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Boric acid attack of the reinforced concrete used in Spent Fuel Pool

Abstract : In the framework of Spent Fuel Pools (SFP) lifetime studies, an investigation of the concrete degradation in aqueous boric acid has been requested by the Electric Power Research Institute. The main goal of this study is to identify the physico-chemical degradation mechanisms involved in a boric acid medium. A well-tested methodology for testing cementitious materials degradation in other solutions (water, sulfate solution…) was applied. This methodology involved an experimental study and computational modeling. For the particular case of boric acid attack, a multi-scale approach was used; concrete as well as its main components (cement paste and aggregates) were studied. The degradation experiments were carried out for three to eight months in 2400ppm boric acid solution. Aggressive solution conditions were maintained by pH regulation and periodical renewal. Characterization concerned the composition of the degradation solution during the experiments, as well as the mineralogical evolution of the degraded cementitious materials at the end of the experiments. Solution analysis was performed by ionic-chromatography and solid characterizations were carried out by the means of XRD and SEM observations. The study of the Portland cement paste degradation shows that the leaching mechanism is driven by diffusion. The degradation kinetics in boric acid is higher than the one in pure water. The process of concrete degradation is more complex; a nonlinear behavior of the calcium leaching over square root of degradation duration can be noticed. Besides, an additive contribution of cement paste and aggregates on the leached elements from concrete is suggested. Nevertheless, the degraded thickness ranges from 2400 $\mu$m to 2800 $\mu$m, which is significantly lower than the one obtained in cement paste at the same duration (3400 $\mu$m). This observation is quite unexpected and could indicate a possible surface dissolution. Finally, reactive transport numerical calculations are carried out with HYTEC platform to be confronted to experimental results. The first results on cement paste degradation are encouraging; the four zones composing the degraded Portland cement paste profile pattern that were identified experimentally are obtained from numerical simulation. This modeling work must be continued to improve the description of the degradation kinetisc on cement paste and to include the modeling of concrete degradation.
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L. Chomat, A. Benakli, P. Le Bescop, J. Wall. Boric acid attack of the reinforced concrete used in Spent Fuel Pool. GLOBAL 2015, Sep 2015, Paris, France. ⟨cea-02509263⟩

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