The Great East Japan Earthquake of magnitude 9.0, considered to be one of the largest earthquakes in the world, occurred on the east coast of northern Japan at 2.46 pm on Friday, March 11, 2011. It generated a series of huge tsunami which struck the east coast of Japan, the highest being 38.9m at Aneyoshi, Miyako with the duration of 3 minutes. Japan moved a few metres east and the local coastline subsided half a metre. The tsunami inundated about 560 sq km and resulted in a human death toll of over 19,000 and much damage to coastal ports and towns with over a million buildings destroyed or partly collapsed. Electricity, gas and water supplies, telecommunications, and railway service were all severely disrupted and in many cases completely shut down. These disruptions severely affected the Fukushima Daiichi nuclear power plant.
Following the earthquake on Friday afternoon, the nuclear power plants at the Fukushima Daiichi, Fukushima Daini, Higashidori, Onagawa, and Tokai Daini nuclear power stations (NPSs) were affected, and emergency systems were activated. The tsunami caused a loss of all onsite and offsite power at the Fukushima Daiichi NPS, leaving it without any emergency power. The subsequent tsunami caused significant damage to at least four of the six units of the Fukushima Daiichi nuclear power station. The resultant damage to fuel, reactor, and containment caused a release of radioactive materials to the region surrounding the NPS. In addition to triggering the tsunami, this earthquake caused thousands of deaths and economic losses approaching $500 billion (USD). Yet, despite the sheer scale of destruction in northeastern Japan, the accident at the Fukushima Daiichi nuclear power station (NPS) has come to define the tragedy for many and has become a momentous event in nuclear power technology.
Of the more than 400 NPPs currently operating throughout the world, accumulating ~16,000 years of reactor experience, >90 percent are light water reactors (LWRs), which produce heat by controlled nuclear fission and are cooled by water. In the United States, all 104 operating NPPs are LWR NPPs. There are two general LWR designs: boiling water reactors (BWRs) and pressurized water reactors (PWRs). In BWRs, the heat generated by fission turns the water into steam, which directly drives the power-generating turbines and the electrical generator connected to them. In PWRs, the heat generated by fission is transferred to a secondary loop via a heat exchanger (steam generator), where the steam is produced and drives the power-generating turbines. In both BWRs and PWRs, after flowing through the turbines, the steam turns back into water in the condenser. The water required to cool the condenser is taken from and returned to a nearby ocean, river, or water supply.
The Japanese NPPs involved in the Fukushima Daiichi accident were BWR NPPs. The following graph illustrates the configuration of a BWR:
In a BWR NPP, the nuclear reactions take place in the nuclear reactor core, which mainly consists of nuclear fuel and control elements. The nuclear fuel rods (each ~10 mm in diameter and 3.7 m in length) are grouped by the hundred into bundles called fuel assemblies power after a few hours, water must be circulated within the RPV to maintain adequate cooling. This cooling is provided by numerous systems. Some systems operate during normal conditions, and some systems, such as the emergency core cooling systems (ECCSs), respond to off -normal events. Normal reactor cooling systems maintain the RPV and temperature and a proper cooling water level, or if that is not possible, ECCSs directly flood the core with more water.
According to the World Nuclear Association, in the last century there have been eight tsunamis in the region with maximum amplitudes at origin above 10 metres (some much more); these having arisen from earthquakes of magnitude 7.7 to 8.4, on average one every 12 years. Those in 1983 and in 1993 were the most recent affecting Japan, with maximum heights at origin of 14.5 metres and 31 metres respectively, both induced by magnitude 7.7 earthquakes. The June 1896 earthquake of estimated magnitude 7.6 produced a tsunami with run-up height of 38 metres in Tohoku region, killing 27,000 people.
In case of the Fukushima Daiichi Nuclear plant, eleven reactors at four nuclear power plants in the region were operating at the time and all shut down automatically when the quake hit. Subsequent inspection showed no significant damage to any from the earthquake. The operating units which shut down were Tokyo Electric Power Company’s (TEPCO) Fukushima Daiichi 1, 2, 3, and Fukushima Daini 1, 2, 3, 4, Tohoku’s Onagawa 1, 2, 3, and JAPCO’s Tokai, total 9377 MWe net. Fukushima Daiichi units 4, 5 & 6 were not operating at the time, but were affected. The main problem initially centred on Fukushima Daiichi units 1-3. Unit 4 became a problem on day five.
