Evaluation of plant behavior during the accident at Fukushima Daiichi Nuclear Power Station
MIYATA Koichi1, and NISHINO Shoichiro2
1. Tokyo Electric Power Company, Chiyoda, Tokyo, 100-8560 Japan (Miyata.Koichi@tepco.co.jp)
2. Tokyo Electric Power Company, Chiyoda, Tokyo, 100-8560 Japan (sho.nishino@tepco.co.jp)
Abstract: The Great East Japan Earthquake that occurred at 14:46 on March 11, 2011, caused all off-site power to be lost at Fukushima Daiichi Nuclear Power Station (NPS), but the emergency diesel generators (EDGs) started up, and the electric power necessary for reactor safety was maintained. At 15:27 the first tsunami was observed, which far exceeded the design basis of the power plant and caused the loss of almost all safety functions of core cooling and containment. As a result, the reactor core was damaged in Units 1 to 3, and hydrogen explosions destroyed the reactor buildings (R/Bs) of Units 1, 3 and 4. Furthermore, since the spent fuel pool (SFP) cooling was also lost, external water was injected into the SFPs.
In Units 1 and 3, since depressurization of primary containment vessel (PCV) through the vent from suppression chamber (S/C) was completed, release of radioactive materials was controlled. On the other hand the PCV vent was not successful in Unit 2. Monitoring data rose sharply before noon on March 15 when Unit 2 PCV pressure dropped considerably and steam was seen coming from Unit 2 R/B. Winds blowing toward the north-northwest direction prevailed that day and rain fell in that direction at night, so it is likely that the contamination in high contamination zones around the plant resulted from a release of radioactive materials from Unit 2 R/B on March 15. Considering the field surveys and the design of the PCV, it is expected that the leak might have occurred at the seal of the PCV top head flange. In addition, as a result of evaluating the release of radioactive materials, the vent operations and the explosions at the R/Bs were not considered a major cause of contamination.
Keyword: core damage; MAAP; hydrogen explosion; release of radioactive materials |