In 2006, with the fear nationally that terrorists could get hold of a small nuclear device, Hawaii held an exercise simulating a half-kiloton detonation in Honolulu Harbor.

The crater at Pier 1, the notional explosion spot, was predicted to be 20 feet deep with a radius of 220 feet. Up to 820 feet away, all buildings would collapse under a more than 300-mph blast wave. Cars on Ala Moana Boulevard would be overturned.

Officials said at the time that such a ground blast would result in 200 immediate fatalities, 3,000 to 8,000 casualties with injuries or radiation, and possibly 4,000 longer-term fatalities.

The threat of a terrorist nuclear bomb replaced fears of Cold War Soviet nuclear bombs heading for Hawaii. Now the concern is changing again, with North Korea threatening Hawaii and the mainland.

The state again is planning for the worst.

Vern Miyagi, administrator with the Hawaii Emergency Management Agency, said North Korea could be developing a 10-kiloton warhead capability. For comparison, the bomb dropped on Hiroshima in 1945 was 15 kilotons.

“So if you have a strike here, like we had in Hiroshima, I mean, there’s not much you can do,” Miyagi said. “If you have the warning time, (the thing to do) is to really find a substantial place and get under it, and that’s all you can do.”

Pearl Harbor and Camp Smith, headquarters of U.S. Pacific Command, would be likely targets, and Miyagi remembers as a kid being told “make sure you are on the other side of the Koolaus if there’s a missile coming.”

The type of explosion also has to be considered, and “whether it’s in the atmosphere or whether it’s on the ground or if it’s offshore,” Miyagi said. “Again, potentially all kinds of effects. Offshore could be a tsunami because of that.”

A nuclear weapon detonated in or above Earth’s atmosphere can create an electromagnetic pulse that can damage electronic devices connected to power sources or antennas. This happened in Honolulu on July 8, 1962, with the test of a hydrogen bomb above Johnston Atoll, 860 miles to the southwest.

The Hawaii Emergency Management Agency, housed in Birkhimer Tunnel in the side of Diamond Head, has an 8-by-20-foot EMP-shielded room that dates back to the Cold War and still has Federal Communications Commission paperwork from 1979 on the door.

The room, which received some updates five years ago, is used to house a high-frequency radio system that can reach back to the mainland if other forms of communication go out.

The United States faces nuclear threats from countries such as China and Russia that are much greater — and far less survivable — than North Korean devices. The Bulletin of the Atomic Scientists in 2015 postulated what would happen if an 800-kiloton Russian warhead detonated over Manhattan: A fireball would vaporize structures beneath it, and a blast wave would crush concrete structures within a couple of miles of ground zero, according to the organization.

Toby Clairmont, executive officer of the Hawaii Emergency Management Agency, said the purpose of Oahu’s fallout shelters was to provide protection from the radiological contaminants in the air — the fallout — for about two weeks. When a nuclear device is detonated, the radiation is actually fairly short-lived, he said.

“And after two weeks you could walk out of those places,” Clairmont said. “In the meantime you’ve got to keep some concrete (as a barrier) so there’s some density there, some thickness, until the radiation decays.”

The city’s Department of Emergency Management said on its web page that “the old yellow signs designating fallout shelters are no longer current.”

Miyagi said Hawaii Emergency Management is an all-hazards agency, “so the point is, whether it’s nuclear events, whether it’s hurricane or tsunami, the same thing that we need to get out to the folks is that they need to have a plan.”

That includes coordinating with family members on meeting spots and communication when and if usual methods are disrupted.

“Know where to go, know what to do, and know when to do it,” Miyagi said.