Modelling of the Degradation of Cement in a Nuclear Waste Repository

Abstract

ABSTRACTThe current UK concept for a low- or intermediate-level nuclear waste repository includes a largely cementitious backfill. The cement provides a high pH environment in which the general corrosion rate of the metal canisters is reduced and the solubilities of many nuclides low. It has previously been assumed that this high pH will exist for a period of 107 years, however cement will degrade due to leaching of the solid components and attack from aqueous species in groundwater. In this paper we describe the preliminary stages of a model of the degradation of cement in a repository. The modelling involves the incorporation of a thermodynamic description of cement[2] into the static code PHREEQE[5]. This is then used in a coupled chemistry-transport model of simple leaching of cement using the code CHEQMATE[4]. This preliminary modelling also provides a useful verification of CHEQMATE as a direct comparison with a THCCDM (a coupled code based on CHEMTRN) model is possible. Results from this preliminary model suggest that the fall in pH due to leaching is slow. The model is sufficiently flexible to form the basis of more detailed investigations of the effect of groundwater interactions on the degradation of cement.