Importance of geosphere in hard rock nuclear waste repositories
|Location||International Geological Congress,oslo 2008|
|Holding Date||04 October 2008|
Hard rock is a suggested host medium for nuclear waste repositories in many countries. For example, the nuclear industry in both Finland and Sweden plan to construct such repositories according to the KBS-3 concept, where the spent fuel is encapsulated in copper canisters with a cast iron insert, surrounded by bentonite clay buffer and deposited at about 500 m depth in the crystalline basement rock.
In order to ensure that the repository is safe, safety assessments are carried out. These aim at describing the future evolution of the repository in its geological environment, to quantify the potential for releases of radionuclides from the deposited waste and to establish the confidence in this assessment.
The geological environment is an important part of the initial state. Field data are obtained from various investigation activities, such as surface mapping, geophysics, borehole drilling, borehole testing or underground mapping.
The field data are interpreted and evaluated into an overall inter-disciplinary Site Descriptive Model (SDM), being a synthesis of geology, rock mechanics, thermal properties, hydrogeology, hydrogeochemistry, bedrock transport properties and surface system properties.
Repository evolution is modelled considering the processes judged to affect the repository and the geological environment. Different time periods are considered e.g. the excavation and operational phase, closure and initial temperate period, a first glacial cycle expected to last about 120 000 years and subsequent glacial cycles. Alternative climate evolutions are also considered.
Radionuclide release could only occur in case the canister integrity is breached, e.g. in case of adverse chemical or mechanical conditions. Furthermore, also if the canister is breached the release from the spent fuel and the migration through the bentonite barrier into the rock is small and limited. The host rock contribution to safety is thus primarily to ensure adequate conditions for the canister and the buffer and only secondarily to ensure good retention for the released radionuclides.
Only some site conditions are really important for safety. These include, the geometry of deformation zones and fractures affecting the mechanical stability of canisters, the rock stress and the intact rock strength affecting the stability of deposition holes, the rock thermal conductivity that together with the repository design affects the repository temperature and the groundwater flow at the deposition hole affecting both the stability of the buffer and the copper canister as well as the release of radionuclides for the case of an intact buffer.
Both the current distribution of groundwater composition as well as the processes effecting this, need to be determined in order to assess the chemical evolution of the barriers. A prerequisite for confidence is also to develop sufficient understanding of the processes and mechanisms governing the general evolution of the site.