Flaw Detection and Characterization of Pressure Tubes from CANDU PHWR using Eddy Current Methods

This paper presents a new type of eddy current transducer, send-receiver type, using the rotating magnetic field method. The forward problem was solved, the experimental results obtained for detection of different types of degradation have proved the capability to emphasize blisters, and garter springs location and tilt as well as hydrogen content. INTRODUCTION The nondestructive testing of metallic tubing from heat exchangers, steam generators, fuel channels is made through eddy current method using inner transducers. In present, many types of eddy current transducers are used, one of them being analyzed in [1]. The aim of this transducers and control equipment development consists in the obtaining of better probability of detection for a imposed reliability coefficient. The perturbation factor which can affect the control results is represented by the totality of noises: vibration of transducer inside the tube, local modification of parameters, without exceed the tolerances imposed to chemical composition, elliptically and diameter modification of tubes, thermal fluctuations, and diverse deposits on tube’s surface due to chemical treatment of residual water, electromagnetic induction in transducer, electronic noises of equipments, etc [2]. To obtain a better probability of detection, the sensibility of transducers shall be big enough for all possible types and orientations of the defects that can appear at manufacturing and exploitation. In the same time, signal to noise ratio must be as better as is possible. 1. THE CONTROL OF PRESSURE TUBES FROM PHWR POWER PLANTS A critical part of pressurized heavy water reactor (PHWR) CANDU 600MW(e) type Nuclear Power Plant is the calandria tube made of austenitic steel, two integral end shields (also made of austenitic stainless steel) with carbon steel shielding balls each, horizontally penetrated by 380 lattice tubes, 380 Zircaloy-2.5 calandria tubes joining the lattice tubes at each position in the lattice and 380 fuel channel assemblies mounted within these lattice sites [3]. The fuel channel assemblies consist of: • Zirconium –2.5%Niobium alloy pressure tubes (6.3m long x 105mm nominal bore x 4.16mm minimum wall thickness to house fuel and pressurized D2O coolant) • AISI type 403 stainless end fittings, with type 410 stainless steel liners • 4 garter springs tube spacers to support each pressure tube within its calandria tube • positioning assemblies for each end fitting • shield plugs for every end fitting to minimize neutron leakage from the fuel channel and to provide axial support to the column of 12 fuel bundles • removable closure plugs to seal each end of the fuel channels and to enable access for refueling by the fuelling machine • Feeder connections. The calandria is filled with D2O, which moderates the fast neutrons, that allows chain reaction to take place. The heat in the fission reactor within the fuel is transferred to the pressurized D2O coolant, which is pumped through the fuel channels. The annular space between the pressure tube and calandria For more papers of this publication click: www.ndt.net/search/docs.php3?MainSource=70 6th International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized Components October 2007, Budapest, Hungary