Prediction of SG tube support plate flow area blockage from thermal-hydraulic analysis

Recently, some nuclear power plants have experienced hydrodynamic instability in steam generators. Instability, if present in the pressurized water reactor steam generator, will result in periodic oscillation of the water level, steam flow, feedwater flow and flow through the circulation loop. In this instability analysis, the major parameters are power level and tube support plate (TSP) flow area. The threshold power is generated with TSP flow area variations. The current method to estimate this blockage rate is visual inspection of the SG interior. In order to improve this method, we focus on SG wide range (WR) level measurements. Recently SG WR level measurements are increasing even though narrow range level maintains constant. To understand this phenomenon, several causes which affect the SG WR level measurements are evaluated. The major reason of this phenomenon is analyzed that the SG downcomer flow rate decreases due to blockage of the TSP flow area. To quantify this effect, the circulation ratio is calculated according to change of the TSP flow area. Most of the pressure drops affecting the SG WR level are then evaluated. Level measurement sensor drift analyses are performed to confirm that this level variance derives from system characteristics rather than from sensor drift. Finally, the blockage rates of the TSP flow area are generated using SG WR level measurements. (author)