Increased Concern for Degradation of Upper Steam Drum Internals Leading to Monitoring and Modifications Considerations

Pressurized water reactor plant steam generator upper internals have been monitored in accordance with accepted industry practice and on schedules based on the history of good inspection results with isolated cases of failures in a small percentage of plants worldwide. The current practice is, in general, to perform a small sample qualitative visual inspection. The causal factors for the historical events are indicators but, in general, not directly applicable to the current rationale for increased quantitative surveillance. There is an emerging trend toward increased indications of degradation with identifiable causal factors similar to but distinct from the traditional failure mechanisms. Primary and secondary moisture separation components and feedwater ring degradation is increasing as calculated by traditional models and according to qualitative and quantitative inspection results. Two established objective criteria for the performance functions for the steam drum components under consideration are: moisture carryover < 0.25% and material loss of 62.5 mils from nominal component thickness of 0.25 inches. These traditional acceptance criteria can be re-established based on an engineering evaluation of failure modes and effects. Two changes that have increased the concern for unacceptable degradation of steam drum components in recent years are 1) the chemistry improvements to reduce flow assisted corrosion (FAC) in the feed train piping with the concomitant reduction in entrained iron transport into steam generators, and 2) the increase in water-steam flow velocities that result from power uprates. The reduction of entrained particulate into the steam generators is positive with respect to fewer corrodents introduced but may have an unintended and previously under-analyzed effect of the transport of entrained particulate into the upper regions of the tube bundle and into the moisture separation components. The first indications of degraded moisture separator components have begun to emerge and warrant consideration of an increased surveillance program for selected plants. This paper discusses how recent changes in the operating parameters and secondary water chemistry control have created conditions for concern for exceeding the traditional moisture carryover criteria and erosion-corrosion of upper steam drum components, especially in light of extended life operation. Examples of recent developments in inspection results and considerations for improved materials, increased and improved surveillance techniques, and replacement strategies will be presented.