Plant operator, Tokyo Electric, released the report based on plant data and employee interviews.
The findings describe chaos in the desperate battle to protect the Fukushima Dai-ichi plant from meltdown.
Workers were not equipped with protective gear on site and had to bring in emergency materials to guard against radiation exposure.
They also struggled with equipment they hadn't been trained on.
Tokyo Electric has been criticized for dragging its feet with venting and sea water cooling following the March 11 tsunami - two crucial steps that experts say could have mitigated the damage.
The report revealed insufficient preparations at the plant that the power company hadn't previously acknowledged. It said plant workers had a disaster drill just a week before the tsunami and "everyone was familiar with emergency exits," but it apparently did not help them cope with the crisis.
The report also said workers borrowed batteries and cables from a subcontractor on the compound to set up a backup system to gauge water levels and other key readings.
Meanwhile, more radioactive water is pooling at the plant. Workers scrambled to restart a key cleanup system, which was shut down Saturday hours after beginning full operations because a component reached its radioactivity limit faster than expected.
More than 100,000 tons of contaminated water at the plant could overflow within two weeks if action is not taken.
The Associated Press contributed to this story