R&D for the geological disposal of medium and high level waste in the Boom Clay


Already in 1974 the SCK•CEN started research on the geological disposal of high level nuclear waste in clay. The Boom Clay was regarded as a potentially suitable host rock for a geological repository. As the Boom Clay is present underneath the technical domain of the SCK•CEN at Mol, SCK took the initiative for the construction of the HADES (insert link naar Hades onder facilities) underground research facility in 1980. The HADES facility is situated at a depth of about 225 m. HADES was further extended in several phases.

In 1981 NIRAS/ONDRAF, the national institute for radioactive waste and fissile materials, was created by law and became operational in 1983. Since then NIRAS/ONDRAF has gradually taken over the responsibility for the management and co-ordination of the R&D on waste disposal. SCK•CEN remained the main research partner for NIRAS/ONDRAF. With the construction of HADES, the researchers gained direct access to the clay at depth which resulted in an important increase in the research both in surface laboratories and in situ in HADES. This R&D led in 1990 to the presentation of the SAFIR 1 report, Safety and Feasibility Interim Report 1, to the government. The evaluation following this report confirmed that geological disposal in clay is a long term waste management option to be further developed.

From 1994 onwards the geological disposal of spent fuel was also studied. From the beginning the studies on geological disposal in the Boom Clay were performed in an international context mainly through the European Commission nuclear science and technology framework programmes on nuclear fission. Especially the research on nuclear waste behaviour, radionuclide migration, engineered barriers and performance assessment was performed in this context.

End of the nighties and early 2000 important extension works of HADES were performed i.e. the construction of the second shaft and the connecting gallery. In 2001 the SAFIR 2 report was presented to the government. The national and international review by the NEA lead to the confirmation of the option of geological disposal in a plastic clay like the Boom Clay and the main orientation of the R&D for the next ten to fifteen years requiring for example the study of all waste types to be disposed of at a similar level.


Currently the research is focused on the demonstration of both the feasibility and the safety with the aim of presenting the Safety and Feasibility Case 1 (SFC 1) asking the government the decision to give NIRAS/ONDRAF the task to find a site, a "go for sitting" decision. The safety of nuclear waste disposal is based on the multi-barrier principle. The application of this principle leads to the design shown in Fig. 1. By 2020 SFC 2 should be ready and should allow a "go for licensing" decision. The license file should be ready by 2025 so that around 2035 the geological disposal installation should be ready for the disposal of medium level waste. As vitrified HLW and spent fuel are heat generating and need to cool down for 50 to 60 years their disposal is only planned after 2050.

Fig. 1

Research topics

While initially the research was mainly focused on geological, hydrogeological and geomechanical studies, gradually new subjects were introduced such as:

  • The study of the compatibility of waste matrices, metal overpack materials and cement based materials with the disposal medium;
  • Radionuclide chemistry and migration;
  • Gas generation and migration;
  • Chemical, hydromechanical and thermal perturbations of the Boom Clay.

The development of experimental studies is complemented by a parallel development of models focusing on geochemical and coupled mass transport models, geomechanical models and mechanistic models for radionuclide sorption and nuclear waste leaching. Currently the research has become very multi-disciplinary (see Fig. 2) and the integration of all these aspects is essential when performing the long term repository safety and performance assessment. Uncertainty and sensitivity analysis is of key importance in this context and helps to focus the research on the most important phenomena. More information on the various research topics that are on-going can be found under research domain disposal of radioactive waste.

Fig. 2

In the HADES Underground research facility (URF) experiments on the subjects indicated above are performed to study possible scale effects and confirm the findings of the laboratory experiments under in situ condition.

The current research is focused on the further development of the assessment basis for the SFC 1. This includes a deeper mechanistic understanding of, for example, radionuclide release from vitrified HLW and spent fuel and sorption on clay to under build the safety statements and long term safety assessments.

The consecutive extensions of the HADES URF have demonstrated the construction of shafts and galleries with industrial techniques. Currently the main focus is the demonstration of the feasibility of the construction of a gallery crossing and a disposal gallery as well as the demonstration of our understanding of the thermo-hydro-mechanical behaviour of the Boom Clay at a decameter scale. Therefore the PRACLAY demonstration and large scale heater test (see Fig. 3) is being installed. The crossing and gallery have already been successfully installed and the preparations for the heater test are ongoing.

Figure 3: Scheme of the PRACLAY heater test in the HADES underground laboratory

Contact: Druyts Frank