– Nuclear Regulation Authority “Reactor4 possibly had 2~3 strong explosions in 311? (Fukushima Diary, Sep 19, 2013):
http://www.nsr.go.jp/committee/yuushikisya/jiko_bunseki/data/0003_03.pdf
On 8/30/2013, Nuclear Regulation Authority released their report about reactor4 investigation conducted from this July to August.
In this report, they concluded reactor4 experienced at least 2 strong explosions in 311.
According to their report, the ceiling and floor of S-W part of the 4th floor had a severe damage. Also, the floor of N-W part of the 3rd floor had a serious damage. The investigating team assumed there were 2 major explosion around these areas.
In addition to it, the 5th floor lost the walls on theS, E and W and the roof. There might have been another explosion there according to their report.