FUEL bundles for Rajasthan Atomic Power Reactor, as per the design, consist  of a  cluster  of 19 elements held together by zircaloy  end  plates  and  spaced  by wire 

helices on the six elements of the inner ring and alte-rnate  elements  of  the  outer ring. Each element is a zircaloy-2 tube 495 min long,1524 mm in diameter and of 0.4 nom wall thickness containing  the  nuclear fuel in 

the form of sintered uranium dioxide pellets and sealed

at both ends. In  order to provide the required spacing between the  elements, zircaloy wire of 1.25 mm dia. is spot-welded  around the elements in the helix form. The wire helices serve to separate the elements to maintain the dimensions of the coolant sub-channels between them 

and to increase the mixing of coolant between inner and

outer sub-channels. Zircaloy  wire  of  1.63 mm dia. is

spot-welded  on  the  outer  elements so  as to provide

bearing surfaces for the bundle  when  it moves through

the fuel channels in the coolant assemblies

Chandramouli, V A

Kumar, J S

Moorthy, K Balarama

English

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Eddy current testing of zirconium wiresV. A. CHANDRAMOULI, K. BALARAMA MOORTHY andJ. S. KUMARFUEL bundles for Rajasthan Atomic Power Reactor,as per the design, consist of a cluster of 19elements held together by zircaloy end platesand spaced by wire helices on the six elements of theinner ring and alternate elements of the outer ring.Each element is a zircaloy-2 tube 495 min long,1524 mm in diameter and of 0.4 nom wall thicknesscontaining the nuclear fuel in the form of sintereduranium dioxide pellets and sealed at both ends. Inorder to provide the required spacing between theelements, zircaloy wire of 1.25 mm dia. is spot-weldedaround the elements in the helix form. The wire helicesserve to separate the elements to maintain the dimen-sions of the coolant sub-channels between them andto increase the mixing of coolant between inner andouter sub-channels. Zircaloy wire of 1.63 mm dia. isspot-welded on the outer elements so as to providebearing surfaces for the bundle when it moves throughthe fuel channels in the coolant assemblies.As in the case of all other reactor materials, thezircalov wire also needs to be inspected 100° to avoidany defective part entering the reactor system. Therequirement of rigid specifications on the quality ofwire, or for that matter any material intended forreactor use, need not be over-emphasised here. Thezircalov wire must be free from defects like seams,internal voids, welded spots, etc., apart from the dimen-sional and metallurgical requirements.Considering the large quantity of the wire to beinspected and since 100% of the wire is to be scanned,the testing procedure adopted must be non-destructiveand fast enough and at the same time capable ofdetecting and differentiating the defects mentionedabove. Radiography, as is well known, is a slow inter-mittent process and also the seams and welded jointscannot be readily detected. Ultrasonic method, on theother hand, intrinsically is not suited for such smalldiameter wire owing to the limitations such as deadzone, resolution, pulse width, etc. Other non-destructivetests such as penetrant test, etc., are suited only forspecific types of defects (which are open to the surface)and hence those methods are not applicable to ourproblem. Eddy current testing has been found to beonly the ideally suitable method to detect the flawsMessrs V. A. Chandramouli, K. 13alarama Moorthy and J. S.Kumar, Bhabha Atomic Research Centre, Bombay.SYNOPSISZircaloy-2 wires of 1.25 min and 1.63 trmi dia. are usedas spacers in the fabrication of nuclear fuel bundles forRajasthan Atomic Power Project. These wires are spot-welded at regular and close intervals on zircaloy-2 fuelsheaths which contain UOz pellets. In order to get aconsistent good quality of spot welds, it is imperative thatthe wires must he free from seams, internal voids,welded joints, etc. Out of the various non-destructive testmethods like ultrasonics , radiography, etc., for qualityevaluation , eddy current testing affords a quick andreliable mode of testing the wire on a 100% basis for thedetection of the above defects. This paper deals with thepreliminary investigations on the eddy current testing ofzircaloy wires. The preparation of various defect stan-dards and the standardisation of' the equipment for thedefects are also described along with the necessaryphotographs and recording charts. The details of thearrangement proposed for automatic inspection of wiresare given at the conclusion of the paper.in the thin wire and being an electro-magnetic method,the testing can be done at a very rapid rate.Principle of eddy current testingThe eddy current test is an electro-magnetic test appli-cable to the examination of electrically conductingtest specimens for detecting any flaw. For eddy currenttesting the word `flaw' has a much wider meaningthan it has for X-ray inspection, ultrasonic inspec-tion, and other forms of non-destructive testing. Notonly cracks and voids can be found, but also differencesin alloy, conductivity, differences in diameter of