Nondestructive imaging inspection of defects for a nuclear fuel plate by using an active laser interferometer

Most research reactors have adopted the use of plate type nuclear fuel, which consists of a fuel core in aluminum alloy. The production quality of nuclear fuel is an important part for an efficient and stable generation of thermal energy in research reactors. Thus, a nondestructive quality inspection for the internal defects of plate type nuclear fuel is a key process during the production of nuclear fuel for safety insurance. Nondestructive quality inspections based on X rays and ultrasounds have been widely used for the defect detection of plate type nuclear fuel. X ray testing is a simple and fast inspection method, and provides an image as the inspection results. Thus, the testing can be carried out by a non expert field worker. However, it is hard to detect closed type defects because the X ray image is made from the density differences of the materials. Thus, an X ray inspection is usually used for inspection of the filling status of nuclear fuel in aluminum alloy. Ultrasonic testing is a powerful tool to detect internal defects including open type and closed type defects in plate type nuclear fuel. The testing can also provide thickness information for each internal plate of the fuel. A high frequency ultrasonic signal of over than 20 MHz is usually adopted because the thickness of each plate in plate type nuclear fuel is about a millimeter to sub millimeters thin. Thus, the inspection process is complicated because an immersion test should be carried out in a water tank. It is also a time consuming inspection method because area testing is based on the scanning of the point by point inspections. In addition, this testing method needs to be carried out by an expert since the detection capability is greatly dependent on the inspector's knowledge.In this paper, an optical imaging technique using an active laser speckle interferometer with periodic thermal power was developed to visually detect internal defects. The laser speckle interferometer is an optical technique used to measure micro changes in the surface topography. This method is sensitive to very small displacement at a resolution of nanometers by superposing the speckle patterns of two different object states. The information of deformation difference among intact areas and defect areas has been widely used for the detection of internal defects in plate specimens. The phase information has been also used when the phases of frequency components in displacement variation among intact areas and defect areas are different during periodic deformation of plate specimens. The developed active laser interferometer measures an interference image, which is a micro deformation image of a specimen surface including the internal defect information. Thus, the inspection can be carried out by a non expert field worker. Here, the surface deformation is induced by a heater supplying thermal wave with the periodic sinusoidal intensities. The inspecting images can be displayed on a monitor in real time, and visual images including internal defect shapes after signal processing to reduce the noises can also be displayed. The experimental results using the developed active laser interferometer to detect the internal defects of a plate type nuclear fuel specimen are described in this paper