March 14th – March 15th, 2011 – Unit 2 Timeline
After the explosion of the Reactor 3 building just before noon on March 14th, workers were unable to open the suppression chamber valve, and were also unable to inject water from the fire engine due to the high radiation fields and equipment damaged from debris.
The malfunctioning valve continued to prevent workers from manually venting the containment vessel to release pressure and maintain injecting seawater into the reactor.
At 11:30 JST workers noted that the reactor water level began to drop. As a result, the nuclear fuel in that reactor was exposed for many hours, and may have caused a breach of the container vessel releasing even more radioactive particles.
At 13:18 JST on March 14th, the reactor core isolation cooling system for Reactor 2 stopped and shortly afterwards, the water level within the reactor continued falling.
At 13:25 JST, workers confirmed that the RCIC function was lost
Around 16:34 JST, the plant manager at Fukushima Daiichi ordered for seawater injection to begin to cool down the damaged fuel in the reactor pressure vessel.
By 18:00 JST, the reactor pressure again began a downward trend.
Approximately 20 minutes later by 18:22 JST the top of the fuel was exposed to the air.
At 18:35 JST, workers tried to open both the large and small S/C vent valves, but were unable as the air pressure from the compressor was insufficient to overcome the forces inside of the containment.
19:20 JST, the fire truck pump stopped after running out of fuel, and was restored at 19:57 JST.
By 19:30 JST on March 14th, the water level readings inside of the Unit 2 reactor were reported to be showing below minimum possible values.
At 19:54 JST, the workers attempted to restart injecting seawater into the reactor from a fire engine.
At 21:20 JST, workers opened 2 safety release valves, the water level recovered, and the reactor pressure decreased. The small S/C vent valve opened slightly, and the vent line was complete except for the rupture disk.
A short while later, around 21:37 JST, radiation levels at the front gate had reached levels where annual limits would be exceeded in 20 minutes.
Around 22:50 JST, pressure levels in the Dry-Well exceeded the maximum usage pressure, and within 40 minutes workers were forced to attempt to vent the pressure out of the system by opening a small valve in the D/W vent. This drywell vent may be attributed to a large radiological release from Reactor 2.
The workers encountered an increasingly difficult situation as they found that the D/W pressure was trending upwards whereas the S/C pressure was stable, meaning they were not unified.
At 23:00 JST, workers are further set back when it is found that 1 safety relief valve was closed because the reactor pressure had increased.
As the S/C pressure was lower than the working pressure of the rupture disk, and D/W pressure still increasing, the workers opted to open the D/W vent valve to vent the reactor.
The Unit 2 reactor pressure vessel lost pressure around one hour before midnight on March 14th, which may indicate the time when more of the melted nuclear fuel may penetrated the pressure vessel.
In the early hours shortly after midnight on March 15th, workers opened up safety release valves in an attempt to vent some of the pressure out through the vent stacks leading from the Unit 2 reactor, but these attempts were unsuccessful.
Instead workers for forced to open a small vent valve which allowed the radioactive materials to dry vent from the D/W. They managed to create the vent line and hold the valve open for a few minutes.
Then at around 06:14 JST, an explosion rocked the suppression chamber and the S/C pressure zeroed, Tokyo Electric Power said that after the explosion inside of the Unit 2 reactor, pressure had dropped in the “suppression pool” — a section at the bottom of the reactor that converts steam to water and is part of the critical function of keeping the nuclear fuel protected.
Government officials at the time to said that there was a very high probability that a portion of the containment vessel was damaged.
The reliability of all of the measurement instruments after the explosion is certainly suspect, and TEPCO workers have continually been plagued by broken gauges and measurement devices.
After that pressure drop occurred, radiation levels outside the Unit 2 reactor were reported to have risen sharply to over 400 millisieverts an hour. Even at that rate, a mere 7 minutes of exposure would effectively cause a worker to exceed the maximum annual dose allowed, and 75 minutes of exposure would likely lead to the subject exhibiting symptoms of acute radiation sickness.
Later that morning, TEPCO announced that it had fixed the broken suppression chamber valve and resumed seawater injections, but admitted that it had detected possible leaks in the containment vessel that prevented water from fully covering the fuel rods.
On the night of the 15th, around 20:00 JST, a majority of the fuel in Reactor 2 is thought to have completely collapsed inside of the reactor pressure vessel, and the explosion which had occurred the day before had further damaged the containment structure.