Castle Bravo

• 1. Introduction

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  Within one second of the Castle Bravo detonation, a fireball almost 4.5 miles across hung over Bikini Atoll. It was visible from Kwajalein Atoll, more than 250 miles away. The test took place at 06:45 local time on the 1st of March 1954, in the Marshall Islands. It was the first shot of Operation Castle, a series of high-yield thermonuclear tests run by the United States. It was also the first American test of a hydrogen weapon design that burned a solid fuel. The device did not behave the way its builders expected. Why did the explosion run so far past every prediction? Who was standing downwind when the weather turned? And how did one morning in the Pacific change the way nations tested their most powerful weapons?

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• 2. Building Shrimp

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  In 1953 the United States Air Force made clear that it wanted lighter thermonuclear weapons, ones its B-47 Stratojet and B-58 Hustler bombers could actually carry. Los Alamos National Laboratory answered with a three-quarter scale version of an earlier design called RUNT, and it named the new design SHRIMP. The planned weight fell from 42000 pounds for the TX-17 to 25000 pounds for the TX-21, which promised a far more practical gravity bomb. The device fired at Bikini was a proof test of the TX-21 and was not yet a finished weapon.

  The earlier Ivy Mike test had burned cryogenic liquid deuterium, and that fuel needed elaborate cooling equipment. SHRIMP used lithium deuteride instead, a compound that is solid at room temperature. The success of this approach made the cryogenic design obsolete, along with its weaponized derivative, the JUGHEAD, which had been slated for the first Castle Yankee shot. Enriched lithium was scarce, so the fuel was nominally 40% lithium-6, with the remainder the far more common lithium-7. The fuel slugs ran from 37 to 40% enrichment, and the weaker slugs sat at the end farthest from the primary. The first of the Alloy Development Plants had only begun production in late 1953. The charge came to about 390 kilograms of lithium deuteride, priced at 4.54 dollars per gram. Its fusion burn efficiency was close to 25.1%, the highest of the first generation of thermonuclear weapons.

  The whole device sat in a cylinder weighing 23,500 pounds, 179.5 inches long and 53.9 inches across. Its ballistic case was 9.5 centimeters of 7075 aluminum, chosen to save weight where other projects of the day used heavy stainless steel.

  At one end sat COBRA, a compact atomic bomb from Los Alamos boosted with deuterium-tritium gas. COBRA had been tested in the Upshot-Knothole Climax event, where it yielded 61 kilotons against an expected range of 50 to 70. That result was good enough to let planners cancel Operation Domino, a series meant to find a suitable primary. Its implosion system weighed only 900 pounds, partly because the charge used Cyclotol 75/25, which was denser than the usual Composition B.

  Inside the ballistic case ran a natural-uranium radiation case about 2.5 centimeters thick. Its inner face carried copper foil about 240 micrometers thick, a cheaper reflector than gold. X-rays from the primary traveled down a channel packed with polystyrene foam to reach the secondary. That gap, the interstage, is among the most closely guarded secrets of any multistage weapon. The secondary itself was steeply tapered, and the shape made the assembly look like the body of a shrimp. Down its center ran a hollow plutonium rod, the spark plug, weighing about 18 kilograms and boosted by about 4 grams of tritium. Boron was placed around the secondary to stop stray neutrons from setting off the spark plug too early. The Castle Koon device named MORGENSTERN lacked boron, and its spark plug fired early and cooked its own fusion fuel.

  The device stood in a shot cab on a one-acre artificial island built on a reef off Namu Island. Two pairs of 40-foot pipes ran from the case to the top of the cab. They carried early light to an arc of 12 mirror towers, which reflected it to high-speed cameras.

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• 3. The Lithium Surprise

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  Roughly 60% of the lithium in the fuel was lithium-7, and the designers at Los Alamos had written it off as a bystander. Their reasoning was that lithium-7 would absorb a neutron, become lithium-8, and then decay into a pair of alpha particles over nearly a second. That is vastly slower than a nuclear detonation, so it should have added nothing.

  Lithium-6 was the expected performer. It absorbs a neutron from the fissioning plutonium and releases an alpha particle and tritium, and the tritium then fuses with deuterium. The flaw lay in the lithium-7 assumption. When struck by neutrons carrying more than 2.47 MeV, lithium-7 does not simply absorb one. It splits into an alpha particle, a tritium nucleus and another neutron.

  The result was far more tritium than anyone had planned. The extra tritium fused with deuterium and produced still more neutrons. That flood of neutrons drove much more fission in the uranium tamper than predicted.

  The yield came in at 15 megatons, give or take 5, well over double what the designers had forecast. That is about 1,000 times the power of the bomb dropped on Hiroshima. It makes Bravo the sixth largest nuclear explosion in history. Only Tsar Bomba at about 50 megatons, Test 219 at 24.2 megatons, and three Soviet tests of roughly 20 megatons in 1962 at Novaya Zemlya exceeded it.

  Lithium-7 had been used only because lithium-6 was scarce and expensive. The later Castle Union test used almost pure lithium-6. Had enough lithium-6 existed, the usefulness of the common isotope might never have been discovered.

