Foreign Ministry officials made a concerted effort to downplay the Chernobyl nuclear disaster in 1986 to promote nuclear power and avoid friction at a Group of Seven summit in Japan, ministry documents showed.

The documents released on Dec. 20 also showed a sense of overconfidence in the safety of nuclear power in Japan that may have led in part to the Fukushima nuclear disaster in March 2011.

Soviet officials announced on April 28, 1986, that a nuclear accident had occurred in Ukraine. It would become the worst nuclear plant disaster in history.

According to the documents, Foreign Ministry officials scrambled to gather information about the nuclear accident ahead of the Group of Seven summit in Japan that started on May 4.

The United States was initially passive about issuing a G-7 declaration that criticized the Soviet Union for the accident.

Washington and Moscow at that time were negotiating an agreement to reduce their nuclear arsenals, and U.S. officials did not want to push the Soviet Union into a corner with criticism about Chernobyl.

Although then Prime Minister Yasuhiro Nakasone wanted a G-7 statement that touched upon the nuclear disaster, Japan and other G-7 members were promoting nuclear energy. So the declaration that eventually emerged downplayed the possible dangers to the environment and human health from the Chernobyl disaster.

The diplomatic documents showed that terms that might disrupt plans to push forward nuclear power generation were gradually deleted from the final statement.

“The confidence of national leaders about the safety of their own nation’s nuclear plants emerges from the documents,” said Kazuhiko Togo, a former Foreign Ministry official who now heads the Institute for World Affairs at Kyoto Sangyo University. “There was likely a sense of overconfidence that the accident happened because it occurred in the Soviet Union.”

The course taken by Japan veered widely from that of European nations regarding nuclear power.

Many European nations were directly hit by radioactive materials from the Chernobyl plant, and public sentiment in those nations quickly turned against nuclear power.

One year after the Chernobyl accident, the Green Party emerged as a political force in West Germany based largely on its anti-nuclear stance. A national referendum in Italy led to a landslide victory for anti-nuclear forces.

However, in Japan, the then Ministry of International Trade and Industry, which was in charge of nuclear power generation, showed a different stance in a statement issued on April 29, 1986, immediately after the Soviet Union announced the accident.

“The accident occurred at a nuclear plant unique to the Soviet Union, and such an accident would be unthinkable in Japan,” the ministry’s statement said.

Public debate on the need for greater safety at Japan’s nuclear plants did not deepen despite cover-ups of problems at a nuclear plant operated by Tokyo Electric Power Co. and an accident at a Hokuriku Electric Power Co. nuclear plant.

“Japan did not think seriously or make preparations whenever it was faced with a nuclear incident,” said Tatsujiro Suzuki, a former vice chairman of the Japan Atomic Energy Commission. “As a result, its failure to learn from its past lessons led to the Fukushima No. 1 nuclear plant accident.”

Japan also failed to keep up with international moves to strengthen the safety of nuclear plants.

In 1988, the International Atomic Energy Agency asked member nations to establish measures to deal with severe accidents on the precondition that such events are possible.

However, Japan did not obligate nuclear plant operators to set up these measures.

The U.N. Convention on Nuclear Safety, which took effect in 1996, carried a provision calling on signatory nations to separate their safety oversight agencies from the agencies that promote nuclear power.

Japan did not fulfill that obligation.

(This article was compiled from reports by Ryosuke Ishibashi, Masanobu Higashiyama and Toshihide Ueda, a senior staff writer.)