Measures to improve robustness of primary containment vessel and operation of filtered containment venting system based on the lessons from the containment leak at Fukushima Dai-ichi Nuclear Power Station Unit 2

Significant leakage of the primary containment vessel (PCV) occurred on March 15, 2011 at Unit 2 of the Fukushima Dai-ichi Nuclear Power Station, causing land contamination over a large area. From the radiation dose rate map of Unit 2 and the temperature trend of the PCV, degradation of the PCV top head flange gasket was identified as the cause of the leakage. The design set point of the rapture disc and operability of the valves in the hardened containment venting system were further identified as factors contributing to the leakage. Based on the lessons from the leakage, the gasket material was improved to have greater heat resistance in a steam environment, the PCV cooling system was diversified to improve reliability, and the operability and operation methods of the PCV venting were improved for greater robustness of the PCV in severe accidents. (author)