  The blast left a crater 6,500 feet wide and 250 feet deep. The mushroom cloud reached 47000 feet and a diameter of 7 miles in about a minute. In under 10 minutes it stood at 130000 feet and 100 kilometers across, still expanding at more than 160 meters per second. The cloud contaminated more than 7000 square miles of the Pacific.

  On the far side of the atoll, many permanent buildings on the control site island were badly damaged. Instruments built to send back data before being destroyed were vaporized instantly, and those meant to be recovered were wrecked by the blast, so little of the hoped-for data was collected. X-rays running through line-of-sight pipes even produced a small second fireball of 1 kiloton at Station 1200.

  The uranium tamper that boosted the yield also made the explosion unusually dirty, a fact that would matter once the wind turned.

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• 4. The Wind Shift

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  Dr. Alvin C. Graves, the Scientific Director of Operation Castle, held the final word on firing, with authority above the military commander. He decided to go ahead under the prevailing winds. In the declassified film Operation Castle, task force commander Major General Percy Clarkson points to a diagram showing that the wind shift was still within the range of "acceptable fallout", though just barely.

  The earlier film of the 1952 Ivy Mike test shows what such a decision looked like. Its narrator, the western actor Reed Hadley, stands on the control ship during the final conference and notes that 20,000 people live in the potential fallout area. He asks whether the test can be aborted. He is told yes, but that it would ruin the preparations for timed measuring instruments. In Mike the fallout landed north of the inhabited area.

  Bravo was different because of heavy wind shear. The wind that had blown north the day before the test steadily veered toward the east.

  The people running the test were caught as well. The firing crew on Enyu, an island of Bikini Atoll, noticed the wind carrying fallout toward them. They sheltered in their firing station, buttoned up for several hours until the outside radiation decayed. A reading of "25 roentgens per hour" was recorded above the bunker.

  The Navy tanker Patapsco had been at Enewetak Atoll in late February and was ordered on the 27th to return to Pearl Harbor at top speed. Without a decontamination washdown system, it could not safely sit in the test area. A cracked cylinder liner slowed the ship to one-third of its full speed. When Bravo went off, it was still about 180 to 195 nautical miles east of Bikini. Fallout began landing on its deck in the mid-afternoon of the 2nd of March. No radiation detectors were aboard, and the dust was first thought harmless, so nobody decontaminated the ship. Total whole-body exposure has been estimated between 3.3 and 18 roentgens.

  Farther east lay the inhabited atolls of Rongelap and Utirik.

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• 5. The Marshall Islanders

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  The heaviest fallout from Bravo was pulverized surface coral, and it settled like dust on Rongelap and Utirik. Within a few days, 86 people from Rongelap and 167 from Utirik were taken to Kwajalein for medical care. The United States Department of Energy later estimated that 253 islanders were affected. They suffered nausea, vomiting and diarrhea, and several weeks later many developed hair loss and skin lesions.

  In 1957 the Atomic Energy Commission judged Rongelap safe and let 82 inhabitants move back. They found that staple foods, including arrowroot, makmok and fish, had vanished or made people ill. They were removed again. In the end, 15 islands and atolls were contaminated. By 1963 thyroid tumors had appeared among Marshall Islands natives, including 20 of the 29 Rongelap children who had been present at the time of Bravo. Many birth defects were also reported. A medical study named Project 4.1 followed the effects of the fallout on the islanders.

  Compensation from the American government began in 1956 and varied with the level of contamination. By 1995 the Nuclear Claims Tribunal reported that it had awarded 43.2 million dollars, nearly its entire fund, to 1,196 claimants for 1,311 illnesses.

  Caesium-137 created a second wave of harm. Islanders who had returned to the testing atoll drank locally grown coconut milk. Plants and trees absorb caesium readily because it behaves chemically like potassium. The islanders showed abnormally high caesium levels in their bodies and had to be evacuated a second time. Meanwhile, ocean currents carried fallout that had landed in the water northwest and southwest.

  Exposure has been linked to a higher likelihood of leukemia and thyroid cancer. The relationship between iodine-131 levels and thyroid cancer is still being researched.

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• 6. The Lucky Dragon

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  Twenty-three crew members of the Japanese fishing vessel Daigo Fukuryū Maru, meaning Lucky Dragon No. 5, were caught in the heavy fallout. They did not expect any hazard. Unlike the firing crew on Enyu, they did not shelter in the hold of their ship or avoid breathing the dust. They developed acute radiation syndrome. Kuboyama Aikichi, the boat's chief radioman, died six months later. Another crew member had a child who was stillborn and deformed.

  The official American position was that bigger atomic bombs did not release proportionally more radioactivity. Officials denied that the crew had been harmed by fallout, even though data from the vessel and international sources said otherwise.

  Sir Joseph Rotblat, working at St Bartholomew's Hospital in London, showed that the contamination was far greater than officially stated. He deduced that the bomb had three stages and that the final fission phase had increased the radioactivity a thousand-fold. He probably reached this by studying tell-tale isotopes in the fallout, such as uranium-237. His paper was taken up by the media. In Japan the outcry grew so strong that diplomatic relations were strained, and some dubbed the incident a "second Hiroshima".

