NF-PRO Deliverable 1 of Component 2. State- of-the-art report.

Abstract

Chapter 1 gives a general view of the different conceptual designs of a Deep Geological Repository (DGR) in granite, clay and salt formations. It also provides information on the possibility of retrievability in the different disposal designs. Chapter 2 is focused on the role and requirements of the different components of the Engineered Barrier System (EBS), giving special attention to the mineralogical, chemical and physico-chemical characteristics of the natural bentonitic clay (FEBEX and MX-80) and ordinary Portland cement. It also presents a brief review of the mineralogical, chemical and physico-chemical characteristics of the C-steel and Cu/stainless steel containers, clay barrier and concrete, the latter being mainly used in clay repositories. The types of pore water, state, location and quantity of the FEBEX bentonite are reviewed. The chemical composition of the pore water in the clay barrier is discussed, as well as the techniques to collect the pore water and the difficulties posed by the small amounts of water collected. The thermohydro- mechanical properties of the clay barrier are also briefly mentioned. The hydration stages of concrete are described together with the accompanying precipitation/dissolution processes of different minerals. The pore water chemistry of concrete is studied, and the degradation of concrete and its influence on the mobility of the radionuclides as well as the resulting high pH plume are briefly described. Chapter 3 gives a general view of the processes that take place during the hydration/saturation states under thermal gradient. It is organized according to the chronological order in which the processes may occur. First, the groundwater will flow through a concrete plug in the case of clay and salt formations causing modifications in the concrete. The groundwater from the concrete or from the granite will interact with the bentonite generating a series of geochemical processes in the bentonite. Finally, the groundwater will reach the canister and corrosion processes will start. The geochemical processes that take place in a salt formation are also reviewed in this section. Chapter 4 reviews the pro and cons of the chemical thermodynamic data bases relevant to nuclear systems. Kinetics is pointed out as being an important uncertainty in the data bases, since reaction kinetics depend on the environment conditions, making any extrapolation difficult. Anyway, the development of kinetic data base is in progress. The most used computer programs to model the near-field geochemical evolution are mentioned, as well as the main processes and reactions taken into account in the geochemical modelling. A brief review is given of the thermodynamic of the main reactions in the near field. Chapter 5 describes the main chemical processes in each of the considered components of the EBS (conceptual model); furthermore, a review of the existing numerical models, and the uncertainties related to present-day models (identification of key chemical processes, critical concepts and missing parameters) are given. Chapter 6 compiles the known and unknown facts and uncertainties in the host rock/ concrete/ bentonite/ canister interfaces and gives a list of those parameters that should be studied to broaden the knowledge and understanding of the geochemical processes in the near field.