Castle Bravo (1954): The Worst US Nuclear Test Accident

Castle Bravo was a United States hydrogen bomb test carried out at Bikini Atoll in the Marshall Islands on 1 March 1954. The explosion produced about 15 megatons, far more than its designers had expected, and winds carried radioactive fallout onto inhabited islands and a Japanese fishing boat. It was the largest nuclear test the United States ever conducted and its worst radiological accident, and it turned nuclear fallout into a worldwide public concern.

Background: the race for the hydrogen bomb

After the Soviet Union tested its first atomic bomb in 1949 (see RDS-1), President Harry Truman ordered work on a far more powerful weapon, the thermonuclear or hydrogen bomb. Such a weapon uses a fission explosion to trigger the fusion of light elements, releasing vastly more energy than the bombs dropped on Hiroshima and Nagasaki.

The first US thermonuclear test, Ivy Mike in November 1952, used liquid deuterium that had to be kept at extremely low temperatures, making it far too bulky to drop from an aircraft. The next step was a “dry” design using solid lithium deuteride, which could be made into a deliverable weapon. Operation Castle, a series of six tests in 1954, was designed to prove it. The tests were run by Joint Task Force 7, a combined military and civilian organisation, under the authority of the US Atomic Energy Commission, which was then chaired by Lewis Strauss. Several of the Castle devices were designed at the Los Alamos laboratory, and their success led directly to the first deployable American hydrogen bombs.

The Marshall Islands were then a United Nations Trust Territory administered by the United States. Bikini’s inhabitants had been moved to other atolls in 1946 to make way for nuclear tests.

What happened

The Bravo device, nicknamed “Shrimp”, was placed on an artificial island on the reef near Namu islet, at the north-west of Bikini Atoll. It was detonated at 6.45 am local time on 1 March 1954.

The scientists had predicted a yield of about 6 megatons, within a range of roughly 4 to 8. Instead the explosion reached about 15 megatons. The fuel contained lithium-6 and the more common lithium-7; the designers had assumed lithium-7 would contribute little, but under intense neutron bombardment it also produced tritium, which fused and greatly increased the yield.

The fireball was several miles across, and the mushroom cloud rose to around 40 kilometres (about 25 miles) within minutes. The blast left a crater roughly 2 kilometres (about 1.2 miles) wide in the reef. Test personnel sheltering in a bunker about 30 kilometres away were trapped for hours by rising radiation.

The fallout

The explosion pulverised coral and sucked it into the cloud, where it became radioactive. Before the test, forecasts had shown winds blowing at high altitude towards the north; in fact winds carried the debris east over inhabited atolls, and officials had decided to go ahead despite reports of a shift.

A few hours later a white, powdery ash began falling on Rongelap Atoll, roughly 160 kilometres (100 miles) east of Bikini. Children played in it, not knowing what it was. About 236 Marshallese on Rongelap, Ailinginae and Utirik were exposed, as were 28 American weather station personnel on Rongerik. The people of Rongelap were not evacuated until 3 March, and those of Utirik the following day. Many suffered burns, hair loss and nausea, and in later years thyroid tumours and other illnesses were found at high rates, especially among those exposed as children.

The Lucky Dragon

The Japanese tuna trawler Daigo Fukuryu Maru (Lucky Dragon No. 5) was fishing well outside the danger zone the United States had declared. Its 23 crew saw the flash, heard the blast minutes later and were covered in ash for hours.

The boat returned to its home port of Yaizu on 14 March 1954, and the crew were diagnosed with acute radiation sickness. Contaminated tuna caused a panic in Japanese markets. The radio operator, Aikichi Kuboyama, died on 23 September 1954; the cause is generally given as complications of radiation sickness, though some US officials attributed it to hepatitis from blood transfusions.

Consequences

Castle Bravo caused a diplomatic crisis with Japan, only nine years after the atomic bombings. In January 1955 the United States paid Japan 2 million dollars without admitting legal liability. In Japan a petition against hydrogen bomb tests, started by women in Tokyo’s Suginami district, gathered many millions of signatures, and the first world conference against atomic and hydrogen bombs met in Hiroshima in 1955. The film Godzilla, released in November 1954, opened with a fishing boat destroyed by a mysterious flash.

Elsewhere, scientists and the public learned that fallout could contaminate huge areas far from a test site. Concern over radioactive strontium-90 in food and milk grew through the 1950s, contributing to the Partial Nuclear Test Ban Treaty of 1963, which ended atmospheric tests by its signatories.

Legacy in the Marshall Islands

The Rongelap people were returned to their atoll in 1957, though it remained contaminated. In 1985, fearing for their health, they left again with the help of the Greenpeace ship Rainbow Warrior. The United States has provided medical monitoring and compensation, including through a Nuclear Claims Tribunal set up in 1988, but many Marshallese consider it inadequate.

Between 1946 and 1958 the United States conducted 67 nuclear tests in the Marshall Islands. Bikini Atoll was inscribed as a UNESCO World Heritage Site in 2010 as a record of the nuclear age.

By the numbers

Myths and facts

Castle Bravo is sometimes described as the largest nuclear explosion ever. It was the largest by the United States, but the Soviet Union’s Tsar Bomba test of October 1961, at roughly 50 megatons, was more than three times as powerful.

It is also often said that the fallout was a pure accident of nature. The unexpectedly high yield was indeed a scientific miscalculation, but weather briefings before the shot had indicated that winds were shifting towards the east, and the test went ahead anyway. Marshallese survivors and their advocates have long argued that the risks to inhabited atolls were not taken seriously enough. Claims that the islanders were deliberately exposed as test subjects, however, have not been proven; what is documented is that they were then enrolled in a long-term US medical study of radiation effects, often without fully informed consent.

Timeline

• 1946, March: Bikini’s inhabitants are moved to make way for nuclear tests.
• 1952, 1 November: Ivy Mike, the first US thermonuclear test, at Enewetak.
• 1954, 1 March: Castle Bravo is detonated at Bikini Atoll.
• 1954, 3 March: Rongelap’s inhabitants are evacuated.
• 1954, 14 March: The Lucky Dragon returns to Yaizu, Japan.
• 1954, 23 September: Aikichi Kuboyama dies.
• 1955, January: The United States pays Japan 2 million dollars.
• 1963, 5 August: The Partial Nuclear Test Ban Treaty is signed.

Why it matters

Castle Bravo showed that hydrogen bombs could poison people and environments far beyond the blast itself. Its victims in the Marshall Islands and Japan made fallout a global political issue, strengthened the movement against nuclear weapons and helped bring about the first treaty limiting nuclear testing.