Long-term experimental evidences of saturation of compacted bentonite under repository conditions

Abstract

This paper summarises the information gathered in the last 15years on the saturation of compacted bentonite obtained from different laboratory-scale tests, a large-scale mock-up test, and a real-scale in situ test, that were performed to simulate the conditions of the bentonite barrier in a high-level radioactive waste repository and to better understand the hydration/heating processes. In all the tests the bentonite used was the Spanish FEBEX bentonite, the maximum temperature in the system was 100°C and the water used was of the granitic type, with low salinity. Some of the tests were running for more than thirteen years.The migration of water vapour in areas affected by the high temperature induced by the radioactive waste decay is very rapid, its extent depending on the actual temperature and bentonite porosity. The water vapour condensates in cooler areas and this causes water content increases in internal zones of the barrier where the liquid water coming from the host rock has not yet arrived. The hydration kinetics is initially quicker when the temperature is high, provided no vapour phase is formed. Nevertheless, the major effect of the thermal gradient on saturation is a delaying of it in the inner parts of the barrier, which can be very persistent and depends on the actual thermal gradient and consequently, on the barrier thickness and boundary conditions.During the transient period in which the barrier is saturating, important changes in the water content and dry density of the bentonite are generated, which induce bentonite density and water content gradients along its thickness. These gradients could eventually disappear once the barrier is fully saturated, depending on the irreversibility of the deformations.The average density of the water in the saturated barrier will be higher than 1g/cm3, due to the predominance of high-density, interlayer water in the compacted bentonite, and consequently, more water than expected, according to calculations made considering the density of free water, would fit in the bentonite pores.The rate of hydration of the barrier depends on the bentonite and surrounding media hydraulic properties (that is, water availability), waste temperature and buffer thickness and geometry.