For the third time in the history of the commercial use of nuclear power, operating a reactor during an emergency proved beyond human abilities, leading to a critical chain reaction of nuclear fission and the meltdown of highly radioactive fuel in the reactor core. The accidents at Three Mile Island, 1979, and at Chernobyl, 1986, constitute the precedents.
☢ At 14:46 on Mar. 11, 2011, the Tōhoku-Chihou-Taiheiyou-Oki earthquake struck the east coast of Japan with magnitude 9. The temblor sent the three at the time power-generating units of the Fukushima Daiichi nuclear power complex operated by Tokyo Electric Power Co. into emergency shutdown known with a SCRAM [first Tokyo Electric Power Co. press release with the title "The Effect of Earthquake Occurred in the Northern Part of Japan(as of 4:30 pm today)", dated Mar. 11, 2011].
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| Excerpt from the scram log of reactor No. 1. The log records the time stamps of the seismic trips and the pursuing actions. Systems to monitor neutron fluxes were activated (RBM - rod block monitoring, APRM - average power range monitor; RWM - rod worth minimizer). The control rods were inserted. Electric power sources were checked. The water level in the reactor pressure vessel (RPV) fell, likely because the main steam line valves closed as part of the SCRAM, shutting off feedwater and the main steam turbine path. RPV pressure built immediately. Within a minute the Reactor Protection System provided makeup coolant, possibly involving the Isolation Condenser. |
In accord with the NHKWorld report, documents released by TEPCO suggest that the reactor vessel pressure significantly diminished within roughly 15 minutes after the automatic engagement of the isolation condenser of the emergency core cooling system (ECCS), but returned to pre-quake levels within another 15 minutes (TEPCO press release with the title "Submission of a report regarding the plant parameters of Fukushima Daiichi Nuclear Power Station at the time of the earthquake to Nuclear and Industrial Safety Agency, Ministry of Economy, Trade and Industry", dated May 16, 2011). The pressure curve below documents this quick turn around. The recording stops within five minutes after the tsunami hit.
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| Excerpted from TEPCO Press Release, May 16, 2011. |
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Control room whiteboard with operator notes on Reactor 1 through the first 30 minutes after the seismic SCRAM. The entry at 15:16 mentions the isolation condenser the first time (courtesy TEPCO).
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☢ At 15:29, a radiation monitor about a mile from reactor unit 1 sounded alarm because of high amounts of radiation according to Yuji Okada, Tsuyoshi Inajima and Shunichi Ozasa's post with the title "Fukushima May Have Leaked Radiation Before Tsunami" published online on Bloomberg May 19, 2011.
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| The above photograph taken May 10, 2011, and added Feb. 27, 2012, shows the pair of steam vent pipes for isolation condenser subsystem A and B, exiting the reactor building of Unit 1 in the middle of the wall immediately under the refueling floor. Note the vent pipe surrounded by debris is clearly open. This is the vent of subsystem A, from which steam was observed emanating in the hours after the quake. Both subsystems were actuated automatically after the SCRAM and shut down by the operator minutes later. Only subsystem A was re-activated before the power outage (courtesy Daisuke Tsuda). |
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| Picture taken from the 4th floor of the Radiation Waste Treatment Facility of the Daiichi Fukushima nuclear power complex showing the north side at approx. 15:42 (TEPCO Archive of May, 2011). |
The loss of coolant led to a complete meltdown of reactor fuel on that very day (TEPCO press release with the title "Reactor Core Status of Fukushima Daiichi Nuclear Power Station Unit 1", dated May 15, 2011). A hydrogen explosion on the following day resulted in a massive release of airborne radiation.
In the weeks since, TEPCO poured tens of thousands of tons of salt and fresh water into the reactor in an attempt to cool the molten fuel slumped at the bottom. Highly radioactively contaminated water is seeping through cracks, broken seals and shattered pipes (Jiji Press post with the title "Over 100,000 Tons of Polluted Water Seen at Fukushima N-Plant" dated May 18, 2011), possibly escaping into the open sea. The situation is by far not yet under control.
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| The oldest are the most vulnerable (schema of Reactor 1, a boiling water reactor). Molten fuel presumably collected at the bottom of the oblong reactor pressure vessel (RPV) at the center. Some may have escaped into the pear-shaped drywell of the primary containment, because indirect evidence suggests that the RPV's bottom has been breached and the containment is damaged according to a NHKWorld News post with the title "No.1 reactor has 4.2 meters of contaminated water" published online May 20, 2011. Moreover, we cannot rule out that molten fuel may have reached the doughnut-shaped suppression chamber of the primary containment seen in cross-section at the very bottom of the drawing. |
The reactors are still smoldering. The crisis is far from over. I shall update the time line of the first-day events leading up to the core meltdowns at Fukushima as more details become available. They crassly reveal once more that in times of extraordinary crisis, when nuclear reactors are impacted by events outside the scope of their design basis, the human mind is prone to fail.
