You may be surprised to know that Germany's wild boars are too radioactive to eat – and Chernobyl may not be solely to blame. Fallout from nuclear weapons testing decades ago during the Cold War is a significant contributor to that radiation, it turns out.

High levels of the radioactive isotope cesium-137 – or caesium-137 for those outside the US – found inside most animal species living in Bavarian forests have declined over time, except for wild swine. Scientists haven't been able to explain this so-called "wild boar paradox," though a team of researchers led by those at the Leibniz University Hannover may be one step closer to figuring out why.

They analyzed 48 wild boar meat samples to potentially pinpoint the source of the contamination. In 88 percent of the samples, the level of radiation from cesium-137 exceeded Germany's food safety limits of 600 Bq/kg. It's believed this radiocesium got into the boars' diet via truffles, which they root out from underground. The cesium-137 sinks down, is absorbed by the fungus, and then gobbled up by the pigs.

Nuclear weapons fallout contributes significantly to the notorious contamination levels in wild boars in central Europe

The radioactive cesium likely came from two possible sources: the 1986 Chernobyl nuclear reactor disaster, and nuclear weapons testing during that century. To check for this, and using a mass spectrometer, the university team calculated the ratio of cesium-135 to cesium-137 in the wild boar, and found the ratio high enough to suggest that, as the scientists put it, "nuclear weapons fallout contributes significantly to the notorious contamination levels in wild boars in central Europe that were previously believed to be dominated by Chernobyl."

A high ratio of cesium-135 to cesium-137 is a telltale sign that the isotopes came from nuclear weapons rather than nuclear reactors. Radioactive cesium-135 is created when xenon-135, a product of the nuclear fission of uranium, decays without absorbing a neutron. In nuclear reactors, the level of xenon-135 is controlled and converted to its stable form of xenon-136 through neutron absorption. In the case of nuclear weapons, however, xenon-135 is left to proliferate, and it leads to higher levels of radioactive cesium-135.

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Since cesium-135 has a longer half-life than cesium-137 – also a product of uranium and plutonium fission – a higher ratio indicates that the source of radioactivity is more likely to come from nuclear weapons than nuclear reactors. It seems to us that the scientists are arguing that the cesium-135-to-cesium-137 ratio is high, and the level of cesium-137 is already known to be high, so that points to nuclear weapons tests as a significant source of the contamination.

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That appears to be the logic, anyway.

"It is the nuclear history that is different," Georg Steinhauser, co-author of a paper about this work [4]published in the American Chemical Society's Environmental Science & Technology journal, told The Register . "Nuclear explosions produce more cesium-135 than nuclear reactors."

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"[Xenon-135] has a large cross-section for thermal neutron capture, resulting in suppressed onset of [cesium-135] under the high neutron flux density of a reactor core," the team explained in their paper.

"By contrast, despite the intense but short neutron flux at the moment of a nuclear explosion, [xenon-135] mostly 'survives' the explosion because most [of its] primary fission products [telerrium-135] and [iodine-13] have yet to decay to [xenon-135]. A nuclear explosion hence yields a relatively high [cesium-135 to cesium-137] ratio, whereas a reactor yields a low ratio."

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The researchers calculated that between 10 and 68 percent of the cesium contamination in the wild boar samples are from nuclear weapons testing. "Although Chernobyl has been widely believed to be the prime source of [cesium-137] in wild boars, we find that 'old' [cesium-137] from weapons fallout significantly contributes to the total level in those specimens that exceeded the regulatory limit," they wrote.

Now, the team is warning against the detrimental long lasting effects of nuclear weapons and nuclear reactor disasters on food safety. Countries like the US, Soviet Union, and UK conducted thousands of nuclear weapons tests during the Cold War from the 1940s to 1990s.

"An important takeaway is that the long-forgotten atmospheric nuclear weapons tests and their fallout still cast a shadow on the environment," Steinhauser told us.

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"Just because they took place 60 years ago doesn't mean that they no longer impact the ecosystem. Monitoring of wild boars will remain necessary (then there is no problem eating them). We must take good care of our planet and avoid any further releases. These may create a snowball effect." ®

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