Chemical evolution of the engineered barrier system (RTDC2)

In all repository designs under investigation in the EU, the near-field has an important safety function as its principal role is to prevent or delay the release of radionuclides from the waste to the host rock. To perform its containment function, the disposed waste is enclosed by various engineered components including the waste canisters, backfills, overpacks seals and plugs. These man-made repository components are part of a multi-barrier configuration and constitute the so-called engineered barrier system (EBS).

Interactions between various components of the EBS will lead to evolving chemical conditions in the near-field environment. The chemical evolution of the near-field system is an important factor with respect to the long-term performance of a geological repository for the disposal of HLW and spent fuel as it determines the solubility, the speciation and the mobility of radionuclides in the near-field. These aspects are investigated in NF-PRO's RTD Component 2.

In general, distinction is made between the following three major phases in the near-field evolution:

• During the initial phase, the near-field system is hydraulically undersaturated as repository groundwater has not yet entered the disposal system. This phase is characterised by high thermal and chemical gradients.

• In the subsequent phase, the near-field has become hydraulically saturated due to the ingress of repository groundwater. At this stage, chemical conditions in the EBS will evolve due to interactions between various components constituting the near-field as well as with repository groundwater. Radioactive decay will lead to a decrease in heat production and result in lower thermal gradients. Chemical interactions in the near-field also will result in less pronounced chemical gradients. This phase is thus characterised by residual thermal and chemical gradients.

• In the final phase, the near-field environment will be fully saturated and will reach a quasi “isothermal stage” with minor geochemical gradients mainly imposed by the surrounding host rock.

NF-PRO's RTD Component 2 investigates key processes affecting evolving chemical conditions in the near-field environment. The focal point of research performed under NF‑PRO is on processes affecting the chemistry of the porewater in the bentonite buffer during the transient phase including alteration processes in the bentonite-based EBS. In addition, NF-PRO includes studies on interactions between the bentonite buffer and corrosion products originating from the decomposition of waste canisters and/or metal-based overpacks.

In repository concepts applying cement-based engineered barriers, the repository groundwater will acquire a chemical imprint resulting in high pH solutions that diffuse in the bentonite barrier. These high alkalinity conditions may lead to the development of new-formed minerals and the transformation of the bentonite buffer. NF-PRO investigates how these changes affect the porosity and radionuclides mobility.

Finally, an important part of the work performed as part of NF-PRO concerns investigations aiming at an in-depth understanding of the impact of evolving chemical conditions in the near-field (bentonite buffer and corrosion products) on the containment function of the EBS as well as on radionuclide immobilisation.