Fukushima Nuclear Accident: Tsunami Induced But Man-Made Disaster

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Fukushima Nuclear Accident: Tsunami Induced But Man-Made Disaster


Modern Tokyo Times

There is no doubt that the Tohoku quake, 9.0 on the Richter scale, with its associated tsunami up to 40 m, has been one of the worst natural disasters in recorded history. The scale of the tragedy is epic. It has been well publicized throughout the world. In relative proximity of the epicenter there were 14 nuclear power plants. All of them shutdown automatically as well as the nearby coal-fire plants. The quake knocked out the Northern Japan power grid, while the resulting tsunami disabled the onsite electric power supplies at the Tokyo Electric Power Company (TEPCO) Fukushima Daiichi six unit nuclear power plant causing a nuclear emergency due to loss of water injection ability.  All six units are Boiling Water Reactors (BWRs), originally designed by General Electric (GE) and later by GE’s Japanese licensees Toshiba and Hitachi, commissioned in the 1971-1978 time frames. Units 1-3 were generating electricity at the quake time (14:46 March 11), while units 4-6 were shutdown for inspections. For units 1-4 the nuclear emergency led to serious nuclear accidents. The author asserts that these accidents were preventable and thus the disaster amounts to tsunami induced man-made disaster. The author faults both the TEPCO management as well as the regulator: Japan Tokyo Electric Power Company (TEPCO)a part of the Ministry of Economy, Trade, and Industry (METI), with its advisory panel: The Nuclear Safety Commission. The plant workers performed admirably in the presence of more than 900 aftershocks, including 60 over 6.0 on the Richter scale and three over 7. The surrounding infrastructure was obliterated with many plant workers losing their homes.

Impact of Quake

The Japanese plants have been designed to operate in high seismicity areas although not as high as the Tohoku quake. There were several seismic upgrades including the one after the 2007 Kashiwazaki quake which raised the standard to1000 gal ground acceleration.  TEPCO raised the seismic standards for the Daiichi plants to 600 gals. However, presumably due to age this standard was not implemented at Daiichi units 1-4. The ground acceleration measured at Daiichi unit 3 was 507 gal compared to the design basis of 441. The structures are very robust, located on solid rock, so they withstood greater than the design basis earthquake. It appears, subject to inspection, that all affected plants remained structurally sound in response to the Tohoku quake.

 Tsunami Impact

With regard to the tsunami the case is, however, very different. The plants were designed for 5.7 m waves, while the Tohoku tsunami impacting the Daiichi site amounted to 14-15 m. At quake time the president and chairman of TEPCO were away from their offices on business trips while the Prime Minister (PM) was testifying in the Diet. At 15:42 the tsunami flooded both the Daiichi and Daini Fukushima sites. At Daiichi it obliterated the service water pumps causing loss of the ultimate heat sink. Twelve diesel generators (DGs), two per unit, were disabled by flooding the fuel tanks thus causing loss of all AC power. The service building didn’t have waterproof doors. Daini site was also flooded but there was no loss offsite power saving the plants from accidents. An abbreviated accident timeline for the Daiichi Units # 1-4 follows.

Batteries, PM Actions

The only source of electric power left were the batteries, which were estimated to last about 8 hrs. TEPCO informed the PM that there would be no problem for 8 hrs which gives them adequate time to bring mobile DGs. With this info in hand the PM addressed the nation at 17:00 and said that tsunami caused a serious nuclear accident but there was no release of radioactivity. At 19:03 the PM declared nuclear emergency and at 21:23 directed local officials to evacuate residents within radius of 3 km from the Daiichi site and advise those in the 3-10 km to stay indoors.

Core Cooling 

The nuclear safety imperative for all light water reactors (LWRs) is to keep the core flooded at all times. In case of BWRs passive core cooling system known as the Reactor Core Isolation Cooling (RCIC) provides core cooling for a limited amount of time. In order for the RCIC to function it is necessary to have the battery power and for the wetwell (containment condensation chamber) temperature to be below less than 100 degrees C. In case of unit #1 the batteries were soaked by the tsunami flooding and failed at 16:36. The core was not cooled for the next 27 hrs resulting in core overheating with associated fuel damage and release of radioactivity from the fuel.  In case of the unit #2, the RCIC operated for ~70 hrs and in case of the unit #3 for ~35 hrs. At about 0:00 on March 12 the unit #1 reactor vessel pressure reached 2.1 of the design pressure while radiation was detected in the turbine building. Clear indication that the core was overheating and that emergency injection of seawater must become the top safety priority since no source of fresh water was available. Unit #1 was the first to require injection of seawater as well as the containment venting.  

Containment Venting 

At 1:30 the head of the Japanese Nuclear Energy Safety Commission (JNES) advised the PM that the unit #1 containment must be vented. At 3:00 Chief Cabinet Secretary Yukio Edano informed the public that final venting preparations were taking place. At 5:44 the PM ordered evacuation to 10 km. At 6:00 the PM inquired as to when venting would start. TEPCO’s response “Venting power source cut off.”  This meant that there was no power for the vent blowers necessitating a manual opening which was difficult to accomplish. At 7:00 the PM arrives at the site angry and questions the TEPCO executive VP “Why don’t you hurry with venting.” He demanded to see the plant manager, liked the guy and wanted to deal with him directly. At 7:50 the power trucks arrived but power couldn’t be connected to the plant as the plugs didn’t fit. At 9:00 the containment venting procedure was initiated. At 11:52 Cs-137 was detected at the main gate.  NISA declared unit #1 core-melt. At 14:30 the containment vent was finally open.

 Unit #1 Hydrogen Explosion

 At 15:36 Hydrogen explosion took place blowing off the steel service floor roof. Four plant workers were injured. Why the containment was vented via the service floor area rather than the stack is unclear. TEPCO didn’t seem to understand that it was a hydrogen explosion as their press releases kept referring to the Nagano earthquake, independent of Tohoku, which took place that day. At 18:25 the PM extended the evacuation to 20 km.

 Sea Water Injection

 The PM ordered pumping of seawater, which TEPCO implemented at 20:20. At 22:15 it was suspended due to an aftershock but resumed at 0:00 March 13. It is incomprehensible as to why TEPCO waited 27 hrs to inject the seawater into the core of unit #1 via the fire extinguisher system. Delay with the containment venting was a reason but not the only one. There was a hesitation and indecision resulting in delays. Injection of seawater meant widespread corrosion and most likely the plant write-off. Presumably the plant manager was not empowered to make such a decision in a Japanese top-down management structure, while the top management didn’t understand that the core was melting so the PM had to act. TEPCO demonstrated a lack of the safety culture.

 Unit #3 Hydrogen Explosion

 At 2:44 batteries in unit #3 were depleted causing no further core cooling. Rapid core uncovering was recorded at 4:15. The containment was vented at 8:41 and seawater pumping initiated at 13:12. It was suspended for both unit #1 and #3 between 1:10 and 3:20 on March 14 due to lack of seawater in seawater pools. At 11:01 the Unit #3 hydrogen explosion took place in exactly the same manner like in case of unit #1, blowing the service floor roof and injuring 11 plant workers. The service floor area contained panels which were not opened showing that little was learned from the first explosion. This mistake was not repeated in case of unit #2.

Unit2 Containment Damage

 At 13:25 the RCIC pump at unit #2 failed. Core meltdown was announced at 18:22, while the seawater pumping was initiated at 20:05. At 22:22 wetwell damage was suspected meaning that highly radioactive water was spilled into the reactor building and from there into the turbine building. It amounted to some 25,000 tonnes of water. This highly radioactive water, which was leaking into the ocean for a while is now being transferred into a centralized waste treatment  building . There was an informal TEPCO intent to evacuate employees. The PM responded: “Is TEPCO planning to abandon roll as electric power company.” The containment was vented at 0:02 on March 15 but there was no hydrogen explosion as the service floor area panels were opened. Later in the morning TEPCO reduced the staff to 50, those needed for the water injection operations only.

