FIV Considerations for BWR Steam Dryers for EPU Operation

Abstract

Steam dryers in Boiling Water Reactors (BWRs), located in the upper steam dome of the reactor pressure vessel, are not pressure retaining components and are not designed and constructed to American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code Section III safety class. Although the steam dryers in BWRs perform no safety function, they must maintain the structural integrity in order to avoid the generation of loose parts that may adversely impact the capability of other plant equipment to perform their safety functions. The majority of licensees of many operating nuclear plants are applying for extended power uprate (EPU), which increases the thermal power output up to 20% above the original licensed thermal power (OLTP). Nuclear power plant components such as steam dryers can be subjected to strong fluctuating loads and can experience unexpected high cycle fatigue due to adverse flow effects while operating at EPU conditions. However, there are some unique challenges for operation of dryers at EPU conditions requiring special considerations to prevent fatigue damage from the effects of flow induced vibration (FIV). This paper examines the FIV considerations and margin recommendations for fatigue stresses due to many uncertainties in the prediction of the fluctuating pressure loading acting on the steam dryer and the structural analysis of the steam dryer, very limited bench marking of the methods, and operating experience with some dryer failures.