A Method for Predicting CT-LIN Contact Using CANDU 6 Inspection Results

Abstract

In the Point Lepreau Generating Station (PLGS), the Liquid Injection Nozzle (LIN) was installed 120 mm below the Calandria Tube (CT) above it. Since the creep sag rate of the fuel channel exceeds that of the LIN, CT-LIN contact may be possible. CT-LIN contact is considered unacceptable since it will result in fretting damage to the CT. Because of concerns about potential CT-LIN contact, various measurements of CT-LIN gap and CT and LIN elevations have been performed in different CANDU 6 reactors. A method for predicting time-to-contact of the CT with the LIN, using inspection data, was developed by CPUS for PLGS in 2005. The method involves using updated CDEPTH fuel channel models to predict CT creep sag rates and using LIN creep sag rates derived from CT-LIN gap measurements. CT-LIN time-to-contact predictions using this method indicated no contact before 200 kEFPH, resulting in the decision to cancel further CT-LIN inspections in PLGS. The same assessment was undertaken for Gentilly-2 (G-2) but is awaiting further inspection results. For CANDU 6 reactors without CT-LIN gap measurements, it is proposed that the LIN initial sag and sag rate deduced from measurements in Wolsong 1 and Wolsong 4 can be used to model the sag behaviour of the LIN. CT-LIN contact predictions can then be performed for any CANDU 6 reactor using: 1. Updated CDEPTH models to predict CT sag versus time in-service 2. Sag measurements from Wolsong Units 1 and 4 to determine LIN sag versus time in-service, or if available, CT-LIN gap measurements. The development of the above method is outlined in this paper. Examples of CT-LIN time-to-contact predictions are presented for PLGS and G-2.