RPV Temperature of Fukushima Dai-ichi 1 号機 [°C]
Date and Time of Last Update:
Radiation Dose at the Bottom of the Primary Containment of Fukushima Dai-ichi 1 号機 [Sv/h]
1 Sv = 100 rem
At this dose, we develop acute radiation syndrome. We would die within minutes staying at the location of the dose measurement above!Acknowledgment
I collected most helpful technical information from this exhaustive scribble Live discussion and are indebted to its many contributors with their diverse views and backgrounds.
Addenda
- This Mainichi Daily News post with the title "No. 1 reactor pressure vessel likely damaged immediately after quake" published online May 26, 2011, reports that “at the reactor, the magnitude 9.0 quake registered an intensity smaller than envisaged under its quake-resistance design.” TEPCO said they suspect, however, that the temblor directly damaged the RPV and/or accessory piping (06/02/2011).
- The National Geographic Daily News published an informative article by Josie Garthwaite with the title "Would a New Nuclear Plant Fare Better than Fukushima?" online on Mar. 23, 2011, which tells us about recent developments in nuclear reactor design thought to improve the ability of preventing a fuel core meltdown after a LOCA which happened to three reactors after the earthquake and tsunami at Fukushima Daiichi Nuclear Power Station. It remains to be seen whether the passive, gravity-driven emergency cooling systems implemented in the most recent reactor generation actually deliver an enhanced functionality compared to the emergency core cooling system that was supposed to be able to cool the reactors at Fukushima solely on battery power still available on that fateful day (06/03/11).
- On Sep. 20, 2007, TEPCO released a press announcement with the title "Results of a general study of potential seismic impact on safety-significant equipment in the Fukushima-Daiichi Nuclear Power Station and the Fukushima-Daini Nuclear Power Station in light of the Niigata-Chuetsu-oki Earthquake in 2007". In the appendix to this release with the title "A summary of the “Report on the results of a general study of potential seismic impact on the main facilities of the nuclear power stations made on the basis of the observational data at the Kashiwazaki-Kariwa Nuclear Power Station”" the company published a graph shown below comparing two acceleration vs. frequency seismic spectral curves.
One curve represents the seismic spectrum recorded at the Kashiwasaki Kariwa Nuclear Power Station during the 2007 earthquake (red). The other represents the design basis spectrum for Fukushima Daiichi Unit 1 (black), apparently intended as reference for the quake spectrum observed at Kashiwasaki Kariwa. Vertical lines indicate what I assume are the resonance frequencies for certain reactor components. The earthquake produced the greatest increases in acceleration above design basis around 0.25 cycles/s, that is near the resonance frequencies for FUEL BUNDLE and SHUTDOWN COOLING SYSTEM PIPE. Roughly equal or higher accelerations were found at 0.40 cycles/s and greater, but no component resonances are provided for this range. The seismic design weaknesses uncovered above may have contributed substantially to the early demise of Unit 1 (06/17/2011).
Comparison of seismic spectra presented in TEPCO press release dated Sep. 20, 2007. - Unit 1 is equipped with two independent isolation condenser subsystems named A and B, only one of which was activated during the shutdown after the quake. According to this excerpt from the Report of the Japanese Government to the IAEA Ministerial Conference on Nuclear Safety dated June, 2011, page IV-39, the operators in the control room were instructed by manual to shut the isolation condenser down, if the temperature in the reactor pressure vessel dropped faster than a prescribed rate:
“The shutoff of the MSIV increased the RPV pressure, and at 14:52 the IC automatically started up. Next, in accordance with the operating manual for the IC, at 15:03 the IC was manually shut down. The manual notes that the temperature decrease rate for the RPV should be adjusted to not exceed 55 °C/h. Moreover, the reactor pressure varied three times between 15:10 and 15:30, and TEPCO performed manual operations using only the A-system of the IC. Note that when the IC is operated, the steam is condensed and cooled, and is returned into the reactor as cold water through the reactor recirculation system. The records of the temperatures at the entrance to the reactor recirculation pump show three drops in temperature, so this is assumed to be the effects of the manual operation of the IC.”
