Unit 1 and Unit 3 are being cooled with seawater injection, their secondary containments blown off by hydrogen explosions and their fuel likely melted. The fight to keep Unit 2 from a similar fate goes on into March 15th.
It became apparent to the operators at this time that there were several abnormalities in their instrument indications. Containment pressure was higher than the rupture disk’s blowout pressure, but the disk remained intact. Drywell pressure was trending upward, but indicated suppression chamber pressure was remaining steady, where both pressure readings should have been equivalent. Two minutes after midnight operators decided to act on the increasing drywell pressure indication and opened a small air-operated drywell vent valve. Operators also rechecked the vent path from containment and discovered that the drywell vent valve had failed closed. They continued to work to find another vent path, but at 06:00 a loud noise was heard in the area around the torus. Initially this noise was thought to be an explosion as the suppression chamber pressure started dropping rapidly. It was later determined that it was likely not an explosion, as there was no indication of a shockwave, and the pressure drop was likely due to a failed instrument. Reactor water level and drywell pressure did not change after the noise and were 110 inches below top of active fuel and 106 psia respectively. An explosion at Unit 4 occurred which resulted in the evacuation of all nonessential personnel, and only 70 people were left onsite. After this point Unit 2 indications were checked periodically, but nothing was logged between 07:20 and 11:25. When containment pressure was checked at 11:25, it indicated 22.5 psia, but there had been no reports of steam escaping the Unit 2 stack and no change to the vent path valve lineups. TEPCO believes there was a breach of containment related to the loud noise, but what form that breach took is still not clear. Seawater injection continued from this point onward, and boron was added to the water source to address criticality concerns. As on Units 1 and 3, priority went to restoring electrical power to the unit’s many systems.
The explosion in Unit 4, occurring around 06:00 in the morning, caught site personnel by surprise. The existence of and accumulation of hydrogen in the Unit 4 building in high enough concentration to cause an explosion was not thought possible given the Spent Fuel Pool (SFP) water inventory at the time that electrical power was lost. The explosion caused many people to think that the SFP water had boiled off and that fuel damaged had occurred, which would account for the hydrogen. Extreme measures were then adopted to try and refill what was believed to be an empty SFP. Helicopter water drops and fire engine sprays into the buildings through the damaged roofs were used. However, once inspected it was discovered that water level in the SFP had never dropped below the top of the fuel and no fuel damage had occurred. The hydrogen had to have come from somewhere else. The most widely accepted theory is that gases backflowed from Unit 3 to Unit 4. Unit 4 shared part of its vent path with the Unit 3 vents. Unit 4’s valves along that portion of the vent path had failed open and there was no backflow damper to stop gases from flowing from Unit 3 to Unit 4, when Unit 3 was vented. TEPCO performed radiological surveys on the Unit 4 Standby Gas Treatment System filters, which revealed higher radiation levels on the portion of the system closest to Unit 3, which supports this theory. No other issues occurred at Unit 4.