Increase of adsorption capacity of single-walled carbon nanotubes by opening holes for application in cryopumps

In International Thermonuclear Experimental Reactor (ITER), coconut-shell activated carbon (CSAC) with a large specific surface area (SSA) has been used in the torus cryopumps to absorb hydrogen isotopes with helium and impurity gases. However, CSAC has irregular morphology and contains high levels of impurities; search for more tritium-compatible adsorption materials is necessary. Single-walled carbon nanotubes (SWCNTs) have higher purity and more regular morphology, and less SSA than CSAC. In this study, opening SWCNTs via slow oxidation in air was performed to increase their SSA. The potentials of treated SWCNTs (t-SWCNTs) as adsorption materials in the tritium-compatible torus cryopump were evaluated. The results show that the SSA of SWCNTs has significantly increased by slow oxidation in air without changing their basic morphology and introducing impurities. T-SWCNTs with the target temperature of 500 °C have the largest SSA, which is larger than CSAC. The impurity contents of t-SWCNTs with target temperature of 500 °C are less than that of CSAC, especially the impurities (P, S, H) harmful to hydrogen isotope separation process in subsequent tritium plants are significantly less than that of CSAC. The results demonstrate that T-SWCNTs are potential candidates for a new cryosorbent in the tritium-compatible torus cryopump.