So, at least part of whatever is steaming on the operating floor of Reactor 3 is coming from inside the Containment Vessel after all.
Here we go again?
From TEPCO's email notice for the press on Reactor 3 operating floor steam, No.14 (latest), 7/26/2013:
Mechanism of steam generation
In addition to rainwater seeped in from a gap in the shield plug and warmed by the Containment Vessel head, there is an observable difference(3 m3/hr) between the amount of nitrogen gas being injected into the Reactor Pressure Vessel and Containment Vessel (16 m3/hr) and the amount of nitrogen gas being extracted (about 13 m3/hr), and it is possible that this gaseous body (3 m3/hr) containing enough water vapor is leaking through the Containment Vessel head. We presume that when the vapor leaks through the gap in the shield plug onto the 5th floor of the reactor building, it is chilled by the air which is relatively colder than the vapor, and is visualized as steam.
Nitrogen injection into the Containment Vessel of Reactor 3 has been on-going since July 14, 2011. Start of nitrogen injection into Reactor 3 on July 14, 2011, only four months after the explosion, marked the completion of "step 1" of the famous "roadmap" to decommissioning.
I wonder whether the difference in the amount of nitrogen going in and coming out has always existed. As per TEPCO's open admission (that no one apparently noticed or cared) during the press conference that the steam rising from Reactor 3 in March 2011 was from inside the Containment Vessel, it looks safe to assume that the difference, if any, has been leaking from inside the Containment Vessel all along.
Whether the leak is through the CV head, as TEPCO says, or through somewhere else (like the open equipment hatch on the 1st floor) remains to be seen, I think.
Now that TEPCO has figured out why the steam is rising, the company will resume the work to clear the debris from the Reactor 3 operating floor to "test the validity of the evaluation", according to the notice.
By the way, the infrared thermography taken on October 14, 2011 seems to show the same location where the steam is rising now. The images use different temperature scales, but in both images the location temperature is between 30 and 35 degrees Celsius. It is just as hot today as it was in October 2011.
Infrared thermography of the location of the steam on July 24, 2013, from TEPCO (labels are mine):
Infrared thermography of the same location (or very close to it), on October 14, 2011, from TEPCO (labels are mine, and the white circle indicating the location):