Engineered barriers

The future deep geological repository will feature a multibarrier system
that will ensure the protection of the radionuclides

The future deep geological repository will feature a multi-barrier system that will ensure the protection of the biosphere from the penetration of radionuclides from the disposed of spent nuclear fuel (SNF).

The natural barrier will consist of the rock mass itself to a depth of around 500 metres beneath the Earth’s surface. The engineered barriers that will complement the natural barrier will consist of the following:

  • The waste disposal package (WDP)
  • The buffer surrounding the WDP in the disposal well
  • The backfilling of the remaining underground spaces

Waste disposal package

Spent nuclear fuel waste disposal packages must fulfil a range of demanding requirements, most of which are based on SÚJB regulations and the Atomic Act (263/2016 Coll) and its various implementing decrees. These requirements must be met both during the operational stage of the repository and, particularly, following the closure of the facility. Nuclear safety must be ensured, i.e. via the removal of residual heat output and the maintaining of sub-critical conditions during operation and the subsequent disposal period, with respect to normal repository development and in the case of accident situations. In addition, protection from exposure to radiation must be assured at all times; the waste disposal packages must prevent the escape of radioactive substances, i.e. the packages must remain tightly sealed during the operational phase and over a time horizon of thousands of years. The lifetime of the sealing function of the waste package will depend on the materials from which it is made and the functioning and mutual interaction of the other engineered barriers.

Buffer and backfill barriers

The sealing and damping barrier or buffer will surround the WDP within the disposal borehole; its main functions will be to protect the WDP and provide for the retardation of radionuclide migration should the WDP become damaged. In order to ensure long-term safety, following closure, the whole of the underground repository will be filled with a backfill material which will not be subject to such strict requirements as those that will apply to the buffer.

Bentonite (a clay material produced by the weathering of the products of vulcanisation) will constitute the basic material of the buffer and backfill. Bentonite was selected since it is a naturally-occurring material that exhibits excellent sealing properties, i.e. swelling ability and very low permeability, which ensure the complete closure of all the so-called technological gaps. A further important feature of this material is its ability to absorb radionuclides and thus retard their potential escape into the biosphere. In order to ensure the maximum performance of the bentonite barriers in the deep repository it will be necessary to compact the material, thus rendering it less permeable, increasing its swelling potential and making it more resistant to external influences. Compaction, however, must not exceed certain limits, since excessive swelling pressure could lead to damage to the WDP. SÚRAO plans to use bentonite of Czech origin.