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Role of Non-Destructive Testing during Manufacture of Zircaloy Structurals and Nuclear Fuel for Power Reactors in IndiaN.Saratchandran, S.V.Swamy, R.K.Srivastava, B.Lakshminarayana, M.Suryaprakash
Nuclear Fuel Complex, Hyderabad, India
Nuclear fuel Complex has been manufacturing critical Zircaloy core structural components and fuel for Pressurized Heavy Water Reactors and Boiling Water Reactors constructed in India since 1971. The quality requirements for any component or assembly that goes into the reactor are necessarily very stringent, because of the enormous failure costs. Therefore, a comprehensive Quality Assurance Program has been put in place to meet this objective. A variety of NDT techniques including Ultrasonic Testing, Radiography, Visible and Florescent Dye Penetrant Testing, Leak Testing, Eddy current Testing and Acoustic Emission Testing are employed at various stages, viz. Raw Material Inspection, In-Process Quality Control and Final Inspection. Feedback from the Reactor sites helped in fine-tuning the techniques of manufacturing and testing.
This paper briefly describes the non-destructive tests employed at various stages of fuel manufacture, R&D in NDT, and the status of indigenous development of automated NDT Systems.
Key Words: Zircaloy core structurals, nuclear fuel, PHWR, BWR, reactors, NDT, Quality Assurance
Nuclear Fuel Complex (NFC) was set up in 1970 as an integrated unit to manufacture fuel and zirconium alloy structurals for Pressurized Heavy Water Reactors and Boiling Water Reactors, the main stay for the Indian Nuclear Power Program. As a part of its integrated role, NFC has taken up the fabrication of Stainless Steel Assemblies for the Fast Breeder Test Reactor (FBTR) and the Prototype Fast Breeder Reactor. Further, NFC manufactures various seamless and seam-welded tubes and pipes in different materials and to different grades, to cater to a wide spectrum of nuclear, fertilizer, petrochemical and other sectors of industry in India. NFC has exported zirconium alloy rods and zirconium oxide. NFC has developed expertise in building Special purpose machines and furnaces for in-house use as well as for other customers.
Non-Destructive Testing (NDT) techniques play a vital role in the comprehensive Quality Assurance (QA) program that has been set up at NFC to assure the highest levels of Quality and customer satisfaction. Visual Test (VT), Penetrant Test (PT), Magnetic Particle Test (MT), Eddy Current Test (ET), X-ray Radiographic Test (RT), Ultrasonic Test UT), Leak Test (LT), Beta Back Scatter Test, Gamma Spectroscopy, and Acoustic Emission (AE) are the main Non-Destructive Tests used in NFC for ensuring the product integrity at various stages.
This paper discusses the expertise developed in the use of these sophisticated NDT techniques for the Quality Control (QC) at various stages of the product manufacturing cycle in NFC, emphasizing the use of NDT in manufacture of nuclear fuel and Zirconium alloy structurals for PHWR and BWR.
ZIRCALOY-2 CALANDRIA TUBES FOR PHWR are manufactured by Vacuum Arc melting, hot extrusion to slabs, hot rolling followed by cold rolling of zircaloy-2 sheets. The slabs, plates and sheets are qualified by VT and UT. The sheets are annealed, tested, machined and are brake-formed into tubular shapes, which are seam-welded using semi-automatic GTAW process. The seam-welds are qualified by RT, and are then seam-leveled followed by annealing. The seam-leveled welds and HAZ are qualified by Visible Penetrant Test.
To improve the reliability and yield, cold pilger process was developed for making Seamless Calandria Tubes. Initial trials have established that the mechanical properties and metallurgical texture are similar to the seam-welded tubes. Irradiation trials are going on presently to enable the modified route of tube making to be accepted.
SQUARE CHANNELS FOR BWR are also seam-welded, seam-leveled and drawn to exact size and shape. The NDT methods employed are similar.
