·Table of Contents
·Industrial Plants and Structures
An Ultrasonic probe for NDT Inspection of fuel Assembly used in Nuclear Power Plant reactorsJ. C. Machado, Z. D. Thomé, A. J. Xavier, J. C. A. C. R. Soares,
COPPE/UFRJ - Federal University of Rio de Janeiro
P.O. Box 68510, Rio de Janeiro, RJ, 21945-970, Brazil
Email : firstname.lastname@example.org
W. G. Silva
UFRN - Federal University of Rio Grande do Norte, Brazil
The ultrasonic transducer must take into account several requirements imposed by the geometry of the assembly as well as the necessary depth resolution between the echo signals reflected from the outer and inner walls of the fuel rod. To be positioned between adjacent fuel rods, the whole transducer thickness can not exceed 2.5 mm. To provide depth resolution, the transmitted ultrasonic pulse length must be shorter than 0.5 ms. The dead zone can not extend beyond 1 mm because the transducer face is placed very close to the fuel rod. In order to comply with these requirements, the piezoelectric element must: be very thin, operate at a high frequency (~ 25 MHz) with a broad spectrum and present low mechanical and electrical quality factors. To satisfy these demands, a PVDF, poly(vinylidene fluoride) piezoelectric polymer film , with 25 mm in thickness and gold electrodes, was used as the transmitting and receiving element. The transducer, with an overall thickness of 2.5 mm, was tested with a fuel assembly prototype of the same type used at a PWR nuclear reactor. It was excited by an ultrasonic equipment (Krautkramer, model USD-15). The returning echoes, from the outer and inner walls of the fuel rod, were clearly detected and differentiated. The transducer was used in a detection of failed fuel assembly, through the presence of water inside the fuel rod. A total of 5,000 echoes was acquired and processed, with the transducer attached to an XY computer controlled electromechanical positioning system. Rods containing water were detected successfully.
The main components of the transducer are: the active PVDF piezoelectric film (FV301926; Goodfellow Cambridge Limited, United Kingdom), the backing layer and the housing. Figure 1 depicts a diagram of the transducer layout viewed in a plane perpendicular to the fuel rod. Due to the cylindrical symmetry of the target to be tested, the transducer was made cylindrically focused and with the focus at the axis of the rod. The housing front face was machined to have a cylindrical groove oriented with the fuel rod axis of symmetry. The gap between the PVDF film and the outer wall of the cladding is filled with water.
|Fig 1: Transducer diagram including the PVDF film, the backing and the housing. Also shown are the cladding of the fuel rod and the mechanical arm to which the transducer is fixed by means of a spring.||Fig 2: Schematic diagram showing the electrodes on each side of the PVDF film.|
Transducer manufacturing and assembling was implemented through the main steps described as follows:
|Fig 3: Picture of the transducer held by the mechanical arm and placed close to a fuel assembly prototype of the same type used at a PWR nuclear reactor.|
Figure 3 presents a picture of the assembled transducer mounted in the mechanical arm and placed close to a fuel assembly prototype.
Once connected to the USD-15 ultrasonic equipment, the transducer was inserted between parallel adjacent fuel rods of a fuel assembly prototype of the same type used at a PWR nuclear reactor. A typical echo signal envelope from the rod is presented in Figure 4(a) and (b) for the rod filled with air and water, respectively. As it can be observed from the figure, the envelope of the echo returning from the rod front wall, as well as the envelope of multireflected echoes (returning from the interface between the rod inner wall and medium inside of the rod) are clearly distinguished. The transducer was able to capture multireflected echoes with a significant difference between the exponential decays for the envelope peak amplitudes obtained with air and water inside the rod.
Fig 4: (a) Envelope of echo signals returning from the rod front wall and from multireflections at the interface between the rod inner wall and the gap between this wall and the medium, air (a) and water (b), inside the rod.
The transducer was submitted to gamma radiation with a dose of 80 kGy and its pulse-echo response continued the same as that before being irradiated.
The ultrasonic transducer presented in this work demonstrates adequate dead zone as well as axial resolution to be used in the detection of failed fuel system. It was designed with a cylindrical focus to optimize the interaction of the transmitted wave with the geometry of the cladding. The transducer was also designed to be attached to a flexible mechanical arm by means of a pair of springs, which allows it to be kept always in contact with a fuel rod wall, when passing in front of it. The geometry and dimensions of transducer, springs and arm, altogether, provide the conditions to accommodate and navigate the transducer between the rods of a fuel assembly, using a computer controlled XY electromechanical positioning system. The arm containing the transducer can be easily substituted for another one in case of transducer failure.
Future work involves the use of this transducer in a real situation with the presence of a fuel assembly used at a PWR nuclear reactor.
|© AIPnD , created by NDT.net|||Home| |Top||