I know it's an old story at this point, but the comment section of my post on highly radioactive debris from the Reactor 3 operating floor piqued my interest again.
The worker who tweeted from Fukushima I Nuke Plant, "Happy", said in his interview with Tokyo Shinbun that right after the explosion of Reactor 3 building on March 14, 2011, there were people who were coated with black soot that came from the explosion. Unlike the explosion of the Reactor 1 building, the Reactor 3 explosion emitted a decidedly dark, black smoke skyward, with what looks like several huge chunks of structure (concrete?) blown up and then falling down.
Video of Reactor 3 explosion; after more than 2 years since it took place, there are still many Japanese who have never seen the footage:
The camera is looking at the Reactor 3 building from west. The flicker does seem like it's from the Spent Fuel Pool or close to it. Here's Reactor 3's SFP (covered, as of 4/22/2013) and the operating floor plan (from Ian Goddard's article that appeared on Lewrockwell.com on 9/3/2011; H/T reader Atomfritz) oriented the same way as the explosion video:
Was the explosion ex-vessel? In one of the very early meetings after the explosion, one of the US NRC people casually mentioned that it was ex-vessel. Others, in an unofficial capacity, have insisted that the explosion was nuclear, from the Spent Fuel Pool that they say went critical.
I'm personally more inclined to Mr. Goddard's theory that it was an ex-vessel steam explosion when the molten fuel escaped the Reactor Pressure Vessel and dropped in the water that had accumulated in the Containment Vessel, and the pressure from explosion lifted the reactor cap flange, gas escaped through the gap and ignited the hydrogen gas generated and accumulated in the Spent Fuel Pool.
If those black smokes were black because they were from the Containment Vessel after the molten fuel dropped, the smokes and the soot from the smoke must have been very radioactive. I wonder if any measurement of radioactivity exists of the smokes or the soot.
(I do believe more contaminated debris will be found when TEPCO attempts to remove them from the area above the reactor well cap. I'm trying to locate the document again, but TEPCO released a while ago the measurement of radiation in all the reactor buildings and turbine buildings at Fukushima I Nuke Plant. In that, at 2 to 3 meters above the area where the reactor well is located, the radiation was over 500 millisieverts/hour. The measurement must have been taken using the crane boom during the air sample collection.)
New York Times reported on April 5, 2011 that "fragments or particles of nuclear fuel from spent fuel pools above the reactors were blown "up to one mile from the units"", quoting the then-confidential NRC report that the paper obtained.
However, the NRC report dated March 26, 2011, later released, says on page 10:
Fuel pool is heating up but is adequately cooled, and fuel may have been ejected from the pool (based on information from TEPCO of neutron sources found up to 1 mile from the units, and very high dose rate material that had to be bulldozed over between Units 3 and 4. It is also possible the material could have come from Unit 4).
Comparing the New York Times description and the NRC report, it looks the New York Times writers took the "neutron sources found up to 1 mile" from the reactor buildings to be fuel fragments.
The way TEPCO reported at that time was that neutron beam was detected at the main entrance of the plant on March 14, 2011, which is located at about 1.5 kilometer, or about 1 mile, west of the Reactors 3 and 4.
Checking TEPCO's revised data from that day (from TEPCO's 2011 data archive), neutron beam was not detected at the main gate on March 14 until 9PM (0.01 microsievert/hr). The explosion was at 11AM. Neutron beam was detected again later that night, at 11:20PM, 11:50PM, 11:55PM, then intermittently during the early hours of March 15, 2011. It seems to me that they coincide more with the dry vent of Reactor 2 that the plant was attempting.
(I have seen this "up to 1 mile" of neutron beam detection morphing into "fuel fragments scattered several miles outside the plant".)
In the Reactor 3 explosion, there were also streaks of white smoke (or steam?) from the lower part of the building, much like the explosion in Reactor 1.
Professor Takashi Tsuruda of Akita Prefectural University thinks that's the steam from the Suppression Chamber, and the Suppression Chamber of Reactor 3 (and 1) is broken. Personally, people like Professor Tsuruda, expert in explosive reactions, should be sitting in a committee at the Nuclear Regulatory Authority trying to figure out what exactly happened at the plant on March 11, 2011 and after.
There are so many loose ends, after more than two years.