Fukushima Daiichi Unit 3 had an explosion that resembled a nuclear bomb detonation. Fukushima Daiichi unit 3's reactor was running MOX fuel, which is uranium based but "enriched" with plutonium. I was never clear how much MOX was stored in unit 3's pool.
The authorities told us all 4 explosions at Daiichi were hydrogen-caused, resulting from ineffective venting after the quick shutdowns of the reactors (the 4th explosion in unit 4 was blamed on hydrogen from unit 3).
UNIT 3 SPENT FUEL POOL
Nuclear Engineer Arnie Gundersen suggested that a prompt criticality occurred in melted fuel located in unit 3's SPENT FUEL POOL:
A. Gundersen (26 April 2011) ‘Gundersen Postulates Unit 3 Explosion May Have Been Prompt Criticality in Fuel Pool,’ Fairewinds, http://fairewinds.com/content/gundersen-postulates-unit-3-explosion-may-have-been-prompt-criticality-fuel-pool, date accessed 26 April 2011The images at Cryptome are illuminating and suggest the explosion at unit was pretty catastrophic: https://cryptome.org/fukushima-dai-ichi-series.htm
Read what John Monger said in the US Nuclear Regulatory Commission (NRC) discussions March 16, 2011 about the state of unit 3's spent fuel pool as documented in the NRC Transcript March 16, 2011 http://pbadupws.nrc.gov/docs/ML1205/ML12052A108.pdf
John Monger page 62: “unit 1 and 2 is boiling down, and unit 3 and 4 is having zirc/water reaction. They believe there is essentially no walls on unit 3. The explosion – I’m sorry – unit 4. The explosion leveled the walls, leveled the structure for the unit 4 spent fuel pool all the way down to the approximate level of the bottom of the fuels. So, there’s no water in there whatsoever.”
I figure the fuel from unit 3's spent fuel pool is all over the globe now. I hope I'm wrong.
TEPCO doesn't talk much today about the fuel in reactor 3's pool other than to say its being cooled, as part of the 300 tons of water injected at the site daily to prevented melted fuel, located in ruptured containment, from having "zirc reactions."
UNIT 3's REACTOR FUEL
From the beginning of the disaster, TEPCO claimed to have no direct knowledge of the location/status of unit 3's REACTOR fuel, although in 2014 the company stated it believed it had all melted and dropped beneath the pressure vessel:
TEPCO: Nearly all nuclear fuel melted at Fukushima No. 3 reactor. (August 7, 2014). The Asahi Shimbun, http://ajw.asahi.com/article/0311disaster/fukushima/AJ201408070055
According to the latest estimate, fuel at the No. 3 reactor began melting at 5:30 a.m. on March 13, and almost all the melted fuel had dropped to a broad area on the bottom of the containment vessel soon after 7 a.m. on March 14.
TEPCO officials said they believe part of the melted fuel still remains inside the pressure vessel, citing the fact that the temperature inside the pressure vessel fell after a later water injection.
The estimated start of the fuel melting is roughly consistent with when neutrons were detected near the front gate of the nuclear plant, according to the officials.
Neutrons were also detected when nuclear fuel started to melt at the No. 2 reactor at midnight on March 14. For this reason, TEPCO estimates that radioactive substances released from the No. 3 reactor emitted neutrons near the front gate.
All of nuclear fuel at the No. 1 reactor is estimated to have melted after the disaster, while around 60 percent is believed to have melted at the No. 2 reactor. TEPCO said it will now consider how to remove the melted nuclear fuel from the No. 3 reactor.
Now TEPCO is telling us that nearly ALL the fuel from the REACTOR is accumulated in the bottom of the containment vessel:
Kohei Tomida. July 20, 2017. Fukushima robot reveals collapsed scaffolding in nuclear reactor http://www.asahi.com/ajw/articles/AJ201707200046.htmlThis narrative is being repeated especially strongly in The Asahi Shimbun:
Almost all of the nuclear fuel in the No. 3 reactor likely melted and dropped from the pressure vessel and accumulated at the bottom of the containment vessel, according to analysis by Tokyo Electric Power Co., the crippled plant’s operator.
Images of fuel debris 1st step in deactivating Fukushima plant. The Asahi Shimbun, July 22, 2017 http://www.asahi.com/ajw/articles/AJ201707220018.htmlDo people remember the explosion of unit 3? It was huge. The building was wrecked. I find it hard to believe that all the fuel from reactor unit 3 resides cooling, in a singular space.
Operator Tokyo Electric Power Co. said the manner in which the solidified material was found within the No. 3 reactor suggested the material is fuel debris.
"It clearly appears to be something that solidified after melting out of the pressure vessel," said one official. "We believe the material emerged after nuclear fuel mixed with structural matter within the pressure vessel."
Past analysis of the No. 3 reactor led to the assumption that almost all the nuclear fuel had dropped through the pressure vessel after burning a hole in the bottom and dripping down. The latest robot survey confirms that is what likely happened.
It was UNIT 3 that appeared to have fires in the fall of 2011. Unfortunately almost all the videos posted at Youtube have been taken down and I only have a few screenshots from that time, saved in this pdf, which I strongly recommend viewing:
I always presumed it was because of the MOX. The low-end estimate of 32 mox assemblies is contained in the reactor core is from France’s Areva, which provided the fuel for unit 3. As the French Fukushima 3/11 Watchdog group points out, the low-end estimate of 32 mox assemblies translates into 5.5 tons of fuel containing more than 300 kg of plutonium: “300 kg is therefore equivalent to 300 billion lethal doses.”
I don't know where the fuel from unit 3's reactor is, but I will say, as a Webcam Watcher, there is a METAPHORIC DRAGON either in or below unit 3 that seems particularly active after the last earthquake a few days ago: