On the thermal impact on the excavation damaged zone around deep radioactive waste disposal

TL;DR

This paper investigates how thermal effects influence the excavation damaged zone in clayey host rocks for radioactive waste disposal, using experimental methods to assess volume changes, pressurization, and self-sealing properties under temperature variations.

Contribution

It introduces a new hollow cylinder apparatus and a laboratory method to measure thermal pressurization, extending understanding of thermal effects on claystones in waste disposal.

Findings

Claystones exhibit thermoplastic contraction under heat.

Thermal pressurization occurs around excavations in claystones.

Claystones retain self-sealing properties at elevated temperatures.

Abstract

Clays and claystones are considered in some countries (including Belgium, France and Switzerland) as a potential host rock for high activity long lived radioactive waste disposal at great depth. One of the aspects to deal with in performance assessment is related to the effects on the host rock of the temperature elevation due to the placement of exothermic wastes. The potential effects of the thermal impact on the excavated damaged zone in the close field are another important issue that was the goal of the TIMODAZ European research project. In this paper, some principles of waste disposal in clayey host rocks at great depth are first presented and a series of experimental investigations carried out on specific equipment specially developed to face the problem are presented. Both drained and undrained tests have been developed to investigate the drained thermal volume changes of clays and claystone and the thermal pressurization occurring around the galleries. This importance of proper initial saturation (under in-situ stresses) and of satisfactory drainage conditions (in spite of the significantly low permeability of claystones) is emphasized, leading to the development of a new hollow cylinder apparatus. It is observed that claystones cannot be considered as overconsolidated clays given that they can exhibit, as the Callovo-Oxfordian claystone does, a thermoplastic contraction. Mechanical and thermal hardening are however observed, extending to claystones the knowledge already gained on clays. A new method of determining in the laboratory the thermal pressurization coefficient is described and the data obtained allow completing existing data in the field. Finally, the hollow cylinder apparatus makes it possible to demonstrate that the good self-sealing properties of clays and claystones can be extended to temperature effects, an important conclusion in terms of performance assessment.