IBL PROJECT

Introduction

Tsukinuno bentonite mine (NW Japan) is a source of Miocene age Na- (at depth) and Ca- (near surface) bentonite. The site holds great potential for studying processes of direct relevance to safety cases (SC) for radioactive waste repositories which will utilise bentonite as part of their multi-barrier safety system.

As such, the 35 bentonite layers in the Tsukinuno mine, varying in thickness from a few cm to ca. 7 m, occurring at various depths, can provide information of relevance to the long-term behaviour of bentonite at physical and temporal scales of relevance to both the buffer and backfill clays to be utilised in a repository for radioactive waste.

The mine and its environs are ideal for studying long-term, safety-relevant bentonite processes, including:

• saturation – natural saturation states of bentonite in differing environments (on the surface and at varying depths underground, dry and wet host rock conditions) for comparison with ongoing short-term, laboratory and underground rock laboratory (URL) tests
• bentonite density changes (swelling and heave) – due to exposure to groundwaters/meteoric waters
• bentonite water interaction processes with fresh and deeper groundwater chemistries – for example, changes in cation exchange capacity (CEC) and exchangeable cation composition (EC)
• bentonite erosion – both under repository-relevant conditions at depth in the tunnels where water conducting features contact the bentonite and under extreme conditions (river erosion at the surface)
• bentonite reaction – with mudstone and siltstone host rocks (of relevance to borehole seals)