Fukushima Daiichi Nuclear Disaster
The Fukushima Daiichi Nuclear Power Plant is located in Japan, which, like the rest of the Pacific Rim, is in an active seismic zone. The International Atomic Energy Agency (IAEA) had expressed concern about the ability of Japan's nuclear plants to withstand seismic activity. At a 2008 meeting of the G8's Nuclear Safety and Security Group in Tokyo, an IAEA expert warned that a strong earthquake with a magnitude above 7.0 could pose a "serious problem" for Japan's nuclear power stations.The region had experienced three earthquakes of magnitude greater than 8, including the 869 Jogan Sanriku earthquake, the 1896 Meiji-Sanriku earthquake, and the 1933 Sanriku earthquake.
Neglect of Safety R&D for use of Robotics:
Following the 1999 Tokaimura criticality accident, there was interest in Japan for developing radiation-resistant robots for use in the event of nuclear accidents.Other countries (e.g. Germany and France) already had them available. The Japanese government budgeted 3 billion yen (US $38 million) for research and development. Several companies produced state of the art prototypes in 2001, which were tested and deemed technical successes. In December 2002, a task force (which included Tokyo Electric Power Company - TEPCO executives) further concluded that the robots were unnecessary: the possibility of Chernobyl-scale disasters was completely discounted and it was thus assumed that human employees- compared to whom the robots had limited speed and range- would still be able to operate in the event of an accident. The program halted, and the prototypes remained in storage until March 2006; some were subsequently donated to Tohoku University. The termination of the program left Japan without functional radiation-resistant robots to send into Fukushima when the crisis began.
As the crisis unfolded, the Japanese government sent a request for robots developed by the U.S. military. The robots went into the plants, and took pictures to help assess the situation, but they couldn't perform the full range of tasks usually carried out by human workers. Following Fukushima, efforts to develop humanoid robots that could supplement relief efforts have accelerated dramatically.
Similarly, pre-Fukushima, Japan's Nuclear Safety Commission said in its safety guidelines for light-water nuclear facilities that "the potential for extended loss of power need not be considered."
The Fukushima I (Daiichi) Nuclear Power Plant consists of six GE light water, boiling water reactors (BWR) with a combined power of 4.7 gigawatts, making Fukushima Daiichi one of the world's 25 largest nuclear power stations. Fukushima Daiichi was the first GE-designed nuclear plant to be constructed and run entirely by the Tokyo Electric Power Company (TEPCO).
Reactor 1 is a 439 MWe type (BWR-3) reactor constructed in July 1967. It commenced operation on 26 March 1971. It was designed to withstand an earthquake with a peak ground acceleration of 0.18 g (1.74 m/s2) and a response spectrum based on the 1952 Kern County earthquake. Reactors 2 and 3 are both 784 MWe type BWR-4. Reactor 2 commenced operating in July 1974, and Reactor 3 in March 1976. The earthquake design basis for all units ranged from 0.42 g (4.12 m/s2) to 0.46 g (4.52 m/s2).
All units were inspected after the 1978 Miyagi earthquake when the ground acceleration reached 0.125 g (1.22 m/s2) for 30 seconds, but no damage to the critical parts of the reactor was discovered.
Units 1â5 have a Mark 1 type (light bulb torus) containment structure; unit 6 has Mark 2 type (over/under) containment structure.In September 2010, Reactor 3 was partially fueled by mixed-oxides (MOX).
There is no MOX (Mixed Oxides)fuel in any of the cooling ponds. The only MOX fuel is loaded in the Unit 3 reactor.
Cooling requirements :
These reactors generate electricity by using the heat of the fission reaction to create steam. When the reactor stops operating, the radioactive decay of unstable isotopes continues to generate heat for a time. This decay and the decay heat that results requires continued cooling.Initially this decay heat amounts to approximately 6% of the amount produced by fission,decreasing over several days before reaching cold shutdown levels.
Exhausted fuel rods that have reached cold shutdown temperatures typically require several years in a spent fuel pool before they can be safely transferred to dry cask storage vessels.
The decay heat in the Unit 4 spent fuel pool had the capacity to boil about 70 tonnes of water per day (12 gallons per minute). On 16 April 2011, TEPCO declared that cooling systems for Units 1-4 were beyond repair and would have to be replaced.
In the reactor core, circulation is accomplished via high pressure systems that cycle water between the reactor pressure vessel and heat exchangers. These systems then transfer heat to a secondary heat exchanger via the essential service water system, using water that is pumped out to sea or an onsite cooling tower.
When the reactor is not producing electricity, cooling pumps can be powered by other reactor units, the grid or by diesel generators or batteries.
Units 2 and 3 were equipped with steam-turbine driven emergency core cooling systems that can be directly operated by steam produced by decay heat and which can inject water directly into the reactor.Some electrical power is needed to operate valves and monitoring systems.
