(Make sure you watch the vids TEPCO took on April 15. Quite an explosion on Reactor 4, for sure.)
Duh. But that's what Yomiuri Shinbun wonders aloud in its article (in Japanese, see the bottom of this post; 1:34AM JST 4/17/2011).
And why does this newspaper wonder NOW? Because, apparently, TEPCO released the side-view picture of the Reactor 4 building. After having seen the aerial photos and videos of the Reactor 4 building with the roof and side panels blown off and the top floor (the 5th floor) in total mess and chaos, Yomiuri is claiming it didn't occurred to them until now: Oh, it may have been a hydrogen explosion, perhaps?
Anyone but the Japanese had probably guessed long time ago that it was a hydrogen explosion, and in the case of the Reactor 4, hydrogen could only come from the Spent Fuel Pool (there was no fuel in the Reactor Pressure Vessel).
To the defense of the gullible Japanese, this is what TEPCO still says (from the English press release, TEPCO's translation) about the Reactor 4. There's no mention of hydrogen:
-At approximately 6:00 am on March 15th, we confirmed the explosive sound and the sustained damage around the 5th floor rooftop area of the Nuclear Reactor Building.
-At this moment, we do not consider any reactor coolant leakage inside the reactor happened.
TEPCO's Japanese press release (the authoritative version) is ever slightly different on the first point, as it says:
- At approximately 6:00AM on March 15th, there was a loud noise, and we confirmed a damage near the rooftop of the Reactor Building 5th floor.
It is the same wording as the press release on March 15 after this "loud noise" was heard.
Now, let's see what the Japanese MSMs were saying about this "loud noise" on March 15. They did report it, and some, including Yomiuri, even quoted experts saying it could be a hydrogen explosion from the exposed fuel rods in the Spent Fuel Pool.
But then they very quickly moved on, and instead of further pursuing this hydrogen explosion aspect, they latched onto the fire that broke out later that day and the next day, and reported extensively about "two holes on the northwest side of the building". They also latched on quickly to the Self Defense Force chief's comment that there was water in the Reactor 4 Spent Fuel Pool - therefore, no fuel rods were exposed, and so no hydrogen explosion was possible.
So, after a month, it is news (again) to a Yomiuri writer/reporter that there may have been a hydrogen explosion.
The media is just as bad as the government, who "just didn't feel like announcing" the breached Containment Vessel and the core melt down in the Reactor 1 on April 12.
The Yomiuri article on April 17 that it for the first time speculates it may have been an hydrogen explosion in the Reactor 4: