The Foreign Ministry secretly conducted a simulation in 1984 to assess damage from a hypothetical attack on a nuclear power plant in a war and concluded that up to 18,000 people would be killed with acute symptoms from radiation exposure, it emerged Wednesday.

The previously secret report also mentioned the possibility of a hydrogen explosion that could follow the meltdown of fuel rods in a nuclear power plant, the exact phenomenon that happened during the 2011 Fukushima crisis.

Anti-nuclear activists slammed the ministry for not publicizing the report earlier, allegedly out of fear that the warnings could fan anti-nuclear sentiment among the public.

“The report should have not been held secret. (The government) should publicize it and consider how it can protect” nuclear power plants, said Hideyuki Ban of the Citizens’ Nuclear Information Center.

The content of the document was first reported Wednesday by the Tokyo Shimbun, which obtained a copy of the 63-page report through the information disclosure law.

The Japan Times confirmed the outline and key conclusions of the report with a senior Foreign Ministry official.

According to Yasushi Noguchi, head of the ministry’s arms control and disarmament division, the ministry asked the Japan Institute of International Affairs, an affiliate of the ministry, to draw up the report after Israel staged an airstrike and destroyed a reactor under construction in Iraq in 1981.

Noguchi said the report concluded that up to 18,000 people would die in the worst-case scenario if the primary containment vessel of a 1 million kilowatt-class reactor in Japan was severely damaged and local residents did not evacuate immediately.

In another scenario, the report also warned that if all power supplies were cut and critical cooling functions lost, fuel rods would melt down. The hydrogen generated from metals used for fuel assembly cladding could then potentially cause an explosion.

This type of hydrogen explosion actually happened and aggravated the meltdown crisis at the Fukushima No. 1 power plant after the massive tsunami of March 11, 2011, knocked out the power supply, killing the critical cooling operation.

Few plant workers and nuclear experts anticipated a hydrogen explosion until one actually ripped through the No. 3 reactor building.

The Tokyo Shimbun alleged the Foreign Ministry did not publish the report because it feared it would fan anti-nuclear sentiment while the government was trying to build more nuclear power plants in the 1980s.

Noguchi said it was not published because it was intended as an internal reference for Foreign Ministry officials.

Ban claimed nuclear plants in Japan are not designed to be robust enough to withstand missile attacks or a suicide airplane crash such as the 9/11 terrorist attacks in New York in 2001.

The Nuclear Regulation Authority now requires that reactor buildings be robust enough to withstand the “intentional crash of a large aircraft” and other terrorist attacks.

But the NRA declines to disclose any more details of the safety regulations, saying they should be kept secret for security reasons.

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