EXPERIENCE WITH HIGH EFFICIENCY ULTRASONIC FUEL CLEANING USING A SIDE-STREAM SAMPLING SYSTEM AT RINGHALS UNIT 4

摘要

Through mid-2014, 22 campaigns of High-Efficiency Ultrasonic Fuel Cleaning (HEUFC) have been performed worldwide, two of them in Ringhals Unit 4 during the first two outages following steam generator replacement (SGR). The main purpose of introducing HEUFC at Ringhals was to reduce the risk of crud induced power shift (CIPS), which was expected to be higher than in prior cycles due to the SGR and a scheduled power uprate. Visual observations indicated that HEUFC removed greater than 90 percent of the deposits that remained on the fuel assemblies following a standard shutdown procedure. Cleaning during sequential outages may significantly improve the understanding of material releases to reactor coolant and improve cycle-specific evaluations and mass balances. Over time continued HEUFC may also reduce the radiation field build up, mainly by removing long-lived nuclides and fissile material. These advantages have led to the decision to introduce frequent fuel cleaning in all Ringhals PWR units during refuelling outages beginning in 2015. During both of the Ringhals HEUFC campaigns, a first-of-its-kind sampling system was used to sample the material removed from the fuel assemblies during cleaning. The objectives of the sampling were to improve evaluation of HEUFC effectiveness, investigate the possible replacement of crud scrape sampling, understand possible source term reduction through frequent HEUFC campaigns, and to estimate material release rates from the new SG tubing. Results indicate that the sampling system during fuel cleaning is more effective in some areas of crud analysis but may not replace crud scraping entirely. It is judged to be a better tool to calculate the total inventory of deposits across an average area between mixer grids (zone), the whole fuel assembly or the entire core. However, the sampling methodology cannot sample "worst case" deposits in small rod areas or at specific elevations, as crud scraping does.