The Dresden Units 2 and 3 have a history of hydraulic transients occurring in the Isolation Condenser System piping. One of the most significant transients occurred following a spurious trip and isolation of the Isolation Condenser when operators restarted the system. Piping support damage occurred on supports in the condensate return line. A detailed RELAP5 M3 analysis of the Isolation Condenser and reactor system was performed to study the thermal hydraulic behavior of the system through system initiation and subsequently to system isolation and restart. Several post isolation restart times were analyzed to provide guidance for operating procedure changes. Isolation valve operation sequence was also considered. Piping segment force time histories in the condensate return line were generated to evaluate the pipe loadings. The loads computed compared favorably with the historical damage observed and demonstrated that allowing sufficient time for post isolation void transients to complete will allow system restart with minimal loading on the system.

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