Study on vacuum outgassing characteristics of hot-pressing Boron Carbide(B4C) for ITER equatorial port

Abstract

There are approximately 33,000 blocks of boron carbide (B₄C) employed as the main neutron shielding material in the International Thermonuclear Experimental Reactor (ITER) Equatorial Port#12 Integration System (EQ#12) in-vessel components. The gases desorbed from the surface and diffused from B₄C internally can adversely affect the vacuum environment of ITER device. Therefore, it is crucial to study its vacuum outgassing characteristics for ensuring the vacuum performance of the ITER device. In this paper, B₄C block with an average density of (2.50±0.01) g/cm³ were prepared via the hot-pressing process. Utilizing a self-developed outgassing rate testing equipment, the outgassing characteristics of two kinds of B₄C samples (non-prebaked and prebaked at 900 °C) were studied by switching between two pumping paths method. Results indicate that after the ITER thermal cycle, the outgassing rate per unit area of all B₄C samples were as low as the order of 10^–9 Pa·m³·s⁻¹·m^-2, the H₂O content is significantly decreased, with H₂ constituting the majority of the outgassing composition, and the majority impurities released were CO and CO₂. Furthermore, samples pre-baked at 900 °C exhibit lower impurity levels, with the outgassing rate per unit area of impurity (CO) being less than 10^–10 Pa·m³·s⁻¹·m^-2, implying that the high-temperature pre-baking treatment facilitates the decomposition of organic molecules and enhances the diffusion of internal gases, thereby further reducing in the outgassing rate of B₄C. This study demonstrates that B₄C treated with rigorous processes can serve as main shielding material in the vacuum chamber of ITER devices, providing valuable reference for the selection of shielding material of future fusion reactors.