Gauge Newsletter January 2020 | Page 43

When the earthquake happened, the reactor 1, 2 and 3 were shut down automatically. Unit 4, 5 and 6 were shut down previously for outages, with unit 4 having been defueled in November 2010. So chain reaction was stopped within seconds of the earthquake but the uranium fuel rods remained very hot. It is called decay heat and it takes 20 -24 hours to cool down the reactor to less than 212 Celsius and achieve the state, which is called cold shutdown. Emergency power generators supplied power for the cooling system due to the loss of offsite power as they designed to do. Things were moving smoothly. With the earthquake, the tsunami warning was issued and they expected a 10 ft tsunami. Fukushima Daiichi was situated 30 ft above sea level and nobody cared about the tsunami. When the tsunami struck the power plant, the waves were massive and they were more than 50 ft high. Both emergency power generators were affected, but there were another 2 emergency generators. Both generators were not affected but both of them had their switches in the basement and both basements were affected by the tsunami. So, both generators were unusable. A few minutes later, the entire alarming system went off and station blackout happened. That means, all the AC and DC power sources of the power plant were not working. Now there’s nothing to stop the meltdown. The people in the control room had no clue that the reactors contain the water or not. If the water in the nuclear reactor was boiled and evaporated, then the fuel rods will start to melt. Engineers collected batteries from vehicles and tried to power on the alarm system. But, since there was no way to stop the meltdown, the Japanese government declared an emergency at the Fukushima Daiichi power plant. On Saturday, 12th march around 6:22 pm, the first hydrogen explosion happened on the service floor of the building above unit 1 reactor containment and injured four people. Around 8:20 pm, residents living within 20 kilometers of the plant were told to evacuate the area. Around 200,000 people had to leave. On 14th March the second explosion happened in the reactor no 2 and authorities began to pump a mixture of seawater and boron into the No.2 reactor to cool its nuclear fuel rods. Early on the 14th, the third hydrogen explosion happened in the reactor no 3 as well, but surprisingly, reactor no 4 also caught fire. This one was shut down before the tsunami and the earthquake. This explosion happened from hydrogen arising in unit 3 and reaching unit 4 by backflow in shared ducts when vented from unit 3. The peak of radioactive release was reported on the 15th and apparently from the explosion of the reactor no 2. To take the reactors into the cold shutdown conditions, they had to pump water and nitrogen until December 2012. GAUGE Magazine University of Peradeniya Page 41