Hydrothermal alkaline stability of bentonite barrier by concrete interstitial wastes

At present, the main source of High Level radioactive Waste (HLW) is the electrical energy production during all the steps of developing. In almost all the countries with nuclear programs, the option for the final management of HLW is the Deep Geological Repository (DGR) based on the concept of multi barrier. According to this concept, the waste is isolated from biosphere by the interposition of confinement barriers. Two of the engineering barriers in the Spanish design of DGR in granitic rock are compacted bentonite and concrete. The bentonite barrier is the backfilling and sealing material for the repository gallery, because of its mechanical and physico-chemical properties. The main qualities of concrete as a component of a multi barrier system are its low permeability, mechanical resistance and chemical properties. With regard to chemical composition of concrete, the alkaline nature of cement pore water lowers the solubility of many radioactive elements. However, structural transformation in smectite, dissolution or precipitation of minerals and, consequently, changes in the bentonite properties could occurs in the alkaline conditions generated by the cement degradation. The main objective of the present work is to evaluate the effect of concrete in the stability of Spanish reference bentonite (La Serrata of Nijar, Almeria, Spain) in conditions similar to those estimated in a DGR in granitic rock. Because of the main role of bentonite barrier in the global performance of the repository, the present study is essential to guarantee its security. (Author)