The reactors proved robust seismically, but vulnerable to the tsunami. Power, from grid or backup generators, was available to run the Residual Heat Removal (RHR) system cooling pumps at eight of the eleven units, and despite some problems they achieved “Cold Shutdown” within about four days. The other three, at Fukushima Daiichi, lost power at 3.42 pm, almost an hour after the quake, when the entire site was flooded by the 15-metre tsunami. This disabled 12 of 13 back-up generators on site and also the heat exchangers for dumping reactor waste heat and decay heat to the sea. The three units lost the ability to maintain proper reactor cooling and water circulation functions. Electrical switchgear was also disabled.
Thereafter, many weeks of focused work centred on restoring heat removal from the reactors and coping with overheated spent fuel ponds. This was undertaken by hundreds of employees as well as some contractors, supported by firefighting and military personnel. Some of the TEPCO staff had lost homes, and even families, in the tsunami, and were initially living in temporary accommodation under great difficulties and privation, with some personal risk. A hardened emergency response centre on site was unable to be used in grappling with the situation due to radioactive contamination.
Three TEPCO employees at the Daiichi and Daini plants were killed directly by the earthquake and tsunami, but there have been no fatalities from the nuclear accident.
Among hundreds of aftershocks, an earthquake with magnitude 7.1, closer to Fukushima than the 11 March one, was experienced on 7 April, but without further damage to the plant. On 11 April a magnitude 7.1 earthquake and on 12 April a magnitude 6.3 earthquake, both with epicenter at Fukushima-Hamadori, caused no further problems. Here is how the World Nuclear Association summarized the accident:
- Following a major earthquake, a 15-metre tsunami disabled the power supply and cooling of three Fukushima Daiichi reactors, causing a nuclear accident on 11 March 2011;
- All three cores largely melted in the first three days;
- The accident was rated 7 on the INES scale, due to high radioactive releases in the first few days. Four reactors are written off – 2719 MWe net;
- After two weeks the three reactors (units 1-3) were stable with water addition but no proper heat sink for removal of decay heat from fuel. By July they were being cooled with recycled water from the new treatment plant. Reactor temperatures had fallen to below 80C at the end of October, and official ‘cold shutdown condition’ was announced in mid-December;
- Apart from cooling, the basic ongoing task is to prevent release of radioactive materials, particularly in contaminated water leaked from the three units; and
- There have been no deaths or cases of radiation sickness from the nuclear accident, but over 100,000 people had to be evacuated from their homes to ensure this. Government nervousness has delayed their return.
The Fukushima Daiichi accident produced radioactive gaseous, liquid, and solid wastes. The gaseous emissions were released in the early days of the accident and have dispersed and decayed to small levels and are no longer a health threat. Based on measurements in November, TEPCO has already declared that significant gaseous releases have stopped and that the temperatures in all three reactors are <75°C (167°F). Liquid waste management and the cleanup and management of the water that was injected into the reactors and SFPs had been a major concern. For many weeks following the accident, rainwater mixed with the water that had been injected into the reactors and SFPs was accumulating in NPS buildings and tanks. As the buildings and tanks filled up, additional temporary storage tanks were brought in to hold the water. In June, the first of two temporary wastewater cleanup systems was started. At the end of 2011, two temporary wastewater processing systems are in service operating at ~90 percent capacity, cleaning more water than is being injected into the reactors and SFPs. Water levels in the buildings are slowly decreasing, and plans are in place to start work in 2012 on a new, more permanent long-term wastewater processing facility. The solid wastes at the Fukushima Daiichi NPS consist of:
- Secondary wastes accumulating as a result of the water treatment processes (such as sludge and filter resins);
- Radioactive particles that were released during the reactor building explosions and drifted away and settled across downwind areas;
- Contaminated rubble and materials from in and around the NPS buildings (including deforestation and other organic debris cleared to make room for storage tanks and buildings; and
- Radioactive nuclear fuel in the SFPs and in the damaged reactors.
