Fukushima nuclear accident

On 11 March 2011, a major nuclear accident started at the Fukushima Daiichi Nuclear Power Plant in Ōkuma, Fukushima, Japan. The direct cause was the Tōhoku earthquake and tsunami, which resulted in electrical grid failure and damaged nearly all of the power plant's backup energy sources. The subsequent inability to sufficiently cool reactors after shutdown compromised containment and resulted in the release of radioactive contaminants into the surrounding environment. It is regarded as the worst nuclear incident since the Chernobyl disaster.
According to the United Nations Scientific Committee on the Effects of Atomic Radiation, "no adverse health effects among Fukushima residents have been documented that are directly attributable to radiation exposure from the Fukushima Daiichi nuclear plant accident". Insurance compensation was paid for one death from lung cancer, but this does not prove a causal relationship between radiation and the cancer. Six other persons have been reported as having developed cancer or leukemia. Two workers were hospitalized because of radiation burns, and several other people sustained physical injuries as a consequence of the accident.
Following the accident, at least 164,000 residents of the surrounding area were permanently or temporarily displaced. The displacements resulted in at least 51 deaths as well as stress and fear of radiological hazards. The evacuation was accused of causing more harm than it prevented. Ten years later over 41,000 people from Fukushima were still living as evacuees.
Investigations faulted lapses in safety and oversight, namely failures in risk assessment and evacuation planning. Controversy surrounds the disposal of treated wastewater once used to cool the reactor, resulting in numerous protests in neighboring countries.
The expense of cleaning up the radioactive contamination and compensation for the victims of the Fukushima nuclear accident was estimated by Japan's trade ministry in November 2016 to be 20 trillion yen.

Background

The Fukushima Daiichi Nuclear Power Plant consisted of six General Electric light water boiling water reactors. Unit 1 was a GE type 3 BWR. Units 2–5 were type 4. Unit 6 was a type 5.
At the time of the Tōhoku earthquake on 11 March 2011, units 1–3 were operating. However, the spent fuel pools of all units still required cooling.

Materials

Many of the internal components and fuel assembly cladding are made from a zirconium alloy for its low neutron cross section. At normal operating temperatures, it is inert. However, above, Zircaloy can be oxidized by steam to form hydrogen gas or by uranium dioxide to form uranium metal. Both of these reactions are exothermic. In combination with the exothermic reaction of boron carbide with stainless steel, these reactions can contribute to the overheating of a reactor.

Isolated cooling systems

In the event of an emergency, reactor pressure vessels are automatically isolated from the turbines and main condenser and are instead switched to a secondary condenser system, which is designed to cool the reactor without the need for pumps powered by external power or generators. The isolation condenser system involved a closed coolant loop from the pressure vessel with a heat exchanger in a dedicated condenser tank. Steam would be forced into the heat exchanger by the reactor pressure, and the condensed coolant would be fed back into the vessel by gravity. Each reactor was initially designed to be equipped with two redundant ICs which were each capable of cooling the reactor for at least 8 hours. However, it was possible for the IC system to cool the reactor too rapidly after shutdown, which could result in undesirable thermal stress on the reactor pressure vessel. To avoid this, the protocol called for reactor operators to manually open and close the condenser loop using electrically operated control valves.
After the construction of Unit 1, the following units were designed with new open-cycle reactor core isolation cooling systems. This new system used the steam from the reactor vessel to drive a turbine, which would power a pump to inject water into the pressure vessel from an external storage tank to maintain the water level in the reactor vessel. The system was designed to operate for at least 4 hours. Additionally, this system could be converted into a closed-loop system, which draws coolant from the suppression chamber instead of the storage tank, should the storage tank be depleted. Although this system could function autonomously without an external energy source, direct current was needed to remotely control it and receive parameters and indications and alternating current was required to power the isolation valves.
In an emergency where backup on-site power was partially damaged or insufficient to last until a grid connection to off-site power could be restored, these cooling systems could no longer be relied upon to reliably cool the reactor. In such a case, the expected procedure was to vent both the reactor vessel and primary containment using electrically or pneumatically operated valves using the remaining electricity on site. This would lower the reactor pressure sufficiently to allow for low-pressure injection of water into the reactor using the fire protection system to replenish water lost to evaporation.

