EPR: Steam generator tube rupture analysis in Finland and in France

S. Israel,
Eurosafe 2006, Paris, 13-14 novembre 2006,
Rapport DSR 144

Different requirements between Finland and France lead EPR designer to define different features (system or action) for management of accidents on Olkiluoto 3 EPR that is under-construction in Finland compared to Flamanville 3 EPR that is foreseen in France. One of these differences concerns the management of Steam Generator Tube Rupture since no primary coolant (liquid and steam) release to the environment is allowed in Finland dislike in France where primary steam releases are not forbidden. This leads to define on Finnish EPR a strategy that anticipates mitigation action compared to French EPR and that only uses the unaffected steam generators. This strategy is intended to reduce the release to the environment. IRSN has analysed an other aspect of the Steam Generator Tube Rupture: the backflow (flow of unborated water from steam generator to the primary circuit). Indeed, if the Reactor Coolant Pumps have been shut down, the creation an unborated water plug because of the backflow could lead to reactivity accident in case of Reactor Coolant Pump restart. IRSN analysis shows that, using the current Olkiluoto 3 SGTR mitigation strategy and very penalizing assumptions, the amount of unborated water transferred to the primary circuit on the Finnish EPR could be higher than on the French EPR in the long term. Discussions are going on between STUK and TVO to finalize the SGTR strategy so that both releases into the environment and risk of backflow can be minimized.