A turning point in the efforts to avert a meltdown at Fukushima Daiichi nuclear power station came in the wake of the blast that destroyed the exterior walls of the crippled reactor...
The Tokyo Electric Power Company, Tepco, had the flexibility to use pressure release valves to vent some steam, even though it was mildly contaminated, because it had taken the precaution of evacuating the local population within a 12 mile radius...
The steam was released from the pressure vessel into the surrounding building and this was consistent with reports that radiation levels had soared to around 1000 times the background level. Officials also said they had detected caesium, an indication that some fuel was already damaged.
The shock wave that can be clearly seen in video of the blast suggests a point ignition source detonated the released hydrogen when it came into contact with oxygen in the air, he said.
Four workers were injured, according to Atsushi Sugimoto of Tepco.
"At this point, we don't know how much radiation has escaped," said Shinji Kinjo of the Nuclear and Industrial Safety Agency. "Should the situation change, the evacuation zone could become larger."
Yukio Edano, Japan's Chief Cabinet Secretary, said the cause of the explosion was a mixture of hydrogen, from steam escaping the core, and oxygen from the surrounding air. He added that the pressure vessel was unaffected and the incident would not be a cause for a large amount of radiation to leak.
Although the concrete cladding disintegrated in a spectacular fashion, Grimston said that the fact that the metal frame of the building was left intact suggests that the explosion was not as violent as it looked...
http://www.newscientist.com/blogs/shortsharpscience/2011/03/massive-explosion-rips-through.htmlpower plant 70 mSV
Press Release (Mar 12,2011)
Plant Status of Fukushima Daini Nuclear Power Station (as of 11pm March 12th
Unit 1 (shut down at 2:48pm on March 11th)
- Reactor is shut down and reactor water level is stable.
- Offsite power is available.
- At 8:19am, there was an alarm indicating that one of the control rods
was not properly inserted, however, at 10:43am the alarm was
spontaneously called off. Other control rods has been confirmed that they
are fully inserted (reactor is in subcritical status)
- Status of main steam isolation valve: closed
- Injection of water into the reactor is done by Make-up Water Condensate
System.
- At 6:08pm, we announced the increase in reactor containment vessel
pressure, assumed to be due to leakage of reactor coolant. However, we
do not believe there is leakage of reactor coolant in the containment
vessel at this moment.
- At 5:22am, the temperature of the suppression chamber exceeded
100 degrees. As the reactor pressure suppression function was lost,
at 5:22am, it was determined that a specific incident stipulated in
article 15, clause 1 has occurred.
- We decided to prepare implementing measures to reduce the pressure of
the reactor containment vessel (partial discharge of air containing
radioactive materials) in order to fully secure safety. This preparation
work started at around 9:43am and finished at 6:00pm.
Unit 2 (shut down at 2:48pm on March 11th)
- Reactor is shut down and reactor water level is stable.
- Offsite power is available.
- Control rods are fully inserted (reactor is in subcritical status)
- Status of main steam isolation valve: closed
- Injection of water into the reactor is done by Make-up Water Condensate
System.
- We do not believe there is leakage of reactor coolant in
the containment vessel.
- At 5:32am, the temperature of the suppression chamber exceeded
100 degrees. As the reactor pressure suppression function was lost,
at 5:32am, it was determined that a specific incident stipulated in
article 15, clause 1 has occurred.
- We decided to prepare implementing measures to reduce the pressure of
the reactor containment vessel (partial discharge of air containing
radioactive materials) in order to fully secure safety. This preparation
work started at around 10:33am and finished at 10:58pm.
Unit 3 (shut down at 2:48pm on March 11th)
- Reactor is shut down and reactor water level is stable.
- Offsite power is available.
- Control rods are fully inserted (reactor is in subcritical status)
- Status of main steam isolation valve: closed
- We do not believe there is leakage of reactor coolant in
the containment vessel.
- We decided to prepare implementing measures to reduce the pressure of
the reactor containment vessel (partial discharge of air containing radioactive
materials) in order to fully secure safety. The preparation woke started at
around 12:08pm and finished at 12:13pm.
- Reactor cold shutdown at 12:15pm
Unit 4 (shut down at 2:48pm on March 11th)
- Reactor is shut down and reactor water level is stable.
- Offsite power is available.
- Control rods are fully inserted (reactor is in subcritical status)
- Status of main steam isolation valve: closed
- Injection of water into the reactor is done by Make-up Water Condensate
System.
- We do not believe there is leakage of reactor coolant in
the containment vessel.
- In order to cool down the reactor, injection of water into the reactor
had been done by the Reactor Core Isolation Cooling System, however,
At 6:07am, the temperature of the suppression chamber exceeded
100 degrees. As the reactor pressure suppression function was lost,
at 6:07am, it was determined that a specific incident stipulated in
article 15, clause 1 has occurred.
- We decided to prepare implementing measures to reduce the pressure of
the reactor containment vessel (partial discharge of air containing
radioactive materials) in order to fully secure safety. The preparation
woke started at around 11:44am and finished at around 11:52am.
Indication from monitoring posts installed at the site boundary did not
show any difference from ordinary level.
No radiation impact to the external environment has been confirmed.
We will continue to monitor in detail the possibility of radioactive
material being discharged from exhaust stack or discharge canal.
There is no missing person within the power station.
We are presently checking on the site situation of each plant while
keeping the situation of aftershock and Tsunami in mind.
A seriously injured worker who had been trapped in the crane operating
console of the exhaust stack was transported to the ground at 5:13pm and
confirmed dead at 5:17pm. We sincerely pray for the repose of his soul.
A worker was lightly injured spraining his left ankle and cutting both
knees when he fell while walking at the site. The worker has returned to
work after medical treatment and rest.
At 10:15pm, an earthquake occurred centered in offshore Fukushima
prefecture. No damage to the plant has been found at this moment.
http://www.tepco.co.jp/en/press/corp-com/release/11031233-e.html 
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