The North Korean nuclear crisis has fueled global concern over how much time is left before Pyongyang masters the technology required to miniaturize nuclear warheads and make the re-entry vehicles needed to deliver them on target.

But a Sept. 3 announcement by the reclusive state raised fears of another kind of attack that completely bypasses those hurdles: an electromagnetic pulse.

In Sunday’s announcement, Pyongyang claimed to have successfully tested a hydrogen bomb and said it had the ability to detonate one at high altitude to generate an EMP — an electromagnetic wave that would fry electronic devices and disrupt communications for hundreds of kilometers around.

A nuclear device detonated 30 km to 400 km above ground could disrupt nearly all types of electronics within range, including computers, power grids and communication systems, experts say. Recovery could take years.

“The electromagnetic pulse generated by a high-altitude nuclear explosion is one of a small number of threats that can hold our society at risk of catastrophic consequences,” concluded a 2008 report by a U.S. commission tasked with assessing such threats for Congress, the president and other key government bodies.

Some experts have questioned the commission’s estimates. Others have argued the North would not stage such an attack because it would immediately be met by a massive, devastating retaliatory attack by the U.S.

But perhaps the most alarming aspect of an EMP attack is that staging one wouldn’t require an accurate ballistic missile. Even a simple balloon would be enough.

“For instance North Korea could make an EMP attack against the United States by launching a short-range missile off a freighter or submarine or by lofting a warhead to 30 kilometers burst height by balloon,” William Graham, chairman of the U.S. congressional commission, wrote in a June 2 article published by 38 North, an authoritative website that specializes in analysis of North Korean affairs. “Even a balloon-lofted warhead detonated at 30 kilometers altitude could blackout the Eastern Grid that supports most of the population and generates 75 percent of U.S. electricity.”

Japan, experts say, looks particularly vulnerable compared with the U.S., which is thought to have already shielded key defense and government facilities from EMPs.

At a news conference Thursday, Chief Cabinet Secretary Yoshihide Suga said Japan would consider measures to do the same.

But a senior government official, speaking on the condition of anonymity, admitted that Japan remains in the very early stages of addressing EMP threats and said Suga’s comment means the government “will just start studying what it can do.”

Retired Major Gen. Takashi Onizuka of the Ground Self-Defense Force has long warned that a nuclear EMP attack could be catastrophic for Japan.

Onizuka headed the GSDF’s Chemical School in Omiya, Saitama Prefecture, Japan’s sole institute specializing in studying defensive measures against nuclear, biological and chemical weapons, from 2004 to 2005.

“Japan hasn’t even recognized the EMP threat until recently,” he told The Japan Times in a telephone interview, adding that he doesn’t believe even the Self-Defense Forces are well-prepared against EMP threats.

According to Onizuka, if a 10-kiloton atomic bomb is detonated 30 km above the Kanto region, it would have a 602-km radius of effect that would cover most of Honshu. At an altitude of 135 km, the blast radius would expand to 1,300 km, covering Hokkaido and Kyushu.

In a 2016 article for the Tokyo-based nonprofit organization Center for Information and Security Trade Control, Onizuka warned that a high-altitude EMP attack would damage or destroy Japan’s power, communications and transport systems as well as disable banks, hospitals and nuclear power plants.

“If the electricity supply for a nuclear power plant is cut off, and the operator cannot deal with the situation by activating an emergency power system or generators, it could lead to an emergency like the Fukushima nuclear power plant accident,” Onizuka wrote.

After the Fukushima core meltdowns, Japan idled all of its commercial reactors. Five have since been allowed to resume operation and the central government is pushing hard for more restarts.

At a meeting of the Lower House Foreign Affairs Committee on May 12, Shunichi Tanaka, chairman of the Nuclear Regulation Authority, admitted that the nation’s nuclear power plants haven’t taken any particular measures against the EMP threat.

Tanaka said the effects of an EMP attack “are not assumed” under nuclear safety regulations and that the NRA would only order the plants to shut down if war broke out.

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