Gentilly-2 Full-Power Operation: History and Future Challenges

Abstract

The Gentilly-2 nuclear generating station performance has been affected, in the recent past, by the reduced safety margins resulting from heat transport aging mechanisms. Margins provided at the design of the CANDU-6 stations, to cater for the in-service aging degradation that was expected to occur, have eroded to a point where remedial actions needed to be taken in order to continue operation at full power. Steam generator fouling was originally considered by plant designers as the single most important aging mechanism affecting safety margins in CANDU PHW reactors. Changes were expected for other heat transport components such as that resulting from pipe roughness degradation and pressure tube diametral creep; however these were not deemed to be unduly detrimental to safety margins and were not explicitly considered in the safety margins determined at the design of the stations. Monitoring of Gentilly-2 plant operating data has indicated that these aging mechanisms significantly affect the heat transport system hydraulic characteristic. As a consequence, effective Regional OverPower (ROP) setpoints have been reduced in order to mitigate the impact of these aging mechanisms on safety margins. Also a number of actions have been taken to restore safety margins and maximize operating reactor power; these actions include replacement of the steam generator bolted divider plates by the welded design, reduction of secondary heat transport steam pressure, refinement in channel selection rules for refueling and steam generator primary side cleaning using Siemens-SIVABLAST. In spite of the success of these actions, safety margins are expected to be further eroded by the recently identified aging mechanism such that operation at full power may no longer be possible in the near future. This paper summarizes the evolution of Gentilly-2 data since first commissioning of Gentilly-2 and presents the relative contribution of the various aging mechanisms which impact on the safe operating envelope. Challenges to the future full power operation of the Gentilly-2 nuclear generating station are identified and avenues are identified for possible corrective actions which could be taken to restore full power operation or to minimize production losses.