Near-field atmospheric dispersion of a gas emitted from a hot source : a comparison between analytical modelling and in situ measurements

Incidents in nuclear facilities can lead to the emission of a radioactive plume dis-
persing into the atmosphere. In such events, the highest radionuclide concentration
is usually located near the source at distances ranging from a few meters to several
hundred meters. It is, therefore, crucial to be able to accurately predict these levels
of near-source concentrations.
One challenge arises from the thermal characteristics of the source, which regulate
the initial dispersion of the plume. In the case of a non-thermal gas release, the
dispersion of the plume is driven by atmospheric conditions, related to wind and
atmospheric instability, and is influenced by local surface characteristics such as
roughness and the presence of obstacles. In contrast, when the gas is emitted from
a hot source such as a fire, the released gas first rises in the atmosphere up to a
so-called ‘injection height’ due to buoyant forces. The injection height is reached at
a certain distance from the source and doesn’t only depends on the properties of the
hot source but also on the atmospheric conditions (e.g. downdraft effects). The gas
then disperses like in a non-thermal gas release.
While CFD modelling can offer an accurate description of the plume dispersion, its
processing speed is not suitable for use in emergency situations. In contrast, existing
analytical models can provide rapid results, but their injection height parametriza-
tions may lack comprehensive coverage. So far, analytical models have rarely been
validated against field measurements, and few field experiments have been conducted
to improve their parameterization.
The goal of this presentation is twofold, first to present a field experiment on the
atmospheric plume dispersal of a gas released from a hot source, and second to
evaluate an analytical model of plume dispersal against the experiment, with a
particular focus on the Atmospheric Transfer Coefficient of the released gas.
The field experiment was conducted in May 2024 on a flat terrain near Vire (Nor-
mandy, France), under unstable and neutral atmospheric conditions.

The source comprised a burner (PYROS) that generated a propane fire with an average heat
release rate of between 450 kW and 750 kW . Helium was injected into the plume
to serve as a tracer gas. During 15-minute observation periods, helium concentra-
tions in the air were measured at ground level at distances from the source ranging
from 40 m to 400 m, as well as at various altitudes, using air sampling points at-
tached to a rope lifted vertically by a drone. Additionally, atmospheric turbulence
characteristics were also measured using ultrasonic anemometers.
The analytical model employs Heskestad’s formulas to determine the fire character-
istics and Briggs’ dispersion parameters to characterise the Gaussian dispersion of
the plume when buoyant forces become negligible.