Igor Gramotkin spoke Thursday at an international conference in Kiev called "Twenty-Five Years of the Chernobyl Disaster: Security in the Name of the Future."
Gramotkin said a large part of the plant remains unexplored and a new "sarcophagus" would be erected over the plant's power unit No. 4, which was destroyed in the 1986 blast, Tass reported.
"Over the past few years, the staff of the plant have examined slightly more than 60 percent of the premises inside the sarcophagus while the rest of the premises are inaccessible because of rubble or high radiation levels," Gramotkin said.
"The absence of information on the character of the nuclear fuel hidden in the rubble poses one of the most serious risks today," he said.
Gramotkin said about 95 percent of the damaged reactor's radioactive fuel remains buried under 200 tons of debris.
The new sarcophagus has a service life of 100 years and will be completed in 2015, Gramotkin said.