Monitoring of LISS Nozzle Proximity to CANDU® Fuel Channels Using the Eddy Current Gap Probe

Fuel channels in CANDU® (CANadian Deuterium Uranium) nuclear reactors consist of two non-concentric tubes; an inner pressure tube (PT) and a larger diameter calandria tube (CT). The gas annulus space between the 6 m long tubes, known as the gap, is monitored from within the PT using a transmit-receive eddy current probe that can detect changes in the environment using electromagnetic induction according to Faraday’s Law. Accurate measurements are required to ensure that contact between PT and CT is not imminent. Fuel channels may also pass over perpendicularly oriented tubes (nozzles) that are part of the liquid injection shutdown system (LISS). The proximity of LISS nozzles not only compromises gap measurement, but flow-induced vibrations from within the moderator could lead to CT deformation and/or fretting at LISS-CT contact. Investigation of the eddy current based gap probe as a tool to measure proximity of LISS nozzles was carried out experimentally. Eddy current response as a function of LISS-PT proximity, the distance between the PT outer diameter and LISS nozzle outer diameter, was determined. When PT wall thickness, PT resistivity, probe lift-off, and PT-CT gap variations spanned typical values found in-reactor this dependence could be used to determine the LISS-PT proximity up to 20 mm with sub-millimeter accuracy. This method has the potential to provide LISS-CT proximity using existing gap measurement data. Obtaining LISS nozzle proximity at multiple inspection intervals could be used to provide an estimate of the time to LISS-CT contact, and thereby, provide a means of optimizing maintenance schedules.