NHK, Dec 17, 2015 (emphasis added): [TEPCO] says radioactive fallout that polluted the environment in mid-March of 2011 was likely caused by a leak directly from a containment vessel of the facility’s No.3 reactor. Officials… on Thursday reported their latest findings on what happened at the plant… They concluded that radioactive contamination of the environment between the night of March 14th and the 16th was likely caused not by the vent operations but failure of the vessel. They said the vessel likely lost airtightness due to heat from nuclear fuel, leading to the direct release of radioactive substances into the environment.
NHK transcript, Dec 17, 2015: [TEPCO] said the substances in one of the reactors probably leaked directly from the containment vessel… They suspect the heat of fuel caused the containment vessel to lose airtightness.
TEPCO, Dec 17, 2015: A significant release of steam from the night of March 14 to March 16, 2011 is believed to have been responsible for contamination to the surrounding environment of Fukushima Daiichi. The investigation reports that the primary containment vessels in Units 2 and 3 did likely lose leakage resistant properties by March 15 and had been in a condition where radioactive materials could leak directly from them. It is therefore presumed the environmental contamination outside Fukushima Daiichi during that period was caused by steam leakage directly from the primary containment vessels and not from the vent.
TEPCO, Dec 17, 2015: Leakage and release of a large amount of steam from the Unit 3 Reactor Building — The PCVs of Units 2 & 3 lost the airtightness in the end, which is confirmed by the fact that steam escaped from the Reactor Buildings. Analysis of the behavior of the pressure in the PCVs and the situation at the time of the accident has revealed that the environmental contamination from the night of March 14 to March 16 occurred by steam leakage together with radioactive materials directly from the PCVs not from the vent.
UBS Investment Research (via WikiLeaks), Apr 2011: Of particular concern was Unit 3, because, since September 2010, the plant had been fueled with mixed oxide, or MOx… Use of MOx heightened fuel risk — Fukushima Daiichi Unit 3 was fuelled with mixed oxide (MOx), which is about 93% uranium and 7% plutonium. This has caused additional worries for TEPCO and the government, because MOx is more radioactively aggressive. We think national nuclear safety reviews might consider restrictions on its use.
Published: December 18th, 2015 at 7:03 pm ET