Migration des isotopes de plutonium dans un aquifère à partir d'une tranchée de déchets à Tchernobyl.

Titre du congrès :Migration '07 Ville du congrès :München Date du congrès :26/08/2007

The study site is located in the territory of shallow waste burials called the "Red Forest" created in 1987 during the ChNPP cleanup operations. The site includes trench #22 and adjacent territories. The 70 m long and 2-3 m deep trench contains radioactive waste (humus topsoil and rests of the contaminated trees) and thus represents the main source of radionuclides and stable chemical elements in the sand aquifer near its location. The trench bottom is periodically flooded by a shallow (3±1 m) groundwater table. The real groundwater velocity in the aquifer is estimated at 10 m/year. Field experiments and laboratory studies were performed to investigate migration processes of plutonium isotopes in the aquifer at the experimental site. The objectives of these experiments were to characterize the spatial distribution and possible migration mechanisms of plutonium in the aquifer, and to obtain the parameters that determine radionuclide retardation. During 2005-2006 experimental investigations were carried out and spatial distributions of plutonium isotopes (239,240Pu, 238Pu), 90Sr and main dissolved ions in aquifer along the groundwater flow were obtained. Radionuclides' specific activities in groundwater depended on the location of the piezometer and varied in the range of 1÷200 mBq/l for 239,240Pu, 0.5÷100 mBq/l for 238Pu and n•100÷n•104 Bq/l for 90Sr. It was found that the spatial features of the distributions of plutonium and strontium specific activities in groundwater were similar, i.e. there was a correlation between the positions of the activity maximums of the radionuclides. The Pu isotopes' plume in the aquifer spreads about 10 m downstream of the radionuclides source. Values of distribution coefficients (Kd) of plutonium in waste material (source) and in eolian sand (saturated zone) were estimated in field conditions. The obtained Kd value in the aquifer (6 L/kg) is three orders of magnitude less than those in the source material ((1-2)103 L/kg). Characterization of the initial radionuclides' composition of waste showed that all plutonium in the aquifer originated from the trench. Ratio of plutonium isotopes (239,240Pu/ 238Pu) was the same in waste material and in groundwater samples. In-situ ultrafiltration of several groundwater samples was carried out. Groundwater samples (2-4 L) were sequentially filtrated through 0.22 μm, 30 kDa and 5 kDa membranes. Obtained size fractionation data on plutonium activities in collected colloids suggest that significant part of Pu (50-80%) passes through the 5 kDa filter. Distribution of plutonium activity between fractions was following: > 0.22 μm - 0.5%; 0.22 μm÷30 kDa - 18.8%; 30 kDa ÷5 kDa - 16.7%; permeate - 64%.