Progresses in Tritium Accident Modelling in the Frame of IAEA EMRAS II

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.

Using a coupled dispersion model to estimate depletion of a tritium oxide plume by a forest

Tritium processing facilities may release tritium oxide (HTO) to the atmosphere which poses potential health risks to exposed co-located workers and to offsite individuals. Most radiological consequence analyses determine HTO dose by applying Gaussian plume models to simulate the transport of HTO. Within these models, deposition velocity is used to assess the sum of all deposition processes acting on the plume. While this may account for vegetative and soil uptake or respiration processes, it may currently lack inclusion of the complex interactions within heterogeneous forested environments. In this complex morphology, dispersion patterns are significantly altered by changing flow regimes above and below the forest canopy and by the transfer of plume material across the canopy boundary. To determine the effects of a heterogeneous forest canopy on an airborne HTO plume, a Gaussian plume model coupled with an advection-diffusion plume model was applied to estimate transport in the free atmosphere above the forest and within the forest canopy and understory. During 2012, wind speed and wind direction measurements taken at 5 heights, ranging from 2-m to 28-m, on an instrumented meteorological tower located in a loblolly pine forest at the Department of Energy (DOE) Savannah River Site (SRS), near Aiken, SC. From these measurements, model predictions were made over a full spectrum of meteorological conditions. Deposition and resuspension velocities were calculated based on the model-predicted flux of plume material across the top of the forest canopy. Additionally, net deposition velocity of the plume material was calculated as the difference between the deposition and resuspension velocities. The 1st and 5th percentile net deposition velocities were estimated to be 0.7 cm s^-1 and 1.2 cm s^-1, respectively.

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

From previous catastrophic fission nuclear accidents, such as the Chernobyl and Fukushima accidents, researchers learnt the lessons that external hazard beyond design basis or human errors could result in severe accidents and multi-failure of the confinements although they were considered as very-low-probability events and not requested to be paid much attention to according to the current nuclear safety regulations. Fusion energy is always regarded as a safe and clean energy. However, massive quantity of radioactivity still exists in the fusion reactor and is possible to be released into the environment. The environmental pollution and potential public consequences due to severe accidents of fusion reactor remain largely unexplored. In this contribution, we intended to investigate the hypothetical accident to envelop the worst but probable consequences of fusion reactor, and compare with historic Chernobyl and Fukushima accidents under assumed environmental conditions. It was demonstrated that, the radiation consequences of a hypothetical fusion accident would be much less severe than fission accidents, e.g. an INES 7 accident could not appear in a fusion reactor, as in the Chernobyl and Fukushima nuclear accidents. However, it would still be disastrous and the publics close to site might be exposed to "potentially lethal" radiation dose.

Atmospheric stability effects on potential radiological releases at a nuclear research facility in Romania: Characterising the atmospheric mixing state

A radon-based nocturnal stability classification scheme is developed for a flat inland site near Bucharest, Romania, characterised by significant local surface roughness heterogeneity, and compared with traditional meteorologically-based techniques. Eight months of hourly meteorological and atmospheric radon observations from a 60 m tower at the IFIN-HH nuclear research facility are analysed. Heterogeneous surface roughness conditions in the 1 km radius exclusion zone around the site hinder accurate characterisation of nocturnal atmospheric mixing conditions using conventional meteorological techniques, so a radon-based scheme is trialled. When the nocturnal boundary layer is very stable, the Pasquill-Gifford "radiation" scheme overestimates the atmosphere's capacity to dilute pollutants with near-surface sources (such as tritiated water vapour) by 20% compared to the radon-based scheme. Under these conditions, near-surface wind speeds drop well below 1 m s(-1) and nocturnal mixing depths vary from ∼ 25 m to less than 10 m above ground level (a.g.l.). Combining nocturnal radon with daytime ceilometer data, we were able to reconstruct the full diurnal cycle of mixing depths. Average daytime mixing depths at this flat inland site range from 1200 to 1800 m a.g.l. in summer, and 500-900 m a.g.l. in winter. Using tower observations to constrain the nocturnal radon-derived effective mixing depth, we were able to estimate the seasonal range in the Bucharest regional radon flux as: 12 mBq m(-2) s(-1) in winter to 14 mBq m(-2) s(-1) in summer.