Overview of the Doel 3 and Tihange 2 Reactor Pressure Vessel Safety Cases

Abstract

During the summer outages of 2012, large numbers of indications were found inside the shell material of the Doel 3 and Tihange 2 reactor pressure vessels (RPV). Therefore both plants remained in cold shutdown with their core unloaded. A series of examinations, tests and inspections were performed leading to the conclusion that the indications are hydrogen flakes and that they do not affect the structural integrity of the RPVs, regardless the operating mode, transient or accident condition. All this was documented in the Safety Case Reports issued in December 2012 and the Safety Case Addenda issued in April 2013. Based on those reports, the Belgian Federal Agency for Nuclear Control (FANC) authorized the restart of both units that went back on-line in June 2013. In parallel, a number of requirements from the FANC were addressed such as the qualification of the applied Ultrasonic Testing (UT) procedure, and mechanical testing of irradiated specimens containing hydrogen flakes. The preliminary tests showed unexpected results regarding the shift in RTNDT (Reference Temperature for Nil Ductility Transition) under irradiation that could not confirm the hypotheses considered in the initial Safety Case Reports. Therefore, the Licensee Electrabel decided to shut down both plants immediately. In order to fully address this concern, the material test programme was extended including several RPV materials and covering additional irradiation campaigns. This led to a modification of the irradiation embrittlement trend curves considered in the structural integrity analysis. In addition, the qualification of the UT procedure led to an updated cartography of the flakes. The structural integrity assessments of both RPVs were revised accordingly. The final Safety Case Reports, confirming the fitness-for-continued operation of both RPVs, were submitted to the FANC in October 2015. FANC allowed restart of both units on November 17th, 2015. The paper gives a historical overview of the Doel 3 and Tihange 2 RPV Safety Cases and explains how the roadmap was built in order to demonstrate the RPV’s structural integrity in the presence of hydrogen flakes.