61 years ago today, the Castle Bravo Shot

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Zero hour for Bravo was at 6:45 a.m. local time on March 1. From the moment the device detonated, many of the observers knew something had gone spectacularly wrong.

The flash from the nuclear explosion was overwhelming, even by the standards of nuclear explosions. Men saw their bones appear as shadows through their living flesh. Streams of blinding light shone through the smallest cracks and pinholes in secured doors and hatches.

Bravo’s thermal radiation was far more intense than expected. More than 30 miles away from Ground Zero on Bikini Atoll, sailors on board Navy ships said the heat was like having a blowtorch applied to their bodies.

The shock wave destroyed buildings supposedly outside of the calculated damage zone. It nearly knocked observation aircraft out of the sky, and caused some men inadvertently trapped in a forward observation bunker to wonder if the explosion ripped their concrete and steel shelter from its foundations and flung it into the sea.

Then there was the fireball.
Film originally produced for classified review by the Eisenhower administration and key members of Congressional oversight committees. It emphasizes the successes of the Castle Bravo shot and makes scant mention of the test’s miscalculations

It was four miles in diameter and hotter than the surface of the sun. The Bravo fireball rose at the rate of 1,000 feet per second, and created a mushroom cloud that eventually topped 130,000 feet above sea level.

“In mere seconds the sailors sensed that something unspeakably wrong was occurring … Battle-hardened men who had served in World War II went to their knees and prayed,” wrote L. Douglas Keeney in 15 Minutes: General Curtis LeMay and the Countdown to Nuclear Annihilation.

“We soon found ourselves under a large black and orange cloud that seemed to be dropping bright red balls of fire all over the ocean around us,” one sailor recounted. “I think many of us expected that we were witnessing the end of the world.”

Later, scientists calculated that Castle Bravo’s yield was actually 15 megatons.

The reason? A “tritium bonus” occurred during the thermonuclear reaction. Cascading neutrons transformed the lithium-7 isotope—that comprised most of the “dry fuel”—into tritium and helium.

Tritium causes extremely energetic fusion.

It was the thermonuclear equivalent of throwing gasoline on a small blaze and producing an instant conflagration.

Bravo’s yield was 1,000 times greater than the Hiroshima bomb, far bigger than the scientists had planned. To make matters worse, meteorological forecasts predicted that high-altitude winds would blow the radioactive fallout away from inhabited areas.

Instead, the wind blew the radioactive cloud toward them.

Fallout from Bravo rained down on ships and sailors. Ships’ captains ordered entire crews below decks, and sealed their vessels for days in an effort to escape contamination. Fallout dusted U.S. service members stationed on nearby Rongerik Island.

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