Nonlinear Ultrasonic Evaluation for Corrosion Assessment of Steel Plates Embedded in Concrete

The growing demand for sustainable electricity production, coupled with global uncertainties, highlights the need for nations to achieve self-sufficiency in producing electricity. Nuclear power, which provides around 30% of Sweden's electricity, plays a critical role in delivering fossil-free electricity. As many nuclear plants are near the end of their designated lifespans, extending their operational periods is crucial to meeting current and future demands.
Ensuring safe nuclear operations is vital. To address this, the Swedish Energy Research Centre, along with the Swedish Radiation Safety Authority and other industry partners, runs a research program focusing on concrete-related challenges in nuclear power. A significant issue within this program is detecting corrosion in containment liners – welded plate constructions that serve as gas-tight barriers to prevent the release of radioactive particles. These liners, sometimes embedded in thick concrete walls of reactor containment buildings, are difficult to inspect. Corrosion can compromise their structural integrity long before visible damage appears, making early detection essential for planning preventive measures.
This thesis evaluates nonlinear ultrasonic methods for detecting and assessing corrosion in embedded containment liners. Ultrasonic testing uses sound waves above the human audible range to image internal structures. However, in concrete, high-frequency sound waves are quickly absorbed, necessitating the use of low-frequency waves, which reduces the ability to resolve small defects. Nonlinear ultrasonic evaluation offers a solution. Defects like cracks cause elastic nonlinearity, leading to waveform distortion. This distortion creates new frequency components in the sound wave, allowing for the detection of defects regardless of the wave's frequency. The strength of these new components relative to the original frequency provides a measure of damage in the material.
The research, conducted on small-scale laboratory specimens, primarily focused on pre-corroded steel plates. The results demonstrate that nonlinear evaluation can locate and assess corrosion in embedded plates, with more severe corrosion yielding stronger nonlinear indications. Additionally, the method shows potential for detecting embedded foreign objects, such as wood, and separations between the plate and concrete. The findings suggest that combining conventional and nonlinear ultrasonic imaging techniques can provide a better understanding of a structure's damage state. However, further experiments on more realistic specimens are necessary to fully evaluate the methods' detection capabilities. These experiments should consider the challenges posed by intrinsic concrete micro-cracking, coarse aggregates, reinforcement, and large dimensions typical in nuclear power plants.