Diffusion of 99-technetium in compacted bentonite under aerobic and anaerobic conditions

Abstract

The main aim of this study was to investigate diffusion of technetium 99 Tc under different conditions. Because technetium represents one of the most dangerous fission products due to its very long halftime and high mobility in aerobic conditions diffusion experiments of technetium (as 99 TcO 4 anion) in Czech bentonite from Rokle locality have been carried out. For performance and evaluation of experiments the through-diffusion method was chosen and apparent (D a ) and effective (D e ) diffusion coefficients were evaluated. The effects of particle mesh-size, dry bulk density and aerobic or anaerobic conditions on diffusion were studied. In the presence of oxygen, technetium occurs in oxidation state VII, as an anion, soluble and mobile in the environment. However, under reducing conditions it occurs in a lower oxidation states, mainly as insoluble oxides or hydroxides. Aerobic experiments were carried out under laboratory conditions and anaerobic experiments were performed in a nitrogen atmosphere in a glove box, to simulate the real underground conditions.

Effect of competing anions on pertechnetate adsorption by activated carbon

“…All technetium isotopes are radioactive, and are produced by neutron fusion of 235 U in a small portion in nuclear reactors (10 -6 %), mainly by b-decay of fusion product 99 Mo (6 %). 99 Tc is released to the environment as pertechnetate [1][2][3][4][5][6][7][8][9][10][11][12][13].…”

“…The adsorption mechanism on AC is not fully known, however some of the results previously obtained indicate that the mechanism involves an anion exchange [10,11,14]. Yoshida et al [32] have attributed ion adsorption by active carbon to the chemical functional groups of chromene, pyrone and lactones present in the surface oxide of active carbon.…”

Through-and Out-Diffusion of Se(IV) and Re(VII) in Compacted Bentonite

“…Electromigration experiments indicated that D a for selenate is range from 1.7×10 -11 to 6.2×10 -11 m 2 s -1 and no reduction of selenate [4]. The degree of compaction is one of the important factors that influence the diffusion behavior of radionuclides in the clay [6]. Increasing clays compacting can change hydration energy, overlapping electric double layers and clay pore size and structure [7].…”