Meltdown Caused Nuke Plant Explosion: Safety Body, Nikkei, March 13, 2011:

The Nuclear and Industrial Safety Agency (NISA) said Saturday afternoon the explosion at the Fukushima No. 1 nuclear plant could only have been caused by a meltdown of the reactor core.

The same day, Tokyo Electric Power Co. (9501), which runs the plant, began to flood the damaged reactor with seawater to cool it down, resorting to measures that could rust the reactor and force the utility to scrap it. …

At a news conference Saturday night, Chief Cabinet Secretary Yukio Edano discounted the possibility of a significant leak of radioactive material from the accident. “The walls of the building containing the reactor were destroyed, meaning that the metal container encasing the reactor did not explode,” Edano said. …

NISA is affiliated with the Ministry of Economy, Trade and Industry.