Still nobody knows what actually happened in reactor4, what caused the explosion.
Nuclear accident investigation committee of the government commented that the hydrogen explosion of reactor4 was caused by both of hydrogen gas from heated SFP4 and reactor3. (cf. Investigation committee “there is a possibility that the earthquake damaged equipment”)
However, Tepco published the report to deny the possibility that hydrogen gas was emitted from SFP4 on 7/24/2012.
Japanese version of the report was not published and they only emphasize that there was enough amount of water in SFP4.
Nothing can be more irrelevant for the criticism of the investigation committee. They pointed out the temperature of the water, not the water level of SFP4.
However, according to the study of JAEA or Tokyo university, when water is so hot that steam air bubble comes up, the amount of hydrogen significantly increased.
Considering the volume of reactor4 building, 13.7m3 of hydrogen gas is enough to make detonating gas.
18.1m3 of hydrogen gas was probably produced within a day from the amount of water in SFP4, which is enough to make detonating gas.
It is therefore rational to think the hydrogen gas from both of reactor3 and SFP4 caused explosion, but quantitative assessment is still impossible.
The report of Tepco’s own shows the water temperature was between 80 ~ 90℃ before the explosion of reactor4.
↓ Tepco’s report published on 7/24/2012.