  The two governments nevertheless reached a quick political settlement. Japan received 15.3 million dollars, and the surviving victims were paid about 5,550 dollars each in 1954. It was also agreed that the victims would not be given Hibakusha status.

  The story did not end there. Records released in 2014 acknowledged that crews of 10 ships had been exposed, though below health-damaging levels. In 2016-45 Japanese fishermen from other ships sued their government for withholding records of their exposure. In 2018 the Kochi District Court rejected the suit. It acknowledged the exposure but could not "conclude that the state persistently gave up providing support and conducting health surveys to hide the radiation exposure".

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• 7. Backlash And Limits

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  Traces of radioactive material from Castle Bravo turned up in Australia, India and Japan, and even in the United States and parts of Europe. Monitors set up a worldwide network of gummed film stations after Operation Castle. Debris tended to stay in tropical latitudes. Outside the tropics, the Southwestern United States received the greatest total fallout, about five times that received in Japan.

  Scientists later caught strontium-90 particles from the test in the stratosphere using balloon-borne air filters, in a study called Project Ashcan. The American magazine Consumer Reports warned that milk was being contaminated with strontium-90.

  The Russell-Einstein Manifesto of the following year pointed straight at Bravo. It said: "we now know, especially since the Bikini test, that nuclear bombs can gradually spread destruction over a very much wider area than had been supposed."

  At a National Security Council meeting in May 1954, President Dwight D. Eisenhower said, "everybody seems to think that we are skunks, saber-rattlers, and warmongers." Secretary of State John Foster Dulles added, "comparisons are now being made between ours and Hitler's military machine."

  Operation Castle as a whole had produced 48.2 megatons against an expected 23. In 1956, Operation Redwing was therefore run on an energy budget that capped total yield at 20 megatons and limited the fission share. Some very dirty fission weapons were still tested. Testers also began replacing the fallout-producing uranium tamper with a clean lead tamper, at the cost of halving the yield.

  In 1958, while preparing Operation Hardtack I, Eisenhower set an unwritten rule that no American test could exceed the 15 megatons of Castle Bravo. John F. Kennedy kept to it even after the 50-megaton Soviet Tsar Bomba test in 1961, and despite pressure from the Department of Defense, the Atomic Energy Commission and the Livermore laboratory. After the 1963 Partial Nuclear Test Ban Treaty, American testing moved underground. The largest underground shot was Cannikin in 1971, at 5 megatons.

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• 8. The Peaceful Atom

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  Lewis Strauss, chair of the Atomic Energy Commission, spoke at a conference six months after Bravo and promised that children would enjoy "electrical energy too cheap to meter". The government's image repair leaned on the Atoms for Peace initiative, launched four months before the test, and on the idea of a peaceful thermonuclear atom. The Soviet Union had already announced that the Obninsk Nuclear Power Plant joined the electrical grid on the 27th of June. The United States achieved a brief grid connection with the BORAX-III reactor in 1955 and a permanent one with the Shippingport Atomic Power Station in 1957.

  Fusion energy drew its own proposals. In 1957, Edward Teller and other Livermore weapons scientists described a gigawatt-level plant that would drop 1-megaton bombs every 12 hours into a 1,000-foot cavity to make steam. Livermore scientist John Nuckolls started a "hectoton group" of 100 tons, which looked for ways to remove the fission primary. He invented the idea of low-yield implosions of deuterium-tritium pellets, now known as inertial confinement fusion.

  Bravo also sits in a race of weapon designs. The Soviet Sloika of 1953 used lithium deuteride but was not a true hydrogen bomb. Fusion gave only 15-20% of its 400-kiloton yield. Ivy Mike reached 10.4 megatons, yet 77% of that came from fast fission of its natural-uranium tamper. In the month after Bravo, Andrei Sakharov hit on what the Soviet Union called his third idea, that the primary's X-rays could compress the secondary before fusion began. His team tested their first staged device in 1955. Rotblat's deduction that Bravo had three stages was itself militarily sensitive.

  The SHRIMP design evolved into the Mark 21 bomb, 275 of which were built. Each weighed 17600 pounds and measured 12.5 feet long and 58 inches across, with a yield of 18 megatons. Production ran until July 1956, and in 1957 the design was converted into the Mark 36.

  In Japan, the test and the poisoning of the Lucky Dragon crew fed antinuclear protests. Bravo is often woven into Japanese media, especially around Godzilla. In the 2019 film Godzilla: King of the Monsters, Castle Bravo becomes the call sign of Monarch Outpost 54, near Bermuda. Donald Fagen's 2012 song "Memorabilia", on the album Sunken Condos, mentions both Castle Bravo and Ivy King.

  In 2013 the Defense Threat Reduction Agency published Castle Bravo: Fifty Years of Legend and Lore. Its focus is exposure on the uninhabited atolls, and it makes no attempt to address in detail what happened at Bikini Atoll itself.

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