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·Methods and Instrumentation
Indigenous Development of Automatic Ultrasonic Test SystemsS.V.Swamy, M.K.Yadav, R.K.Srivastava, N.Saratchandran
Nuclear Fuel Complex, Hyderabad, India
Ultrasonic Testing is one of the many non-destructive test techniques used in Nuclear Fuel Complex to ensure the integrity of fuel and critical core structural components for nuclear power reactors. In the last few years, work has been initiated in the area of indigenous development of automatic ultrasonic test systems, to automate manual methods and to introduce ultrasonic testing in new areas of quality control. Ultrasonic immersion testing system for Zirconium alloy billets and high-speed tube rotation type ultrasonic test system have been successfully developed. A fully automatic Go - No Go type ultrasonic immersion test system for qualification of end cap welds in Fuel elements is at an advanced stage of development.
This paper covers the design and operational features of automatic ultrasonic testing systems.
Key Words: Ultrasonic Testing, Zirconium alloy, Fuel elements, Automatic Ultrasonic Test Systems
Manual methods of UT, suffer from the disadvantages of
Imported NDT systems are expensive and are difficult to service. In the last few years, some new techniques of UT which can replace destructive techniques like metallography were developed through in-house R&D.
It was thus decided to take up indigenous development of -
The methodology adopted for the development of each of the above systems is briefly described below.
Zirconium Alloy Billets are hot extruded from vacuum arc-melted ingots and are machined after beta-quenching, a heat treatment process where the billet is heated to ~ 1050o C and quenched into water. The billets are of 150 mm or 230mm diameter and range in length from 400 mm to 800 mm depending upon the application. Presently normal beam contact ultrasonic testing is being carried out with a 1.5 mm Flat-Bottomed Hole as the defect standard and macro-etching of plackets is used to qualify the beta-treatment.
A mechanized UT system to carry out immersion UT of the billet and simultaneously give information about the beta-treatment through attenuation of the ultrasonic signals has been designed and fabricated.
|Fig 1: View of Roller and Shaft Assembly inside the tank||Fig 2: Overall View of Billet Testing System|
The salient features of the system are given below.
Based on these concepts, detailed engineering was done and the system has been fabricated. An overall view of the system is presented in Fig.2. The system is in an advanced stage of trials before permanent installation in the shop floor.
In the early seventies, a fairly sophisticated ultrasonic tube testing system was imported for testing the fuel cladding tubes of PHWR and BWR type. The thin-walled tubes of zirconium alloy Zr-2 or Zr-4 are 15.2 X 0.4 mm and 14.3 X 0.88 mm, respectively. The system was of the tube-rotation and helical indexing type with a through put speed of ~ 1.5 m/min. Later, the electronic modules of the system were changed from the thermionic valve to the transistor type. The system has been successfully operated all these years but its efficiency has come down. A very high-speed probe-rotation type system Rota-25, was imported with a much higher throughput of ~ 8 m/min. This has been the mainstay of the fuel tube-testing programme all these years. However, spares are not easily available for this system. Thus a need for a new ultrasonic tube testing system was felt and it was decided that with the existing technological base in India, Tube-rotation type systems are simpler to build and operate. Thus specifications were drawn up for a high-speed ultrasonic tube testing system of the tube-rotation type incorporating the latest hardware and software features, with a designed throughput of ~2 m/min. The system is at an advanced stage of development and is expected to become operational in 2 to 3 month's time. The salient features of this system are:
The fuel elements of PHWRs 15.2 mm dia. and nearly 490 mm long are produced by encapsulating Uranium Oxide UO2 pellets in zircaloy fuel tubes and sealing both the ends by resistance welding of zircaloy end plugs to the tube. These welds are extremely critical since weld failure can lead to contamination of Primary Heat Transport System with radioactive fission products. These welds have been traditionally qualified by metallography, which being destructive can only be applied on a sample basis. In the recent years, considerable work has been done at NFC by colleagues to successfully develop an Ultrasonic Technique for the detection of discontinuities in the weld [2,3]. It was therefore decided to subject all the end cap welds of PHWR fuel elements to UT to improve the reliability of the welds going into the reactor. Since the volume of the testing is very large, automation is absolutely essential to meet this demand. A fully automatic Go - No Go type Ultrasonic Test System has been conceived and preliminary design of the system finalized. Salient features of the System are given overleaf:
The system is in an advanced stage of design and is expected to take shape in about 6 to 8 months time. The designed cycle time is ~ 2 minutes per element. Defective elements will be scanned in a separate system to decide the acceptability or otherwise of the defects.
In case of fuel elements for Boiling Water Reactors, the fuel element is ~ 4.5 m long and is made of a zircaloy fuel cladding tube containing slightly enriched UO2 pellets and closed at both ends with Zircaloy end plugs which are welded to the tube using Gas Tungsten Arc Welding (GTAW) process. X-ray radiography is the present NDT method for evaluation of the welds. A group of elements welded at one end are radiographed at one time. The film-based technique, though sensitive is slow and any process instabilities will not be immediately revealed. Work carried out by Shri M.Suryaprakash, , has established the feasibility of using ultrasonic immersion testing for defect detection. Since the volume of testing is moderate, about 4000 elements per year, a semi-automatic system is being developed. The salient features of the system are:
Indigenous development of ultrasonic test systems is an important activity taken up at Nuclear Fuel Complex, Hyderabad, India, to protect the manufacturing programme from the uncertainties of imports and to bridge the technological gap between imported and hitherto available indigenous systems.
The authors are grateful to Dr. C. Ganguly, Chief Executive of Nuclear Fuel Complex for his keen interest and encouragement in this nationally important endeavor and for giving permission to present this paper at the 15th WCNDT at Rome, Italy. They also thank their numerous colleagues who have helped with discussions and assistance in various aspects of this multi-disciplinary work. They also acknowledge the efforts of the indigenous manufacturers, who have eagerly accepted the challenge.
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