Overview and Objectives

An important part of the performance and safety assessment of disposal systems for radioactive waste and spent nuclear fuel in deep geological formations is to evaluate the impact on repository performance of the coupled effects of mechanical deformation, fluid and gas flow through the repository, and thermal loading from the decaying waste. It was recognized early on in such assessment that to be able to conduct such an evaluation, there was a need to enhance the theoretical background and to develop models capable of simulating coupled thermo-hydro-mechanical (THM) processes. More recently, chemical (C) processes have also been added to enable the study of fully coupled THMC processes in geosystems. The term &quote;coupled processes&quote; implies that each process potentially affects the initiation and progress of all other processes. Thus, the response of a rock mass to radioactive waste storage cannot be predicted with confidence by considering each process individually or in direct succession. In the field of rock mechanics and rock engineering, many studies have been made on binary couplings TM and HM, but for the repository performance problem, it is essential to study and be able to predict processes with THM coupling, and even full THMC coupling. Such coupling is still a major challenge to the science and engineering community, in part because relevant effects need to be better understood and described with constitutive relations, but also since the processes have widely different characteristic temporal and spatial scales.

In 1992, recognizing the need to address the modeling challenges related to coupled THM and THMC processes, DECOVALEX was initiated as an international cooperative project of nuclear waste organizations, including both implementers and regulators, and associated research and modeling teams. Since then, the project has been operating in several four-year phases, each phase featuring a small number (typically three to six) modeling challenges of importance to radioactive waste disposal, together covering a large number of theoretical, numerical, laboratory and field studies. With seven successfully concluded project phases between 1992 and 2019, DECOVALEX-2023 is the current project phase running from 2020 through 2023. As the project has made significant advances in modeling THM and THMC processes over the past two decades, its main objectives have not changed:

• To support development of numerical simulators for THM and THMC processes in geological systems
• To investigate and implement suitable algorithms for THM and THMC modelling
• To compare model calculations with results from field and laboratory experiments
• To design new experiments to support code and model development
• To study the application of THM and THMC modelling to performance and safety assessment of nuclear waste repositories

One of the most important characteristics of DECOVALEX is the emphasis on frequent interaction between teams and on a cooperative supportive environment. Project workshops are held about twice per year, and task-specific discussions may be held in between. At the workshops, in-depth discussions and exchanges of information and scientific knowledge are made on the different scientific disciplines and alternative approaches. Through comparisons of results from the research teams, much insight has been gained, not only on the effects of coupled processes, but also on the strengths, weaknesses, and adequacies of the various approaches and computer codes. It is also an excellent way for all the participants to learn from each other and collaborate to address issues of common interest.