The chief of the Fukushima No. 1 nuclear plant said in testimony before his death that he feared the core meltdowns he was trying to contain in March 2011 would cause catastrophic damage to eastern Japan, government documents show.

"Our image was a catastrophe for eastern Japan," Masao Yoshida told a government panel probing the Fukushima nuclear crisis. "I thought we were really dead."

Yoshida wanted his testimony to remain confidential after his death because the account might contain mistakes from the confusion created by the crisis triggered by the massive earthquake and tsunami of March 11. But leaks to two major Japanese dailies prompted the government last week to announce its intent to disclose most of the documents, which detail the drama that took place during the worst nuclear disaster since Chernobyl.