Fukushima Nuclear Disaster: How the Cooling Was Lost

The Fukushima nuclear disaster began not with the earthquake, but with the tsunami that flooded the backup generators 50 minutes later. A wave nearly 15 metres high overwhelmed defences built for 3.1 metres, cutting power to the pumps that remove decay heat from shut-down reactors. Three cores melted over three days, not from runaway fission, but from leftover heat with nowhere to go. On 11 March 2011, a magnitude-9.0 earthquake shook Japan's coast and automatically shut down Fukushima Daiichi's three operating reactors. The chain reaction stopped within seconds, exactly as designed. But a shut-down reactor still generates decay heat—the radioactive byproducts inside the fuel keep decaying and releasing energy. At the moment of shutdown, that heat is still about 6.5% of the reactor's full operating power. One hour later, it falls to about 1.5%, but removing it still requires active cooling. The tsunami arrived roughly fifty minutes after the earthquake, just as decay heat was falling but still far from harmless. The wave overtopped the seawall and the plant's ten-metre ground level, flooding every emergency diesel generator except one. All AC power was lost at units 1 through 5. With no electricity to run the pumps, cooling water began to boil away from the reactor vessels. As water levels dropped, fuel temperatures rose past the point where the metal cladding and then the fuel itself began to melt. All three operating cores melted within the first three days. The melted fuel in Unit 1 breached the primary containment vessel. Hydrogen explosions damaged the reactor buildings on units 1, 3 and 4—not nuclear explosions, but a chemical reaction from overheated fuel igniting in air. The accident was rated level 7 on the International Nuclear and Radiological Event Scale, the maximum level, because of the radioactive releases during days four to six. The earthquake and tsunami themselves killed over 15,000 people. From the nuclear accident at Fukushima Daiichi, the World Health Organization recorded no acute radiation injuries or deaths. Japan's official record lists one death confirmed as caused by radiation: a worker diagnosed with lung cancer four years after the accident. Fukushima was not a reactor running out of control. It was a loss of cooling to heat that never stopped, after the part everyone worries about had already shut down. SOURCES World Nuclear Association — https://world-nuclear.org/information-libr... American Nuclear Society — https://www.ans.org/pubs/reports/fukushima... World Health Organization — https://www.who.int/news-room/questions-an... What question about this accident would you want answered next? #FukushimaNuclearDisaster #JapanNuclearDisaster #WorstNuclearDisaster Chapters: 0:31 What did the earthquake do to the reactors? 1:31 Wasn't the plant built to survive a tsunami? 2:30 What happened when the wave arrived? 3:35 Why did the flood matter if the reactors were off? 4:36 Why would a shut-down reactor still need power? 5:35 How fast does that leftover heat fall? 6:34 What happened once cooling actually stopped? 7:35 Did the melted fuel stay inside the reactor? 8:26 Where did the explosions come from? 9:30 How bad did regulators judge this to be? 10:34 How many people actually died from the accident? 11:40 So what actually failed at Fukushima? Narrated by an AI voice. Pictures are AI-generated illustrations unless credited above.