The Great East Japan Earthquake on March 11, 2011 triggered huge tsunami waves that devastated the northeast region of Japan along the Pacific coastline. The Tokyo Electric Power Company (TEPCO) owned Fukushima Daiichi Nuclear Power Plant (Fukushima-1) survived the earthquake, however, not the tsunami that followed. Four of the 6 reactor units underwent Station Blackout. Unit 5 lost all its own AC power, however, it shared AC power with Unit 6. Units 1, 3, and 4 had hydrogen explosions that destroyed their reactor buildings, and even worse, 1, 2, and 3 had core meltdowns to release a large amount of radioactive material to their surroundings.
The accident was rated Level 7 on the International Nuclear Event Scale, the worst level defined by International Atomic Energy Agency (IAEA). Reports and papers have been published by a number of entities including the Japanese Diet, Government, TEPCO, IAEA, and more. They give detail explanation of how the accident developed into a nuclear disaster explaining the direct and background causes and faults made after the accident broke out. Finding the accident process, i.e., how it happened, and its causes of why it happened, are the most important first steps in accident analysis.
Figuring out how to prevent similar events in the future, or even if it is possible to do so, however, is equally important for our future. We started our study in 2014 to find what actions TEPCO could have taken before the accident for preventing it from growing into a catastrophe. Then in February 2015, we set the goal of our study group to find answers to the following two questions:
A. Was the huge tsunami, induced by a huge earthquake, predictable at Fukushima-1?
B. If it was predictable, what preparations at Fukushima-1 could have reduced the severity of the accident?
In response to our invitation to experts in the nuclear field, active and retired people gathered from academia, manufacturers, utility companies, and even regulators. After a series of tense discussions, we reached the conclusions that:
Aa. Tsunami of the level that hit Fukushima-1 in 2011 was well predictable, and,
Ba. The accident would have been much less severe if the plant had prepared a set of equipment, and most of all, had exercised actions against such tsunami.
Preparation at the plant to prevent the severe accident consisted of the following items 1 through 7, and drills in 8:
1. A number of 125Vdc and 250Vdc batteries,
2. Portable underwater pumps,
3. Portable AC generators with sufficient gasoline supply to run the pumps, and
4. High voltage AC power truck
This set applied only to this specific accident. For preparing against many other situations that could have taken place at Fukushima-1, we recommend having, in addition, the following equipment and modifications.
5. Portable compressor to drive air-operated valves for venting,
6. Watertight modification to RCIC and HPCI control and instrumentation,
7. Fire engines for alternate low pressure water injection after vent (Fukushima-1 had three).
Just making these preparations would not have been sufficient. Activating valves with DC batteries, for example, takes disengaging the regular power supply lines and hooking up the batteries.
8. Drills against extended loss of all electric power and seawater pump
This item 8, on and off-site drills was the most important preparation that should had been made. All other necessary preparations to save the plant in such cases would have followed logically.
Modeling of Spray System Operation under Hydrogen and Steam Emissions in NPP Containment during Severe Accident
