Behaviour of concrete structures as lining for geological disposal facility of radioactive waste

In Belgium, ONDRAF/NIRAS (the federal agency responsible for managing radioactive waste and enriched fissile materials in Belgium) and SCK CEN (Belgian Nuclear Research Centre) have been conducting research on the concept of a deep geological repository (DGR), within poorly indurated clays, as a solution for the long-term management of low-level and intermediate-level long-lived waste and high-level radioactive waste. The Belgian reference option for disposal of such waste is a DGR located at 400 m. A deep geological repository consists of a network of disposal galleries or tunnels in which the radioactive waste is placed. These galleries are connected by so called access galleries from the ground surface via vertical shafts. After the emplacement of radioactive waste, the tunnels are backfilled and closed. During the operational phase, these tunnels must provide a safe working environment for a few decades and even more when considering the possibility of retrievability.

The feasibility of constructing galleries in deep poorly indurated clays is investigated in the HADES Underground Research Laboratory where different tunneling technologies have been used. The Connecting Gallery built in 2002 consists in a segmental concrete lining in which three rings are equipped with embedded vibrating wire strain gauges to follow the deformation of the gallery. Different environmental conditions are acting on this structure. Firstly, the overburden pressure from the overlying Boom Clay deforms the lining in a lying-egg shape. Secondly, the intrados is exposed to a very low humidity environment thereby creating a strong hydraulic gradient across the lining thickness with the extrados in contact with Boom Clay. Under these conditions, long term creep and shrinkage/swelling processes drive the evolution of mechanical stability of the material. The primary objective of this PhD is to enhance existing theoretical and numerical approaches to better understand the evolution of concrete under these environmental conditions. Although there is sufficient data for the concrete lining material, some additional but minimal experiments will be necessary to support the creep and shrinkage/swelling model development. Structural scale simulations will then be carried out to study the mechanical stability of the connecting gallery lining and to assess its evolution for the next decades. To support this work, strains recorded in the concrete segments for over 22 years will be made available, which will facilitate the back-calculation of the external pressure acting on it. This last point is very important, as it enables us to determine the pressure acting on future repository galleries and, consequently, to optimize a DGR. Therefore, this PhD study concerns with the short- and long-term performance of concrete lining of the connecting gallery and, more broadly, to disposal galleries.

The overall scientific objectives of the PhD are:

1. Development of a new model for creep and shrinkage for blended cementitious concrete within a Hydro-chemical-mechanical framework, with extensive verification, and validation against experimental data from a recent PhD, supplemented with literature data.
2. Application of the model to estimate the long-term stability of connecting gallery lining, including model updating and uncertainty quantification.