Unit #4 Spent Fuel Pool Explosion

 At 6:14 an explosion took place in the Spent Fuel Pool (SFP) of unit # 4 causing a hole in the reactor building. Many attributed it to yet another hydrogen explosion but that may not be the case. The pool is located at the fifth floor of the reactor building so it is conceivable that the quake could have pushed water out of the pool. Unit #4 SFP had 1,331 spent fuel assemblies, almost as many as units # 1-3 put together because the whole core was removed into the pool due to the core shroud replacement. Due to loss of all AC power no cooling was provided to any of the pools prior to this explosion. It can be assumed that the SFPs received little attention from the plant personnel as they were too busy handling multiple meltdown scenarios. This brings up the subject of vulnerability of multi-unit sites to severe accidents when extra staffing is vital and could not be provided due to infrastructure damage. A visual inspection by remote controlled camera on April 29 has shown no significant fuel damage.

 Who was in Charge?

The PM called the cabinet meeting at 15:30. The upshot was to set up integrated headquarters at TEPCO. TEPCO went along. The PM met with TEPCO executives and asked “What do you intend to do?” President Shimizu replied “We will make every effort to protect Fukushima Daiichi.” After 3 hrs with the TEPCO executives the PM fell asleep seated. Later on he told aides “Bring all info to me. I will make decisions.” At this point in time he assumed the full responsibility compatible with the Japanese law, which lays out measures to be taken by the government in case of a nuclear emergency in order to facilitate necessary inter-agency coordination. Mismanagement phase of the accident turned into a management with missteps.

 An Accident Summary

Mismanagement phase of the accident ended on March 16. Between March 11 and 16 three partial core meltdowns took place plus reactor building damage in unit #4. In units #2 and #3 there appears to be either containment or pressure vessel damage. It was thought that fuel damage in unit #1 amounted to 55%, unit #2 35% and unit #3 30%. My guess is that the percentages are higher. However, the corium mass, which ended at the bottom of the reactor vessel appears to be adequately cooled. Nonetheless volatile and soluble fission products, such as I-131, Cs-134 and Cs-137, were released to the environment and amounted to ~1/10 release compared to the Chernobyl accident in 1986. Despite severity of the accidents there were no early fatalities or even early injuries. This includes the plant workers. 21of them were exposed to >100 mSv, 2 to 200-250 mSv, which should be harmless to human health. Units # 1-4 are a total plant write-off and must be decommissioned over the next decade or so with the cost likely to exceed $10 billion. This writer disputes the finding that the Fukushima accident should be classified is in the same class as Chernobyl (level 7). More likely it is Level 6.

Initially, severity of the accidents was not grasped and no adequate staff was provided to handle probably the most complex accident in the commercial history of nuclear power. TEPCO and NISA seem to have believed that such an accident scenario was not credible and thus no adequate precautions were in place. There was obvious a lack of emergency drills as several missteps took place. The response boiled down to hesitation, delays, and indecision. The coordination between TEPCO and NISA were inadequate. The Japanese law must be amended to address clearly a division of responsibility between the utility and the government agencies. The PM was mad at both of them and hired his own advisers. Some of these inadequacies were of course recognized as the public apologies were offered. The media in Japan has zoomed in on cozy regulator to utility culture, the culture of complicity or an amakudari system. An example was given that 68 former government bureaucrats assumed executive posts in utilities. In addition, the author wishes to point out a lack of safety culture as well as stifling Japanese top-down culture, which doesn’t empower lower levels to make decisions in these types of situations when timing of decision making is crucial. Management training courses including in particular responses to severe accidents, beyond the design basis should also be on the agenda. Once the damaged plants have been stabilized and brought to the safe shutdown level, an independent team of international and national experts should be assembled to provide a thorough accident assessment and propose applicable lessons learned. This should then be shared with operators of the remaining 442 power reactors operating in over 30 countries worldwide as well as 109 forthcoming. A due attention should be paid to multi-unit site in excess of, say, four units. 

      * Writer has the PhD in nuclear engineering and is a retired nuclear safety specialist.


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