Neutron activation analysis of unstable bitumen-salt-solvent mixtures.

Impens N., Dekelver S., Vermaercke P., Bruggeman M.,

Fonteyne A., Vos L., Sneyers L, Van Laer J.

SCK•CEN, Boeretang 200, B-2400 Mol, Belgium.

In the past, blown bitumen (R85/40) has been used as a matrix for several types of radioactive waste salt particles smaller than 40µm. A weight ratio of 60/40 bitumen/waste was applied. In order to study the feasibility to recover the maltenes from the waste, it is first mixed with a 50 % aromatic solvent. Then the precipitation of asphaltenes is induced by adding a paraffinic so-called non-solvent. We investigated the settling of asphaltenes in several bitumen – solvent – non-solvent mixtures. Similar experiments were carried out with non-radioactive bituminised waste simulates. When applying similar bitumen - solvent – non-solvent ratios, asphaltenes show much higher settling velocities in the presence of the salts. To recover the maltenes, the precipitate containing the major part of radionuclides and asphaltenes is separated from the dissolved maltenes. The aim is to minimise the salt (radionuclides) present in the maltenes. As a function of the waste simulate – solvent – non-solvent ratio, the amount of salts in the maltene fraction was determined gravimetrically in an earlier study.

To improve the detection limit and refine the elemental analysis of the salts in the maltene fraction we opted for a nuclear analysis method.  We use neutron activation in the BR1 reactor of the bituminised waste simulate samples, to activate the elements composing the salts, which produces several radionuclides. After adding solvent and non-solvent, we can  follow the settling of these radionuclides by gamma spectrometry as a function of time and sample height. In this paper, we present the experimental setup and the analytical methodology.