What happened at Fukushima?

On Friday, March 11, 2011, one of the largest earthquakes in the recorded history of the world occurred on the east coast of northern Japan. This earthquake also generated a major tsunami, causing nearly 20,000 deaths. Electricity, gas and water supplies, telecommunications, and railway service were all severely disrupted and in many cases completely shut down. These disruptions severely affected the Fukushima Daiichi nuclear power plant, causing a loss of all on-site and off-site power and a release of radioactive materials from the reactors.

After the earthquake, Units 1, 2, and 3 shutdown automatically (SCRAM), per design. The power generators “tripped”. Movement of the plant’s foundation “exceeded design basis earthquake ground motion” (DBEGM) in Units 2,3, and 5. This disabled offsite power systems, but no serious damage to onsite safety systems.

After the loss of power, heat generation due to fission of uranium stops with SCRAM, but heat generation due to radio-active decay of fission products continues. Power was needed to pump water and cool the core, so emergency diesel generators provided power to the Emergency Core Cooling Systems (ECCS). After the earthquake, the reactors were isolated and stable.

About 40 minutes after the quake, a tsunami hit the plant. The design basis for tsunami height was 5.7 m (17.1 ft), but the actual maximum tsunami height was 15 m (45 ft). Since the seawater pumps were submerged by the tsunami, the residual heat removal systems (which releases the residual heat inside the reactor to the seawater) and the auxiliary cooling system (which releases the heat of equipment to the seawater), lost their functions.

As a result of the tsunami, all AC power was lost for Units 1 -5, and unit 6 retained one operating generator. The Reactor Core Isolation Cooling (RCIC), followed by High Pressure Coolant Injection (HPCI) system, were operated manually and/or with power from back-up batteries. When the back-up batteries failed, the pressure of the containment structures began rising rapidly, due to the decay heat producing steam in reactor pressure vessel. Steam relief valves were opened to relieve pressure, but the steam had already discharged into wet well. This caused a decrease in coolant level in the reactor pressure vessel.

The containment pressure began to rise, to around 130 psi, due to a build-up of hydrogen, nitrogen and water vapor. It was only designed for 73 psi. Both Units 1 and 3 exploded, and the steel frame roofs were destroyed. The concrete building surrounding the units remained intact, while seawater was injected to cool the reactors.

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