What Happens Inside an Exploding Nuclear Reactor? | Unveiled

VOICE OVER: Ashley Bowman WRITTEN BY: Caitlin Johnson
When a nuclear plant goes into meltdown, it can lead to some of the most devastating disasters in human history. In this video, Unveiled discovers exactly what happens inside an exploding nuclear reactor - including at the three most famous meltdowns of all time; Fukushima, Three Mile Island and Chernobyl...

What Happens Inside an Exploding Nuclear Reactor?

By the mid-20th century, we’d already been experimenting with radioactive materials for numerous decades, leading to the development of everything from nuclear bombs to indispensable x-ray machines. But it wasn’t until the ‘50s that atomic energy for power became a reality, and the first nuclear reactors were developed and put into use. In the following years, however, questions about the safety, and long-term effects, of nuclear reactors have only grown in importance.

This is Unveiled, and today we’re answering the extraordinary question; what happens inside an exploding nuclear reactor?

There have been numerous nuclear accidents over the years, from experiments gone wrong to radioactive sources being stolen and causing contaminations elsewhere. But there have only been three large-scale nuclear reactor meltdowns: Three Mile Island in Pennsylvania in 1979 (a partial meltdown), the Chernobyl Disaster in Ukraine in 1986, and the Fukushima Daiichi meltdown in Japan in 2011. Of these, both Chernobyl and Fukushima are considered number 7s on the International Nuclear Event Scale, which is the highest an accident can be. Despite being the same ranking, however, the Chernobyl disaster is still widely considered the worst nuclear accident in human history due to the number of direct deaths it caused, and the volume of radioactive material dispersed into the environment – which was significantly larger than at Fukushima.

There are different kinds of nuclear reactors, but the most common generate energy primarily via fuel rods of an inherently unstable uranium isotope, uranium-235. This isotope, when its nucleus is purposely split by a free neutron in the reactor core, produces a reaction called nuclear fission, which generates large amounts of energy. This energy creates heat, which evaporates water and produces steam, which in many nuclear plants then turns a large turbine, and the turbine generates electricity. In other words, these plants make power the same way, say, a coal plant does, with the crucial difference of course being the raw material. Nuclear plants can generate much more power with much less fuel, and are technically environmentally “cleaner” — in terms of pollution, though not radioactive waste — provided they work as they’re supposed to.

Though the causes vary wildly, from preventable design flaws to human error to an earthquake, reactor meltdowns happen for just one reason: the reactor ceases to function properly and the radioactive fuel rods inside “melt down” because nothing is cooling them. This happens because nuclear fission is, unsurprisingly, very very hot; a fully-functioning nuclear reactor core is around 572 degrees Fahrenheit, for instance. Because of this, a coolant system — usually water-based — is necessary to make sure it doesn’t overheat. The 2011 earthquake and resulting tsunami that affected Fukushima was strong enough to cut off power to the plant, meaning its coolant system failed and three of the six reactors went into meltdown. At Chernobyl, a shift of workers carried out a complex safety test they weren’t properly prepared for that involved simulating how well the reactor would manage in a power cut — ultimately meaning again that a coolant system failed. When a meltdown happens, the nuclear reaction goes completely out of control as the core begins to rapidly decay, which can sometimes result in an explosion.

Explosions at nuclear plants are very rare, however. At Fukushima, the meltdowns caused a different reaction, which created hydrogen gas that was eventually able to ignite and explode, but this was still nothing compared to the explosion at Chernobyl’s Reactor 4. This enormous explosion that blew apart the plant happened for multiple complex reasons due to negligence and lack of attention to safety protocols, leading to a massive buildup of pressure, exacerbated by a design flaw with the control rods. Inside the reactor, these control rods, made of boron, were inserted to shut down the reaction, but their graphite tips briefly, yet consequentially intensified the already precipitous release of energy. The resulting explosion was so huge, the roof of the plant building was completely destroyed, and large concrete structures had to built to contain the area.

An exploding nuclear reactor is vastly different from an exploding nuclear bomb. Despite being designed to explode and cause destruction, no atomic bomb can ever have effects as long-lasting or far-reaching as a nuclear plant accident if it were truly out of control. This is because of the volume of radioactive fuel. Little Boy, the bomb dropped on Hiroshima in 1945, contained 141 pounds of enriched uranium, while just one reactor at Chernobyl contained over 30,000 pounds in its fuel rods. At the same time, though there’s significantly more nuclear material in a reactor, it’s thankfully rare that it gets dispersed over a large area; Chernobyl was an ostensibly unique case, with many particular factors contributing to the explosion and fire; the Chernobyl Exclusion Zone is still 1000 square miles in size to this day. And a lot of care has also had to be taken to try and prevent groundwater tables from being contaminated by radioactive water in Fukushima. Nuclear reactor accidents simply have many long-reaching consequences.

One thing nuclear events of all shapes and sizes can produce is radiation sickness. In order to develop Acute Radiation Syndrome, which is radiation sickness caused by a high dose over a short period of time, a human needs to be exposed to 0.7 Grays of radiation or higher. It’s been estimated that Alexsandr Akimov, the shift supervisor of Reactor 4 during the Chernobyl accident, was subjected to a lethal dose of 15 Grays during his work to try and contain the disaster, which killed him within two weeks. If you were exposed to an exploding nuclear reactor and didn’t die as a result of the explosion, you’d almost certainly develop ARS and be dead anyway in a matter of weeks – and it’s not a pleasant death. Some of the early symptoms of radiation poisoning are diarrhea and vomiting, while later the radiation burns get so severe that your skin may slough off. This is the proper term for what an animal does when it sheds its skin, only humans aren’t, of course, designed for this, so it is a horrible way to die.

Radiation also makes you more susceptible to secondary infections, made even worse by the fact your skin is covered by open sores and you’re plagued by internal bleeding. The short-term death toll of the Chernobyl disaster has stood at 54 since 2006, though this is still a point of great contention and it’s impossible to know the number of long-term deaths due to exposure to radiation and the toxic cloud that spread from the site of the explosion, though it is often estimated to be many thousands.

While at its best, atomic energy is powerful and efficient, at its worst it can create some of the most violent and deadly incidents in human history. And that’s what happens inside an exploding nuclear reactor.