Japan's Nuclear and Industrial Safety Agency approved opening the double-entry doors, deciding it would not cause more environmental harm, Kyodo News reported.
TEPCO said it would send workers into the reactor building about 4 a.m. Monday to measure radioactivity levels.
The utility said even if radioactive substances are released by opening the doors, the dose within the plant would be below unsafe levels, the Japanese news agency said..
On Thursday, TEPCO workers installed pipes connecting the No. 1 reactor building with a ventilator to filter out radioactive substances and allow safe entry. The ventilation was stopped and some of the pipes removed Sunday night.
The door opening will let workers start building a new cooling system for the reactor, whose cooling was knocked out by the March 11 earthquake and tsunami.
Meanwhile, the company said radioactive strontium at 100 times normal levels was found in soil inside the plant, and the temperature at the No. 3 reactor's pressure vessel was rising again.