from the hydrogen gas that came from Reactor 3 when they did the vent of Reactor 3. TEPCO says in the handout for the press on November 10 that the gas, instead of going up the exhaust stack (oh no here we go again, the stack...) into the air, went to Reactor 4's air duct on the 4th floor.
To prove their case, TEPCO released the photographs on November 10 taken on the 3rd, 4th, and 5th floors of Reactor 4 on November 8. The heaviest damage is on the 4th floor with totally destroyed and disfigured air duct pipes. The rebar of the floor of the 5th floor is lifted upwards, while the floor of the 4th floor is pushed downward. It seems plausible enough that an explosion (there may be more than one, judging by the number of "fires" that Reactor had on March 15 and 16) took place on the 4th floor.
Photographs taken inside Reactor 4 before were rather neat and orderly. But this set of photographs shows the inside to be just as badly destroyed as Reactor 3. Or for that matter, as the top floor of Reactor 1 as seen in the video I posted before.
5th floor: Rebar blown to vertical position
5th floor: Rebar of the floor lifted upward
5th floor: Floor lifted upward
4th floor: Wreckage of air duct
4th floor: Wreckage of air duct, on the floor
4th floor: Floor pushed downward (could be seismic damage?)
3rd floor: Wreckage of air duct
For the complete set of photos, go to TEPCO's "Photos for Press" page, here.
The sequence of the explosive events in Reactors 3 and 4 is this, then:
TEPCO did the vent of Reactor 3 at 9:20AM on March 13. The hydrogen gas supposed to have flowed into Reactor 4 through the air duct, to the 4th floor.
Reactor 3 blew up at 11:01AM on March 14. The duct connecting Reactor 3 to the exhaust stack (shared by Reactors 3 and 4) was destroyed in the explosion and disconnected from the stack.
After nearly 2 days after the hydrogen gas from Reactor 3, as the result of the vent, had filled the air duct on the 4th floor of Reactor 4, Reactor 4 had a hydrogen explosion at 6:12AM on March 15.
The problems I have are these:
Why did the hydrogen gas from Reactor 3 go to Reactor 4 to begin with, instead of out to the stack and into the atmosphere?
Why didn't Reactor 4 blow up when Reactor 3 did, if it was filled with hydrogen gas? Instead, it waited another day till it finally blew.
It does look like a hydrogen explosion, as I don't see any evidence of high temperature. The Spent Fuel Pool of Reactor 4 looks pretty much intact.
Any insight, readers?