During the operational lifespan of nuclear power plants, the accidental leakage of boric acid solutions is a potential event that can lead to the corrosion of concrete structures. This corrosive attack induces degradation in the macroscopic mechanical properties of concrete, posing a significant threat to the long-term structural integrity and operational safety of the facility. Consequently, a profound investigation into the mechanisms and patterns of this degradation is of paramount engineering importance for the accurate assessment of the long-term safety and service life of nuclear power plant structures. The general deterioration of concrete in aggressive environments is a well-studied field. The specific effect of boric acid as a prevalent chemical in nuclear power plant primary circuits on the microstructural and macro-mechanical performance evolution of concrete needs to be investigated. This work was to systematically investigate the degradation of concrete subjected to boric acid environments via integrating experimental tests with theoretical analysis. The primary objectives were to quantitatively characterize the deterioration of both microstructural and macro-mechanical properties in order to elucidate the influence and coupling effects of key factors (i.e., immersion time, boric acid concentration, temperature and water/binder ratio) and ultimately to develop predictive models for assessing the service life of concrete structures under the corrosive conditions.

A combined approach of experimental testing and theoretical analysis was employed to systematically investigate the degradation of concrete in boric acid environments. Concrete specimens with varying mix proportions, specifically different water/binder ratios, were prepared and exposed to boric acid solutions at different concentrations and temperatures, simulating a range of potential leakage scenarios. A comprehensive set of tests was conducted at regular intervals to monitor the evolution of concrete properties for an extended period (i.e., up to 360 d).

The experimental program included non-destructive and destructive tests to capture both microstructural changes and macro-mechanical degradation. Ultrasonic pulse velocity measurements were utilized as a non-destructive technique to probe the internal microstructural integrity and its evolution, providing an indicator of the extent of damage and micro-cracking induced by boric acid attack. Destructive tests were performed to quantify the mass loss, axial compressive strength loss, and splitting tensile strength loss. The mass loss rate was calculated to assess the leaching of cementitious components and overall material loss. The axial compressive strength loss rate and splitting tensile strength loss rate were determined to evaluate the degradation of key mechanical properties critical for structural load-bearing capacity.

Based on the extensive dataset obtained from these tests, a detailed quantitative analysis was performed. The effects of individual factors, immersion time, boric acid concentration, temperature, and water/binder ratio on the performance degradation indicators were analyzed. Furthermore, the interactions and coupling effects among boric acid concentration, temperature, and water/binder ratio were investigated. For the current volume of experimental data, a bilinear function approach was adopted for the preliminary analysis of these coupling effects. Finally, empirical prediction models for mass loss rate, axial compressive strength loss rate, and splitting tensile strength loss rate were proposed based on the experimental data, forming a basis for service life assessment.

The experimental results reveal significant trends in concrete degradation under boric acid attack. The microstructural deterioration quantified through ultrasonic pulse velocity measurements shows consistent intensification with increasing boric acid concentration, temperature, and water/binder ratio. The relative ultrasonic pulse velocity is decreased by 15.49%–111.46% after 360 d, while the ultrasonic pulse velocity-time curve slope is reduced by 25.98%–134.37%, indicating an accelerated micro-damage. The factor interactions exhibit weak coupling characteristics. A distinctive two-phase behavior emerges across all the measured parameters. During initial immersion (i.e., 0–90 d), the negative values are obtained for mass loss rate and strength loss rates due to pore-filling effects of corrosion products and ongoing hydration. Beyond 90 d, all the parameters are positive and show monotonic increases, reflecting net material loss and structural deterioration. The degradation rates significantly amplify at higher boric acid concentrations, elevated temperatures, and increased water/binder ratios. Based on the comprehensive data, empirical predictive equations were proposed for mass loss and strength degradation, enabling quantitative service life assessment of concrete structures in nuclear power plants under a boric acid exposure.

The results revealed a clear two-phase degradation mechanism, driven by microstructural evolution. During the initial immersion period (i.e.,0–90 d), the formation of calcium metaborate crystals in the near-surface region that could not be fully corroded introduced a dominant filling effect. This led to a temporary increase in density and strength, manifested as negative mass and strength loss rates. Beyond 90 d, the development and progression of a fully corroded zone became a prevailing factor. This process could cause a sustained microstructural deterioration, which was effectively characterized by ultrasonic pulse velocity measurements. The deterioration intensified with increasing boric acid concentration, temperature, and water/binder ratio. Consequently, the mass loss and strength loss rates both became positive and exhibited a monotonic increasing trend. The empirical models were developed to predict the mass loss and strength loss, providing a reliable basis for assessing the structural performance and remaining a service life of concrete structures in nuclear power plants, thus offering valuable tools for durability and safety assessment.