Electrochemical potential monitoring in the feedwater at the St. Lucie 2 PWR

Reducing conditions are necessary in nuclear steam generators to minimize local corrosion phenomena, e.g., stress corrosion cracking. To achieve this goal, the electrochemical potential (ECP) should be maintained below the threshold at which these corrosion mechanisms occur. Ideally, in-situ ECP measurements in the steam generator would allow for monitoring and controlling oxidant ingress to maintain the desired reducing conditions. Unfortunately such measurements are difficult to implement. However, oxidant ingress can be monitored and controlled via ECP measurements in the high pressure feedwater. A feedwater ECP monitoring system was installed at the St. Lucie 2 PWR Florida Power and Light Company to assess this approach. Variations in feedwater ECP and corrosion product transport were monitored. As chemistry was varied, Alloy 600 ECP was shown to depend on the feedwater hydrazine/condensate dissolved oxygen ratio. Operation at hydrazine/oxygen ratio above 6 was necessary to maintain a low ECP