Ultrasonic monitoring of stress and cracks of the 1/3 scale mock-up of nuclear reactor concrete containment structure.

To evaluate the stress level and damage of a reinforced concrete containment wall and its reaction to pressure variations, we implemented successive ultrasonic experiments on the exterior surface of the containment wall in the gusset area for three consecutive years. During each experiment, the pressure inside the containment wall increased gradually from 0 MPa to 0.43 MPa and then decreased back to 0 Mpa.From the analysis of the ultrasonic coda waves obtained in the multiple scattering regime, we performed Coda Wave Interferometry to calculate the apparent velocity changes in the structure (denoted by $dV/V_a$) and Coda Wave Decorrelation (DC) measurements to produce 3D cartographies of stress and crack distribution. From three source-receiver pairs, located at the top, middle and bottom of the experimental region, we observe that coda waves dilate, shrink and remain almost unchanged, respectively. This corresponds to the decreasing, increasing and invariant pressure inside the concrete. The comparison of three years' results demonstrates that the variation of $dV/V_a$ and DC under the same pressure test increases through the years, which indicates the progressive deterioration and aging of the concrete. From a large collection of source-receiver pairs at different times, the spatial-temporal variations of $dV/V_a$ and DC are then used to produce a map of the structural velocity and scattering changes, respectively. We observe a decreasing velocity on the top part and an increasing in the middle one, which is in line with the $dV/V_a$ analysis. The reconstructed scattering changes (or structural changes) highlight the active region during the inflation-deflation procedure, corresponding to the opening and closing (and sometimes the development) of cracks. The larger magnitude in 2019 than in 2017 indicates the increasing damage in the concrete.