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Author:	Stéphane Baize
Date:	2016-03-11 00:26:00
Blog:	Stephane on Blogger Earthquakes, geology and related topics... but not only
URL:	http://stephaneonblogger.blogspot.com/2016/03/five-years-after-m9-tohoku-earthquake.html

Summary:

Five years ago, one of the heaviest nuclear accident (Fukushima Daiichi Nuclear Power Plant) was caused by a natural disaster, the M9 Tohoku earthquake and related tsunami. This event has significantly influenced the nuclear safety approach with respect to external hazards.Japanese Nuclear Power Plants (from Wikipedia). Red star depicts the M9 Tohoku earthquake epicnter.In March 2011, one of the biggest earthquakes ever recorded (M9) occurred off Japan, causing very strong shaking (with acceleration exceeding 1g and reaching 2 g) and triggering a big tsunami (with tsunami height up to 35 m locally). Earthquake and related-tsunami caused a lot of damages, casualties, and many deaths.You can find a page with many information on the earthquake, its tectonic setting, its effects, and so on the dedicated EMSC/CSEM page.A video and picture collection of damages related to the tsunami:  https://vimeo.com/itic/japan_tsunami_2011The size of the earthquake was "unpredicted" and is considered by many as an extreme event. Actually, this statement is mainly based on the fact that hazard was, along the Japanese coast, assessed with historical M8 events. Since this, the international community is exploring the resilience of nuclear plants to "extreme events" (earthquakes and floodings) which could occur at very low probabilities, beyond the historical catalogs. According to a IAEA report released in september 2012, "one of the lessons learned is that seismic hazard assessments based on historical data are not sufficient to capture low frequency seismic events. Investigations to collect prehistoric data are needed".Ground shaking associated with the M9 Tohoku quake (after NIED) and tsunami (yellow-red bars) and run-up (blue bars) heights (after Mori et al., 2011, Geophys. Res. Letters).Along the Pacific shoreline exposed to the Tohoku earthquake and related tsunami, the Tokai NPP, to the south, experienced a tsunami wave height of around 5 meters but its slightly higher protection wall prevented major flooding of the plant. At Onagawa NPP, the tsunami height was very high (around 13 m), but the protection seawall was higher. Despite a similar tsunami height in Fukushima Daiichi (13 m), the seawall was under-designed (5.7 m high).In the first months after the disaster, there has been a controversy about the relative role of shaking and tsunami flooding on the loss of reactors' cooling. A paper in The Independent (August 2011) defends the hypothesis that severe damages to reactor buildings and pipes were observed before the tsunami flooding, and even that radiation leakage started. In the 2015 IAEA report relating the accident, it is stated that "the earthquake damaged the off-site power lines and switchyard equipment and caused extensive damage to the infrastructure at and around the site". But this report also describes that the loss of off-site power triggered the activation of emergency diesel generators and batteries, which effectively provided power and started to cool the reactors. This emergency system was nonetheless drowned and destroyed by the tsunami flooding and core melting started.The tsunami wave is flooding the nuclear power plant site (Fukushima Daiichi), overwhelming the emergency generators which could not contribute to colling reactors, causing reactors explosion and melting. Picture TEPCO.The Japanese government created the Reconstruction Agency which role  is to "accelerate structural reconstruction and revitalization of the affected areas". You can find on its webpage (http://www.reconstruction.go.jp/english/) the status and progress of recovery. Currently, among the 470,000 initial evacuees, around 60,000 are still living in temporary housing.According to a recent paper in Science American, 5 years after the accident, the "Crippled Fukushima reactors are still a danger", and many questions about environment and health are still open. According to numbers reported by another paper in Scientific American, 3,200 people could die resulting from suicides and ailments during evacuation, and 1,000 deaths from cancers due to radioactive leaks are predicted.Just after the disaster, all the plants (22) have been shutdown and the Japanese government decided to decommission the reactors of Fukushima Daichi. As of beginning of 2016, only 2 nuclear power plants (3 reactors) have been restarted; the last reactors to have been accepted to reoperate are located at the Takahama (Source: Nuclear Energy Institute).Among the 22 sites, some are, or have been, under inspection for "geological" purposes: for instance, around the Tsuruga and Oi sites, tectonic investigations have been performed to conclude on the age of the last activity of nearby faults. The Tsuruga site is located nearby the active Urasoko Fault (AIST Fault Database). Until 2013, National authority (NRA) and operators argued about the activity of the secondary fault segments running beneath the reactors, which statement (on the basis of Japanese regulation) depends on the age of last faulting event. An independent panel of experts finally concluded that there was no positive evidence of recent (<125,000 years) fault segments' activity (see operator site). This contentious situation, mainly due to the regulatory context, could be overtaken by another approach, involving probabilistic assessment and expert panel elicitation process (e.g. Chapman et al., 2014).The operator decided to close the site, because of economic issues (World Nuclear News).Location of the Tsuruga reactors with respect to the fault segments (source http://ajw.asahi.com/).At Fukushima Daiichi NPP, five years after the disaster (2016), the site recovery is still on-going and, according to the operator TEPCO, decommissioning may last for decades. Japan also provided an updated report of the situation together with a roadmap to recovery. This report was analyzed by the International Agency (IAEA). See link1 and link2.To get info from the operator workplan for decommissioning, see this video: video1.Here is another video published by the operator TEPCO (in January 2016) to present the current situation at Fukushima Daiichi NPP: video2.A big challenge is to deal with the contaminated water, issue which is addressed through three actions: remove the sources of contamination, isolate water from contamination and prevent leakage of contaminated water. The contaminated water is stored in massive tanks. According to the operator, water has been cleared from the most dangerous radionuclide (Cesium, Strontium), but further treatment will be necessary to remove others. Tritium would remain an issue, because its extraction from water may pose problems for massive volumes.Dealing with the water contamination, the operator plans to use the main following means, to avoid ground water flow through the contaminated area:- A land-side impermeable wall is planned to be operational in the next months (2016). The soil will be frozen with a refrigerant injected as deep as 30 m. This wall role is to block the underground water flow into the contaminated area of unit 1 to 4.- The building of the sea-side impermeable wall has been completed in 2015. This wall is assumed to stop the flow of contaminated water from the reactor area into the ocean.Sketch of the main facilities for water management According to a report published in 2015 and reported by Scientific American, the fuel could be totally removed from the Fukushima site in 2020-2021 and the plant would be decommissioned between 2051 and 2061.The Fukushima disaster also had consequences all over the world. After 2011, the group of national safety authorities of Europe (ENSREG) launched the "stress tests" to check whether the safety standards used at the time of plant licensing were sufficient to cover unexpected, extreme and low probability events, including earthquakes and floodings. The countries, via their national authorities, presented a Stress Test report which was then reviewed by European experts. These gave recommendations which implementation is under the country responsability (European Commission).To come back to "natural hazards", the French authority asked to the operators to investigate the seismic hazard following a probabilistic approach, accounting for long recurrence times, low probabilities and severe events from which the crucial part and systems of the plant (hardened safety core) must be protected (see Autorité de Sûreté Nucléaire page in French and an abstract in English). In Spain, the authority pointed out the necessity to take into account the paleoseismological data for relevant active faults in the vicinity of plants (see ENSREG).

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earthquake