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