Spent Fuel Pool Cooling
Due to the impact of the tsunami, Units 1–6 and the common spent fuel pool (SFP) all lost cooling capacity. There was no emergency situation with the reactors, but the fuel energy deposition was large, and there was concern about the condition of the Unit 4 SFP that eventually led to the hydrogen explosion. The day after the explosion (March 16), a TEPCO employee accompanied a Self-Defense Force (SDF) helicopter pilot, and according to the employee, the pool water level was maintained.
SDF helicopters sprayed water onto Unit 4, while firefi units from the SDF, Tokyo Fire Department, and the National Police Agency hosed it down. Later, as a long-term stable measure for injecting cooling water, a large size concrete pump vehicle was used. (Cooling water injection into Unit 4 began on March 22.)
Dealing with the Unit 4 SFP was an extremely important turning point in preventing the spread of the disaster.
Units 1 and 3
It is assumed that when the fuel inside the reactor was damaged, hydrogen was generated as a result of zirconium-water reaction, which then leaked out and remained in the reactor building, finally resulting in hydrogen explosion.
The exact route by which the hydrogen escaped into the reactor building is unknown, but it is assumed that leak-proof seals on the head of the PCV and hatch joints where machinery and personnel enter and exit were exposed to high temperatures and may have lost their functionality.
Another possibility is that it may have escaped from the PCV vent line via the standby gas treatment system (SGTS) line into the reactor building, but the results of investigating the condition of the Unit 2 SGTS show that the volume of hydrogen that could travel this route is limited, and therefore, the major source of hydrogen for the explosion must have leaked directly from the PCV into the reactor building.
There are no indications of damage to the fuel in the SFP, and as the process of radiolysis of the water in the pool can only generate small amounts of hydrogen, the fuel inside the SFP is not being considered as a possible cause of the explosion.
The results of investigating conditions of the Unit 4 SGTS and the field investigation of conditions inside the Unit 4 reactor building lead to the hypothesis that the hydrogen that caused the explosion was the Unit 3 PCV vent gas that traveled through the SGTS pipes into Unit 4.
There are still unclear issues and some observed phenomena that cannot be confidently interpreted. For example, the reason why the reactor core isolation cooling (RCIC) system of Unit 2 lost its function still remains unknown. Also, concerning earthquakes and tsunami, there are some issues for academic researchers to tackle, such as the mechanism of earthquakes of this historically huge scale occurring in the same district and causing massive tsunami.
Discovering root causes for loss of the safety equipment function improves knowledge about existing system functionality and thus enhances safety. Fuel removal and prevention of generating contaminated water are crucial for decommissioning Fukushima Daiichi NPS.
In order to cope with these issues, it is essential to grasp the damage mechanisms as well as the current situation of debris in the reactors and containment vessels (PCV). Even the issues not directly related to accident progression may provide clues to enhancing safety as a result of examining them.
-  SFP generally has fuels with small decay heat, therefore rapid accident progression is not considered. However, fission product released in case of fuel damage is large since there is no containment vessel for SFP.