Abstract
In all the investigations of corrosion and leaching of radioactive waste containers for final storage in rock salt, the assumption was made of direct intrusion of brine to the waste container or to the waste matrix.
Previous studies that assume waste containers are free-standing in brine (and a self-consuming brine) do not adequately describe the actually occurring conditions and sequences for the case of heat-producing waste. The occurring processes, like crystallization, resolving, evaporation, and condensation, will lead to a possible build-up of a protective layer around the waste container, or to cavity formation in the vicinity of the heat generating source.
Previous considerations of long-term safety of final repositories with heat-producing waste in a salt dome incorrectly assumed the rock salt fill material will stay homogenous, and do not take into account a change in the fill properties (especially porosity and permeability) caused by a brine intrusion and the resulting salt resolution and crystallization.
Investigations of possible sub-processes occurring under final repository conditions in rock salt on a small-area, short-period scale (including modelling) were prepared. Various laboratory experiments, under normal pressure as well as under increased pressure conditions, and with different temperature gradients and moisture content, were carried out to analyse the behaviour of the system, to identify the main processes and to determine the essential influencing factors.
The experiments show compacted continuous zones (the initial porosity will be reduced from approximately 35% to approx. 15–20% or even less) and loosened areas (porosity increasing 50 to 100%). Local densities of up to ρ = 2.04 g/cm3, with a porosity of 8%, were found. Temperature gradients can initiate convection processes, especially in the non-compacted areas, which in turn will result in further resolving and dissolving of salt. In completely brine-saturated porous-salt, the growth of salt crystals takes place along the temperature gradient, which creates salt deposits in colder zones and large brine filled cavities in warmer regions.
Transferring the observed phenomena to repository conditions, it can be postulated:
1. In case of a brine-filled repository section, the salt transports toward the colder section, and in addition, gravity could allow the container to sink downwards into the rock salt. – This effect, under certain circumstances, could result in serious safety issues.
2. The compaction of salt in the colder repository section (roof) might prevent further brine inflow into the repository section after some time has elapsed.
3. All processes are dependent on the rate of brine-inflow and the content of gaseous and liquid phases in the disposal gallery.
