High radioactivity has prevented workers from entering the reactor buildings at the plant, some 140 miles northeast of Tokyo, since hydrogen blasts rocked four of its six reactor buildings on March 12.

The plant's operator, Tokyo Electric Power Co. (TEPCO), said workers are trying to install ventilation equipment to Unit 1 to reduce air radiation levels.

As soon as radiation drops to permissible levels, expected in four to five days, specialists will attempt to restart the reactor's cooling system, knocked out by a powerful earthquake and tsunami on March 11.

A TEPCO spokesman said radioactivity levels remain high in the building, and teams will enter the crippled nuclear facility for short periods on rotation.

TEPCO expects radioactivity levels at the plant to stabilize within six to nine months. It also says radioactivity levels in the air around the plant will drop significantly within three months.

The twin natural disaster in northeast Japan in March left more than 25,000 people dead or missing. Thousands more were forced to evacuate their homes in the ensuing nuclear disaster.