A seismic SCRAM following a temblor of beyond design basis magnitude causing obvious massive damage should have unequivocally overridden this instruction (07/08/2011). - RADIATION EMBRITTLEMENT, corrosion, fatigue and other wear dictate the rate of temperature change at which the reactor pressure vessel (RPV) can be cooled down at given pressure and temperature without cold-shock damage. The permissible rate is re-assessed periodically as part of the reactor coolant system (RCS) PRESSURE AND TEMPERATURE LIMITS REPORT or PTLR for short. NUREG-1433 Vol. 1, Rev. 1, with the title "Standard Technical Specifications General Electric Plants, BWR/4", technical specification 5.6.6, details the report's requirements for the reactor type of Units 2, 3 and 4 at Fukushima Daiichi Nuclear Power Station. Similar regulations apply to the reactor type of Unit 1 as well. As a common rule, with increasing age more time is needed to cool the RPV down safely.
According to the Amendment to the Facility Operating License for Oyster Creek Nuclear Generating Station filed by Amergen Energy Co., LLC, with the Nuclear Regulatory Commission (docket no. 50-219) on Nov. 27, 2006, p 3.3-6, the RPV of a BWR of similar type and age as Fukushima Daiichi's Unit 1 can be safely cooled at 300 °F/h, or 149 °C/h, in an emergency shutdown. In accord, Unit 1 could have reached cold shutdown, that is an RPV temperature of less than 100 °C, in two to three hours after the seismic SCRAM on March 11, 2011. However, the plant operators determined that the RPV was cooling down too fast. In an effort to prevent damage, they turned off the isolation condenser that provides coolant to the RPV during shutdown. After the tsunami arrived, they could not restart this system. After four decades of service, the vessel's wear may have prevented a timely shutdown at this dire moment, a circumstance that may pertain to any aging nuclear power reactor (07/22/2011). According to the update of the Government of Japan provided the International Atomic Energy Agency with the title "Additional Report of Japanese Government to IAEA - Accident at TEPCO's Fukushima Nuclear Power Stations Transmitted by Nuclear Emergency Response Headquarters, Government of Japan, 15 September 2011", p II-80, the operator manual for Reactor 1 limited RPV cooling to only 55 °C/h (10/21/2011). - The footage below was recorded by TEPCO engineers inspecting Unit 1's reactor building floor with the isolation condenser of Reactor 1 in October, 2011. This must have been their first visit since a hydrogen explosion destroyed the refueling floor above one day after the earthquake and tsunami (TEPCO press release video -36 s, dated Oct 21, 2011). Note the CC button on the bottom bar. A friend was so kind to translate a good part of the conversation the visitors had into English (annotations are bracketed in curved and curly braces). The radiation levels were quite high on the third floor near piping and valves for the isolation condenser (at ca. 13:00). Exposed to 100 milliSv/h or more, we would develop symptoms of acute radiation syndrome in ten hours or less!
- A most exhaustive description of the isolation condenser is provided in section 6.3.2.1 of Part 6 of 7 - GE BWR/4 Advanced Course R-504B. According to this document, the operator did not have to fully close the isolation condenser valve on the condensate return line. Rather, he could have throttled the valve. Maintaining some condensate flow might have preserved the systems functionality (11/30/2011).
- TEPCO's evaluation of the walk-down of Unit 1 shown in the video above was published in a press release with the title "Fukushima Nuclear Accident Analysis Report (Interim Report)" on Dec. 2, 2011. I quote from page 40:
“3) Results of Unit 1 IC walk-down [Attachment 6-8 (3)]
・ The main unit of the IC installed in the Unit 1 reactor building, main pipes, and valves were visually investigated to confirm whether or not there was any damage that could cause the reactor to lose its cooling water. Since the inside of the PCV could not be entered, main body, pipes and valves outside of the PCV were investigated.
・ On the 4th floor of the reactor building where the main unit of the IC is installed, a hole was made on the north-side ceiling due to the hydrogen explosion on the 5th floor. Some of the insulating material at the top part of the IC's north side was scattered among the rubble and considered to have been blown off by the explosion. Furthermore, the insulating material on the south side of the main unit of the IC was also severely torn off and it had fallen down, which was on the reactor building equipment hatch (opening on the floor) side. It is considered that the hydrogen explosion on the 5th floor blasted through the opening and damaged the insulating material on the IC. None of insulating materials on the 3rd or 2nd floor was found to have been torn off or scattered.
・ No damage was found on the main unit of the IC. No ruptured pipes, leakage from flange sections, and broken valves were found. Also, no trace by a blast of the high pressure steam from the reactor was found.
