Lessons Learned and Recommendations Derived
The overview of the TEPCO's Fukushima Daiichi nuclear power station (NPS) accident is depicted in Fig. 12.1 [also see Chap. 2 in this volume—eds.]. The essential lessons from the accident are described in . The lessons learned concerning the reactor design and operation states of view and the recommendations dedicated thereof are described in this section.
Fig. 12.1 Lessons learned from the Fukushima Daiichi nuclear power station accident 
The accident was caused by the big tsunami. It flooded the reactor and turbine buildings. The emergency diesel generators (DGs) lost their function and all AC power supply was lost. The loss of function of the emergency diesel generators were caused by flooding of power supply panels and diesel generators themselves as well as loss of heat sink of the DG cooling by the flooding of sea water pumps. The anti-seismic design of the plant worked well by the improvements after the big earthquakes in Kobe and Chūetsu-oki. The movements of multiple regions in the seabed caused the big earthquake of March 11, 2011. The acceleration on the base mat of the reactor building is, however, predicted well by the standard acceleration for safety grade system.
The safety systems did not lose function by the earthquake. The height of the tsunami was, however, underestimated. It is the most important direct reason for the initiation and progression of the accident. The tsunami was caused by the slides along the boundary of continental plates. The interaction of tsunami waves from multiple origins appears to make the waves high.
It is necessary to develop imagination of natural hazards and its combinations that may potentially cause severe accidents. For example, big hurricanes and typhoons cause extreme high tides that floods large area. The combination of external fires, tsunami and earthquakes may cause difficulty in the availability of the emergency power supply, cooling water and accessibility of the plants.
Emergency Power Supply
The external power of the TEPCO Fukushima Daiichi plants was lost by the failure of transmission lines by the earthquake. The emergency DGs and some batteries were flooded. Both AC and DC power were lost. The capacity of the remaining batteries was exhausted. The safety systems and instrumentation systems lost their functions. Units 5 and 6 of the site survived with the electricity from an air-cooled emergency DG. The loss of DC power caused difficulty for the operators to know the condition of the plants and conduct timely actions.
It is necessary to enhance the reliability of both AC and DC power supply against external events and provide sufficient power in case of severe accidents. In case that they are lost, alternative power supplies need to be provided for the plant.
Loss of Heat Sink
Loss of ultimate heat sink is the important lesson of the accident as well as loss of emergency power. Damage of seawater pumps by the tsunami caused multiple failures of functioning of pumps and heat exchangers needed for cooling and dumping heat into the sea.
Provision of protective measures such as bunkering of important components and/ or alternative cooling devices as well as the water source is necessary.