I happened on this 25-minute video in TEPCO's video archive (Japanese), instead of the usual Photos and Videos Library. There is no document explaining the purpose of the video or the findings from the video.
It focuses on the heavily-damaged northwest corner of the Reactor 3 operating floor, as you have seen in this aerial view (from TEPCO's photos and videos library, 2/14/2014, red circle added, click to enlarge):
The video camera dips below the shredded metal bars and concrete of the operating floor:
In the video, the section that is covered with metal sheets is where the Spent Fuel Pool is. TEPCO plans to construct a structure over the Reactor 3 building to install the crane and the fuel handling machine to remove the spent fuel assemblies. This video survey is probably related to the plan, to assess the structural integrity of the section in order to build the structure around the building.
Or so I thought at first, until I remembered a togetter I read in August.
What was beneath the operating floor in the northwest section?
One of the people who have diligently followed the news and press conference on the Fukushima I NPP accident, @mtx8mg "koajisashi", has the answer in his/her togetter from 8/10/2014: PLR-MG, or Primary Loop Recirculation System Motor Generator, which occupied almost the entire length of the west side of the Reactor 3's 4th floor:
And what happened there?
"koajisashi" reminds us in the togetter of the March 23, 2011 fire in the Reactor 3 building, with black smoke seen rising vigorously (see my post on 3/23/2011; TEPCO called it "gray smoke"). At that time, the exact location of the fire was not reported, and the time and date when the fire started was not known (or reported) either. Black smoke was seen rising from the Reactor 3 building on and off until the evening of March 23, 2011.
But in the meeting on March 24, 2011, the location of the fire was identified as PLR-MG in Reactor 3's 4th floor. It was never reported by the media, as far as I know. Reading what was reported in the meeting now, I can see why it was not reported back then. According to the meeting report on March 24, 2011 in the NISA archive (translation is mine),
・黒煙の原因と考えられているオペフロ下の MG セットから隣接している SFP の壁まで２ｍ程度。黒煙発生時の熱によって壁の強度の劣化が懸念されるため検討を行った。SFP 壁は厚さが１８５ｃｍ、RC 造で、鉄筋は壁表面にもっとも近いもので８ｃｍの深さにある。鉄筋は３００－４００℃で影響を受け始めるが、学会の耐火試験データに基づき評価したところ、３５０℃に達するのは４時間程度の時間が必要であり、壁の強度に大きな影響はないものと評価した。
At 11PM last night [3/23/2011], conducted visual survey and thermography measurement and concluded that the burning had subsided.
The cause of the black smoke is considered to be the MG set [PLR-MG] beneath the operating floor. SFP is about 2 meters away from [part of] the MG set. As there was a worry about deterioration of the SFP wall strength due to the heat generated when the black smoke was rising, we evaluated the data. The SFP wall is 185cm (about 6 feet) in thickness, made of reinforced concrete. The reinforcing bars closest to the surface of the wall are at 8 centimeters from the surface. The reinforcing bars start to be affected by heat at 300 to 400 degrees Celsius. According to the fire-resistance data by the [relevant] scientific society [no mention of which one], it would take about 4 hours [of burning] for the temperature to reach 350 degrees Celsius, and we concluded there would be no major effect on the strength of the SFP wall.
As I wrote above, no one knows exactly how long the MG was burning. In fact, the same NISA meeting report, on March 23, 2011, says the black smoke started to gush out at 4:20PM on March 23, 2011, and as of 9:30 the black smoke was still rising. So the fire may have been burning for at least 5 hours on March 23, 2011.
The news, if the details like these had been reported by the media at that time, should have made people very nervous. And this is probably why TEPCO video-surveyed the area in detail, and also why TEPCO seems very eager to remove the spent fuel assemblies from the Reactor 3 Spent Fuel Pool, despite the mess and damage of the Reactor 3 building.
The structural integrity, not of the northwest section per se, but of the Spent Fuel Pool itself, may be the issue that concerns TEPCO.