・ Judging from the above, it was confirmed that there was no damage to the IC equipment located outside of the PCV that could have caused loss of reactor cooling water.
・ In addition to this field walk-down, the positioning status of IC valves and IC water level were also checked. It was confirmed that Valve 2A and Valve 3A of the Subsystem-A were open, and Valve 2B and Valve 3B of the Subsystem-B were closed. Not only that, both Subsystem-A and Subsystem-B that make up feed valves to the IC were also confirmed to be closed. The IC field water level gauges (cooling water) indicated 65% for the Subsystem-A and 85% for the Subsystem-B. This was confirmed to match the instrumentation in the MCR (main control room).”
The December interim report provides the most detailed, presently available account on the operators' efforts to actuate the IC in the hours after the quake on pp. 53 (02/09/2012).
- I found the radiation survey of Unit 1's 4th floor conducted during the October walk-down in this TEPCO report with the title "Unit 1-3 core about the state of Fukushima Daiichi Nuclear Power Station (Nov. 30, 2011)". The dose rates are expressed in [mSv/h] in the resulting survey map below. The twin boilers on the left are the isolation condenser heat exchangers inspected in the video. The left boiler (A) belongs to subsystem-A which was utilized in the hours after the quake. The right boiler (B) of subsystem-B was left idle. The video begins with the men inspecting the southside (bottom) of the isolation condenser (02/15/2012).
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| Schematic drawing of a boiling water reactor 3 (BWR-3) similar to Fukushima Daiichi Nuclear Power Station Unit 1, showing the twin boilers of the isolation condensers right under the refueling floor on the left side. This must be the approximate location visited in the video (courtesy Letsbereal's comment #461 on Prison Planet Forum). |
- In this TEPCO handout with the title "Evaluation of operating conditions of Isolation Condenser, Unit1, Fukushima Daiichi Nuclear Power Station" dated Nov. 22, 2011, the company reconstructs the use of the isolation condenser on the day of the earthquake and tsunami in greatest detail to date. According to TEPCO, both subsystems of the isolation condenser were engaged after its automatic actuation following the reactor scram until the operator shut the isolation condenser down. Therefore, subsystem B was in use approximately from 14:52 to 15:03 on March 11 (05/11/2012).
- On page 24 of TEPCO's press handout with the title "Fukushima Nuclear Accident Investigation Report (Interim Report – Supplementary Volume)" dated Dec. 2, 2011, the company states:
“ At around 15:03, reactor pressure dropped quickly in conjunction with IC started up, and the IC’s return line’s isolation valves (MO-3A, 3B) were temporarily “fully closed” after it was determined that the pressure vessel temperature drop rate of 55 degree C/h as stipulated in the operating procedures could not be adhered to. Other valves were left open and on standby. After this it was determined that one IC system was sufficient for controlling reactor pressure at around 6 to 7MPa and the subsystem-A was assigned with this task. The reactor pressure was then control by opening and closing the return line’s isolation valve (MO-3A).
Although the above shutdown procedural actions have been pointed out by some as an operation error, the operators conducted the operations as stipulated in the operating procedures.”
However, on page 16 of their study of potential consequences of a hypothetical station blackout at Browns Ferry Nuclear Power Station in Alabama with the title "Station Blackout at Browns Ferry Unit One - Accident Sequence Analysis", Cook and others (1981) suggest that “a depressurization at the Technical Specifications limit of 55.6°C/h (100°F/h) will produce a gradual reduction of the drywell ambient temperature; a rapid depressurization will produce a faster drywell temperature reduction, but is probably not necessary. Certainly, if the operator perceives developing difficulties with the in-drywell equipment such as the relief valve solenoids, he should convert an ongoing 55.6°C/h (100°F/h) depressurization into a more rapid one.”
The operator's guide used at Fukushima should have included this provision (05/17/2012)! - If the isolation condenser of Fukushima Daiichi Nuclear Power Station Unit 1 was designed like that at Oyster Creek Generating Station in the US, a filling of shell water in one train of the condenser would have lasted 45 minutes of operation without makeup water (Grant and others, 1997). Depending on the particular design of the reactor, condensate water from a tank, demineralized water or fire water could have been used for makeup. In US reactors, a condensate water tank filling alone could have extended condenser operation up to 6 hours. Because TEPCO re-activated only one isolation condenser train and makeup water may have been unavailable as a consequence of the station blackout, the shell water in the active condenser train could have completely boiled off at 16:30 on Friday, Mar. 11 (09/02/2013).
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