NDT methods in nuclear power plants
In a previous post, we discussed how non-destructive testing methods play a vital role in assessing the integrity of concrete structures. NDT methods help identify potential weaknesses or structural deficiencies early so power generation companies can address them before excessive downtime or loss of power generation capabilities occur. Such methods ensure the safety of the power plant and serve as proof to government agencies that the company is proactive in its monitoring and regulatory compliance programs.
While vital for testing of concrete structures, NDT techniques are just as critical to testing mechanical systems and components such as boilers, heat exchangers, and piping systems. NDT can also be used to verify the integrity of welds and other types of components such as valves and nozzles. These techniques can be applied to a variety of nuclear plant inspections, including metallic and nonmetallic materials.
“NDT techniques are critical to testing mechanical systems and components.”
Such testing can be carried out during new plant construction, routine maintenance or when a particular component is coming to the end of its lifecycle. No matter the situation, testing and inspection are vital to plant operation.
NDT can assess the current condition of a component or system and allow personnel to evaluate whether it is fit for service and the remaining service life. The International Atomic Energy Agency said such evaluation, which can be done with minimal downtime, can help plant personnel develop strategic plans for system maintenance that extend the lifecycle.
According to the IAEA, NDT life extension and life assessment services include:
- Monitoring corrosion of equipment and structures
- Evaluating corrosion damage
- Analyzing equipment integrity
- Evaluating fitness for service
Types of testing
The U.S. Nuclear Regulatory Commission lists six of the most common methods of NDT, including:
- Visual inspection – Can use a variety of magnification techniques and relies on an inspector’s sight and experience to locate defects.
- Magnetic particle testing – This technique is used to detect imperfections on the surface of “ferromagnetic material,” the IAEA says. A magnetic field is induced into a particular structure and then dusted with iron particles. The magnetic field highlights the imperfections where the iron particles gather, making it easy for inspectors to see the flaws.
- Ultrasonic testing – High-frequency sound waves are used to detect flaws or changes in material properties. The IAEA stated pulse echo is one of the most frequently used methods, where sound is introduced into a material and the bounceback of waves, or the echo, helps locate flaws.
- Liquid penetrant testing – Can be used for metals, plastics and ceramics to detect cracks or surface abnormalities. The object being tested is coated with a dye, then a developer is applied. Together, the dye and developer bring out the surface imperfections.
- Electromagnetic or eddy current testing – An electrical current is generated in an object composed of conductive material through an alternating magnetic field. Disruptions in the current flow indicate the presence of defects or flaws, such as hairline cracks, according to the IAEA.
- Radiography – Uses X-rays or some other radioactive source to take an image to find defects in materials. It is used much in the same way X-rays are used for medical purposes.
Nuclear plants must supply uninterrupted power to the electrical grid. However, safety is of utmost importance, both that of the general public and plant employees. NDT techniques are proven to fulfill both objectives. Such techniques also meet the goals of enhancing reliability of plant assets. New NDT methods are being developed and used in a variety of applications to achieve more accurate results to aid in safe and reliable operation.