100th day!
I. Interview report on countermeasures taken soon after the earthquake disaster at Fukushima Daiichi
Because of the station blackout, which crippled the central control rooms, the process computer data are limited only before the arrival of tsunami. Therefore the operators’ reports should be one of the few source of information to understand what was happening during the time period between March 11-15.
Late yesterday, TAPCO released a 42 page report summarizing the operators’ observation and actions taken during this period. Currently this report is being translated into English. http://www.tepco.co.jp/en/press/corp-com/release/11061805-e.html “Press Release (Jun 18,2011): First Response at Tohoku Chihou Taiheiyou Oki Earthquake in Fukushima Daiichi Nuclear Power Station”
It is interesting to find many episodes, however, I am a bit disappointed. In view of its importance, the TEPCO should have organize a series of internal meetings so that all of the operators recall each other what has happed at that time and record the results into a document. The current report appears to be drafted by each operator independently. Therefore, the report is more focused on those topical issues, that have been attracting attention of mass media, such as whether there were some delay in venting or sea water injection, instead of trying to record the events from scientific point of view. Therefore, even after reading through the document a few times, there remains much frustration.
However, I confirmed my observation that one of the remarkable characteristics of this disaster is superposition of “station blackout”, “crippled emergency DC power sources” and “loss of ultimate heat sink.” Previously, I was not paying much attention to the second point, assuming that the battery power should have been available until it drained. Without charging, the battery power will dry out in several hours. However, at the Fukushima Daiichi, most of the battery power seems to have been lost soon after the arrival of tsunami. Although no detailed report as to the exact timing of the loss of the battery power, let me try to reconstruct the event sequence since the arrival of the tsunami by referring to a PSAR of a plant similar to 1F1as well as the available process computer report. TEPCO provided a new illustration of 1F1, by showing the boron injection line, as attached. This line was used for water injection to the reactor vessel.
(1) The sea water pumps located near the shore were submerged under the the tsunami and resulted in grounding shorts. This signal should have been sent to the 6.9 kV metal clad switch gear and tripped the breakers connected to the Emergency Buses.
RHSW A trip at 15:37
RHSW C trip at 15:37
-> The actual arrival time of the tsunami was 15:37
(2) Diesel generators tripped through interlocks, resulting in some time differences.
D/G 2A 15:37:30 trip
D/G 2B 15:42 trip
D/G 2C 15:37:30 trip
D/G 2D 15:41 trip
-> The “station black out occurred at 15:42 in 1F1 and 15:41 in 1F2.
(3) Emergency DC power was crippled due to flooding into the 1F1 turbine hall.
125V DC Bus (I) failed soon after 15:42.
125V DC Bus (II) failed around 15:50 (guessed by knowing a failure of the emergency light in the control room.)
125V DC Bus (HPCS) failed around 15:50 (guessed by knowing Instrumentation and Control System became unavailable)
-> The 125V DC buses were available soon after the “station blackout” but failed very soon.
(4) The 6.9kV Emergency Buses were also flooded, since an urgent insulation check revealed wide range of grounding. Only survived electric panel was a 480V power center of 1F2. Water injection was performed through boron injection line (SLC).
(5) Operators tried to recover the essential I/C by gathering batteries and cables
RPV Level indicator recovered at 21:19
Emergency lighting in the 1F1/1F2 control rooms recovered at 20:49 by dispatching a small generator.
Therefore, the “station blackout” and “crippled emergency DC power” are different. With the total loss of both AC and DC power source, the plant became uncontrollable. I appreciate the operators’ effort to steer the impossible plant manually and successfully survived until today.
Also, I believe it is essential to confirm which systems failed for reconstruction of the accident sequence such as attempted above. I hope TEPCO will precise this kind of information.
II. Recapping on the water purification system
A new failure in a rupture disk was found in one of the front end SMZ (surfactant modified zeolite) sorption tower, which is designed to remove oil ingredient and technetium
http://www.tepco.co.jp/nu/fukushima-np/images/handouts_110617_04-j.pdf
TEPCO’s explanation was that it is likely due to repeated open/close of valves, however, it can also be due to radiolytic decomposition of water. TEPCO flushed the entire system by using a low contaminated water and started to experiment by changing the sorption material. They explained, in the press conference, that an unexpected high dose rate is perhaps due to sludge and oil ingredients in the real highly contaminated water. TEPCO also released a new handout of the sorption tower, showing a hydrogen release line as well as 7 inch thick shielding. Since the front end SMZ sorption line also employs zeolite, it may absorb several tens of percent of cesium, which also emits a significant intensity of gamma ray. Even with 7 inches of lead shielding, the attenuation is only 8E-4 of doses without shielding, since the relaxation length is approximately 2.5 cm for gamma. Therefore, it is still not surprising to see exceeding 4 mSv through just five hours of operation. Even by changing into a the efficient zeolite for sorption of cesium, a similar problem is likely be repeated in the herschelite sorption towers. It still needs to replace the sober everyday to stay within the 4 mSv guideline. I continue to be concerned that this system is not operable without remote maintenance provisions. TEPCO needs to make a quick decision, since the highly contaminated water may start leaking as early as at the end of this month.
