Ever since the Chernobyl Nuclear Power Plant exploded in 1986, scientists have been studying the area to learn about the effects of  radiation on the ecosystem.   Radiation obviously isn’t great for your health, so it was good news that  the levels of contamination found in the animals nearby that disaster have been decreasing for decades.

Except for one strange outlier.   Researchers noticed that  wild boars around Chernobyl have had the same levels of radioactive isotopes in their bodies for years. Which flies in the face of everything we know about how radioactive decay and ecosystems are supposed to work.

Scientists called it the Wild Boar Paradox. And this year, they may have finally solved the mystery.   [ intro music ] The Chernobyl disaster  happened in Northern Ukraine in 1986 when a meltdown at the plant released about 5% of the  reactor’s radioactive core into the air and the surrounding area. 31 people died as a direct result of the meltdown, and while it’s hard to say  how many cases of cancer resulted from the Chernobyl disaster, experts put that number  somewhere in the thousands. This was the largest radioactive  disaster in Europe’s history.

As soon as it happened, the surrounding area was evacuated and turned into what’s now called  the Chernobyl Exclusion Zone. But the animals and plants in the  Exclusion Zone couldn’t leave, and for years after the disaster, scientists had no idea how they would be affected. The exclusion zone has been studied ever since, and we are still learning new things from it about how radiation interacts with ecosystems.

Basically, we wanted to understand how long it takes nature to  recover after a disaster, and how long it takes until  exposed areas are safe again. And these studies have looked  at other European regions, too. They’ve had to, since the explosion at Chernobyl caused radioactive contamination to spread all over the continent.

Close monitoring of the soil and  all the living things in the area revealed that over the years, the amount of radioactive contamination decreased. In most things, anyway. Over the years, scientists noticed that their surveys had a consistent anomaly – even when all the other animals were beginning to return to normal, the wild boars remained radioactive.

This is the Wild Boar Paradox - why are these guys still  radioactive after all this time? We actually have a good  idea of how radiation works, at least in a closed system. Like, we know that radioactive  atoms break down at a steady rate, which is known as the substance’s half-life.

See, radioactive decay is random - a given radioactive atom  might decay in a millisecond, or in a hundred years. But if you’ve got enough atoms of the element, you can calculate its overall rate of decay. Some radioactive elements  are more stable than others, meaning they have longer half-lives.

The three most dangerous  isotopes released at Chernobyl were Iodine-131, Strontium-90, and Cesium-137. Iodine-131 only has a half-life of 8  days, so it’s basically all gone by now. But the half-lives of Strontium-90 and  Cesium-137 are 29 and 30 years, respectively.

That means that 30 years after the disaster, we’d expect the amount of radiation around  Chernobyl to have decreased by approximately half. But that’s in a closed system. Radiation levels don’t always decrease at that consistent rate, at least, not everywhere all at the same time.

That’s because these radioactive elements are also moving through the ecosystem. Plants take up contaminated water, animals that eat those plants accumulate that radiation in their bodies, and the cycle continues. So while the full amount of radioactive substance in an ecosystem is going down as it decays, there will be different amounts of contaminants in all the different places.

Radiation also filters down into ground water, and eventually, it’s too far down for most organisms to get to. But put a pin in that, because we’re going to come back to this idea. The rate at which radiation moves through  animals is called the biological half-life.

For Cesium-137, that’s about 70 days. That means if you eat food tainted with Cesium-137 and survive, half of it should be out of  your system in around 70 days. If you noticed , that biological half-life of Cesium-137 is much shorter than its ordinary half-life.

That’s because it isn’t just going away due to decay - the animal is also actively excreting it along  with all the other stuff it expels from its body. That said, the animals at Chernobyl didn’t just get one radioactive dinner - they’re eating radiation over and over again throughout their lives. So every time they get a  dose of radioactive dinner,   more time gets added to the half-life counter.

Complicated? Very. Ecosystems are incredibly complex, with a lot of moving parts and a lot of variables.

But while the exact numbers can vary, the consistent thing about  radiation levels in the environment is that it should decrease over time, at the same rate as its half-life or faster. And we see that in these plants and animals. For instance, we do see a decrease in average  radioactive contamination in deer in Europe over time.

Even if we can’t always determine exactly  how quickly the radiation is decreasing, we do see a consistent downward trend. And that is again true of  most things in this vicinity - except for those pesky boars. Unlike the deer, the amount of Cesium-137 in the boars has remained unusually high.

Even though it’s been more than 30  years since the Chernobyl disaster, the average amount of Cesium-137 in the boar meat they’ve been testing hasn’t gone down at all. And since 30 years is the  half-life of that Cesium-137, this finding does seem to break the laws of physics. But a 2023 study suggests  that the wild boar paradox only appears to be a paradox, because of a crucial piece of information that was being left out.

Turns out, these boars aren’t just radioactive because of the Chernobyl disaster. Their particular radiation spike goes back much longer, decades longer in fact. See, back in the 1950s and ‘60s, the Soviet Union conducted quite a few atmospheric nuclear weapons’ tests.

Of course, people were concerned about radioactive contamination from those tests, but it was thought that the fallout from the Chernobyl explosion was a much larger problem. But today, researchers were able to show that a large proportion of  the cesium contamination in boar meat came from those weapons tests, and not from Chernobyl. They used a new method, which allowed them to better detect the remnants of Cesium-137 decay.

Remember what I said about radioactive decay? Yeah, those atoms don’t  just disappear into nothing. They turn into something  called a daughter product, which is an isotope that has  decayed out of a parent atom.

Daughter products can be stable, non-radioactive elements, or another different radioactive element with its own half-life that  someday will break down into different daughter products, because nothing in this world is simple. Anyway, those daughter products can also reveal things about  the radioactive parent material they came from. Specifically, how long that parent material had been there.

In this case, researchers used the ratios  of those daughter products to model how long the cesium had been decaying, and thus, which source the cesium came from. Which is how they knew the cesium contamination was older than Chernobyl, and figured out that the weapons tests were the real culprits. But why is that cesium only  showing up in the boars?

Well It turns out, the answer has to do with mushrooms. Fungi are major movers of  environmental contamination, and radiation is no exception. They spread in networks known as mycelial webs, and they exchange water and  nutrients across those webs.

Mycelial webs drink up contaminated water at a deeper level than plant roots can get to. Because of this, they can access deeper sources of radiation, too. The mushrooms in this area have such deep networks that they’re still soaking up contamination from all that atmospheric weapons testing.

They’ve only just started to grab the Chernobyl cesium too, because it’s taken this long for that cesium to filter into the deepest groundwater. And boars love eating those mushrooms, so that radioactive contamination has been transferring into  them through their dinners. Wild boars have been eating a source of food that has way higher environmental contaminants than anything else in the area, which is what’s keeping their internal radiation levels so high.

So there it is - the solution to a decades-old paradox, found right under our noses. Wild boars do not break the laws of physics. This research doesn’t just help solve a mystery, it’s incredibly helpful for  lots of other investigations into ecosystems contaminated by radiation, and how those dangerous materials will continue to circulate through food webs going forward.

All that, and we didn’t even need to worry about mutant radioactive super-boars. Not yet, anyway. I have seen Teenage Mutant Ninja Turtles though so I know it’s possible.

Thanks for watching this episode of SciShow! And, an extra-big thanks to our patrons over on Patreon for their support. One out of every 10,000 of  our viewers is a patron, it’s a really rare person, there aren’t that many of you and we really couldn’t keep  making these videos without you.

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