ZIRCONIUM - 2.5% NIOBIUM PRESSURE TUBES FOR PHWR are produced by two-stage hot extrusion of Quadruple Vacuum Arc-melted Zirconium-2.5% Niobium Ingots into mother blanks, which are annealed and pilgered in 2 stages to the final dimensions. The tubes are finally stress-relieved. The tubes are designed for a life of 20 years in the reactor and carry the fuel and heavy water at a temperature of 3000 C and 110 kg/cm2 pressure. Since any premature failure of these tubes results in drastic reduction in power production or may even lead to an early replacement of the tubes, the criticality of the tubes is very high. To ensure that the quality level of these tubes is very high, VT, UT and ET are extensively used followed by Pressure Test.
The earliest PHWRs used Zircaloy-2 as the material for the pressure tubes. In India, the Reactors at Rajasthan, Madras, Narora and the first unit at Kakrapar are of this type. From the second unit of Kakrapar, Zirconium - 2.5% Niobium was used as the material but the ingots used were of double-melted type. In line with international trends to improve the fracture toughness and the life of the tube, specifications have been revised downwards w.r.t the levels of chlorine, hydrogen etc. To achieve these revised limits, quadruple melting has been adopted and other process changes have been made in tube manufacturing route. Table No.1 gives the stages of manufacture and the NDT methods used at each stage.
|Component / Stage of Manufacture ( in the order from finished tube to the ingot)||Type of NDT used||Remarks|
|Coolant Tube after autoclaving at 4000 C, 10 kg/cm2, 36 hrs||VT on 100% basis, UT and ET on sample basis||UT and ET carried out on full length tubes on sample basis to check for crack opening due to stress-relieving|
|Coolant Tube after 2nd pass pilgering||VT, ET, UT||I.D. Visual by rigid borescope. ET by I.D. differential bobbin coil. UT by immersion testing with angle beam and normal beam probes.|
|1st pass pilgered and stress-relieved blank (100 X 4.4 mm)||VT, UT|
|Extruded and stress-relieved Blanks (125 X 9 mm)||VT, UT|
|230 mm dia. Billets||VT, UT||Mechanized immersion Ultrasonic Test system under final trials. UT will also qualify beta-quenching|
|Table 1: NDT Methods at various Stages of Manufacture of Zr-2.5% Nb Pressure Tubes|
Because of the method of manufacture, the types of defects likely to be encountered are transverse cracks and laminations (from any shrinkage cavities present in the ingot). In addition, localized surface defects on the inner and outer surfaces are likely. Visual examination of both O.D and I.D is very important. In addition Eddy Current Test of the I.D gives additional protection from surface and subsurface defects which could be missed in the Visual Test. Angle beam and normal beam Ultrasonic Test in an automated test system with recording facilitates the detection and elimination of internal discontinuities.
PHWR FUEL BUNDLE consists of 19 fuel elements welded together with endplates at both ends. Each element is 495 mm long, ~15mm in diameter and is made of thin-walled zirconium alloy, Zircaloy-4 tubes, containing sintered natural Uranium Dioxide pellets. The tube is closed at both ends by zircaloy end caps, which are resistance welded to the tube. Small appendages called spacer pads are welded to the O.D. of all elements, by spot welding, to ensure inter-element gap. The outer elements are also provided with bearing pads to provide bearing surface to the assembled bundle when inserted and pushed by sliding into the coolant channel of PHWR. The stages of manufacture and the NDT method used are given in Table.2.