The Japanese government and TEPCO published a December 21, 2011, roadmap for the decommissioning of the Fukushima Daiichi Nuclear Power Station facilities entitled, “Mid-and-Long-Term Roadmap toward the Decommissioning of Fukushima Daiichi Nuclear Power Station Units 1-4, TEPCO,” (Roadmap). The Roadmap identifies three phases of decommissioning:
- Phase 1 consists of preparation for spent fuel removal activities from the Unit 1 through Unit 4 SFPs. Research into the removal of spent fuel and fuel debris (damaged fuel remaining in the reactor vessels) will be performed during this period, with a target of spent fuel removal beginning sometime in late 2013;
- Phase 2 targets a period up to 10 years in which spent fuel will be removed from the four SFPs, with spent fuel removal beginning in 2013 from Unit 4. As lessons are learned in the removal of fuel from each subsequent facility, those lessons, coupled with the unique challenges presented by each unit, will determine the timeframe by which all fuel will be removed from SFPs; and
- Phase 3 addresses the removal of fuel debris from the reactor vessels and final decommissioning activities, lasting from 30 to 40 years from the beginning of Phase 1.
By agreement with the Government of Japan, the IAEA conducted a preliminary mission to find facts and identify initial lessons to be learned from the accident at Fukushima Daiichi and share this information across the world nuclear community. To this end, a team of experts undertook this Fact Finding Mission from 24 May to 2 June 2011. The results of the Mission were reported to the IAEA Ministerial Conference on Nuclear Safety at IAEA headquarters in Vienna in June 2011. This report included the following 16 lessons to be learned from this accident:
- There is a need to ensure that in considering external natural hazards:
- The siting and design of nuclear plants should include sufficient protection against infrequent and complex combinations of external events and these should be considered in the plant safety analysis – specifically those that can cause site flooding and which may have longer term impacts;
- Plant layout should be based on maintaining a “dry site concept”, where practicable, as a defence-in-depth measure against site flooding as well as physical separation and diversity of critical safety systems;
- Common cause failure should be particularly considered for multiple unit sites and multiple sites, and for independent unit recovery options, utilizing all on-site resources should be provided;
- Any changes in external hazards or understanding of them should be periodically reviewed for their impact on the current plant configuration; and
- An active tsunami warning system should be established with the provision for immediate operator action.
- For severe situations, such as total loss of off-site power or loss of all heat sinks or the engineering safety systems, simple alternative sources for these functions including any necessary equipment (such as mobile power, compressed air and water supplies) should be provided for severe accident management;
- Such provisions as are identified in Lesson 2 should be located at a safe place and the plant operators should be trained to use them. This may involve centralized stores and means to rapidly transfer them to the affected site(s);
- Nuclear sites should have adequate on-site seismically robust, suitably shielded, ventilated and well equipped buildings to house the Emergency Response Centres, with similar capabilities to those provided at Fukushima Daini and Daiichi, which are also secure against other external hazards such as flooding. They will require sufficient provisions and must be sized to maintain the welfare and radiological protection of workers needed to manage the accident;
- Emergency Response Centres should have available as far as practicable essential safety related parameters based on hardened instrumentation and lines such as coolant levels, containment status, pressure, etc., and have sufficient secure communication lines to control rooms and other places on-site and off-site;
- Severe Accident Management Guidelines and associated procedures should take account of the potential unavailability of instruments, lighting, power and abnormal conditions including plant state and high radiation fields;
- External events have a potential of affecting several plants and several units at the plants at the same time. This requires a sufficiently large resource in terms of trained experienced people, equipment, supplies and external support. An adequate pool of experienced personnel who can deal with each type of unit and can be called upon to support the affected sites should be ensured;
- The risk and implications of hydrogen explosions should be revisited and necessary mitigating systems should be implemented;
- Particularly in relation to preventing loss of safety functionality, the robustness of defence-in-depth against common cause failure should be based on providing adequate diversity (as well as redundancy and physical separation) for essential safety functions;
- Greater consideration should be given to providing hardened systems, communications and sources of monitoring equipment for providing essential information for on-site and off-site responses, especially for severe accidents;
- The use of IAEA Safety Requirements (such as GS-R-2) and related guides on threat categorization, event classification and countermeasures, as well as Operational Intervention Levels, could make the off-site emergency preparedness and response even more effective in particular circumstances;
- The use of long term sheltering is not an effective approach and has been abandoned and concepts of ‘deliberate evacuation’ and ‘evacuation-prepared area’ were introduced for effective long term countermeasures using guidelines of the ICRP and IAEA;
- The international nuclear community should take advantage of the data and information generated from the Fukushima accident to improve and refine the existing methods and models to determine the source term involved in a nuclear accident and refine emergency planning arrangements;
- Large scale radiation protection for workers on sites under severe accident conditions can be effective if appropriately organized and with well led and suitable trained staff;
- Exercises and drills for on-site workers and external responders in order to establish effective on-site radiological protection in severe accident conditions would benefit from taking account of the experiences at Fukushima; and
- Nuclear regulatory systems should ensure that regulatory independence and clarity of roles are preserved in all circumstances in line with IAEA Safety Standards.