Unit 1 was equipped with a different cooling system, the "Isolation Condenser" or "IC", which is entirely passive. This consists of a series of pipes run from the reactor core to the inside of a large tank of water. When the valves are opened, steam flows upward to the IC where the cool water in the tank condenses the steam back to water, and it runs under gravity back to the reactor core. For reasons that are unclear, at the beginning, Unit 1's IC was operated only intermittently during the emergency. However, during a 25 March 2014 presentation to the TVA, Dr Takeyuki Inagaki explained that the IC was being operated intermittently to maintain reactor vessel level and to prevent the core from cooling too quickly which can increase reactor power. Unfortunately, as the tsunami engulfed the station, the IC valves were closed and could not be reopened automatically due to the loss of electrical power, but could have been opened manually.
Two emergency diesel generators were available for each of units 1â5 and three for unit 6.In the late 1990s, three additional backup generators for Units 2 and 4 were placed in new buildings located higher on the hillside, to comply with new regulatory requirements. All six units were given access to these generators, but the switching stations that sent power from these backup generators to the reactors' cooling systems for Units 1 through 5 were still in the poorly protected turbine buildings. All three of the generators added in the late 1990s were operational after the tsunami. If the switching stations had been moved to inside the reactor buildings or to other flood-proof locations, power would have been provided by these generators to the reactors' cooling systems.
The reactor's emergency diesel generators and DC batteries, crucial components in powering cooling systems after a power loss, were located in the basements of the reactor turbine buildings, in accordance with GE's specifications. Mid-level engineers expressed concerns that this left them vulnerable to flooding.
Fukushima I was not designed for such a large tsunami, nor had the reactors been modified when concerns were raised in Japan and by the IAEA.
Fukushima II was also struck by the tsunami. However, it had incorporated design changes that improved its resistance to flooding, reducing flood damage. Generators and related electrical distribution equipment were located in the watertight reactor building, so that power from the electricity grid was being used by midnight.Seawater pumps for cooling were protected from flooding, and although 3 of 4 initially failed, they were restored to operation.
Safety Hazard Areas:
Central fuel storage areas:
Used fuel assemblies taken from reactors are initially stored for at least 18 months in the pools adjacent to their reactors. They can then be transferred to the central fuel storage pond.Fukushima I's storage area contains 6375 fuel assemblies. After further cooling, fuel
Many of the internal components and fuel assembly cladding are made from zircaloy because it is relatively transparent to neutrons. At normal operating temperatures of approximately 300 Â°C (572 Â°F), zircaloy is inert. However, above 1200 degrees Celsius, zirconium metal can react exothermically with water to form free hydrogen gas.The reaction between zirconium and the coolant produces more heat
2. The Disaster:
The Fukushima nuclear disaster was an energy accident at the Fukushima I Nuclear Power Plant, initiated primarily by the tsunami of the Tohoku earthquake on 11 March 2011. The damage caused by the tsunami produced equipment failures, and without this equipment a loss-of-coolant accident followed with three nuclear meltdowns and releases of radioactive materials beginning on 12 March. It is the largest nuclear disaster since the Chernobyl disaster of 1986 and the second disaster (after Chernobyl) to be given the Level 7 event classification of the International Nuclear Event Scale.
The plant comprised six separate boiling water reactors originally designed by General Electric (GE) and maintained by the Tokyo Electric Power Company (TEPCO). At the time of the earthquake, reactors 4, 5 and 6 were shut down in preparation for re-fueling. However, their spent fuel pools still required cooling.Immediately after the earthquake, the electricity producing reactors 1, 2 and 3 automatically shut down their sustained fission reactions, inserting control rods in what is termed a SCRAM. Following this legally mandated "safety precaution" which ceases the reactors' normal running conditions, the reactors were unable to generate power to run their own coolant pumps. Emergency diesel generators came online, as designed, to power electronics and coolant systems, all of which operated right up until the tsunami destroyed the generators for reactors 1â5 due to their location in unhardened low-lying areas. The two generators cooling reactor 6 were undamaged and were sufficient to be pressed into service to cool the neighboring reactor 5 along with their own reactor, averting the overheating issues that reactor 4 suffered.
The largest wave in the tsunami arrived some 50 minutes after the initial earthquake. The 13 meter tall wave overwhelmed the plant's seawall, which was only 10 m high,with the moment of impact being caught on camera. Water quickly flooded the low-lying rooms in which the emergency generators were housed. The flooded diesel generators failed soon afterwards, cutting power to the critical pumps that must continuously circulate coolant water through a Generation II reactor for several days to keep the fuel rods from melting down following the SCRAM event, as the ceramic fuel pellets in the fuel rods continue to generate Decay heat even after the fission process has terminated. The fuel rods will become hot enough to melt themselves down during the fuel decay time period if no adequate cold sink is available. After the secondary emergency pumps (run by back-up electrical batteries) ran out, one day after the tsunami, 12 March, the water pumps stopped and the reactors began to overheat due to the high decay heat produced in the first few days after the SCRAM (diminishing amounts of this decay heat continue to be released for years, but with time, passive cooling through water convection in a pool is sufficient to prevent fuel rod melting).
