On March 11, a magnitude 9 earthquake (at depth of 15 miles) caused an automatic shutdown of 11 nuclear power plants on northeast coast of Japan. (Added 2015 Sept 24, event happened in 2011!) Japan topography The earthquake
A subsequent tsunami (45 feet high) swept the Fukushima Dai-chi nuclear power plant site. The earthquake and tsunami provoked widespread devastation in northeastern Japan. Here we concentrate on Fukushima reactors.
Earthquake response. Three of the 6 reactors were shut down for routine refueling and maintenance; reactor 4 fuel downloaded to unit 4 spent fuel pool.
Three operating reactors responded normally to earthquake power trip, inserting all control rods into reactors. Emergency diesel generators started at all six units, providing ac power to critical systems. Summary: response to seismic event appears to be normal.
Tsunami response 40 min later. First large wave exceeded site design protection from tsunami by ~27 feet. It and multiple additional waves extensively damaged the site, including complete loss of power at units 1 thru 5; unit 5 retained one diesel generator.
Response to catastrophic, unprecedented emergency. In near total darkness with limited instrumentation and controls, operators cross-tied unit 6 generator to provide ac to units 5 and 6, and eventually achieved a stable cold shutdown.
Operators experienced repeated loss of fuel cooling: unit 1 after several hrs, unit 2 after 71 hrs, unit 3 after 36 hrs. The exposed fuel was damaged and the units have yet to achieve stable, cold shutdown condition. Explosion further damaged units 1, 2 and 3 facilities, especially secondary containment structures.
Hydrogen explosions. Exposed fuel generated hydrogen that moved from (? damaged ?) primary containment into secondary containment. Hydrogen explosions caused substantial damage to containment. How unit 4 was damaged by explosion is unclear.
SBO Station Blackout SCRAM rapid shutdown of reactor: rapid insertion of all control rods that reduces reactor heat in tens of seconds LOOP Loss of offsite power (needed by RPS below) Stages of planned responses to "contingencies" (incidents) RPS reactor protection system prevents contingencies from happening ECCS emergency core-cooling system, responds to contingencies if they do happen: set of interrelated safety systems Important Systems: HPCI high pressure coolant injection system; 1st line of defense in cooling core [depends on power from steam system] RCIC reactor core isolation cooling system; faster than HPCI can dumps lots of water on core and keep dumping. Powered by reactor's high pressure steam, using only batteries for operating valves.
Background. Combined massive earthquake and devastating tsunami exceeded any external events in current plant design. It challenges mitigation capabilities. At Fukushima there was no effective contact with external agencies that were in any case completely swamped by effect of earthquake.
A list of the challenges is daunting. The current NRC regulatory framework is largely aimed as design-basis events.
Origin of current NRC regulation. Design-basis events were a central element of safety approach almost 50 years ago; US Atomic Energy Commission formulated the idea of requiring safety system to address a prescribed set of anticipated operation occurrence and postulate accidents. Additionally design-basis requirements included a set of external challenges. That approach was used in licensing the current generation of nuclear power plants in 1960s and 1970s.
The Nuclear Regulatory Commission (NRC) from the 1980s maintained design-basis approach, adding requirements to address each new issues as it arose. Typically the items were placed outside of design-basis events as individual requirements. Samples are
Defense in Depth. The Near-Term Task Force Review of Insights from the Fukushima Dai-iche Accidents rejects the NRC patchwork approach. It recommends a new approach it calls "Defense in Depth. It would create multiple independent and redundant layers of defense for potential failure and external hazards so that no single layer is exclusively relied on to protect the public and environment. The Task Force has an extensive Appendix showing how defense in depth addressed protection from design-basis, natural phenomena, mitigation of consequences of accidents and emergency preparedness.
So far nothing has happened. The near-term report will be followed and a longer document. But then?
Changing esitmate of radiation and evacuation zone.
September 2011 Radiation levels frum Fukushim.jpg
Drone shot: unit a sfter explosion for unknown reasons.
Drone shot of unit 4 four days later; no explanation for greater damage.
Danger of overreacting.
Other are still building reactors. Can everyone be right.
Finally there are "plans" in US for big and small reactors.
Japanese government sends reports to IAEA. On September 7, 2017:
In other words, there has been no satisfactory clean up. The posted reports stretch back to March 25, 2011. URL:https://www.iaea.org/newscenter/focus/fukushima/status-update.
The guardian has an article March 8, 2017, with the headline "Dying robots and failing hope: Fukushima clean-up falters six years after tsunami" with subhead "exploration work inside the nuclear plant's failed reactors has barely begun, with scale of the task as `almost beyond comprehension.' A picture of the current site has the caption: "Cleaning up the Fukushima Daiichi nuclear plant is expected to take 30 to 40 years. URL:https://www.theguardian.com/world/2017/March/09/fukushima-nuclear-cleanup-falters-six-years-after-tsunami.