On-site backup power

Station operators switched the reactor control to off-site power for shutdown, but the system was damaged by the earthquake. Emergency diesel generators then automatically started to provide AC power. Two EDGs were available for each of units 1–5 and three for unit 6. Of the 13 EDGs, 10 were water-cooled and placed in the basements about 7–8 m below the ground level. The coolant water for the EDGs was carried by several seawater pumps placed on the shoreline which also provide water for the main condenser. These components were unhoused and only protected by the seawall. The other three EDGs were air-cooled and were connected to units 2, 4, and 6. The air-cooled EDGs for units 2 and 4 were placed on the ground floor of the spent fuel building, but the switches and various other components were located below, in the basement. The third air-cooled EDG was in a separate building placed inland and at higher elevations. Although these EDGs are intended to be used with their respective reactors, switchable interconnections between unit pairs allowed reactors to share EDGs should the need arise.
The power station was also equipped with backup DC batteries kept charged by AC power at all times, designed to be able to power the station for approximately 8 hours without EDGs. In units 1, 2, and 4, the batteries were located in the basements alongside the EDGs. In units 3, 5, and 6, the batteries were located in the turbine building where they were raised above ground level.

Fuel inventory

The units and central storage facility contained the following numbers of fuel assemblies:

Location	Unit 1	Unit 2	Unit 3	Unit 4	Unit 5	Unit 6	Central storage
Reactor fuel assemblies	400	548	548	0	548	764	N/A
Spent fuel assemblies	292	587	514	1331	946	876	6377
New fuel assemblies	100	28	52	204	48	64	N/A

Earthquake tolerance

The original design basis was a zero-point ground acceleration of 250 Gal and a static acceleration of 470 Gal, based on the 1952 Kern County earthquake. After the 1978 Miyagi earthquake, when the ground acceleration reached 122 Gal for 30 seconds, no damage to the critical parts of the reactor was found. In 2006, the design of the reactors was reevaluated with new standards requiring the reactors to withstand accelerations ranging up to 450 Gal.

Venting systems

In the event of an emergency, operators planned to pump water into the reactors to keep them cool. This would inevitably create steam which should not be very radioactive because the fuel would still be in the primary containment vessel. Therefore, the steam would manually be released by venting valves to prevent a high pressure explosion.

Accident

Earthquake

The 9.0 M_W earthquake occurred at 14:46 on Friday, 11 March 2011, with the epicenter off of the east coast of the Tōhoku region. It produced a maximum ground g-force of 560, 520 and 560 Gal at units 2, 3, and 5 respectively. This exceeded the seismic reactor design tolerances of 450 Gal, 450 Gal, and 460 Gal for continued operation, but the seismic values were within the design tolerances of unit 6.
Upon detecting the earthquake, all three operating reactors automatically shut down. Due to expected grid failure and damage to the switch station as a result of the earthquake, the power station automatically started up the emergency diesel generators, isolated the reactor from the primary coolant loops, and activated the emergency shutdown cooling systems.

Tsunami and loss of power

The largest tsunami wave was 13–14 m high and hit approximately 50 minutes after the initial earthquake, overtopping the seawall and exceeding the plant's ground level, which was above sea level.
The waves first damaged the seawater pumps along the shoreline, and 10 of the plant's 13 cooling systems for the emergency diesel generators. The waves then flooded all turbine and reactor buildings, damaging EDGs and other electrical components and connections located on the ground or basement levels at approximately 15:41. The switching stations that provided power from the three EDGs located higher on the hillside also failed when the building that housed them flooded. One air-cooled EDG, that of unit 6, was unaffected by the flooding and continued to operate. The DC batteries for units 1, 2, and 4 were also inoperable shortly after flooding.
As a result, units 1–5 lost AC power and DC power was lost in units 1, 2, and 4. In response, the operators assumed a loss of coolant in units 1 and 2 and developed a plan in which they would vent the primary containment and inject water into the reactor vessels with firefighting equipment. Tokyo Electric Power Company, the utility operator and owner, notified authorities of a "first-level emergency".
Two workers were killed by the impact of the tsunami.