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An Ultrasonic probe for NDT Inspection of fuel Assembly used in Nuclear Power Plant reactors J. 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 : jcm@peb.ufrj.br W. G. Silva UFRN - Federal University of Rio Grande do Norte, Brazil Contact

Abstract

The present work describes the design and manufacturing of an ultrasonic transducer to be used in non-destructive inspections of nuclear fuel assemblies. A PVDF, poly(vinylidene fluoride), piezoelectric polymer film, with 25 mm in thickness and gold electrodes, was used as the transmitting and receiving element, operating with a frequency of 25 MHz. The transducer has an active area of 3.0x6.0 mm and a total thickness of 2.5 mm. It was designed to be positioned at the space (2.5 mm) between adjacent fuel rods and to provide adequate depth resolution as well as a dead zone depth less than 1 mm. When excited by an ultrasonic equipment (Krautkramer, model USD-15) and tested with a fuel assembly prototype of the same type used at a PWR nuclear reactor, the returning echoes, from the outer and inner walls of the fuel rod, were clearly detected and differentiated. The transducer is part of a failed fuel detection system, and was used to detect the presence of water inside the fuel rod. A total of 5,000 echoes was acquired and processed, with the system able to detect successfully the rods containing water.

INTRODUCTION

Efficient and safety operations of nuclear power plants require routine inspections of failed fuel assemblies, such as those with flaws in the cladding of rods containing nuclear fuel. In Nuclear Pressurized Water Reactors (PWR), the presence of flaws allows water to enter the rods and to occupy the gap, previously containing gas, between the cladding and the nuclear fuel. In this case, ultrasound is one of the techniques for the detection of failed assemblies. It is based on the differentiation between the echoes returning from the interface between the inner wall of the cladding and a medium containing gas or water [1].

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 [2], 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.

METHOD AND MATERIALS

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:

1. A flat layer disc (10 mm diameter and 1 mm of thickness) was prepared with a mixture of epoxy and tungsten powder to work as the backing layer of the PVDF film. Two important characteristics were provided by this backing layer: absorption of the ultrasonic wave transmitted into the opposite direction, avoiding echoes from fuel rods on the back side of the transducer, and minimization of the transmitted ultrasonic pulse length, with the consequent improvement of axial resolution. One of the surfaces of this backing layer was machined to have a cylindrical groove to accommodate the PVDF film.
2. A rectangular piece of PVDF film (11x3 mm) was prepared with gold electrodes on both sides and displayed according to Figure 2. This provided a PVDF active surface (8x3 mm) with electrodes coincident on both sides. The center conductor of a subminiature coaxial cable (RF cables MCX; RS Components International, United Kingdom) was connected to the side of the PVDF film back electrode using an electrically conductive silver epoxy (RS catalog No. 496-265; RS Components International, United Kingdom). Thereafter, the PVDF film was glued to the backing layer using epoxy (Araldite XGY 1109 and hardener XR 1951, 10:1 in weight; Ciba-Geigy, Brazil).
3. The outer conductor of the subminiature coaxial cable was soldered to the stainless steel housing, using a lead/tin soldering wire compound.
4. The PVDF film and backing layer disc were inserted into the housing and aligned with respect to the housing face groove axis of symmetry. This was accomplished experimentally, positioning an empty rod in contact with the housing and filling the gap between the front PVDF face and the outer rod wall with an ultrasonic gel. The transducer was excited with the USD-15 ultrasonic transmit/receive equipment and the echo from the rod front face observed. Fine adjustments on the orientation of the PVDF film were done until an echo with maximum amplitude and better resolution, as shown in Figure 4-a, resulted. At this moment, the back face of the backing layer was glued to the housing using a fast curing epoxy.

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.

5. A small amount of epoxy was poured on the PVDF front face and along the aperture window of the housing to seal the transducer and avoid water to be in contact with the PVDF film back face.

Figure 3 presents a picture of the assembled transducer mounted in the mechanical arm and placed close to a fuel assembly prototype.

RESULTS

Before using the transducer together with ultrasonic system designed to detect failed fuel rods, it was submitted to different conditions of temperature and pressure. There was no detected difference on the echo signals from an empty rod when the transducer was exposed simultaneously to 60°C and a pressure of 2 atm (operating conditions at the power plant). The transducer was also exposed to 80°C during several hours. Again, this condition did not modify the transducer performance.

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.

CONCLUSIONS

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.

REFERENCES

1. W. C. A. Pereira, Z. D. Thomé, J. M. Seixas, W. Soares Filho and M. C. Bossan, "Ultrasonic pulse-echo method for failed rod detection based on neural network", Presented at the 15th World Conference on Non-Destructive Testing, Rome, Italy, 15-25 October, 2000.
2. N. Murayama, K. Nakamura, H. Obara and M. Segawa, "The strong piezoelectricity in polyvinylidene fluoride (PVDF)", Ultrasonics, pp. 15-23, 1976.

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