RELABEN – Stress relaxation of bentonite at high temperatures

Project Dates: September 2025 – September 2028

Project Status: Ongoing

Website: N/A

The long-term stability of the hydromechanical properties of the bentonite barrier at high temperature is one of the key conditions for the proper functioning of the bentonite barrier in radioactive waste disposal. Recent experimental research carried out within the EURAD WP HITEC was focused on bentonite behaviour under high temperatures above 100°C. The conclusion points out that there are no major mineralogical or chemical changes, but unexpected evolution of bentonite swelling pressures with time was observed. The clear decrease of swelling pressure of heated bentonite with time, referred to as stress relaxation, was monitored and its irreversible decrease after subsequent cooling was observed. The phenomenon of stress relaxation at high temperatures was identified by different research groups from different countries participating in the HITEC WP using different test setups and different bentonites. However, the specific investigation of the time-dependent behaviour of bentonite was not a specific task of the HITEC project, it was found as a by-product of the experiments performed and was therefore not investigated in more detail. Thus, this project will focus on experimental investigation of stress relaxation of bentonite at high temperatures, its constitutive modelling and finite element evaluation of its effect on barrier performance.

The project aims to:

• Carry out research to gain more detailed insight into the phenomenon of stress relaxation of bentonite at high temperatures
• Clarify whether stress-relaxation is a general behavioural feature of bentonite, or whether it is specific to some bentonites only
• Clarify the expected relationship between relaxation and creep
• Gain a more complete picture of rate effects (including creep tests) which will enable constitutive model development and validation
• Develop constitutive models accounting for stress relaxation and creep.
• Quantify the effect of stress relaxation on barrier performance using finite element modelling.