thewire, eccentricity of tubing, etc. can be found andmeasured.In eddy current testing, high frequency alternatipgcurrent is caused to flow in the test coil which, inconsequence, produces magnetic field in the vicinityof the coil. If a conducting test specimen is introducedinto this field, currents known as Foucault or Eddycurrents are induced in the specimen by electro-magneticinduction, The quantity or the path of the eddycurrents depends upon various factors such as the188Chandrantouli et a! : Eddy current testing of zircaloy wi'esI Eddy currant wire tester with recorderconductivity, magnetic permeability, size, defects, etc.,of the specimen. These eddy currents react on theexciting coil (or upon a secondary coil used as asensing device) and affect its impedance. Variations ineddy currents are caused by flaws as defined above)and so cause variations in the impedance of the coil.These impedance variations can be analysed, and theirmagnitude and phase can give information concerning2 Appearance of a trace when the coils are balanced3 Test wire with 2% variation in diameterthe identification and severity of the flaw. Like mostother non-destructive tests, the eddy current test is anindirect evaluation of the characteristics of the testspecimen. Hence, the effects of these eddy currents areobserved and compared to those caused by currentswhich flow or which would flow in a standard oracceptable specimen under similar set of conditions.Three general types of coils are in common use.Most common one among them is a circumferentialcoil through which the test part passes and this typeis also called feed through coil. In the feed throughdifferential coil type, there are two coils having aprimary and a secondary each. The coils can be placedeither co-axially wherein one part of the specimen (wire)is compared with another as the specimen is pushedthrough the coils, or in different axes wherein a standardacceptable specimen is placed inside one coil and thespecimen under test is pushed through the other. Theprimaries of the two coils are connected in series anda high frequency alternating current is made to passthrough them. The secondaries are connected differen-tially so as to get the difference between the inducedvoltages in the secondaries. The other types are calledthe probe and bobbin coils and are not describedhere.Test procedure and resultsDescription of the unitThe eddy current wire tester (Fig. 1) model FW 201supplied by Messrs Magnaflux Corporation, has beenused in our preliminary studies on zircaloy wire. Theunit has a fixed frequency oscillator tuned to 625 ke/s.The feed through differential coil set having the two1 89Chandramorrli et al : Elily current tectine of zircalor wires4(a) Tent wire with 10 thou diameter holecoils in different axes is plugged into the unit. Specialteflon guides have been made to suit the zircaloywires under investigation. The unit has DC outputfor DC recorder attachment and a Dynograph stripchart recorder has been used to record the traces. Anoscilloscope is provided with the equipment for avisual indication of the defects. The X-plates areenergized by the sweep generator at the same frequencyas that of the alternating current through the pri-maries of the test coil. The Y-plates are fed with then4(b) Test wire with 24 thou diameter hole1905 Test wire with a butt geld joint (at a reduced sensitivity)differential output from the secondaries . Balancingcircuits are provided in the unit both resistive andinducive so as to make the differential secondary out-put zero when the wire specimen in both coils are"identical".Defect standardBefore proceeding with the routine testing of wires,it is imperative that the eddy current unit is standardisedusing wires with simulated defects . The normal defectstandards suggested are drilled holes, surface notchesand welded and filed joints. The unit can be standard-ised with any specified defect standard and the thres-hold level can then be adjusted such that, in theactual wire testing any defect indication more thanthat of the defect standard can be made to triggeran alarm . The indications can also be continuouslyrecorded while the wire is pushed through the coils.In some units there will be a provision to actualspraying of paint at the defective portion of theobject under test.Prelitninarjv investigations0-4 mm dia. hole drilled through and through andperpendicular to the axis of the wire, 04 mm dia. holedrilled half way, good electron beam welded and filedjoint and a fine notch cut on the surface have beenmade as defect standards for 1.25 mm dia. wire. 0.4 nomdia. hole drilled half way at one place and drilledthrough completely have been made as defect standardsfor 1.63 mm dia. wire.The eddy current unit is put on and is allowed forstabilization. Proper connections are made for therecorder and the recorder also is put on. Through onem^III111IMir^^llss^}Iw 11^1111A^ q ° 11 71 7Chandramouli et al.: Eddy current testing of zircaley wiresf- 9I. Trace of defect corresponding'to Fig. 4(b)2. „ Fig. 4(a)3. Fig. S6 Tipical recorder chartof the coils, a round defect-free) sample of 1.25 mmwire is kept and through the other defect standard ispassed. Initially a sound portion of the detect standardis balanced against the other wire.