Identification of Oxygen-Depleting Components in MX-80 Bentonite
- PDF / 107,832 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 6 Downloads / 210 Views
1124-Q05-02
Identification of Oxygen-Depleting Components in MX-80 Bentonite Torbjörn Carlsson and Arto Muurinen VTT Technical Research Centre of Finland, Otakaari 3J, Espoo, FI-02044 VTT, Finland
ABSTRACT After closure, the near-field of a nuclear waste repository contains large amounts of oxygen in tunnels and deposition holes. The bentonite buffer/backfill will contain oxygen as a gas phase in unsaturated pores as well as dissolved gas in porewater. The redox conditions in the bentonite filling after post-closure will change towards reducing conditions. In the initial stage, the development of the redox state is mainly governed by the depletion of oxygen. The main mechanisms of oxygen depletion in the bentonite are: 1) diffusion into the surrounding rock and 2) reactions with accessory minerals and by microbial aerobic consumption of organic matter [1,2]. The reactions leading to oxygen depletion are not, however, well understood. The objective of this work was to gather new information concerning oxygen depletion in MX-80. This was done by measuring oxygen depletion and changes in the redox state in suspensions of 1) MX-80, 2) a heavy fraction of MX-80, or 3) a light fraction of MX-80. INTRODUCTION The KBS-3 concept for the long-term disposal of spent nuclear fuel aims to keep the waste isolated in deep crystalline bedrock. In the Finnish case, the depth of the planned repository is between 400 and 700 meters [3]. Briefly, the spent fuel is placed in copper-coated iron canisters, which are placed in bentonite-filled holes in the bedrock. Details concerning the KBS-3 concept are found in, e.g. [3]. The final choice has not yet been made concerning the bentonite to be used, but MX-80 bentonite has long been used as a reference material in scientific research and it has also been used in the present study. One topic of concern in the safety assessments of nuclear waste repositories is the redox condition, which may have an impact on, e.g., waste canister corrosion and radionuclide sorption and migration [4]. The conditions in the repository will be oxidizing directly after closure, but the redox conditions will thereafter slowly change towards being less oxidizing and, finally, reducing. Lazo et al. [5] mention that the redox changes are closely related to questions like: What happens to O2 initially present in the backfill/buffer at repository closure? How long does it take before oxygen is consumed in the buffer? Oxygen depletion in a repository had been modelled previously, with the results indicating almost complete oxygen depletion after 7-290 years [6]. However, since experimental confirmation was lacking, Lazo et al. started a series of experiments where oxygen depletion was studied in MX-80 and Montigel suspensions. One of the findings was that dissolved oxygen (DO) disappears within a few days. The objective of the present study was to take a first step towards the identification of those components that are involved in oxygen consumption in MX-80 suspensions. This was done by dividing MX-80 into a heavy
Data Loading...
