithium ceramics.

[Method] Li₂TiO₃-5%Pb and Li₂TiO₃-10%Pb powders were synthesized by using solution combustion method, which is a simple, efficient and low-cost powder preparation technology. To adjust the porosity of ceramic sintered bodies, different proportions of polyvinyl alcohol (PVA) solutions were added to the powders as pore-forming agents. The PVA content was precisely controlled to prepare three groups of ceramic sintered bodies, with porosities of 7.6%, 10.1%, and 13.4%. The prepared ceramic samples were subjected to high-temperature deuterium gas exposure experiments at 573 K for 4 h to simulate the tritium absorption process in fusion reactors. After deuterium exposure, thermal desorption spectroscopy (TDS) measurements were carried out at a heating rate of 10 K·min⁻¹, resulting in the TDS spectra of HD, HDO and D₂O. Besides, the Li₂TiO₃-Pb ceramic pebbles were introduced into the Kyoto University Research Reactor (KUR) to perform the neutron irradiation experiment. KUR is a light-water moderated tank-type reactor. For this experiment, the KUR was operated at 1 MW and the Pn-2 pneumatic system was used to transport the ceramic samples into the reactor core. The thermal neutron flux is 5.5×1012 n·cm⁻²·s⁻¹, the fast neutron flux is 1.2×1012 n·cm⁻²·s⁻¹, and the epithermal neutron flux is 2.2×1011 n·cm⁻²·s⁻¹. After neutron irradiation, the samples were transported to Shizuoka University. The tritium release experiment was performed using the tritium-TDS system. The tritium release was recorded by using proportional counters, while the release amount was measured by using liquid scintillation counters.

[Results] According to deuterium TDS results, the HD release peak appeared at at 900 K, while the release temperature of HDO and D₂O was 700 K. The HD is a gas form of deuterium, while HDO and D₂O are water forms of deuterium. The desorption temperatures of HDO and D₂O were significantly lower than that of HD, indicating that deuterium in the form of water molecules is easier to desorb at lower temperatures. For ceramic samples with different Pb contents, the deuterium desorption temperature of  Li₂TiO₃-10%Pb was lower than that of  Li₂TiO₃-5%Pb, confirming that the increase of Pb content can further promote tritium release at lower temperatures. For ceramics with different porosities, the TDS spectra of deuterium gas showed a distinct main peak, while the TDS spectra of deuterium water had two release peaks at 460 K and 600 K. The release rate on the high-temperature side decreased significantly with the increase in porosity. Neutron irradiation experiments showed that when the Pb content was increased to 10%, tritium started to desorb at low temperature. The tritium desorption rate was slightly increased. The tritium production rate per unit volume was expected to increase because tritium is heavy.

[Conclusions] The chemical and structural properties of  Li₂TiO₃-Pb can be controlled using the solution combustion synthesis method. The introduction of Pb can effectively reduce the tritium release temperature and promote tritium release at lower temperatures. The deuterium desorption rate as gas-form HD can be increased by the addition of Pb. The deuterium desorption shifts to a lower temperature range. Porosity has a significant impact on both water-form and gas-form deuterium desorption. The deuterium desorption from lithium ceramics in the high-temperature range can be reduced. For the tritium release, the addition of Pb facilitates the tritium desorption at low temperatures. The tritium release generation amount per unit volume can be expected. The tritium retention rate can be reduced by the addition of Pb.

Key words: tritium breeder; lithium titanate; hydrogen isotope; deuterium; porosity