Cs and Pu migration through engineered and natural barriers

Abstract

Performance assessment of radioactive waste disposal requires modelling of long-term migration of radionuclides through the engineered barriers and the geological environment. The chemical complexity of sorption-desorption processes is usually reduced to integrated parameter distribution coefficients (K d ). There are a great number of publications on K d determination, however, the existing data on K d of radionuclides on different geological materials are for general understanding only and are not very useful for performance assessment since the changes of the geological conditions result in a variability of K d values by two orders of magnitude. In order to obtain realistic sorption data sets for safety-relevant radionuclides present in a cement/concrete based repository some preliminary studies were carried out. The development of sorption database for the near-surface repository was started with measurements of cesium and plutonium K d values. Samples of loam available at the Galilaukė site (Quaternary deposits) and Triassic clay from industrial Šaltiškiai quarry selected as a candidate for the engineered barrier of Lithuanian near-surface repository were taken for laboratory investigations. Several experiments were performed in order to determine the chemical composition of cement water which could originate from infiltration of precipitation and from contact of groundwater with concrete. More than 100 batch sorption experiments were conducted with two clay samples. Cs and Pu K d values were determined under a wide range of geochemical conditions. Changes in the geochemical conditions resulted in the variability of Cs and Pu K d values.

Kinetics of Cs sorption to clay minerals

“…In sorption experiments performed using the Šaltiš-kiai clay and using groundwater with low caesium concentrations (C(Cs) = 2.30·10 −10 mol/l), which were close to that usually found in the natural environment, Cs K d values ranged from 7000 to 10000 ml/g [22]. The lowest Cs K d values were found in experiments with synthetic rainwater (Table 1), however, an increase in Cs K d values was observed during 45 days (Fig.…”

Retention of cesium, plutonium and americium by engineered and natural barriers

“…About 50 % of the Pu(IV) spiked was found in the pentavalent oxidation state in the GW (pH = 7.8) samples and was attributed to the oxidation by MnO 2 colloids probably present in the GW. In GWCW solutions Pu oxidation was not observed [9]. where ≡SO denotes surface sites on the iron oxides [17].…”

“…K d values of Cs and Pu were determined from three replicates. Details of experiment are described in previous publication [9]. The association of sorbed Cs, Pu and Am in geological material was determined using sequential extraction procedure (Table 2) [10,11].…”

“…Pu (IV) was used in sorption experiment [9]. The oxidation state purity of Pu (IV) stock solution was analyzed by solvent extraction at pH 0.5 using 0.5 M TTA as extractant [8].…”

Application of raw and modified pine biomass material for cesium removal from aqueous solution

“…Alkaline solution pH was studied because Cs speciation and precipitation at high pH is quite negligible (Lujanien _ e et al, 2005). From the results of the experiments in Fig.…”