A Chinese nuclear expert said Sunday that the Fukushima nuclear power plant explosion was under control, but the ensuing aftershocks were the biggest threats to Japan.

Zhao Zhixiang, a scientist with the China Institute of Atomic Energy, said in an exclusive interview with Xinhua that Japanese authorities' information release showed the reactor building containment was kept intact in spite of the hydrogen gas explosion.

"In that case, only the radioactive gas could spill out as was detected around the reactors. It's not the nuclear fuel leakage like Chernobyl," Zhao said.

"We still can't judge whether the reactor core melt down or not, but in Three Mile Island's case, the reactor core melt-down didn't bring significant harm to the environment," he said.

Chernobyl Accident took place in 1986 in Ukrainian and was rated as level 7, the top nuclear accident on the international nuclear event scale. The Three Mile Island Accident happened in 1979 in the United States was rated level 5.

The Fukushima nuclear accident was rated by Japanese authorities as level 4. However, Zhao urged Japan and potentially affected countries to keep alert about the possible implications from aftershocks and take preventive measures.

"The damaged reactors are vulnerable to any further shocks," Zhao said. "We have to pay close attention to it."

More than 300,000 people were evacuated in six Japanese prefectures around the nuclear plant area, according to government tallies.

Zhao said that the nuclear plant is usually 8.0-magnitude quake-proof, but Japan saw this time the 9.0-magnitude quake accompanied by tsunami.

"International atomic energy community will definitely re-examine its current nuclear policy after Fukushima, especially concerning the plant site, quake-proof ability, secondary disaster affection and the power system."

The power system was ill-functioned in Fukushima, which leveled up the reactor temperature and thus caused the hydrogen explosion, Zhao said.

China's operational nuclear plants are technically superior to the second-generation plant like Fukushima. It uses the passive residual heat removal system that can reduce explosion risks, and brings relatively more nuclear safety, Zhao said.