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Within the nuclear industry, non-destructive testing (NDT) plays an important role in maintaining the safety and constant inspection of the most critical parts of nuclear power plants at a cost-effective rate.
In 2014, it was estimated that nuclear power plants accounted for approximately 11% of the world’s electricity production; therefore, maintenance of such an important industry can often be associated with high costs to ensure the safe and reliable production of power from these sources.
NDT serves as a basic technique in detection flaws of all sizes, as well as the constant monitoring of material performance and degradation. The following parts of a typical nuclear power plant are monitored by NDT:
- Valves
- Concrete
- Nozzles
- Vessel head welds
- Vessel welds
- Piping components1
The principal NDT techniques used to ensure the reliability and safety of nuclear power plant operations include ultrasonic measurements, eddy current measurements, radiographic testing, acoustic techniques and several others.
Ultrasonic Testing
Of the NDT methods used in nuclear power plants, ultrasonic testing accounts for the most commonly applied as it is primarily used to detect leaking fuel rods and end plug weld control. By sensing the presence of coolant between the cladding and the fuel, ultrasonic methods can accurately measure and determine the characteristics of the cladding in order to provide information to the technician on the complete failure of the system in this area of the plant2.
When ultrasonic waves are applied to the material within the nuclear power plant, the cladding will typically decay as a result of the energy radiation present within the environment. Any reduction of the wave propagation indicates the presence of water leaking from beneath the cladding.
Hitachi Ltd, a Japanese-based technological company has developed a number of unique ultrasonic techniques that can be applied for nuclear power plant applications. For example, through a number of applied pulse voltages, phased array ultrasonic testing is a type of inspection technique that focuses its applied ultrasonic beam to create an image of the material that accurately reflects the size and performance of the material.
The Hitachi software used to control the dimensions to be focused on in a given image is particularly unique, as it provides information on the direction of the applied wave. The inspected materials that can particularly benefit from this Hitachi tool include materials or welds composed of carbon steel, stainless steel, aluminum, carbon graphite, ceramics and even carbon-fiber-reinforced plastic (CFRP).
Industrial and Nuclear Radiography
Industrial radiography is a portable way to identify and inspect welds within pipelines, pressurized vessels, and important structural components. When prompted, a radiographic camera releases radiation from a radioactive source material that is enclosed within the heavily shielded device. A detector then picks up an image captured by the radiation, which can then be analyzed by trained personnel in order to determine the quality and safety of the weld.
Nuclear radiography, which is similar to that which is employed for industrial purposes, but instead utilizes a neutron radiation source, is capable of conducting a better interaction between the camera and the organic material of interest as compared to the gamma sources used in industrial radiography3.
This property allows for a better imaging capability to be permitted for more dense materials, such as metals. Therefore, the detection of corrosion and other moisture-based damage, such as that which occurs following the damage induced by a leak, typically utilizes neutron radiography as its chosen method of detection within the nuclear power plant industry.
Radiography testing within the nuclear power plant industry can also provide important information on the position, depth, amount, dimensions and configuration of the needed reinforcements, damage present within steel parts and thickness measurements.
Sources and Further Reading
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