The post is for my own record and for those who missed the news in July this year about the steam rising from a gap near the shield plug on the operating floor of Reactor 3 at Fukushima I Nuclear Power Plant.
For more posts on the subject, go here.
0. The steam looks like this:
TEPCO's hypothesis from July is that it is part rainwater part leak from the Containment Vessel.
1. The steam was first noticed in July this year, and since then it has been observed numerous times when and after it rains in the geographical area where the plant is located in Fukushima Prefecture (Futaba-machi and Okuma-machi on the Pacific Ocean). It has probably been there since after the March 2011 explosion. Why wasn't the steam noticed earlier? Most likely because of piles of debris on the operating floor, which looked like this from the top, until September 2012 when TEPCO/Kajima started to remove the debris:
(March 24, 2011)
(Video taken on 11/5/2011)
2. Infrared images, taken on July 24 this year, from TEPCO (labels are mine), showing a warm spot with a red X (34.3 degrees Celsius, or 93.74 degrees Fahrenheit) right where the steam was (and still is, occasionally) rising:
3. The air dose rates (gamma radiation) on the operating floor of Reactor 3, particularly around the shield plug, are very high (the latest measurement from November and December this year, presented at the December 26, 2013 meeting to discuss progress of the "roadmap toward decommissioning" - PDF file in Japanese):
The upper numbers: air dose rate in millisievert/hour, at 5 meters off the floor
The lower numbers: localized surface rate in millisievert/hour, measured by a survey meter fitted with collimeter, at 50 centimeters off the floor
(I marked the location of the steam with a blue X.)
But it is more likely that the high air dose rates are not from the occasional weak steam from the spot as seen in the infrared images above, but from the contamination from the fallout in March 2011, from several vents including dry vent (releasing the highly radioactive gas inside the Containment Vessel to outside, without having it go through water to reduce radioactive materials) and the March 14, 2011 explosion of Reactor 3 and steam/smoke afterwards. The steam/smoke in March 2011, as TEPCO casually admitted during the regular press conference on July 24, 2013 (there was no follow-up question from the press), came from a breach in the Containment Vessel of Reactor 3, "as you all know".
As a piece of that evidence,
4. Nuclides analysis result of the radioactive materials in the air, July 25, 2013 shows radioactive cesium in the air where the steam is observed on the operating floor of Reactor 3 is two orders of magnitude lower than the density limit by the nuclear regulation, at 3.3E-05 Bq/cm3 (3.3x10^-5 Bq/cm3 or 0.000033 Bq/cm3, or 0.033 Bq/L, or 3 Bq/m3) for cesium-137, too low to account for several hundred millisieverts per hour radiation:
5. Reactor 3's Spent Fuel Pool is filled with debris big and small, but there are fuel assemblies inside the pool which are cooled with water (water temperature at 11.2 degrees Celsius as of 12/27/2013, from TEPCO's handout for the press.
Video taken in May 2011:
Video taken in September 2012:
Video taken in February 2013:
Again, photos of the Reactor 3 operating floor: (from TEPCO's photos and videos library) March 16, 2011:
March 24, 2011:
July 11, 2012:
September 21, 2012 (SFP, when TEPCO started removing the debris):
May 25, 2013:
October 10, 2013: