Conference Proceedings Paper
PROXIMITY MEASUREMENTS OF LISS NOZZLES TO CANDU FUEL CHANNELS USING THE EDDY CURRENT GAP PROBE
11th International Conference on CANDU® Maintenance and Nuclear Components - 2017 Oct. 01-04
P. Bennett (Queen's University/ RMC)
R. Underhill (Royal Military College)
J. Morelli (Queen's University)
T. Krause (RMC)
CANDU (CANadian Deuterium Uranium) nuclear reactors employ up to 400 horizontal fuel channels, each consisting of two initially concentric tubes. An outer calandria tube (CT) contains a smaller diameter pressure tube (PT) and the annulus space between the tubes, maintained by 4 spacers, is called the gap. Sag causes the tubes to become non-concentric and physical separation between the tubes ensures hydrides, which can lead to potential delayed hydride cracking of the PT, do not develop. Therefore, accurate measurements are required to ensure that contact between PT and CT is not imminent. The gap is monitored from within the PT using the gap probe, which is an eddy current based inspection probe operated at multiple frequencies. The proximity of remote perpendicular tubes, which run beneath the fuel channels, known as liquid injection shutdown system (LISS) nozzles, is a factor affecting gap measurement. The proximity of LISS nozzles confounds gap measurements and contact of the LISS nozzle could lead to fretting and deformation of the CT, due to moderator flow-induced vibrations in the LISS nozzle and CT. Proximity of LISS nozzle to CTs is currently measured optically and costs ~$1 million dollars per inspection campaign. Using gap probe measurements the effect of LISS nozzle proximity to the fuel channel was investigated experimentally and a relationship between eddy current response and LISS- PT proximity was obtained. When PT wall thickness, probe lift-off and PT resistivity variations are not present the observed relationship can be used to predict the LISS-PT proximity up to 25 mm with sub-millimetre accuracy. This method has the potential to provide LISS-CT proximity using existing gap measurement data. Obtaining the LISS nozzle proximity at multiple inspection intervals can provide an estimate of the time to LISS-CT contact, which could be used to optimize remediation schedules.
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