The man who has battled to stabilize Japan’s crippled nucler plants for the past nine months abruptly stepped down today and was hospitalized with an undisclosed illness.

Masao Yoshida, 56, did not disclose his illness Monday, but told plant workers that doctors detected it during a recent medical checkup, and advised that “he concentrate on the treatment.”

Plant operator TEPCO said it could not release the amount of radiation Yoshida had been exposed to at the plant, citing privacy issues.

Yoshida has been at the helm of efforts to stabilize the crippled Fukushima Dai-ichi nuclear plant reactors since a catastrophic tsunami triggered the worst nuclear disaster since Chernobyl.

Nearly nine months later, the 12 mile radius around the plant remains uninhabitable, contaminated with high levels of radiation. More than 80,000 people remain displaced.

News of Yoshida’s illness comes just weeks after TEPCO opened the Fukushima plant to reporters for the first time since the March 11 catastrophe, saying the reactors had finally stabilized. During the press tour, Yoshida told journalists he expected to die several times during the first week of the nuclear crisis. When he witnessed the hydrogen explosions at reactors 1 and 3, he “thought it was all over.”

Takeshi Takahashi, who has been in charge of nuclear power plant operations at TEPCO’s headquarters, will succeed Yoshida.