Simulation of concrete deterioration in Finnish rock cavern conditions for final disposal of nuclear waste

Abstract

In the Finnish disposal concept concrete-made engineered barriers ensuring the safety of low-and intermediate-level waste repositories in rock caverns are designed to remain serviceable for hundreds of years. With regard to the ageing of the concrete, the lifetime of the disposal facilities will be divided into the periods before and after the closing of the caverns, when the concrete will be exposed to two different sets of environmental conditions.The research pointed out that the ageing of concrete in the disposal conditions is a complex physiochemical phenomenon that cannot be predicted by diffusion models based only on Fick's second law for periods considerably exceeding the period covered experimentally. A thermodynamic model, which considers ion-ion and ion-cement hydrate interactions as well as the properties of the gas phase in multi-ionic transport in concrete, was introduced in the study. A statistical methodology was presented for evaluating the effect of uncertainties on the results. A range of experimental data extracted from sulphate-resistant Portland cement concretes exposed to natural carbonation or the penetration of aggressive substances under submersion for 13 years in a controlled environment was used for evaluating the thermodynamic model. The theory of the model was shown to give a plausible basis for estimating the long-term ageing phenomena of concretes, particularly in practical applications where the design periods of structures are hundreds of years. The thermodynamic simulation method also makes it possible to observe the latent factors involved in the deterioration of the concrete. The method considered is general and can be applied to different concrete mixes by defining their casespecific initial values.Generally, the sulphate-resistant type of concrete with a water-to-cement ratio of 0.43 performed satisfactorily during the simulated period of 500 years, but the initiation of the corrosion of the steel reinforcement cannot be excluded. The leaching of calcium-bearing components from the concrete affects the strength of the concrete. However, the main detrimental reactions of the concrete take place at a depth of less than 50 millimetres from the exposed surface, which is typically a relatively small part of the whole structural thickness and, especially, of the total set of multiple technical barriers.Statistical analyses of the factors and the consideration of uncertainties in the simulation results can be used for improving the reliability of the thermodynamic model. The role of the aggregates in the deterioration of the concrete should also be evaluated in the future and more sophisticated models for the alteration of calcium silicate hydrate are necessary if the timescale is significantly extended beyond 500 years. Tiivistelmä Suomalaisessa loppusijoituskonseptissa matala-ja keskiaktiiviset ydinjätteet sijoitetaan kalliotiloihin, joissa olevat betonirakenteiset vapautumisesteet suunnitellaan useiden satojen vuosien käyttöiälle. Betonirakenteen ikääntymistä voidaan aj...

Neural network based hygrothermal prediction for deterioration risk analysis of surface-protected concrete façade element

“…The chloride ion concentration in concrete is often estimated by adopting a simplified Fick's second law of diffusion based analytical formula described by Equation (2.8) [62]. The foremost limitations of Equation (2.8) are [15,36,63,64]: i) the surface chloride content is invariant; ii) the nonsteady diffusion coefficient remains constant; and iii) the 𝐷 𝑛𝑠𝑠 is assumed to be uninfluenced by different 𝐶 𝑠 .…”