III. Recapping on removing contamination from 1F2 reactor building
Today, at 8:51 PM of June 19, TEPCO opened a half way the double door of the 1F2 Reactor Building. TEPCO explained the possible radiation effects to the local municipal administration. If no significant radiation effect would be observed, the door will be opened in full early next morning.
Well, let me stop here,
Genn Saji
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(Previous e-mail sent at 10:57 PM on June 18 as Earthquake (99))
Dear Colleagues:
99th day!
It is actually 100th day since the earthquake and 99th days since I started this daily update. In a sect of Buddhism, the 100th day is the time people should stop grieving for the dear persons who passed away and it is the time to move forward. Many mothers who lost their children have been trying to find the lost children in debris everyday, looking for just a piece of something related to their children, however, many of them were saying that they will not keep doing anymore after this day.
I. Recapping on the water purification system
Although the system went into a production run, it has to be stopped at 0:54 on June 18, only after 5 hours, since the dose rate of the skid reached to 4 mSv/hr, a projected guideline for replacement of the sorption material. The Kurion Inc. was projecting that such an operation will become necessary in a month. TEPCO is investigating the causes of why the dose rate became so high in just 5 hours, however, there is no prospect in sight. I do not think it is surprising at all, since the contact dose rate of the highly contaminated water was as high as 1000 mSv/h. When the cesium is concentrated in the sorption material, it can reach to a level of 10Sv/hr very easily even at a distance of 3 meters by operating just one day, according to my “back of an envelope” estimation, computed for the most highly contaminated water sampled from the 1F2 basement of the turbine hall, according to TEPCO 11:03:06.pdf (showing Cs-134: 2.6E+06 Bq/ml; Cs-137: 2.8E+06). There can be some kind of misunderstanding among the team working on the purification system for the dose rate estimation. I believe it is urgent to skip this stage by removing the sorption materials or by bypassing and connect directly to the AREVA system, since this one seems to incorporate some kind of remote handling consideration. It appears that the co-deposited sludge is designed to be separated through “coagulation settling process” discharged into the waste liquid tank, although no details have been disclosed from AREVA as to the details of this facility. Even in this case, the critical issue is whether the waste liquid tank is well shielded or buried underground. A general view of this stage can be seen in:
http://www.tepco.co.jp/en/news/110311/images/110604_05.jpg .
II. Recapping on removing contamination from 1F2 reactor building
I was amazed to find a rapid drop of the concentration of air samples taken from 1F2 reactor building, however, the most recent results indicating that there was some sampling problem in the previous measurement. The most recent one returned back to a level not so good, although the results cleared the TEPCO’s guideline concentration which necessiate wearing a mask with an air supply bomb. The most recent results are shown in
http://www.tepco.co.jp/nu/fukushima-np/images/handouts_110618_01-j.pdf
III. Recapping on the status of the "Roadmap towards Restoration from the Accident at Fukushima Daiichi Nuclear Power Station"
Details of the assessment are posted in:
http://www.tepco.co.jp/en/press/corp-com/release/11061702-e.html
In reviewing through this report, I found that one of the most serious issue is finding possible leakage locations in the Primary Coolant Boundary and the Containment Boundary. It is obvious that most of the water being injected into the reactor pressure vessel should be leaking before reaching to the final destination. To confirm my thesis, I calculated the most recent decay heat (at 100 days after the earthquake), a necessary water injection rate to remove the decay heat and the amount of water being injected. The results are summarized below, indicating that as much as 70-80 % of water is leaking and simply increasing the volume of highly contaminated waste water:
If there were no leakage, TEPCO could have injected sufficient amount of water to bring the temperature down below 100C. TEPCO seems to be confident that the leakage is through the melt-through at the bottom of the reactor pressure vessel. For me, the recent photos taken inside of the reactor buildings indicate that the “internal hydrogen explosions” have occurred in some magnitude as in the case of the Hamaoka Unit 1 hydrogen explosion/pipe rupture accident, introduced in Earthquake (70).
IV. City-wide radiation health assessment in Fukushima.
I have introduced this effort in Earthquake (77). The Fukushima Prefecture Government is preparing to initiate radiation health assessment for their entire population of 2.02 million people, as early as this summer. Prior to this, a preceding radiation health assessment will be started this month for the highly exposed member of the public of 28,000 people, in paticular those member of the public who work oudoor, such as farmers and construction workers. They lived in the “scheduled (and organized)” evacuation zone, including Iidate-mura, Kawamata-cho and Namie-machi (a part of the residents are in the “vigiliance (off-limit)” zone).
Well, let me stop here tonight.
Genn Saji
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