Masked Eddy Current Probe for Detection of Rover Fuel Element Coating Cracks.

A masked probe has been designed to provide eddy current inspection of the protective coating quality on Rover fuel elements. The probe design and electronic circuitry for the inspection equipment are described. Typical results of inspections are shown and discussed. INTRODUCTION In the development of Rover fuel elements, it became desirable to know the condition (number and size of cracks) of the eiement coating. The elements being investigated are six-sided rods about 52 inches in length and have 19 coolant channels that are about 0.1 inch in diameter. They are generally composed of a graphitemetallic carbide compound which is formed, cured, and graphitized. The interior of the coolant channels is coated with vapor deposited niobium or zirconium carbide to provide a protective layer. This report covers the efforts of the US and EMT Section of Group GMX-1 in developing an eddy current test to do this coating evaluation. The application of this test is only applicable to circumferential cracks in the coating. METHOD It was felt that the circumferential crack frequency in "good" elements would be less than 20 cracks per inch. The first attempt made to count these cracks was tc make a physically narrow coil of the standard bore proi,? configuration which is an insertion probe with the coil ;ixis parallel with the bore axis. Several coils were built but did not give satisfactory results; they lacked the desired resolution, and were of such low impedance that the frequency required to develop a reasonable signal was very high in the order of 25 to 50 MHz. Also, at these frequencies any movement of the cables caused the capacity variation effects to be very large as compared to the desired signal. In April 1969, Argonne National Laboratory released a report describing work rione with conductive masked probes in pulsed eddy systems.' I: was felt that the masking could concentrate the field into a narrow band such that individual coating cracks could be resolved if two adjacent coat cracks were separated by about 0.050 inch. The GMX-1 eddy current systems used in the past have been CW and it was decided to attempt to use a CW system on this test. The mask designed is shown la Fig. 1. The window shown is 0.005 inch was made the following widths: 0.020 inch, 0.005 inch, ( 1 ) C. W. Cox and C. J. Renken, The Use of the Conducting Mask in Pulsed Electromagnetic Testing, Argonne National Laboratory Report ANL-7172, April 1969. 084* DIA STAINLESS STEEL HYPODERMIC TUBING 005' SLIT 9 n r BRASS MASK \ TIP. BRASS OR AIR CORE COIL ,qo3"WIRE oa*ID " FORMED TO ID OF MASK ABPUF1ER 1 " tWIO lk COIL FILTCH S M J HIOH • A M 7ILTDI ADJUST 10-9) %-r-r' RECOROEP '~ 1