Cs (Cesium) deposition and precipitation amount on 15 March. The cyclone produced a few millimeters of rain in areas on Honshu Island engulfed by the FD-NPP plume, which led to 137Cs washout. Precipitation was strongest (6mm) near FD-NPP, which produced particularly large deposition amounts of up to nearly 1000kBqm-2 in the vicinity of FD-NPP.
Our simulation suggests that this was the main deposition event over Japan for the entire duration of the disaster. It was due to an unfortunate combination of three factors: (1) the highest emissions of the entire duration of the accident occurred during 14–15 March, (2) the winds transported these emissions over Japan, and (3) precipitation occurred over eastern Japan. Luckily, it did not rain (also confirmed by radar data) exactly at the time when – according to our simulation – the highest concentrations were advected over Tokyo and other major Japanese cities. In such a disastrous scenario, much higher 137Cs deposition in the major population centers would have been possible.
The report, "Independent Investigation Commission on the Fukushima Daiichi Nuclear Accident," is based on interviews with more than 300 officials, utility executives and TEPCO employees. ... The report quotes Kan's chief Cabinet secretary, Yukio Edano, as experiencing a "demonic scenario in my head" in which four of the plant's six reactors exploded, setting off more explosions at a nearby plant, with massive releases of radiation, according to reports from Japan.
"If that happens, Tokyo will be finished," Edano recalled thinking at the time, according to the foundation report. Edano's public statements at the time did mentioned those fears.