Phenomenon of a Fast Migration of Plutonium Radioisotops in Ground Water: Colloids or Soluble Form?

Abstract

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. Radionuclide 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 specific activities distributions of plutonium and strontium in groundwater were similar, i.e., there was a correlation between the positions of the radionuclides activity maximums. The Pu isotopes' plume in the aquifer spreads about 10 m downstream of the radionuclides source. Characterization of the initial radionuclide 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. Obtained size fractionation data on plutonium activities in collected colloids suggest that significant part of Pu (50–80%) was associated with low molecular weight fraction (<5 kDa).