Crippled by a tsunami of unprecedented scale, one of the reactors at Japan’s Fukushima Daiichi nuclear power plant is in danger of melting down. To save it, workers must do the unthinkable: Vent radioactive steam into the atmosphere, to avoid a larger catastrophe as explosive gasses build up...

In March 2011, one of the largest earthquakes in history struck eastern Japan, followed by a massive tsunami. This one-two punch then caused a third disaster—a total loss of power at the Fukushima Daiichi nuclear power plant. Employees at the plant found themselves scrambling to cool down the...