I recently had the pleasure to attend a lecture by Randy Beatty (who has been affiliated with the International Atomic Energy Agency [IAEA] for some time), focusing upon the IAEA’s perspective on Fukushima, particularly in terms of our current understanding of the sequence of events (which is still evolving) as well as the IAEA’s interactions with the government of Japan and TEPCO (the owner of the Fukushima reactors).
- Following the 9.0 earthquake, each of the reactors was shut down, halting the fission reaction
- Backup power from diesel generators came online to operate coolant pumps to remove decay heat from the reactor until these were wiped out by the incoming tsunami;
- Battery backup power came online to operate these pumps until such power was depleted
- Water inside the reactors began to boil away as it heated up.
- As water boiled away, the fuel rods heated up.
- Water interacted with the extremely hot zirconium cladding, producing free hydrogen (which lead to the hydrogen explosions)
- As the fuel rods heated up, the cladding failed, releasing radioactive fission product gasses. The fuel is also believed to have partially melted when it became uncovered by water, releasing more radioactive materials from the core.
- Cooling was restored by injecting seawater into the reactors to quench the decay heat from the rods and prevent further melting.
- March 11, 2:46 PM: External power lost, emergency diesel generators begin to supply power
- March 11, 2:52 PM: Emergency cooling systems (isolation condenser) started
- March 11, 3:37 PM: All AC power lost
- March 11, around 5:00 PM: Fuel exposed, core melt begins
- March 12, 5:46 A.M.: Begin of freshwater injection from fire extinguishing line
- March 11, 2:47 PM: External power lost, backup diesel generators start up
- March 11, 2:50 PM: Emergency cooling system (Reactor Core Isolation Cooling system – RCIC) starts up
- March 11, 3:11 PM: All AC power lost
- March 14, 1:25 PM: RCIC operation stops
- March 14, around 6:00 PM: Fuel exposed, core melt begins.
- March 14, 7:54 PM: Seawater injection from a fire extinguishing line begins
- March 11, 2:47 PM: Loss of external power, start-up of emergency diesel generators
- March 11, 3:05 PM: Startup of emergency cooling system (RCIC)
- March 11, 3:41 PM: Loss of all AC power
- March 12, 11:36 AM: RCIC stops due to loss of power
- March 12, 12:35 PM: Startup of HPCI (high pressure core injection) system as a backup cooling measure
- March 13, 2:42 AM: Stop of HPCI
- March 13, around 8:00 AM: Fuel exposed, core melt begins
- March 13, 9:25 AM: Startup of freshwater injection into core from a fire extinguisher line
|Fuel stored in the spent fuel pool at Fukushima Dai-ichi Unit 4. (Image credit: IAEA)|
|(Left): Standby pipe from Units 3 and 4; (right) Enlarged view of junction (Image credit: IAEA)|
|Map of air dose plume release from Fukushima. Red: 9-91 microSv/hr, Orange: 9.5-19 microSv/hr, Yellow: 3.8-9.5 microSv/hr, Green: 1.9-3.8 microSv/hr, Blue: 1.0-1.9 microSv/hr, Indigo: < 1.0 microSv/hr (Image credit: IAEA)|
Air measurements conducted around the area of the plants indicate that much of the plume containing radioactive materials traveled northwest (i.e., due to wind); doses outside the main plume spread to the northwest were found to be relatively negligible. Likewise, with the exception of the plume “tail” in the northwest, nearly all of the elevated dose rates in air were concentrated within a radius of 30 km from the plant, calling into question the logic of NRC Chairman Jazcko’s order for Americans within 50 miles (around 80 km) to evacuate.
|Evacuation areas around Fukushima. (Image credit: IAEA)|
|INES accident classification scale (Image courtesy of IAEA)|
|Proposed safety systems upgrades (click for larger version; image credit: IAEA)|