Alkaline Degradation of GMZ Bentonite as HLW Barrier in China

Gaomiaozi (GMZ) bentonite is a suitable buffer material for underground repository barrier of high-level waste (HLW) in China. Strong alkaline cementitious material formed from the corroded lining concrete by groundwater will penetrate into bentonite buffer, meanwhile, the temperature of the barrier will rise under the radiant heat from the decay of the radionuclide. The laboratory tests were conducted to simulate the diffusion of KOH solution into bentonite under elevated temperature conditions and investigate the changes in mineral composition and microstructure of GMZ bentonite by X-ray diffraction (XRD), nitrogen adsorption and scanning electron microscopy (SEM). The results confirmed that alkaline dissolution of montmorillonite minerals in GMZ bentonite is increased with both pH value of the alkaline solution and the temperature. For example, montmorillonite content decreased from initial 44.4% to 25.9% in severe situation. The degree of alkaline corrosion of bentonite is inversely proportional to the distance between the contact interface of bentonite and alkaline solution and visible macro fissures were observed by naked eyes at the interface. Within 2mm of the surface, the wing-like gels produced by montmorillonite hydration were dissolved apparently; and beyond 4mm, the bentonite retained its original microstructure. The results reveal the adverse effects of alkaline corrosion of GMZ bentonite on the long-term performance of buffer barriers should be paid great attention.