41 minutes to meltdown

The largest earthquake in Japan's recorded history. Reactors 1, 2, and 3 automatically shut down, SCRAM procedure activates as designed. The reactor systems are functioning correctly. The plant has survived the earthquake intact.

The earthquake severs all external grid connections. Emergency diesel generators activate automatically, 13 units across the site. The generators are located in the basement. They are working. Cooling water is circulating. The plant is stable.

Forty-one minutes pass. Operators monitor the reactors. The plant is managing the shutdown correctly. The 2008 study's warning, that a large tsunami would flood the basement generators, exists in a document filed somewhere inside TEPCO. No emergency measures are taken for the generators in this window. The decision had never been made.

The tsunami overtops the 5.7-metre seawall by more than eight metres. Seawater floods the turbine buildings and the basement. The wave is 1.7 metres shorter than the engineers predicted in 2008. The direction of the error offers no comfort.

Seawater reaches the basement generator rooms. Twelve of thirteen emergency diesel generators are submerged and short-circuit within minutes. The one surviving generator serves Unit 6 only. Units 1, 2, and 3 lose all AC power. Station blackout.

DC batteries provide limited cooling instrumentation. Operators have approximately eight hours before the batteries are exhausted and all cooling monitoring ceases. The reactors continue generating decay heat. Without active cooling, fuel damage begins. The clock is running.

DC power fails in Unit 1. All cooling instrumentation goes offline. Operators lose visibility into the reactor core. Water levels are falling. Fuel rods are beginning to be exposed. What happens next inside the reactor will not be fully understood for years.

Exposed zirconium fuel cladding reacts with steam to produce hydrogen gas. The hydrogen ignites. The outer reactor building explodes. Reactor 1 core has melted. The evacuation zone is expanded from 3km to 10km, then to 20km. 154,000 people begin leaving their homes.

A more violent explosion destroys the Unit 3 reactor building. Debris is scattered across the site, injuring workers and hampering emergency response. Reactor 3 core has melted through. Mixed-oxide fuel adds to the radioactive release.

Reactor 2 loses cooling. The core melts. A partial containment failure follows, possibly the largest direct release of radiation across the three units. The corium, molten nuclear fuel mixed with structural material, has burned through the reactor vessel.

The eventual cost of the Fukushima disaster, cleanup, decommissioning, compensation, lost energy revenue, litigation, exceeds one hundred billion dollars. The cost of raising the seawall and relocating the generators in 2008 was estimated at approximately one billion dollars. The 2008 study had been correct. The organization had been given three years to act on it.