Geomechanical modelling of the integrity of the salt barrier and the structural safety at the Morsleben radioactive waste repository (ERAM)
Country / Region: Germany
Begin of project: January 1, 1994
End of project: December 31, 2025
Status of project: March 3, 2023
Image of the 3D finite element model, with the idealised geological homogenous zones in the southern field
During the operational and post-operational phases of the Morsleben repository, the rock salt formations enclosing the mined rooms need to function as a hydraulically tight barrier against the infiltration of solutions from the overlying rock formations. Assessing the integrity of the salt barrier is a task within the exploration and evidence preservation programmes as well as the decommissioning concept for the ERAM repository for radioactive waste. An undisturbed salt barrier is considered to be hydraulically tight. Mechanical damage and the accompanying increase of permeability typically occurs as a consequence of geomechanical stress affecting the rock mass due to mining. The degree of damage can therefore be considered as an indicator of the permeability of the salt barrier.
The integrity assessment is based on geomechanical modelling which incorporates data from in-situ measurements and local observations.
The models simulate the stresses and the potential damage to the rock mass during the operational and post-operational phases of the repository induced by previous mining activities, and by the ongoing convergence of remaining rooms. The modelling results are examined with respect to the “dilatancy” and “fluid pressure” criteria. The integrity of the salt barrier is verified when an adequately thick zone of rock salt between the top of the salt structure and the excavations fulfils both criteria
The ERAM repository comprises two mines: Bartensleben and Marie. Both the geology and the excavations in these sections show a high 3D complexity and therefore it is useful to carry out separate modelling investigations for selected representative areas. In the case of the Bartensleben mine, the following four zones were selected with respect to the emplacement of waste material until 1998 and the degree of excavation:
- “Südfeld” (southern field, emplacement zone),
- “Westfeld” (western field, emplacement zone),
- “Ostfeld” (eastern field, emplacement zone),
- “Zentralbereich” (central part, zone with the highest degree of excavation).
In the case of the Marie mine, the following three zones were selected with respect to the degree of excavation:
- ”Lager H” (infiltration of solutions),
- “Zentralbereich” (central part, high degree of excavation),
- “Bunte First” (infiltration of solutions).
The decommissioning concept planned for the Morsleben repository includes extensive backfilling of selected rooms with salt concrete. This backfill material has a mechanically supporting effect and therefore a favourable long-term influence on the integrity of the geological barrier.
Calculation results 10,000 years after excavation (1940) for the “dilatancy” (left) and “fluid pressure” (right) integrity criteria. The fulfillment of the two criteria was achieved over the specified time period
Source: BGR
The long-term development of the barrier integrity at the southern field of the Bartensleben mine was investigated for a period of 10,000 years taking into consideration the decommissioning concept (as of 2005). The complex geological structure and the steep arrangement of excavations in the southern field were implemented in a simplifying model consisting of homogenous zones (zones with the same material properties). In a following step, a three-dimensional numerical model was developed approximating the geological structure and the mining situation in this part of the mine. The results of the numerical simulations were evaluated to assess the integrity of the salt barrier.