Fig. 2. The Baker nuclear test – 25 July 1946.

The Baker (explosion) nuclear test, which was carried out on 25 July 1946 at Bikini Atoll, was one of two nuclear detonations under Operation Crossroads, was the fifth nuclear weapon detonation in human history, and was the first ever to be detonated underwater.

The objective for the US government was to evaluate the effects of the shockwave, thermal component and radiation on a fleet of 57 ships and a number of animals (mainly pigs, goats and rats) located within, and on a small number of the vessels.

The device, a 21Kt TNT equivalent, plutonium implosion-type nuclear weapon similar to Fat Man that was detonated over the Japanese city of Nagasaki by the United States on 9 August 1945, was suspended 27m (90 feet), from a landing craft halfway between the surface of the water and the seabed, which was around 55m (180 feet) in depth at the target location.

The detonation at Bikini Atoll took place at 08:35 local time and was unique in that the initial flash was barely observed at ground level as the target area was underwater. The energy released, generated a vertical column of water, approximately 600m in diameter, interacting with the atmosphere and creating a shockwave and condensation cloud which expanded spherically from the centre.

Fig. 3. Shockwave and condensation cloud.

In the immediate seconds following detonation, the landing craft above the device, was vaporised, and a total of ten ships sank, including the German heavy cruiser Prinz Eugen, which sank five months after the test, due to damage from the underwater shockwave; it was not possible to repair the vessel at the time, due to high levels of radiation.

The explosion produced a shallow crater in the seabed 9m (30 feet) in depth, and approximately 600m in diameter, coinciding with the diameter of the water column. Due to the configuration of the Baker test (under the surface of the water), most of the fission products were contained in the local environment, within the lagoon and target ships that survived the initial shockwave, which were so heavily contaminated they were unable to be boarded (after hosing down with saltwater) for ten days.

Of the 6.2kg of plutonium in the device, approximately 1.4kg (3 pounds) underwent fission and mixed with (approximately) two million tons of seawater and seabed sand that were lifted into the column. The explosion ejected more than twice as many neutrons as there were fission events at about, 2.9 neutrons per fission, most of which were consumed in the production of more fission until the fission fell off, and the remaining uncaptured neutrons escaped.

Many of the radiation effects and levels of contamination were unexpected by the scientists conducting the experiment, which was compounded by a lack of any viable decontamination procedure and cleanup plan in advance of the test.

Although radiation levels have consistently dropped since the last nuclear test on 22 July 1958, a survey in 2016 found radiation levels on Bikini Atoll as high as 639 mrem yr−1, which is well above the established safety standard for habitation. However, scientists at Stanford University reported “an abundance of marine life apparently thriving in the crater of Bikini Atoll” in 2017. Research is ongoing on how marine organisms are surviving in an elevated radiation environment, which could lead to improved understanding of cancer and increased longevity.

In late 2019, a team of oceanographers, geologists, marine biologists and archaeologists, headed by the survey team-leader Dr Trembanis from the University of Delaware, have returned to Bikini Atoll to carry out a comprehensive survey of the former nuclear bomb test site to record environmental changes and map the seafloor.

The survey team had initially expected that the crater would be covered by sediment over time, however, the Baker test crater is still present, along with the twisted remains of the target ships.

Fig. 4. Sonar map showing the depression crater of the Baker test and sunken ships in the lagoon.

Using sonar, the survey team mapped the depression which is approximately 800m across falling off to about 10m in depth from the natural seabed.

At the American Geophysical Union meetingheld on 9 December 2019 in San Francisco, Dr Trembanis told BBC News, “We wanted to draw back the curtain and be able to really reveal this scene. “We were using advanced sonar technology; we could paint the entire scene. It’s a bit like visiting the Grand Canyon with a flashlight versus going in the middle of the day and illuminating the whole area.

“We could start to see the arrangement of the ships; we could see how they were aligned relative to each other, and we could see that this crater still remains – nature is still showing us this wound that it received from the bomb.”

Remarkably, the crater has a rippled structure that looks a bit like rose petals. It’s evidence of all that material initially thrown into the sky then falling back down through the water column and spreading out across the seafloor.

Part of the motivation for the survey was to understand the continuing environmental impacts better. Although radiation levels are much reduced, there is an ongoing pollution problem coming from the sacrificial ships.

Fig. 5. Scan of the aircraft carrier USS Saratoga – 2019.

These vessels – old units from the US, Japanese and German navies – were not prepared with the expectation that they would become artificial reefs. If that was the intention, they would have been stripped down.

operational. That meant they were fuelled and even had munitions aboard.

“As we were mapping, I could know without looking up when we were near the [US aircraft carrier] Saratoga, because we could smell the bunker fuel; it was so heavy and is still streaking out.

“The Nagato – which was the Japanese flagship that [Admiral Isoroku] Yamamoto used to plan the attack on Pearl Harbour – had a streak of fuel coming out from it for many miles.”

As the ships continue to disintegrate in the water, this pollution could become a much bigger problem, Dr Trembanis said.

Fig. 6. Wreck of the aircraft carrier USS Saratoga – 2018.

Below, Video of the Baker nuclear test explosion and shockwave.


Notes:
1. A total of 23 nuclear weapons were detonated at various sites on Bikini Atoll by the United States between 1946 and 1958 producing a total fission yield of 42.2Mt.
2. Further monitoring and investigation is due to be undertaking in 2020 to determine future environmental concerns resulting from climate change.
Links:
Mean External Gamma Exposure above Background (mrem/yr).
Measurement of background gamma radiation in the northern Marshall Islands.