First-Principles Mechanistic Insights into Steam-Enhanced Tritium Desorption from Nuclear Graphite

Area of Science:

• Nuclear Engineering
• Materials Science
• Physical Chemistry

Background:

• Tritium removal from irradiated nuclear graphite is critical for nuclear waste management.
• Steam addition enhances thermal treatment for tritium removal, but the mechanism is unclear.
• Understanding tritium removal mechanisms is essential for optimizing nuclear graphite reprocessing.

Purpose of the Study:

• To investigate the mechanisms of tritium removal from nuclear graphite using steam (H2O) as a carrier gas.
• To elucidate the promotion mechanism of steam in tritium removal via density functional theory (DFT).
• To compare the efficiency of direct isotope exchange versus adsorption-desorption pathways.

Main Methods:

• Employed density functional theory (DFT) calculations to model tritium removal processes.
• Investigated two primary mechanisms: direct isotope exchange and adsorption-desorption.
• Calculated activation energies for tritium removal via both pathways.

Main Results:

• Direct isotope exchange has high activation energies (3.18–5.10 eV), linked to C-T bond strength.
• Adsorption-desorption mechanism involves H2O adsorption and subsequent desorption of tritium as tritiated water (HTO).
• Activation energies for adsorption-desorption (2.01–3.52 eV) are significantly lower than direct exchange, indicating higher efficiency.

Conclusions:

• The adsorption-desorption mechanism, facilitated by steam, is the dominant pathway for tritium removal from nuclear graphite.
• Tritium is primarily desorbed as tritiated water (HTO) through its combination with hydroxyl groups from steam.
• These findings offer crucial theoretical insights for optimizing thermal treatment methods for tritium remediation in nuclear graphite.