Sat. Aug 24th, 2019

US nuclear bombs still pollute several Pacific islands 60 years later | Science


It had not been a year since the bombs on Hiroshima and Nagasaki ended the WWII, when the US started its nuclear test program in the Marshall Islands, then under US administration. Between 1946 and 1958, atomic weapons exploded in this archipelago in the middle of the Pacific. 60 years later, an exhaustive independent study reminds us that sea bottoms, soils and even the fruits of atolls such as Bikini or Enewetak accumulate radioactive particles well above the permitted levels and, locally, in concentrations higher than those measured in areas affected by Chernobyl or Fukushima disasters.

"Until now there have been no independent investigations of radioactive contamination and its consequences," says the Spanish Mónica Rouco, who was deputy director of the Project K = 1, the nuclear studies center of the University of Columbia (USA), when its scientists conducted a series of scientific missions to the Marshall Islands between 2015 and 2018. Until now, the only studies carried out in the former Spanish colony on the effects the nuclear test had been done by government scientists and military, especially the Lawrence Livermore National Laboratory. "The Government of the islands did not trust much of their data," Rouco adds.

Scientists from the K = 1 Project were able to analyze the levels of environmental gamma radiation in successive campaigns. first results published in 2016, and more recently also the concentration of several radioactive elements, such as plutonium-238, americium-241 or cesium-137 in samples of soils and sea beds and fruits collected by all the atolls and islands that supported the bombs or their radioactive fallout.

The US conducted 67 nuclear tests in the Marshall Islands, including its two most powerful hydrogen bombs

The Marshall Islands are made up of thirty atolls and several islands. The nuclear tests of the USA were concentrated in two of them, those of Enewetak and Bikini, located to the north. Many of the explosions took place inside the lagoons but there were some on small islands that were vaporized. Although the 67 bombs account for only 6% of all US nuclear tests, they released more than half of the megatons: 108.5 Mt of the 196 Mt. One megaton equals the energy released by one million tons of trinitrotoluene or TNT .

The authors of the new study, also published in PNAS, they measured the gamma radiation in a dozen islands of four atolls, those already mentioned and those of Rongelap and Utirik. In these there were no trials but they received their radioactive fallout even being far up to 600 kilometers. They also took hundreds of soil samples to measure the concentration of five radioactive elements. From the bottom of the Bikini lagoon, where the US military exploded Castle Bravo, their largest thermonuclear bomb, they took 129 cylinders of the sediment layer.

"Our study of the crater of Castle Bravo It is the first systematic investigation with a large enough number of samples to obtain a map of the extent of contamination by different radioisotopes, "the current director of the K = 1 Project, Ivana Nikolic-Hughes, co-author of the study, comments in an email. In this ground zero species, there is little trace of plutonium-238 and cesium-137, but there is a high activity of three other elements, plutonium-239.240, americium-241 and bismuth-207, all also radioactive. concentration multiplies by 10 or up to 100 that detected in other areas of the Marshall.

As for environmental gamma radiation, the worst stops are the Bikini Islands, of the homonymous atoll, and Naen, in Rongelap. In both cases dozens of measurements reach and exceed five millisieverts (unit that measures the dose of radiation absorbed by living matter). In comparison, the natural radiation a human being receives per year is around 2.4 mSv, according to a guide from the Nuclear Safety Council.

But the worst is on the ground. Although they only detected the presence of plutonium-238, they did find four other radioactive isotopes and very high concentrations. Take an example: The US established the maximum security limit for one of them, americio-241, the figure of 1,110 beckerels per kilogram of matter, in this case land (the beckerel is the unit of nuclear activity of a radioactive isotope) . On the island of Naen, they reached 3,090 Bq / kg. Of other elements, such as cesium-137, in Bikini came to measure 7.140 Bq / kg. Although localized, they are numbers that exceed, and by far, those recorded in areas near Chernobyl a decade after the explosion of reactor number 2 or those measured after the tsunami that dismantled the Fukushima power plant.

"For each radioisotope (Am-241, Cs-137, Pu-238, and Pu-239,240) we sought to compare the values ​​we obtained with the available standards and / or concentrations that have been measured in other regions of the world affected by the radiation caused by humans, "explains Nikolic-Hughes. "In particular, the comparison of the concentrations of Pu-239.240 with the values ​​measured in the regions affected by the Fukushima and Chernobyl accidents indicates that they are significantly higher in some of the northern Marshall Islands," he adds. But the director of the Project K = 1 clarifies that to be able to compare it would be necessary to have many more records of the different sources and radiation types of the areas to be compared.

Cesium-137 is present in fruits at concentrations that exceed the maximum limits established after Fukushima

In 2018, the research went further in the search for other sources of radiation risk: they stopped in the possibility that it would get into the body with contaminated food. In these islands, the vegetable part of the local diet consists almost exclusively of coconuts and pandanos, a fruit reminiscent of pineapple. The members of Project K = 1 collected two hundred of both fruits from eleven of the islands punished by bombs or radioactive fallout.

Here they measured the presence only of cesium-137. "It is extremely soluble, it quickly combines with the surface layer of the soil and is captured by the roots of the plants," recalls Rouco. Following the Fukushima accident, the Japanese authorities set a maximum of 600 Bq / kg for cesium-137 in fruits. Some of the coconuts and pandanos harvested in Bikini exceeded 3,700 Bq / kg.

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