[hen the artificial defects are passed through the secondcoil and the corresponding oscilloscope and recorderindications are noted. Figs. 2 to 5 show oscilloscopeindications for the wire with and without variousdefects. The splitting as well as the inclination of thehorizontal line can be observed in these defects. It mayalso be noted that the oscilloscope indication obtainedwhile passing the welded joint through the coil, isactually much more and for representation in thefigure the sensitivity had to be cut down by twosteps as compared to the other type of defects. Simi-larly the test conditions are standardized for I.65 mmdia. wire. Typical recorder chart is as given in Fig. 6.It may be noted that the pattern of indications onoscilloscope are different for different types of defects.In a normal production line, wire is fabricated startingfrom a given ingot through different main stages ofprocessing such as extruding and/or rolling into smallersections which are further drawn to the specific sizes.In such cases like the ones we contemplate, thestandard flaws that can be expected are dimensionalvariations and defects like cracks, voids or seams. Sincethe type of coils that we are using are of differentialtype, other variables (that could affect eddy currentdistribution) such as hardness, composition, heat treat-ment conditions, are balanced out. As such it will beeasier to a good extent to resolve the type of defectsdepending on the type of patterns indicated on theoscilloscope. The magnitude of the defect can only berecorded on the chart and it is not possible to differ-entiate the type of defect from recording trace.For production line inspection, the zirclaoy wire tobe inspected will be mechanically driven through thetest coil from one intake spool to the other spool,keeping a short length of standard acceptable type ofwire (same dimension as that of the test wire is expectedto be) in the other test coil. Good reproducible resultshave been obtained when the speed of the testing is of theo.der 50 mm/sec and there is a scope for improvement.Fddy current testing on other materialsIt may be of interest to note that eddy current testingtechniques are widely used for evaluation of non-ferrousproducts like copper or brass wires and other sections.The units selected shall have an appropriate frequencydepending upon variables like size, type and resistivityof the material.These techniques are widely employed in other coun-tries even to ferrous materials, incorporating D.C.saturation in addition to the normal requirements toovercome the effects of magnetic permeability of thetest objects. Units such as Magnatest eddy currentsystems are commercially available for the detection ofseams in cold drawn carbon or alloy steel wire ; forautomatic testing of hot-rolled cars of 25 mm dia. andlarger to detect longitudinal defects such as seams, laps,stringers, slivers, scabs, etc.Multifrequency, and phase sensitive instruments arealso available for special jobs like coating or platingthickness measurments, sorting non-ferrous or ferrousmaterials, checking the heat treatment conditions.SummaryZircaloy wires, that are used as a spacer and bearingpads on the fuel pencils in the fabrication of RAPP fuelbundles, will have to undergo one hundred per cent non-destructive examination. The wires should be free fromdefects like voids, cracks, seams, etc. besides meetingthe dimensional and other requirements. The eddycurrent inspection techniques are presently the best andappropriate ones over the other conventional non-destructive testing techniques such as radiography,ultrasonics. The general principle involved in the testingand the equipment employed for the test are describedwith the test results along with the photographs andrecording traces wherever necessary..AcknowledgementsAuthors are grateful to Dr N. Kondal Rao, Head,Atomic Fuels Division, Bhabha Atomic Research Centre,for his keen interest in the work, helpful suggestionsand for his kind consent to publish this report.References1. ASTM 'Symposium on Non-destructive Testing in the field ofNuclear Energy' : Special Technical Publication No. 223, 1967,2. Balaramamoorthy, K. and Chandramouli, V. A. : Non-destructive Testing in the Nuclear Fuel Fabrication, Paperpresented at Eighth ISI Convention, 1963.3. McGonnagle. W. J.: 'Non-destructive Testing', McGraw-HillBook Co., 1961.191Chandramouli et al.: Eddy current testing of zircaloy wiresDiscussionsMr U. P. Mullick (Institute of Consulting Engineers , Mr V. A. Chandramouli '.Author) : Our Eddy CurrentCalcutta) : From which country was obtained the Eddy wire tester model FW 201, has been manufactured byCurrent wire tester model FW 201 supplied by M's. Institut Dr Forster, Reutlingen , West Germany, forMagnadux Corporation ? Maguaflux Corporation, Chicago, Illinois, U.S.A.192

Eddy Current Testing of Zirconium Wires

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Eddy Current Testing of Zirconium Wires

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