According to a Kyodo news report, extremely high radiation levels were measured in unit 1 of Fukushima 1 nuclear power station the night after the quake, hours before radioactive steam was first vented from the containment vessel at 04:00 the next morning (2011-03-12).
Workers entered the No. 1 reactor building during the night to assess the damage only to hear their dosimeter alarms go off a few seconds later, sources at Tokyo Electric Power Co. said. Since they thought the building was filled with highly radioactive steam, the workers decided to evacuate.
Based on the dosimeter readings, the radiation level was about 300 millisieverts per hour, the source said, suggesting that a large amount of radioactive material had already been released from the core.
The source of the steam was believed to be the No. 1 reactor’s overheated pressure vessel.
But for that scenario to hold, the pressure in the reactor would have to have reached enormous levels — damaging the piping and other connected facilities. It should have taken much more time to fill the entire building with steam.
A source at Tepco admitted it was possible that key facilities were compromised before the tsunami.
According to the AREVA presentation by Dr Matthias Braun, the reactor isolation condensor in unit 1 stopped working at 16:36 on 2011-03-11, 55 minutes after the diesel generators had been knocked out by the tsunami. After that, pressure from boiling water built up in the reactor pressure vessel, from where it was vented into the containment via water held in the pressure suppression chamber.
It was only when pressure in the containment had built up far beyond its design limit that steam was first vented from it at 04:00 on Saturday, March 12. Radiation levels of hundreds of millisieverts per hour in the building hours before that suggest that steam from the pressure vessel or containment escaped hours before it was supposed to have come out.
Direct seismic damage to pipes, valves or other components from the quake at 14:46 is a plausible explanation for a premature radiation release while the containment was supposedly still serving its function of holding back radioactivity from the environment. The local intensity of the quake was either close to or in excess of the design specification the reactors had been built to. For example, maximum seismic acceleration at unit 3 was measured at 507 gal in East-West direction, versus the 441 gal it was designed for.
So how does it matter if parts of Fukushima prefecture became uninhabitable because of broken steam pipes or because of flooded diesel generators? Isn’t the outcome the same? It’s actually a very important difference: Nuclear power companies in Japan are now taking measures to evaluate tsunami risks in plants around the country. Units 4 and 5 of Hamaoka NPP in Shizuoka were recently shut down because they’re located right on top of a quake fault line, but the plan is to restart them around 2014, after tsunami defenses have been beefed up around them. The problem is, even if — thanks to raised sea walls — the diesel generators were not knocked out by sea water, but the violent shaking were to destroy some other vital part of reactor such as cooling pipes, the outcome could still be the same.
Unit 4 blown up by leak from adjacent unit 3
The Wall Street Journal reports that TEPCO now think the hydrogen explosion that destroyed the service floor above the spent fuel pool in unit 4 was caused by hydrogen leaking from the adjacent unit 3, which also exploded:
Tepco also released its analysis of a hydrogen explosion that occurred at unit No. 4, despite the fact that the unit was in maintenance and that nuclear fuel stored in the storage pool was largely intact.
According to Tepco, hyrogen produced in the overheating of the reactor core at unit 3 flowed through a gas-treatment line and entered unit No. 4 because of a breakdown of valves. Hydrogen leaked from ducts in the second, third and fourth floors of the reactor building at unit No. 4 and ignited a massive explosion.
Adjacent units 3 and 4 are connected, for example by sharing a venting tower for the release of radioactive gases, which is located between them.
Nuclear expert Arnie Gundersen believes it is plausible that the particularly violent explosion that destroyed the top of unit 3 may have involved a criticality event (i.e. a chain reaction) triggered by the well-publicised hydrogen explosion.