“Outline of the Fukushima Daiichi Accident. Lessons Learned and Safety Enhancements”

Abstract. On March 11, 2011, an earthquake and subsequent tsunamis off the Pacific coastline of Japan’s Tohoku region caused widespread devastation in Japan. As of June 10, 2016, it is reported that a total of 15,894 people lost their lives and 2,558 people are still unaccounted for. In Fukushima Prefecture, approximately 100,000 people are still obliged to live away from their homes due to the earthquake and tsunami as well as the Fukushima Daiichi accident.On the day, the earthquake and tsunami caused severe damages to the Tokyo Electric Power Company (TEPCO)’s Fukushima Daiichi Nuclear Power Station (NPS). All the units in operation, namely Units 1 to 3, were automatically shut down on seismic reactor protection system trips but the earthquake led to the loss of all off-site electrical power supplies to that site. The subsequent tsunami inundated the site up to 4 to 5 m above its ground level and caused, in the end, the loss of core cooling function in Units 1 to 3, resulting in severe core damages and containment vessel failures in these three units. Hydrogen was released from the containment vessels, leading to explosions in the reactor buildings of Units 1, 3 and 4. Radioactive materials were released to the atmosphere and were deposited on the land and in the ocean.One of the most important lessons learned is an importance to prevent such large scale common cause failures due to extreme natural events. This leads to a conclusion that application of the defense-in-depth philosophy be enhanced because the defense-in-depth philosophy has been and continues to be an effective way to account for uncertainties associated with risks. From the human and organizational viewpoints, the final report from the Investigation Committee of the Government pointed out so-called “safety myth” that existed among nuclear operators including TEPCO as well as the government, that serious severe accidents could never occur in nuclear power plants in Japan.After the accident, the Nuclear Regulation Authority (NRA) was established on September 19, 2012. The NRA very urgently developed and issued the new regulatory requirements on July 8, 2014, taking into the account the lessons learned from the accident. It is noted that the NRA issued the Statement of Nuclear Safety Culture on May 27, 2015 which clearly expressed the NRA’s commitment to break with the safety myth.This paper briefly presents the outline of the Fukushima Daiichi accident and summarizes the major lessons learned having been drawn and safety enhancements having been done in Japan for the purpose of giving inputs to the discussions to be taken place in the Special Invited Session “Fukushima, 5 years after”.

 In Unites 5 and 6, an air-cooled EDG of Unit 6 survived and it supplied AC power to both units.
 Then, about 40 min.later, the tsunami arrived at the site. Hydrogen leaked from RPV to PCV and then to the RB, and exploded there. We should have been prepared for "Low frequency, high consequence events" beyond design basis.
 Actually, the Defense-in-Depth (DiD) had not been taken into consideration for external events.
 Watertight doors were not implemented, Seawater pumps were not protected.AM measures were not effective, …  Training / drills assuming external events had not been conducted. DiD has been and will continue to be an effective way to account for uncertainties.
 Effective independence between different layers of defense  More "diversity" and "independence" rather than "redundancy" "Tsunami risk had been recognized":

Lack of Agility
 The Seismic Design Review Guide was revised in 2006 and a requirement was newly introduced against tsunami.So-called "backcheck" was in progress, but its progress was very slow.
 "Uncertainty allows for wishful thinking" + "Safety myth"  Lack of agility, lack of safety culture Diet Report  As the regulatory agency was aware of TEPCO's delaying of countermeasures, but did not follow up with any specific instructions.Nor did they properly supervise the back-check progress.
 When new findings indicate the possibility of a tsunami exceeding previous assumptions, the operator is required to quickly implement countermeasures, rather than taking time to clarify the scientific basis for that possibility through studies of sediment .... "Complacency" together with "Bureaucracy" allowed "Safety myth" to prevail, having let "Continuous improvement" cease.

