BWR Steam Dryer Issues and Lessons Learned Related to Flow Induced Vibration

Abstract

Steam dryers in Boiling Water Reactors, located in the upper steam dome of the reactor pressure vessel, are not pressure retaining components and are not designed and constructed to the provisions of Section III of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code. As such, these components do not correspond to any specific safety class referenced in the Code. 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. Therefore guidance from Section III of the ASME Code is utilized in the design and fabrication of replacement dryers as well as for design modifications of the existing dryers for extended power uprates. The majority of licensees of operating nuclear plants are applying for EPU, which generally increases the thermal power output to 20% above the original licensed thermal power. 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 related to steam dryer operation at EPU conditions requiring special considerations to prevent fatigue damage from the effects of flow induced vibration. This paper examines the issues and lessons learned related to FIV considerations during EPU reviews of BWR steam dryers.