HARD SLUDGE AND DENTING IN THE SECONDARY SIDE OF PWR STEAM GENERATORS

摘要

The gradual corrosion of PWR (Pressurized Water Reactor) feedwater, condensate and drain systems results in the inevitably liberation and transport of corrosion products. In addition, other contaminants and impurities are introduced with the makeup water, auxiliary feedwater and by condenser leaks. All these compounds can eventually be incorporated into the secondary side of the steam generators (SGs) and transformed into deposits on the SG surfaces, causing the degradation of the thermal performance of the plant and lowering its achievable electrical output. These deposits are found as scale on tube surfaces, blockages in tube support flow passages and sludge piles on the tube sheet and the tube support plate crevices. Among all these deposits, sludge piles consolidating and hardening on the tube sheet and the tube support plate crevices are especially detrimental due to tube deformation or denting that can promote stress corrosion cracking (SCC). Hard sludge accumulation on the tube sheet, tube denting and some SCC have been found in Spanish nuclear power plants with Siemens KWU 61W/D3 steam generators. In this work, we purpose a model for the tube denting production as consequence of the consolidation and hardening of the sludge piles accumulated on the tube sheet of the steam generator throughout data obtained from the more recent operating cycles of these plants i.e., chemical characterization of the sludge and denting evolution.