Etude de la spéciation du 60Co dans les effluents de l'usine de retraitement de combustibles irradiés de La Hague ; devenir après rejet dans les eaux de la Manche
thèse de doctorat de l'Université de Paris XI-Orsay, soutenue le 19/03/1999.
60Co is produced as an activation product and is present in the low level aqueous radioactive waste released from the La Hague plant. At present, the concentration in the sea (non filtered at 0,45 mm) at the Goury site are close to or even below, the detection limit : 0,2 mBq.l-1
The Co speciation depends on the type of effluent considered : in the effluent A ("active"), the cobalt is in the form of a stable trivalent complex ; in the effluent V (to be checked), the cobalt is in majority (50% of the activity release) in the form of particles (>0,45 mm), and then in the form of two soluble species : ionic divalent (Co2+) and some stable complexes. The evolution of the reprocessing techniques used does not affect the speciation. So, since the nuclear reprocessing plant started at the La Hague plant in 1966, the chemical species discharged in the sea shows time variation related to the evolution of the type of effluent discharged. 'Mus, since 1994, the particles of cobalt arc the main species discharged in the Channel (the V effluents represent more than 85% of the total 60Co activity released).
The effect of instantaneous dilution into the marine conditions involving a variation of pH, oxydo-reduction, ionic strength, a gradient of salinity, doesn't interfère with the evolution of the chemical species discharged. Nevertheless, during the discharge of the V effluent, the main constituents of the sea water (Mg2+ and Ca2+) go through a precipitation. 'Mis comes with the coprecipitation of the ion 60Co and with the particles of cobalt (complexes are not affecte« and it can be responsible for an increase in the concentration in the particles.
The chernical behaviour of the cobalt in the Channel is différent from those of conservative element such as antimony. The ionic cobalt and the particles have a small dispersion in the water (cobalt bas a very high particle/dissolved distribution factor, it is a non-conservative radionuclide). This sedimentary stock can involve a second source of activity. The power of complexation of the Channel's filtered water is very small (contrary to the rivers like the Seine which has a lot of organic material).
Because of the weak activity of the dissolved phase and the weak percentage of complexes, insufficient evidence (even if the reactivity between the complexes and the sedimentary particles are small) has, as yet, been obtained to confirm that the presence of stable complex significantly increases the mobility and dispersion of 60Co arising from the discharge. However, with a strong dilution, 99% of complexes disappear (conversion of trivalent complex to the divalent ionic forin which is more particle reactive form) between 9 and 44 months.