Around the National Diet Bldg., Tokyo
Raector Unit No.2 of Fukushima Daiichi
(updated on July 24, 2012)
The largest source of radioactive material discharged from the Fukushima Daiich nuclear plant is its No. 2 Unit.
The highest level of discharged doses occurred with some explosion in No.2 Unit on March 15, 2011 as the following figure shows. Note that it was four days after the M9.0 earthquake and the great tsunami attacked the Tohoku Region (northeast Japan), including the Fukushima Daiichi plant.
Trend of Doses at Locations in Miyagi Pref. North of Fukushima Daiichi (with distance specified)
Let's check how the time line of the Fukushima Daiichi accident was reported publicly.
March 14
01:20 - Infusion of sea water to Unit No.1 and No.3 was stopped.
11:01 - An explosion occurred at the No.3 Reactor Unit.
13:25 - Unit No.2 lost a cooling function.
16:34 - Sea water was poured into Unit No.2.
19:45 - All the fuel rods in Unit No.2 were exposed to the air.
21:37 - 3.130 mSv/hour was recorded at the main entrance to the Daiichi plant.
(1 mSv/year is the statutory allowable dose level for citizens in Japan.)
22:07 - 9.6 uSv/h was recorded at a location 10 km south of the plant.
(This level of doses is 260 times larger than the ordinary level at the location.)
23:39 - Pressure in the containment vessel of Unit No.2 increased abnormally rapidly.
March 15
00:00 - Dry vent was performed for several minutes, but no pressure discrease was measured in Unit No.2.
00:00 - 95.7 uSv/h was recorded in Fukushima "Daini" Nuclear Plant 12 km south of Daiichi.
01:11 - Pressure inside the pressure vessel of Unit No.2 fell from 1.44 MPa to 0.92 MPa.
02:30 - Sea water was poured into Unit No.3.
03:00 - Pressure inside the reactor vessel of Unit No.2 exceeded the design pressure.
04:20 - 140 uSv/h was recorded in Fukushima "Daini" Nuclear Plant 12 km south of Daiichi.
06:10 - An extraordiniary sound was emitted from Unit No.2 where pressure decreased rapidly.
06:14 - White smoke was discharged from Unit No.3.
08:25 - White smoke was discharged from Unit No.2.
09:00 - 11.93 mSv/h was recorded at the main entrance to the Daiichi plant.
http://ja.wikipedia.org/wiki/%E7%A6%8F%E5%B3%B6%E7%AC%AC%E4%B8%80%E5%8E%9F%E5%AD%90%E5%8A%9B%E7%99%BA%E9%9B%BB%E6%89%80%E4%BA%8B%E6%95%85%E3%81%AE%E7%B5%8C%E7%B7%AF
As nuclear reactors were stopped and circulation of cooling water was also stopped (due to loss of electricity) in pressure vessels which contained nuclear fuel rods, the action (called "vent") to release gas (mostly vapor contaminated with radioactive material leaking from fuel rods) inside each pressure vessel became necessary in order to decrease pressure inside the vessels and prevent breakage of them. But it was not successful with No. 2 Unit, though it was well performed with No.1 and No.3 Units.
From this stage, there are two theories on how the accident progressed:
Theory I
So, it became urgent to release the gas inside the pressure vessel of No.2 Unit to the pressure suppression chamber situated under the reactor containment vessel which contained the pressure vessel. To carry out this operation, SR valves should be opened. But these valves did not work well. The gas whose pressure was dangerously increasing inside the pressure vessel could not flow through the SR valves and pipes connecting the pressure vessel to the pressure suppression chamber.
From this stage, there are two theories on how the accident progressed:
Theory I
So, it became urgent to release the gas inside the pressure vessel of No.2 Unit to the pressure suppression chamber situated under the reactor containment vessel which contained the pressure vessel. To carry out this operation, SR valves should be opened. But these valves did not work well. The gas whose pressure was dangerously increasing inside the pressure vessel could not flow through the SR valves and pipes connecting the pressure vessel to the pressure suppression chamber.
So, the pressure of vapor continued to increase due to heated nuclear fuel rods from which fuel started to be melted down to the bottom of the pressure vessel. Then an explosion occurred in the pressure vessel of No.2 Unit around 6:00 a.m. on March 15. From fractures of the vessel, a large amount of radioactive material was released to the air through the containment vessel and the housing building. This is believed to be the process of the core accident that resulted in the largest leakage of radioactive material from the Fukushima Daiichi plant.
