Insights into potential consequences of fusion hypothetical accident, lessons learnt from the former fission accidents

Insights into near-surface distribution characteristics of multi-form tritium with consideration of atmospheric buoyancy and gravitational deposition

Tritium contributes majority to the total airborne radioactive effluents from the nuclear facility because of its considerable production and difficulty in separation. Tritium inventory in the fusion reactor would reach an unprecedented magnitude which brings new safety concern. After being released into the atmosphere, inconsistent atmospheric dispersion behaviors might appear regarding different physicochemical forms such as gaseous state HT, gaseous-aerosol-droplet state HTO. In this study, atmospheric dispersion characteristics of multi-form tritium were investigated based on the computational fluid dynamics method validated by multi-fan type wind tunnel experiments. Species transport model and discrete phase model were used to describe atmospheric dispersion of gaseous and aerosol-droplet state tritium, respectively. Deposition velocity was predicted for gaseous and aerosol-droplet state tritium with different particle sizes. Conditions for describing the changes of particle diameter and its influencing on near-surface tritium distribution due to condensation were provided. The results show that buoyancy effect would strengthen along with the increasing gaseous tritium mass fraction in the airborne effluents. We also indicated that obvious gravitational deposition would appear once gaseous HTO was transformed into droplet state HTO with the particle diameter larger than 20 μm. Both the atmospheric buoyancy and deposition phenomenon would result in a quite different near-surface tritium distribution.

HTO activity concentrations in soil and plants in the vicinity of Qinshan Nuclear Power Plant Base

To explore the distributions of tritiated water (HTO) in soil and plants after routine release of tritium, this study focused on the Qinshan Nuclear Power Plant Base(QNPPB) using both soil samples at different depths (0-2 cm, 0-5 cm, 5-10 cm, 10-15 cm, 15-20 cm, and 20-25 cm) and vegetation samples (including leaves and grasses) from eight sampling points. Then, HTO activity concentrations were measured in all samples. The results showed that the range in HTO activity concentrations in the leaves was 24-39.5 Bq/L and in the grasses was 27-85.5 Bq/L.HTO activity concentrations in the topsoil sample (0-2 cm) from the Qinshan Phase III were greatest. At a same sampling point, the HTO activity concentration was the greatest in the topsoil sample. In most case, except for the top soil layer, HTO activity concentration in different depth soil layer differed slightly. Overall, the HTO activity concentration in leaves and grasses were lower than that in the soil at the same site, and the ratios of HTO activity concentration in plant to soil were found to vary between 0.38 and 2.1, with a mean value of 0.79. This result suggests that some parameters used in the specific activity model proposed to predict tritium activities provide a conservative estimate for the QNPPB site; it should be revisited to better account for the behavior of HTO in the actual environment.