Model for Predicting LISS Nozzle Proximity to CANDU Fuel Channels Using Eddy Current Measurements

Horizontally oriented fuel channels (FCs) in CANadian Deuterium Uranium (CANDU) nuclear reactors are composed of an inner pressure tube (PT), surrounded by a larger diameter calandria tube (CT). A transmit-receive eddy current probe is used to measure gap between the two tubes from within the PT. Reactors also contain liquid injection shutdown system (LISS) nozzles, which are perpendicular to and beneath some rows of FCs. Due to fuel bundle weight and high operating temperatures (inlineFormula 2 -LpyC), FCs sag over their lifespan, approaching the LISS nozzles. In the case of LISS-CT contact, there is potential for CT damage. LISS nozzles that are close to FCs also compromise eddy current PT-CT gap measurements due to sensitivity of the probe to the additional conducting structure. Finite element method (FEM) software was used to model eddy current probe response from a nearby LISS nozzle. FEM simulation results, calibrated on only the gap response, were found to be in good agreement with previously collected experimental data. The FEM model was used to simulate a training dataset for a predictive LISS-PT algorithm based on a modified Gram-Schmidt multiple regression. This algorithm displayed excellent agreement with the same algorithm that was trained using experimental data, both having comparable root-mean-square error (RMSE) when applied to the same experimental validation data. The FEM model demonstrates the potential for examining historical in-reactor gap data, where calibrations that include the presence of a nearby LISS nozzle are not available, as well as providing cost savings by augmenting training data sets with simulated results.