Theory II
So, it became urgent to release the gas inside the pressure vessel of No.2 Unit to the pressure suppression chamber situated under the reactor containment vessel. To carry out this operation, SR valves should be opened. But these valves did not work well. They worked several times only intermittently.
When the valves could be opened, the vapor flowed from the pressure vessel to the pressure suppression chamber. However, the chamber lost a cooling function. The temperature of contaminated water in the chamber reached soon 100 degrees Centigrade.
So, radioactively contaminated vapor leaked from the chamber to the reactor containment vessel; it then went out of the housing building to the open air. It resulted in the largest discharge of radioactive material from the Fukushima Daiichi plant, which turned to be the main cause of the Fukushima Daiichi nuclear accident.
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Though Unit No.1 and No.3 underwent a hydrogen explosion, their pressure vessels and containment vessels were not broken. No fatal doses were discharged from these Units.
On the other hand, Unit No.2 did not suffer a hydrogen explosion, but its pressure vessel or pressure suppression chamber was damaged to have cracks from which radioactive material included in heated and high-pressure vapor was released to the outer air to cause the vast radioactive contamination around the Fukushima Daiichi nuclear plant.
Anyway Japanese experts think that if the SR valves could have worked well, the No. 2 Unit pressure vessel would not have been damaged. Or, if the cooling function of the pressure suppression chamber of Unit No.2 had worked sufficiently well, no decisive leakage of radiation doses could have happened from the housing building of Unit No.2. If so, today 160,000 Fukushima people now being evacuated lived in their homes and home towns.
And we have to be alert to a future similar accident, since the same type of SR valves are used in many other nuclear power plants in Japan.
(The SR valve has reportedly problematic structure that prevents its opening operation when pressure inside a connected vessel has increased so much.)
But why could not the vent action be successful with the No.2 Unit? And, why, in Unit No.2, could not the SR valves work well or the cooling function of the chamber work satisfactorily? It is still unknown.
So, it became urgent to release the gas inside the pressure vessel of No.2 Unit to the pressure suppression chamber situated under the reactor containment vessel. To carry out this operation, SR valves should be opened. But these valves did not work well. They worked several times only intermittently.
When the valves could be opened, the vapor flowed from the pressure vessel to the pressure suppression chamber. However, the chamber lost a cooling function. The temperature of contaminated water in the chamber reached soon 100 degrees Centigrade.
So, radioactively contaminated vapor leaked from the chamber to the reactor containment vessel; it then went out of the housing building to the open air. It resulted in the largest discharge of radioactive material from the Fukushima Daiichi plant, which turned to be the main cause of the Fukushima Daiichi nuclear accident.
*** ***
Though Unit No.1 and No.3 underwent a hydrogen explosion, their pressure vessels and containment vessels were not broken. No fatal doses were discharged from these Units.
On the other hand, Unit No.2 did not suffer a hydrogen explosion, but its pressure vessel or pressure suppression chamber was damaged to have cracks from which radioactive material included in heated and high-pressure vapor was released to the outer air to cause the vast radioactive contamination around the Fukushima Daiichi nuclear plant.
Anyway Japanese experts think that if the SR valves could have worked well, the No. 2 Unit pressure vessel would not have been damaged. Or, if the cooling function of the pressure suppression chamber of Unit No.2 had worked sufficiently well, no decisive leakage of radiation doses could have happened from the housing building of Unit No.2. If so, today 160,000 Fukushima people now being evacuated lived in their homes and home towns.
And we have to be alert to a future similar accident, since the same type of SR valves are used in many other nuclear power plants in Japan.
(The SR valve has reportedly problematic structure that prevents its opening operation when pressure inside a connected vessel has increased so much.)
But why could not the vent action be successful with the No.2 Unit? And, why, in Unit No.2, could not the SR valves work well or the cooling function of the chamber work satisfactorily? It is still unknown.
Finally, most of Japanese experts now think that Tokyo Electric Power Corporation (TEPCO) engineers in Fukushima Daiichi could succeed in preventing a hydrogen explosion or fatal damage on the pressure vessel of Unit No.2 on March 15, 2011, but they failed in preventing the decisive leakage of radioactive material from Unit No.2 which must accounts for 90% of doses recorded in Fukushima Prefecture and as far as in other locations on the earth.
Of course, if the nuclear power generation plant had not been hit by the 15-meter-high tsunami on March 11, 2011 (40 minutes after the occurrence of an M9.0 earthquake in the North Pacific Ocean some 150 km far from the plant), those reactor units in Fukushima Daiichi would not have been crippled so badly.
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