As workers struggled to supply power to the reactors' coolant systems and restore power to their control rooms, a number of hydrogen-air chemical explosions occurred, the first in Unit 1, on 12 March and the last in Unit 4, on 15 March. It is estimated that the hot zirconium fuel cladding-water reaction in reactors 1-3 produced 800 to 1000 kilograms of hydrogen gas each, which was vented out of the reactor pressure vessel, and mixed with the ambient air, eventually reaching explosive concentration limits in units 1 and 3, and due to piping connections between units 3 and 4, or alternatively from the same reaction occurring in the spent fuel pool in unit 4 itself,unit 4 also filled with hydrogen, with the hydrogen-air explosions occurring at the top of each unit, that is in their upper secondary containment building.Drone overflights on 20 March and afterwards captured clear images of the effects of each explosion on the outside structures, while the view inside was largely obscured by shadows and debris.
3.Hazards and Ailments:
There have been no fatalities linked to short term overexposure to radiation reported due to the Fukushima accident, while approximately 18,500 people died due to the earthquake and tsunami. However approximately 610 are estimated to have died due to workers' exposure and the evacuation of residents near the power plant. Estimates of the total human fatalities caused by the nuclear accident are up to 10,000, maximum cancer mortality and morbidity is calculated to be respectively 1,500 and 1,800. In addition, the rates of mental illnesses among evacuated people rose fivefold compared to the Japanese average.
In 2013, the World Health Organization (WHO) indicated that the residents of the area who were evacuated were exposed to low amounts of radiation and that radiation induced health impacts are likely to be low.In particular, the 2013 WHO report predicts that for evacuated infant girls, their 0.75% pre-accident lifetime risk of developing thyroid cancer is calculated to be increased to 1.25% by being exposed to radioiodine, with the increase being slightly less for males. While the risks from a number of additional Radiation-induced cancers are also expected to be elevated due to exposure caused by the other low boiling point fission products that were released by the safety failures. The single greatest increase is for thyroid cancer, but in total, an overall 1% higher lifetime risk of developing cancers of all types, is predicted for infant females, with the risk slightly lower for males, making both some of the most radiation-sensitive groups.Along with those within the womb, which the WHO predicted, depending on their gender, to have the same elevations in risk as the infant groups.
A screening program a year later in 2012 found that more than a third (36%) of children in Fukushima Prefecture have abnormal growths in their thyroid glands. As of August 2013, there have been more than 40 children newly diagnosed with thyroid cancer and other cancers in Fukushima prefecture as a whole. In 2015, the number of thyroid cancers or detections of developing thyroid cancers numbered 137.However whether these incidences of cancer are elevated above the rate in un-contaminated areas and therefore were due to exposure to nuclear radiation is unknown at this stage.Data from the Chernobyl accident showed that an unmistakable rise in thyroid cancer rates following the disaster in 1986 only began after a cancer incubation period of 3â5 years, however whether this data can be directly compared to the Fukushima nuclear disaster is still yet to be determined.
A survey by the newspaper Mainichi Shimbun computed that of some 300,000 people who evacuated the area, approximately 1,600 deaths related to the evacuation conditions, such as living in temporary housing and hospital closures have occurred as of August 2013, a number comparable to the 1,599 deaths directly caused by the earthquake and tsunami in the Fukushima Prefecture in 2011. With the exact cause of the majority of these evacuation related deaths not being specified, as according to the municipalities, that would hinder the deceased relatives' application for condolence money compensation.
4. Investigation & Findings:
On 5 July 2012, the Japanese National Diet appointed The Fukushima Nuclear Accident Independent Investigation Commission (NAIIC) submitted its inquiry report to the Japanese Diet.The Commission found the nuclear disaster was "manmade", that the direct causes of the accident were all foreseeable prior to 11 March 2011. The report also found that the Fukushima Daiichi Nuclear Power Plant was incapable of withstanding the earthquake and tsunami. TEPCO, the regulatory bodies (NISA and NSC) and the government body promoting the nuclear power industry (METI), all failed to correctly develop the most basic safety requirementsâsuch as assessing the probability of damage, preparing for containing collateral damage from such a disaster, and developing evacuation plans for the public in the case of a serious radiation release. Meanwhile, the government appointed Investigation Committee on the Accident at the Fukushima Nuclear Power Stations of Tokyo Electric Power Company submitted its final report to the Japanese government on 23 July 2012.A separate study by Stanford researchers found that Japanese plants operated by the largest utility companies were particularly unprotected against potential tsunami.
TEPCO admitted for the first time on 12 October 2012 that it had failed to take stronger measures to prevent disasters for fear of inviting lawsuits or protests against its nuclear plants.There are no clear plans for decommissioning the plant, but the plant management estimate is thirty or forty years.A frozen soil barrier is being constructed in order to prevent ongoing exposure of running groundwater with melted down nuclear fuel.