|Component / Stage of Manufacture ( in the order from finished bundle to the individual components)||Type of NDT used||Remarks|
|Finished fuel bundle||VT, LT||Helium Leak Testing|
|Individual fuel element|
After bearing / spacer pad welding After end cap welding
VT, LT, UT
|AE and UT based methods being developed as substitutes for metallography and mechanical testing of welds|
|Fuel Tube before being filled with pellets, after graphite coating on I.D.||VT, Beta Back Scatter Test for graphite thickness measurement||ET under development as a substitute for Beta Back Scatter Test|
|Fuel Tube before cutting to length||VT, UT (for defects, wall thickness and dimensions)||Both tube - and probe - rotation type systems in use for UT. Tube - rotation type high speed system under indigenous development|
|Zircaloy mother blanks||VT, UT||UT done by contact testing|
|Zircaloy billets for extrusion||VT, UT||UT by contact testing. An automated immersion testing system is under final trials|
|Zircaloy Sheets for End Plates, Spacer and Bearing Pads||VT, UT, PT for the sheet blanks for end plates||Manual methods because of low volume of testing|
|Zircaloy end plugs||VT||ET was used for detecting fine pin-hole type defects and for sorting grade mix-up.|
|Zircaloy bar for end plugs||VT, UT, ET||Coils for ET developed in-house|
|UO2 pellets||VT, Density Measurement||Laser based systems for dimensions and density developed. Laser Scanning Systems for Visual Testing of pellets under development|
|Table 2: NDT Methods at various stages of manufacture of PHWR Fuel Bundle|
Since the integrity of the fuel bundle depends primarily on the integrity of the end-cap weld, efforts have been intensified to improve the quality of the weld and the reliability of defect detection. Metallography was the only method earlier available. A few years ago UT technique for evaluation of the endcap welds has been developed. Recently studies have been initiated to integrate results of Acoustic Emission Test (AE), Metallography, Ultrasonic Testing and Helium Leak Testing to develop an integrated picture in the case of end-cap welds. AE, UT and mechanical testing are similarly being studied together in the case of bearing and spacer pad welds.
BWR FUEL BUNDLE is made of 36 elements, each nearly 4.5 m long. The elements are made of Zircaloy-2 tubes, ~ 14mm X 0.8 mm and the pellets are of 3 different low enrichment levels. Zircaloy end plugs using GTAW close the ends. VT, RT and LT are the NDT methods employed for ensuring the weld integrity. Sealed elements are checked for their isotopic purity by gamma spectroscopy.
A study was carried out to substitute RT by UT for end-plug welds. Work has now been initiated to indigenously integrate the probe positioning mechanisms and Ultrasonic Pulser-Receiver to obtain real time A- B- and C- Scan Presentations. This is expected to improve the reliability of the defect detection through UT.
From various considerations, it has become important for NFC to take up the development of Automated NDT Systems in the above areas, with the support of Indian Industry, which has developed very high capabilities for development of Precision Scanners, Material Handling Systems, PC-based Ultrasonic Flaw Detectors or Software for Integration of such Sub-Systems. Work is currently on hand to develop
PROJECT ON INTELLIGENT PROCESSING OF MATERIALS has been conceived by Department of Science and Technology. In collaboration with scientists of Indira Gandhi Center for Atomic Research, Kalpakkam, India, studies have been conducted in application of Acoustic Emission (AE) and Thermography to End Cap / Bearing Pad / Spacer Pad welding of Fuel elements and Calandria Tube welding. Studies are also planned to study effect of Beta-quenching of Zircaloy on Ultrasonic Velocity.
OTHER IMPORTANT APPLICATIONS OF NDT are:
A rich and varied experience and expertise has been accumulated over several years of successful application of sophisticated NDT methods, other testing and analytical techniques. This and the insight gained by comprehensive Failure Analysis using appropriate methods has enabled NFC to extend Quality Assurance (QA) services to other organisations. The services include Quality Control during Fabrication, In-Service Inspection etc., of High Temperature Reactors, Heat Exchangers, Steam Generators etc., and Pressure Vessels. The client list includes
NDT plays a vital role in the QA program at NFC. Over the years, a very strong inter-disciplinary team of qualified and experienced scientists and engineers has been created. The rich and varied experience and expertise is being extended to other organisations.
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