Tritium removal from nuclear graphite base on hydrogen study

[Background] Nowadays, there are big amount of irradiated nuclear graphite waiting to be decommissioned worldwide. Due to their high content in nuclear graphite, tritium and ¹⁴C are the two main radionuclides that should be specially treated before final disposal. The optimal desorption temperature for ¹⁴C was reported to be 700℃. [Purpose] As tritium is the isotope of hydrogen with similar physical and chemical properties, in order to investigate the desorption process for tritium in nuclear graphite, this study investigates the absorption of hydrogen at 350℃ and desorption of hydrogen from 400℃ to 700℃ in three types of nuclear graphite. [Methods] Samples of three types of nuclear graphite were subjected to flowing hydrogen at 350℃ for hydrogen absorption, then the absorbed hydrogen was desorbed by thermal treatment from 400℃ to 700℃. The hydrogen concentration of the outlet gas was measured by gas chromatograph. [Results] According to our experiments, three types of nuclear graphite have different amount of absorption, but the following desorption experiments showed similar trend with some differences on the amount of desorption varied with time. The amount of absorption of the domestic nuclear graphite NG-CT-10, Japanese nuclear graphite IG-110 and German nuclear graphite NBG-18 were measured to be 6.7 × 10^-3 mL · g^-1, 9.3 × 10^-3 mL · g^-1 and 9.12 × 10^-3 mL · g^-1, respectively, and the amount of hydrogen that is chemically absorbed in graphite were 3.2 × 10^-3mL · g^-1, 3.0 × 10^-3 mL · g^-1 and 0.92 × 10^-3 mL · g^-1 respectively. The difference in the amount of absorption could be due to the difference in the physical properties and synthetic process. Moreover, the absorbed hydrogen start to desorb effectively after the temperature was raised to 700℃. For the three types of nuclear graphite investigated in our research, the amount of desorption at 700℃ was different: 7% of hydrogen was desorbed in NG-CT-10 graphite, 13.5% for IG-110 and 70% for NBG-18 nuclear graphite. [Conclusion] Based on our findings, the amount of stability absorbed hydrogen in domestic nuclear graphite NG-CT-10 was the highest, hence amount of tritium absorbed could also be the highest in NG-CT-10 nuclear graphite. According to the model of hydrogen absorption in nuclear graphite proposed by Atsumi, the hydrogen desorbed at 700℃ were the hydrogen absorbed at the edge surface of graphite crystallites. To fully desorb tritium and ¹⁴C effectively at 700℃, the carrier gas during desorption should be varied. (authors)