Fukushima Daiichi accident

[Note of HIAD event validator: this event reports the nuclear accident which affected the Fukushima Daiichi nuclear power plant. It is a very concise text, based on the ICHEME summary, and it focuses on the hydrogen dimension. For a thorough analyses of the complex series of events happened following the earthquake and the tsunami, see for example the reference volumes issued by the IAEA [ISBN 978-92-0-107015-9, available at
https://www.iaea.org/publications/10962/the-fukushima-daiichi-accident]

The 11 March 2011, 3 of the 6 nuclear reactors of the plant were operating, while the other 3 were shut down for maintenance. When an earthquake of magnitude 9.0 earthquake on the Richter scale occurred, the 3 operating reactors shutdown. The shutdown occurred automatically and as designed. Since the earthquake damaged also the external electrical power to the plant, the emergency diesel generators started up to provide continuity of power to critical systems, such water cooling pumps.

Approximately 41 minutes later, the plant was hit by a 15 m tsunami which damaged the sea cooling water pumps for the main condensers and auxiliary cooling circuits (including the residual heat removal system). It also inundated the diesel generators, the electrical control systems and battery providing additional power.

Without any type of cooling, all 3 reactor cores partially melted within 3 days. There were no in-core steam explosions, but hydrogen was released and exploded breaking open 3 buildings, causing 13 injured people and releasing radioactive material to the environment.

More than 100,000 people within 20 km of the site had to be evacuated. This accident was eventually declared a Level 7 (“severe accident”) on the International Nuclear Event Scale (INES).

The initiating causes of the hydrogen explosions was the overheating of the core and the generation of hydrogen by chemical reaction between the zirconium of the fuel cladding and the over-heated water. Additional hydrogen was possibly formed by radiolysis of hot water in spent fuel ponds.
The hydrogen passed from the reactor pressure vessel into the primary containment vessel and escaped further to the reactor buildings, where it found an ignition source and ignited.

The triggering factor to the ROOT CAUSES was an earthquake and a tsunami of exceptional magnitudes. The extension of the consequences was due to:
1) Inadequate risk assessment during design, which used historical data instead of recent seismic and weather data),
2) Failure to promptly implement tsunami countermeasures after maximum expected tsunami flood levels were reassessed in 2002 and found to exceed design basis levels for
the plant (Japan believed its nuclear power plants were so safe that an
accident of this magnitude was not credible),
3) Inappropriate plant layout: the electrical equipment with a critical safety function were in the turbine hall basements,
4) Inadequate operating and emergency procedures and crisis management,
7) Inadequate regulatory system, due to the conflict of interest between government, nuclear regulatory body and the operating company.

reactor cooling water pump,
nuclear rods,
reactor buildings

The estimated costs of the accident includes onsite and offsite aspects.
The fatalities were indirect: 2259 (mainly elderly) people died during the evacuation process

People within a radius of 20 km of the site and in other designated areas were evacuated, and those within a radius of 20–30 km were instructed to shelter before later being advised to voluntarily evacuate. Restrictions were placed on the distribution and consumption of food and the consumption of drinking water.