The components of the Engineered Barrier System (EBS) for radioactive
waste disposal are simple and well understood materials that are
expected to remain intact for very long time periods. In the safety
assessment of a deep geological repository for radioactive waste,
it must be assumed that, after a certain time, these barriers will
fail. The lifetime of the EBS components varies depending on the
local conditions and the disposal concept. This may be between tens
of thousands of years in some cases, to up to one million years
in others. Additionally, other materials such as monitoring equipment,
grouts, cements, geopolymers and ceramics may also be used in the
construction of a repository and these materials should also be
studied for their durability.
Currently, experiments are performed in the laboratory to examine
the dissolution rates of materials and the results extrapolated
over the repository lifetime. The potential problems with these
experiments are that they may not simulate true in situ conditions
and the time scales are in the order of weeks to months.
Although, the use of natural analogues to demonstrate the stability
of these materials over long time scales is useful, it is difficult
to relate the findings of the analogue studies to the repository
safety case due to the large uncertainties in the initial boundary
conditions associated with natural studies. However, a few cases
exist where materials have been placed in situ for periods of years
(e.g. Ballidon, agricultural test station, UK), but these are few
and far between and the boundary conditions are far from those expected
in a deep geological repository for radioactive wastes.
The long time scales foreseen in the planning of the Grimsel Test
Site offers an opportunity to begin long-term tests of a wide range
of materials under repository relevant conditions.