NHK, Aug 6, 2014 (:50 in): Tepco officials found more problems. After the accident, crews used fire engines to inject water into Reactor 2. But the water may have reacted with the fuel inside to produce hydrogen, raising the temperature significantly — and that sped up the meltdown.
Tepco Press Release, Aug 6, 2014: Unit 2 Pressure Increase and Fuel Melting — In an inquiry into the sequence of events leading to the melting of fuel in the Unit 2 reactor core, the report concludes that the insufficient injection of cooling water into the reactor core by fire engines actually accelerated the melting. According to the report, steam generation from the injection of water at a time when the reactor’s core had been exposed caused a zirconium-water reaction, in turn generating hydrogen and large amounts of heat, causing pressure in the reactor to rise.
‘RESTRICTED DISTRIBUTION’ – Institute of Nuclear Power Operations, ‘Best available but unverified information’ (pdf), Mar 20, 2011: Unit 2 – Emergency [...] 3/15/2011: 06:14 JST – JAEA reported an explosion in the vicinity of the suppression pool tube. Reactor water level was reported at 2.7 m below top of active fuel at the time. Containment pressure dropped to atmospheric. Suppression pool pressure went off-scale low. EPRI relayed information attributed to TEPCO that a defect in the unit 2 torus is preventing flooding of the drywell. The torus may have been breached. — 8:14 JST: Identified that pressure in the pressure suppression chamber decreased from 3 atmospheres to 1 atmosphere. Monitoring post readings (location unknown) increased from 960 mSV/hr (960,000 microSv/hr) [...] Monitoring post reading at Kitaibaraki [...] is 4.87 [...] approximately 100 times larger than ordinary [...]
The next day Kitaibaraki’s radiation dose was 300 times normal:
Monitoring post in Kitaibaraki City, Ibaraki Prefecture (.xls)
73 km SSW of Fukushima Daiichi
- 3/16/2011 – 11:40am JST @ 15.80 microSv/hr
- 3/15/2011 – 04:00am JST @ 4.87 microSv/hr
- 3/15/2011 – 02:00am JST @ 0.20 microSv/hr
- 3/15/2011 – 12:00am JST @ 0.05 microSv/hr