SUMMARY

We present a laser-based measurement system for the detection of subsurface anomalies such as delaminations and voids but also features such as anchor plates. The system aims to assist on one hand with the generation of Building Information Modeling compliant data of nuclear power plants for the decomissioning phase and on the other hand with the risk assessment of nuclear waste disposal sites. The measurement system substitutes the mechanical hammer with a strong pulsed laser and detects the induced vibrations with a laser microphone to detect these structural changes. Two exemplary measurements for the detection of delaminations and anchor plates are shown to demonstrate the general applicability.

KEYWORDS

Resonance Inspection, Delamination, Anomaly Detection, Laser-Doppler Vibrometer

INTRODUCTION

The decommissioning of nuclear plants is a topic of increasing relevance and is crucial to ensure the safe and responsible disposal of radioactive materials while minimizing the environmental impact.

Based on an accurate and comprehensive digital representation of the plant’s infrastructure, Building Information Modeling (BIM) plays a vital role in this process by facilitating efficient planning, risk assessment, and project coordination during the decommissioning phase [1,2].

Measurement solutions, or at least promising approaches, for automated digitization of the geometry and surface structures already exist and typically involve mobile laser scanners and cameras. The resulting 3D and reflection data of the surface can, for example, be evaluated using deep learning methods to identify relevant features.

However, some important features such as anchor plates, pipes, or delaminations are often not visible on the surface and cannot be detected using these methods.

A second application is the monitoring and safety assessment of nuclear waste disposal sites. This typically involves inspecting the natural and engineered barriers for structural integrity [3]. Especially for

disposal sites in existing salt mines, the risk of roof fall can be high. Therefore, the detection of fracture formation, spalling, and delaminations is crucial [4].

Currently, the detection of subsurface anomalies is mostly performed manually using tactile techniques. This results in time-consuming and labor-intensive inspections, subjective measurements, and a low degree of digitization.

METHODS

The measurement system mimics the concept of exciting mechanical (resonance) vibrations by manual hammering and analyzing the resulting acoustic waves to detect anomalies, for example, a hollow sounding area.

This manual hammering process is still the state-of-the-art technique in many application fields to detect non-visible anomalies, such as delaminations, anchor plates, insufficient material thicknesses, or defects such as small cracks.

Our measurement system completely replicates the hammering process using lasers. The mechanical impact is induced by a strong pulsed laser. The Nd:YAG Laser emits up to 10 laser pulses per second with a pulse duration of 6 ns and an energy of almost one Joule each. The laser pulses are focused on the surface that is being inspected, and due to the high energy density in the focus, a plasma is being created. The rapid expansion of the air due to the plasma generation leads to a shockwave that acts as a mechanical impact on the surface. A photograph of such a plasma on a concrete surface is shown on the right of figure 1.

Naturally, the energy that is converted into mechanical vibration of the surface is many orders of magnitude lower compared to a physical hammering impact.

Detecting these vibrations via human hearing or a regular microphone is not possible due to their low amplitude. Instead, a second continuous-wave laser is being deployed in a laser-doppler vibrometer (LDV), also known as laser microphone. The LDV measures surface movements approximately 2 cm next to the excitation point by analyzing the wavelength of the backscattered light, which, in case of a moving/vibrating surface, is shifted compared to the emitted light due to the Doppler effect.

Both laser beams are steered by a mirror that can be tilted along two axis to capture the surface to be inspected. The distance to the surface is measured by a LiDAR sensor to automatically focus the two lasers, even when there are varying distances or discontinuities.

A camera captures the surface which is then visualized in the control software. Here an area of interest and a measurement grid, for instance, 2 cm by 2 cm, can be selected.