Last April, a new sea water sampling campaign was conducted by the LRC (radioecology laboratory) of IRSN as part of the DisVer programme (vertical dispersion of radionuclides in the sea), which completes the campaign conducted in October 2010, the measurements of which have just been completed. The aim of the DisVer programme is to validate the computations of a three-dimensional model simulating the dispersion of a plume of radioactive pollutants.
The key objective of the DisVer project (vertical dispersion) is to validate the MARS model, a three-dimensional hydrodynamic pollutant dispersion model jointly developed by Ifremer and IRSN. It enables the dispersion of liquid discharges during the minutes and hours following a discharge to be simulated, from the sea floor to the surface, as a function of currents, tides and winds. Validation is based on a series of three sea water sampling campaigns (2009, 2010 and 2011) around the Areva reprocessing plant in La Hague in France, a partner in the project. The tritium present in authorised releases from the facility is used as a tracer of the dispersion of the plume. This geographic area is characterised by strong tides generating intense and complex currents.
The Disver11 campaign, conducted in April 2011, was carried out in a study zone extending up to 10 kilometres from the outfall, in a region where depths and bathymetric gradients are important (from 30 to 100 metres depth). Thus, on board the CNRS/INSU vessel “Côte de la Manche”, the IRSN team was able to collect 13,400 samples over 10 vertical levels, between the surface and a depth of 50 metres, using a system developed by IRSN.
The Disver10 campaign, conducted in October 2010, focused on the zone near to the emission point, between 100 metres and 2 kilometres, when the plume is not yet homogeneous vertically. 5,600 samples were then collected over 10 vertical levels, between the surface and a depth of 30 metres.
The analyses of this campaign have now been completed and the initial results reveal that the plume of radioelements has extensive spatial and temporal variability, with concentration gradients ranging from 5 to 7000 Bq/l.
Initial results of the Disver10 campaign (©IRSN)
Representation of tritium concentration measurements (in Bq/l) along sections perpendicular to the axis of propagation of the plume (box in the bottom right hand corner). Y-axis: depth. X-axis: distance to the axis of propagation.
Each section corresponds to a series of samples taken at a given time.
In order to represent this variability, the precision of the models must be adapted with an elementary computing mesh of less than 30 metres. This implies representing turbulent diffusion at this scale, a fundamental challenge that needs to be met to correctly represent the evolution of a plume. Turbulent diffusion is in fact responsible for the rapid spreading out of the plume (more than 100 metres wide, 300 metres from the emission point), the formation of several distinct lobes and the temporal variability of the structures observed (concentrations varying by a factor of one hundred in ten minutes). The 2011 measurements have just started and should continue for more than a year, since 50 minutes of counting are necessary for each sample measurement.