Hydro-mechanical interactions in bentonite engineered barriers (INBEB)
Engineered bentonite barriers are a key component of many repository designs for the confinement of high-level radioactive waste and spent fuel. It is therefore important to know the state of the barrier at the end of the transient period, especially the degree of homogeneity at that stage. In this regard, it should be noted that:
It is inevitable that various types of heterogeneity will be present at the end of construction and that heterogeneity will evolve during the transient period.
Average dry density is not sufficient to characterize the state of the barrier; maximum hydraulic conductivity may be controlled by the zone with the lowest dry density.
Swelling pressure will not only depend on the average dry density achieved but of the wetting/deformation history of the barrier as well.
Heterogeneity may be generated/enhanced by hydration and/or thermal effects.
The objective of Task D is to enhance the current knowledge of the evolution of a bentonite engineered barrier during the transient period, with particular reference to the prediction of its final heterogeneity. Both isothermal and non-isothermal conditions will be addressed. To achieve an appropriate understanding, it is necessary to simulate appropriately the behaviour of the engineered barrier by means of HM and THM coupled numerical analyses. This will require: (1) an increased understanding of material behaviour and properties; (2) an enhanced understanding of the fundamental processes that lead to barrier evolution and homogenization; and (3) improved capabilities for numerical modelling. As homogenization is critically dependent on the mechanical behaviour of the bentonite, the development and adoption of appropriate mechanical constitutive laws will be a key issue in this Task.
The results of two long term tests, EB and FEBEX in situ, are available. In both of them the final state of the barrier has been directly observed after dismantling. The same material (FEBEX bentonite) has been used in the two tests. They provide an appealing range of test conditions:
The EB Experiment (Mont Terri): (1) Total duration: 10.7 years; (2) Isothermal test, HM interactions; (3) Artificial hydration: barrier is saturated at the end of the test; and (4) Barrier type: Combination of pellets and bentonite blocks. (Mayor et al, 2007; (Figure 1)
The FEBEX "in situ" test (Grimsel): (1) Total duration: 18.4 years; (2) Non-isothermal test, THM interactions; (3) Natural hydration; and (4) Barrier type: Bentonite blocks (ENRESA, 2000; Figure 2)
The work to be performed in Task D has been initially structured into the following stages:
Stage 1 (9 months): Operational period of the EB experiment. Modelling of the EB experiment during hydration (10.7 years) against the data from monitoring system.
Stage 2 (9 months): Post-dismantling period of the EB experiment. Computation/ prediction of the final state of the barrier against the data from dismantling.
Stage 3 (9 Months): Operational period of the FEBEX "in situ" test. Modelling of the FEBEX "in situ" test during the first 5.0 years of heating up to and including first dismantling. Check against instrumentation data and first dismantling observations.
Stage 4 (6 months): Post-dismantling period of the FEBEX "in situ" test. Modelling of the FEBEX "in situ" test until the end of the experiment including final dismantling. Check against instrumentation data and final dismantling observations.
Stage 5 (6 months): Final synthesis and reporting.
It should be noted that the final form of Stages 3 and 4 will be reviewed and potentially modified depending on the progress of the task and the availability of the measurements of the FEBEX experiment.
Czech Republic: SURAO (ING)
Taiwan: Tai Power (NCU/TP)
For further information, please contact the task leader, Antonio Gens.
ENRESA (2000) FEBEX project. Full-scale engineered barriers experiment for a deep geological repository for high level radioactive waste in crystalline host rock. Final report. ENRESA, Madrid.