Editor's Note: This is part of IEEE Spectrum's ongoing coverage of Japan's earthquake and nuclear emergency.
Yukio Edano, the Japanese government’s chief cabinet secretary, said in a nationally televised press conference in Tokyo that at 3:36 p.m. today an explosion occurred in a reactor of the Fukushima Dai-1 plant and caused the outer wall of the structure containing the reactor to collapse. He said that water in the reactor dropped when the pumping system failed, causing steam to be generated which filled the space outside the reactor and the inner walls of the outer structure. This in turn generated hydrogen which mixed with the steam and caused the explosion.
“Because there was no oxygen in the container, there was no explosion in the container, so there was no damage,” said Edano. “And there was no great amount of radiation leaked.”
Earlier, he said TEPCO had begun venting steam from the container to reduce the growing pressure inside. As a result, the density of radiation around the structure rose to the level of 1015 microsieverts produced in 1 hour. “After the explosion, the pressure inside the container fell and remains at a very low level,” says Edano. At the same time, the density of radiation also fell to 860 microsieverts, and by 6.48 p.m. it had fallen further to 70.5 microsieverts.
TEPCO has now decided to flood the container with seawater to bring down the temperature. After the government consulted with other experts and TEPCO, it gave its consent to what is “an unprecedented step,” says Edano.