Tritium can be released into the environment in a loss of vacuum (LOVA) scenario in a fusion reactor. The simulation of the atmospheric dispersion behaviour of tritium is one of the core components of the assessment of the radioactive consequences.

This study aims to analyse the behaviour of tritium dispersion in the atmosphere after a fusion reactor accident.

Based on the Gauss model and the Pasquill stability classification method, an analytical model of tritium dispersion was developed for transient cases considering the effects of gravitational settling, smoke lifting, and wind speed, etc. The calculation of the model for dry settling at the ground boundary was improved by adding ground reflection coefficients to the Gauss model. Finally, the Canadian tritium release experiment and the tritium release accident at the Savannah River plant in the United States were used to verify the applicability of the model.

Verification results show that the accuracy of the developed model is the same as that of UFOTRI and the HotSpot 3.0 code. For the LOVA scenario of International Thermonuclear Experimental Reactor (ITER), the atmospheric dispersion behaviour of tritium is obtained for multiple release heights, different wind velocities and tritium phased releases.

The phased release of tritium results in two highly radioactive regions along the downwind direction, and the increase in release height and wind speed will enhance the atmospheric diffusion behaviour of tritium and thus reduce the accumulation of radioactivity in the near field.