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Observed max.acceleration of seismic ground motion at Fukushima-Daiichi exceeded that of the design basis by 26%.Occurred at 14:46 on March 11, 2011  9.0 Mw: largest ever recorded in Japan  Rupture duration: 170 sec  Epicenter: 38 o 10''N and 142 o 86''E, 23.7km in depth  Reverse fault type near the Japan Trench Edited to figure in "1 st Government Report (June) 2011)" A huge tsunami was generated due to a large slip, more than 56m, near the Japan Trench. Multi-segment rupture including "seismic gap area (no seismic record) " Due to rupture of the bonded interface, the North America Plate recoiled and the strain energy was released, which generated the huge tsunami.Sugino, et al., NRA, 24 Feb, 2016 In Fukushima Prefecture, approx.100,000 people are still obliged to live away from their homes due to the earthquake/tsunami as well as the Fukushima Daiichi accident.(website of the Fukushima Prefecture) Evacuation order: (1) Return is difficult (red), (2) Habitation is restricted (yellow), (3) Preparing for lifting orders (Green)  Yellow and Green areas will be lifted no later than March 2017. As of June 2016, the number of evacuees has been reduced from 470,000 to 155,000.Still 51,000 people live in temporary housing. Casualties due to earthquake/tsunami (as of Dec.1, 2011)  Deceased: 15,840  Injured: 5,951 Interim Report from Government's Investigation Committee (Dec.2011) Yasui, METI, "Causes and Countermeasures", March, 2012.

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Units 1 to 3 automatically scrammed on seismic reactor protection trips.The earthquake led to loss of offsite power.All the emergency diesel generators (EDGs) automatically started.DC power was available in Unit 3.  In Units 1 to 4, all the EDGs became inoperable (SBO) together with loss of DC power.*It also caused LUHS as well.

Diet` s 
Report: NAIIC : The National Diet's Fukushima Nuclear Accident Independent Investigation Commission, Reported to the Diet in July 2012 TEPCP's "Trial Calculations" of Tsunami New findings / minor opinion  In 2002, HERP* pointed out the possibility of a tsunami earthquake occurring anywhere along the Japan Trench including the gap area could not be ruled out.In 2008, TEPCO did hypothetical calculation assuming a M 8.2 earthquake occurring off the coast of Fukushima Pref.Max.～15.7 m  TEPCO also conducted a Jogan (869)-type tsunami simulation Max.～9.2 m  However, TEPCO decided only on a plan to ask JSCE* for investigation such as tsunami deposit survey, rather than take any immediate measures.Seismicity Gap Area (no record of large earthquake):  Major opinion: Majority of scientists believed the shallow plate boundary along the Japan trench was not able to store a large amount of strain (coupling is weak).*Headquarters for Earthquake Research Promotion *JSCE : Japan Society of Civil Engineers IAEA Fukushima report, Technical Volume 2/5 (2015) Message from Chairman  … this was a disaster "Made in Japan."Its fundamental causes are to be found in the … Japanese culture: our reflexive obedience; our reluctance to question authority; … and our insularity.Organizational issues …  … actual relationship lacked independence and transparency, … In fact, it was a typical example of "regulatory capture," … Lack of expertise  … the two incorporated technical agencies advising NISA, namely, JNES and JAEA, have been too rigidly tied to NISA …. Conclusions  … The lack of expertise resulted in "regulatory capture,"… They avoided their direct responsibilities by letting operators apply regulations on a voluntary basis.Regulatory Aspects: "Regulatory Capture" Pointed out in the Diet's Report "Myth of Safety"  … a fundamental problem of the inability to capture such crises as a reality that could happen in our lives; this, in turn, is the result of a myth of safety that existed among nuclear operators including TEPCO as well as the government, that serious severe accidents could never occur in NPPs in Japan."Basic Assumption" in the IAEA DG Report  Because of the basic assumption that NPPs in Japan were safe, there was a tendency for organizations and their staff not to challenge the level of safety. The reinforced basic assumption among the stakeholders about the robustness of the technical design of NPPs resulted in a situation where safety improvements were not introduced promptly.Human / Organizational Aspects: "Myth of Safety" Pointed Out in the Government's Report Final Report from Government's Investigation Committee (July 2012) /www.nsr.go.jp/data/000067048.pdf

Unit 1 Unit 2 Unit 3 14:46 on Mar. 11: Shutdown due to earthquake LOOP
: Loss of off-site power