As of the date when the IAEA report was finalized and presented in June of 2011, there was no confirmed health effects have been detected in any person as a result of radiation exposure from the associated nuclear accident. A realistic assessment of doses to individuals would require the establishment and implementation of appropriate health monitoring programmes, especially for the most exposed groups of population. Japan has created an expert group to deal with dose assessments and health surveys of residents. This group includes staff from the Fukushima Prefecture and Medical Universities, including Hiroshima and Nagasaki Universities.
It is understood that around 30 workers at the Fukushima plant have been exposed to radiation exposures of between 100 and 250 mSv, although very recent information indicates that some higher internal doses may have been incurred by some workers in the early days. Doses between 100 and 250 mSv, although significant, would not be expected to cause any immediate physical harm, although there may be a small percentage increase in their risk of eventually incurring some health effects. Monitoring programmes of workers, especially those in the group of higher doses and for internal exposures, are necessary as soon as possible to assist in eliminating any uncertainties and to reassure workers.
Three workers are reported to have suffered suspected radiation burns (non-stochastic effects) on their feet/legs from inadvertent exposure to heavily contaminated water in a turbine basement. After hospital treatment they were released after four days with reported no long-term likelihood of significant harm. Early on two workers on site were confirmed as dead (from other than radiation exposure) and several injured. A further worker was reported on 14 May 2011 to have died.
While there appear so far to have been few radiological health consequences the societal and environmental impacts of the accident have been extensive and far reaching, with tens of thousands of people being evacuated from around the plant, some foodstuffs and drinking water restrictions, and significant contamination of the sea. In addition there has been great public anxiety, both in Japan and internationally, about the possible health and other impacts of the radioactivity released. Finally, the economic impact of the failure of the plant is very significant.
The Japanese government has set up an independent Investigation Committee, consisting of scientists, lawyers and others who have no significant connection with the Japanese nuclear industry to examine the details and determine responsibilities.
- American Nuclear Society – Fukushima Daiichi: http://fukushima.ans.org/
- The World Nuclear Association – Fukushima Accident 2011: http://www.world-nuclear.org/info/fukushima_accident_inf129.html
- IAEA – Mission Report – The Great East Japan Earthquake Expert Mission: http://www.google.ca/url?sa=t&rct=j&q=iaea%20international%20fact%20finding%20expert%20mission%20fukushima&source=web&cd=1&ved=0CCEQFjAA&url=http%3A%2F%2Fwww-pub.iaea.org%2Fmtcd%2Fmeetings%2Fpdfplus%2F2011%2Fcn200%2Fdocumentation%2Fcn200_final-fukushima-mission_report.pdf&ei=C81QUKycOJPRiAK5_oCYDQ&usg=AFQjCNHyk6RragQjEvM6NJGclsTlY0IcaQ
- NRC Understanding of the Structural Stability of the Fukushima Daiichi: http://www.google.ca/url?sa=t&rct=j&q=nrc%20understanding%20of%20the%20structural%20stability%20of%20the%20fukushima%20daiichi&source=web&cd=1&sqi=2&ved=0CCEQFjAA&url=http%3A%2F%2Fpbadupws.nrc.gov%2Fdocs%2FML1218%2FML12187A003.pdf&ei=PIhSUIGzEKvFiwKZvIHYDw&usg=AFQjCNH02RBydzd-8ikexZm94_myyQXL7A