The problems that led to core meltdown and fuel damage at Unit 1 of the Fukushima-Daiichi nuclear plant began as a direct result of the impact of the tsunami and not a loss of coolant from pipe failure caused by the earthquake, a report by plant operator Tokyo Electric Power Company says.
The Fukushima Nuclear Accident Independent Investigation Commission of Japan’s Diet (parliament) had raised the possibility that the accident may have been the result of a loss of coolant because of earthquake- induced component damage and not the result of a loss of emergency power because of the tsunami, Tepco said in a statement.
If correct, this assumption would have contradicted the prevailing understanding that the facility had weathered the 9.0 magnitude earthquake, an important consideration for future designs incorporating seismic safety principles.
But the new report says Unit 1 survived the earthquake intact. Data recorded by wave metre records and other instruments, along with photographic sequences of the incoming tsunami, make it clear that the loss of emergency diesel generator power caused by the tsunami, and the resulting failure of the cooling systems, caused the accident, Tepco said.
Tepco said the report was less conclusive on why water injected into Units 1, 2 and 3 from fire trucks in the immediate aftermath of the tsunami when cooling systems had failed was insufficient to cool the reactor cores and prevent meltdown.
It is possible, the report says, that the water found its way into other systems and failed to reach the core. Because of this, an investigation into the actual amount of water injected into the unit and its impact on the progress of the accident will be “a focus of continued study”.
The new report is the first progress report on Tepco’s continuing investigation into the causes of the crippling of three of the facility’s reactor units after the earthquake and subsequent tsunami of March 2011.
Units 1, 2 and 3 at the six-unit plant were in commercial operation at the time of the earthquake and tsunami and all suffered reactor core, fuel and containment damage.
The other three units did not suffer fuel damage. Unit 4 was offline and was not loaded with fuel, but the reactor building was severely damaged by a hydrogen explosion. Units 5 and 6 were offline, but were still fuelled.
The Tepco report is online: