While channel surfing (or, rather, streaming-service surfing) recently, I came across a documentary on the Chernobyl disaster, Chernobyl: Secrets, Lies, and Untold Stories. Though I watched it on HBO Max, it seemed like typical History Channel fodder--and the writing, research, and editing of this particular documentary left a lot to be desired.
I'm going to focus on one particularly-egregious statement made during the program:
"Some scientists feared that if the nuclear fuel burned through the concrete floor to reach water tanks below, then a nuclear explosion would occur. One estimate said the blast could equal five million tons of TNT."
There are simply so many things wrong with that statement that it's difficult to tease them all apart!
But before I dig into them, I need to cover a few of the basics of nuclear physics (don't be scared; I'll keep it very high level).
Some Nuclear Physics Basics
It's been less than a century since some of the most fundamental discoveries of science that are necessary to understand why the program's statement was so wrong. First, you almost certainly learned in school that all matter is made up of atoms, and you probably learned that atoms are themselves are made up of smaller particles including protons, electrons, and neutrons--the latter not discovered until 1932 (!) by British physicist Sir James Chadwick. The neutron as you'll see is essential to understanding nuclear fission and with it why the statement made on Chernobyl: Secrets, Lies, and Untold Stories was so incredibly and fundamentally incorrect.
The number of protons in each atom determine what element that atom actually is; hydrogen has a single proton, carbon has 6 protons, and on the opposite end of the periodic table, uranium has 92 protons and plutonium 94. Protons have a positive electrical charge and sit in the atomic nucleus along with any neutrons (more on this in a moment), while electrons with their negative charges surround the nucleus. While protons define which element an atom is, electrons define how it interacts chemically with other atoms.
That leaves neutrons, which have no electrical charge (they're neutral, hence their name), weigh individually weigh about the same as a proton. Like protons, they're packed together into the nucleus and contribute to each atom's mass (weight, for laypeople). Recall that the number of protons is always the same for atoms of each element--one proton makes hydrogen, two makes helium, six make carbon, and so on and so forth. But the number of neutrons may vary, which doesn't change which element something is, or how it reacts chemically. Hydrogen, for example, can be a single proton and electron (which is the most common form of hydrogen), or it can include one or two neutrons. Atoms of the same element but with different numbers of neutrons are known as isotopes.
I'm trying to keep this as high level as possible, so without getting too deep into the weeds of nuclear physics, understand that there are forces holding the nucleus together, but as that nucleus gets larger (either because it's a heavier element and thus has more protons, or, most especially, because it has more neutrons), it becomes less stable and more prone to breaking apart.
Radioactive Decay
Atoms that contain extra neutrons often undergo what's called radioactive decay, undergoing changes that over time make them more stable. Again, without getting into too much detail, this decay releases energy, and that energy, or radiation, is what makes radioactive substances dangerous. There are several types of decay, and they can change one atom into another (by impacting its number of protons), or from one isotope of the same element to another (by impacting its number of neutrons).
Nuclear Fission and the Splitting of the Atom
Some large atoms are so unstable that they can break into large pieces (each a completely different atom), rather than just emitting energy and undergoing a relatively minor change through radioactive decay. This is where neutrons come into the picture.
Remember that neutrons increase the weight of a nucleus without changing it into a different element, and likewise increase the instability of that nucleus. An atom of Uranium-235, which contains 92 protons (like all uranium atoms) and 143 neutrons is teetering on the edge of instability, so adding another neutron turns it into the super-unstable Uranium-236, which almost immediately shatters.
One way it can break up is into an atom of Barium-144 (56 protons, 88 neutrons) and an atom of Krypton-89 (36 protons, 53 neutrons). If you do the math, you'll see that totals 92 protons between the two (the same as we started with), but only 141 neutrons. What happened to the other 2 that Uranium-235 started with, or the 1 we added?
They went rocketing off on their own! (Along with a lot of energy and thus heat, released when breaking the forces that held everything together.)
This particular fission reaction produces 3 high-speed neutrons, each of them ready and able to hit another Uranium atom and start the process over again--the "chain reaction" you probably heard in school, where each fission causes at least another fission to occur. If just 1 of the 3 strikes another Uranium atom and causes it to split, the chain reaction will keep going; the physics term used is that the reaction is critical, which usually requires a "critical mass" of fissionable material. Produce less new fissions, and the reaction fizzles out; produce more, and it accelerates in a positive feedback loop, rapidly generating more heat and energy.
Fast Neutrons vs. Slow Neutrons
You might be rolling your eyes by now at the physics lecture, but one more important point to grasp is that the neutrons produced during fission are fast. At high speed, they can easily travel beyond the nuclear fuel without striking another fissionable atom, with the impact (no pun intended) that they go to waste. (It's a bit more complicated than that, but I'm trying to keep this explanation as high level as I can.)
Enter what's called a moderator. When those free, speedy neutrons hit something, there's a chance they'll just be absorbed (atoms like those of cadmium and boron are great at sucking up free neutrons), but when hitting other atoms (like hydrogen or carbon), the neutrons bounce off--and slow down in the process. These reflective substances are called moderators, and by slowing down (and not absorbing) speedy neutrons produced by fissioning Uranium-235, they enable those neutrons to hit and fission more uranium atoms.
Okay--enough physics for the moment. It's time to pivot back to the egregiously-wrong statement presented by the documentary program, and my explanations should make a bit more sense.
There Was Zero Chance of a "Nuclear Explosion"
First, the most gross factual mistake: There was never, ever, ever a chance that a "nuclear explosion" would occur. Physics simply does not work that way, and no matter how poor the reactor design or grave the accident, there's not going to be a nuclear explosion. Let's stop for a moment to consider why.
Nuclear explosions require bringing together a critical mass of fissile material in a very short period of time, using highly-enriched uranium or plutonium and not typical reactor-grade fuel. You need a critical mass so that each fission causes at least one additional fission, and you need to achieve that critical mass in a very short period of time lest the heat and energy produced by that fissioning force the whole thing apart before it can create a nuclear explosion.
Without getting too far into the weeds of nuclear weapon design, this is achieved by high explosives forcing together the necessary materials in a fraction of a second. The simplest atomic bomb design, which the United States dropped on Hiroshima at the end of World War II, achieved this by shooting one chunk of highly-enriched uranium down a gun barrel into another target piece of uranium, slamming them together with explosive force and speed. Almost all bombs designed since then implode hollow spheres of plutonium--again, something that requires incredible speed and explosive force. This happens in fractions of a second, and soon after, the tremendous heat and energy blows the whole thing apart--stopping any further fission as the remaining uranium or plutonium atoms suddenly find themselves too far apart to blast each other with any more neutrons.
With a nuclear reactor, there's simply no way to shove enough fissionable material together and keep it together long enough for it generate a nuclear explosion.
There were however two things that were legitimate concerns regarding molten nuclear fuel dropping into a large reservoir of water, neither of them being nuclear explosions:
A Steam Explosion
Hot, molten fuel would cause the water in those bubbler tanks to boil very, very quickly, producing a lot of steam and pressure with nowhere to go. This steam would expand and do everything it could to spread out, forcing everything around it to expand along with it with explosive force. Instead of being concentrated in radioactive lava deep inside the reactor's wreckage, a huge amount of intensely-radioactive waste would be spread into the air--both by the physical violence of the explosion and the fact that many radionuclides would dissolve into the water and thus be carried away by the steam itself.
Coupled with this was the risk of another hydrogen explosion, triggered as the intense radioactivity of the fuel along with its heat and chemical reactions with water broke down that water into hydrogen and oxygen. As any kid who sat through a middle school science lab or watched Mr. Wizard (I'm dating myself here) knows, hydrogen is flammable and explosive, and with all that oxygen nearby and heat, it's going to go boom.
These would have been disastrous consequences which would have spread much more contamination and could have wreaked havoc on the other, still-operational reactors on site--something we saw at Fukushima. The fallout would have spread over a much larger area of Europe.
Re-criticality of the Fuel
The second risk of dropping the reactor's fuel into a large pool of water is nuclear in that the fuel could start fissioning again, just like it was in an operating nuclear reactor core, producing more heat and radioactivity. This would not lead to a nuclear explosion, but it would be an unconfined, active nuclear reactor, broadcasting intense radiation outward and generating more radioactive waste. The ensuing heat and radiation surges would also generate more steam and associated risks of steam and hydrogen explosions.
Why this is circles back to the brief lesson in nuclear physics I laid out above.
This is where water comes in: Water has a lot of hydrogen in it, and each time a speedy neutron strikes a hydrogen atom, it bounces off and slows down a bit. And the slower a neutron gets, the more likely it is to strike and fission a uranium or plutonium atom. Most nuclear reactors rely on moderators like water or graphite to slow down those speedy neutrons so that they can trigger more fissions--maintaining that "criticality" where the reaction continues, producing heat that is used to generate electricity.
The melted nuclear fuel burning its way through the Chernobyl reactor's concrete floors was highly radioactive, but it wasn't generating a self-sustaining chain reaction of fission. The neutrons it generated were too fast and energetic. But dumping that fuel into a large pool of water would slow down those neutrons, and they'd start triggering more fissions in an ever-increasing chain reaction.
It's physically impossible for water to create the kind of chain reaction that generates a nuclear explosion--but it's still not great for the molten fuel to continue to generate its own heat and radiation, bathing everything nearby in a lethal barrage of x-rays, gamma rays, beta rays, alpha particles, and neutrons (which can cause common materials to themselves become radioactive).
Five Million Tons of TNT "Estimate" Deserves a LOL
The notion that anyone "estimated" that the fuel falling into the water beneath the reactor would generate an explosion equivalent to 5 million tons of TNT--that's 5 megatons--is absolutely nonsensical.
Five megatons is a moderate-sized thermonuclear weapon. For comparison, the bomb that destroyed Hiroshima during World War II was 15 kilotons, equivalent to 15 thousand tons of TNT, or less than 1/300th the claimed 5 megaton explosion. The largest nuclear warhead in the United States' arsenal today is "only" 1.2 megatons.
And no, I'm not going to explain how thermonuclear weapons work--it would take us well off into a lengthy tangent--other than to say there is physically no way to use just nuclear fission to achieve an explosion of that magnitude; it requires nuclear fusion, which is a completely different process and which does not occur inside nuclear reactors.
So either these unnamed "scientists" who provided the "estimate" were somehow suggesting a thermonuclear explosion--something again which was completely, entirely impossible--or they were overestimating how large of a steam explosion could be generated by several orders of magnitude.
For comparison, one of the worst nuclear disasters in history, the Kyshtym disaster (also in the Soviet Union) involved a massive chemical explosion within nuclear waste storage tanks, an explosion with the force of... 70 tons of TNT. Not kilotons, not megatons. It was a massive explosion that spread nearly as much nuclear waste across parts of Russia as did the Chernobyl disaster, yes, but it's insane hyperbole to talk about thermonuclear-sized explosions here. The largest conventional bomb the United States has used, the "mother of all bombs" or MOAB, explodes with the force of about 11 tons of TNT--nearly 500,000 times less powerful than the 5 megatons the program mentioned.
I can't fathom even a massive steam and combined hydrogen explosion reaching 5 megatons. Even had the entire core fallen at once into the water and transferred all of its residual heat to the water at once (defying the laws of physics on several levels), it wouldn't be on the same order of magnitude as 5 megatons.
If I were inclined to be generous, it's possible that someone suggested that the amount of additional fallout generated by the molten fuel striking the water and triggering a steam explosion would have been similar to that spread by a 5 megaton thermonuclear explosion, or maybe 5 megatons worth of smaller nuclear bombs set off together. Certainly there are literal tons of radioactive materials that would have been thrown into the air from a steam explosion of that scale, but there are issues with such a comparison, too.
First and foremost, the amount of fallout from a given nuclear bomb explosion varies significantly on factors like whether the explosion occurred close to the ground or in mid-air, and the configuration of the bomb itself. An explosion closer to the ground will kick up a lot of dust, much of it made radioactive by the intense neutron bombardment of the explosion itself. Likewise, a bomb that generates most of its yield from fission rather than fusion will generate more radioactive fallout--what portion of the yield comes from the "dirty" fissioning of the heavy uranium shell or "tamper" surrounding the core thermonuclear bomb, and how efficient the bomb is, mean that two different bombs with the same yield but different designs can generate widely different amounts of fallout. So does that "fallout equivalent to a 5 megaton nuclear bomb" explanation assume a ground burst of an exceptionally "dirty" weapon, or a "cleaner" airburst of an efficient bomb getting most of its yield from fusion? Who can say, when we're talking "one estimate" made by an unnamed someone?
That leads me to the second point: That it's more accurate to talk about the amount of radioactivity released, not the yield of the bomb in explosive force, when describing fallout from either a bomb or a reactor disaster. Units like curies and becquerels that describe how many radioactive decays occur per second for a given atomic isotope are used--though yes, they're a challenge to grasp ("What does it mean that 85 petabecquerels of cesium-137 was released by Chernobyl?") and I can see the appeal of, "Well, compare that to fallout from, say, the bomb dropped on Hiroshima!"
However, we're talking about a documentary, where statements should be as accurate as possible; had the program wished, they could have said, "Estimates of the fallout would have exceeded 300 times that released from the bombing of Hiroshima!" and it would have been both relatable to a general audience and more accurate.
Finally, fallout from a reactor and a nuclear bomb are actually quite different things, and in some ways, the fallout from a relatively "clean" nuclear bomb could be less bad than from an accident like Chernobyl or the Kyshtym disaster. Nuclear fuel that's spent a long time in the reactor--as was the case at Chernobyl--includes large quantities of isotopes that have high environmental burdens, persisting in the soil, air, and water, as well as within our bodies--whereas those produced by a nuclear bomb tend to decay away fairly quickly. Plus, a large reactor like Chernobyl's Unit 4 has literal tons of radioactive fuel contained within it, whereas even a huge thermonuclear bomb contains only a few dozen pounds of radioactive material (absent that produced by the explosion, e.g. irradiated soil and rock from a ground-level explosion). Spreading those tons of fuel out over a large area isn't great.
As it stood, the disaster released and spread at least half of the most dangerous radioactive isotopes contained in the core, with 60% of the radioactive iodine-131 and 40% of the cesium-137 spread as fallout. That means that at worst, twice as much fallout from these isotopes could have been released, though spreading it dissolved in water that was then vaporized and hefted aloft would have meant it traveled farther and affected a larger area. (Only about 3-4% total of the fuel itself escaped into the environment, which was still over 6 tons.)
Who Are these "Scientists" Cited?
Using weasel words like "some scientists" and passive voice ("one estimate said") easily hide a great many issues, including making it difficult to identify and challenge those claims. Were these scientists nuclear physicists, for example? And how many are "some?" Who estimated the effects would be equivalent to 5 megatons of TNT?
As I've already pointed out, there are physical impossibilities that anyone with a basic grasp of nuclear physics should have grasped, much less anyone of expertise. And while even the experts get things wrong--scientists truly didn't know how melted fuel from a reactor the size of Chernobyl's Unit 4 would behave, with legitimate concerns that the melted fuel might concentrate into a compact, efficient improvised reactor as it collected in the bottom of the structure, generating more and more heat through re-criticality and fission and thus burning deeper into the ground, potentially contaminating ground water and triggering more steam explosions along the way--I find it unfathomable that they'd have ever come up with a scenario of a nuclear explosion.
The program did feature several actual scientists and experts, including eyewitnesses who had been at Chernobyl and who participated in the disaster response. These segments were by and large very good, and what I'd expect to see in a documentary. One British scientist showed a re-creation of the "corium," or molten reactor fuel mixed with concrete, steel, and other parts of the reactor that the blazing-hot fuel melted, a natural segue into talking about the "Elephant's foot" (a famous bit of hardened fuel "lava" found spilling from pipes deep in the reactor's basement) and the dangers of a steam explosion had that molten fuel struck the water beneath the reactor.
Instead, we got the factually wrong scaremongering weasel-worded statement that drove me to write this blog entry.
Conclusions and Thoughts
I've been fascinated by the history of both nuclear energy (and nuclear weapons) for a long time, including the often-unknown stories of nuclear disasters. Yes, everyone has heard of Three Mile Island, Chernobyl, and Fukushima; fewer of the Kyshtym disaster, or the SL-1 steam explosion in remote Idaho (which killed three soldiers), and likely even less the Tokaimura fuel reprocessing accident (which killed two employees), the "demon core" criticality accidents that killed two American scientists, or the countless far more minor incidents that fill the unknown histories of nuclear fission.
From that standpoint, I applaud a documentary that delved deeply into the former KGB files surrounding the Chernobyl accident. Revealing hidden aspects of the events that led up to the disaster, along with the human notes of the response, is fascinating.
Nuclear physics is a complex subject in a complex branch of science that few of us spend much time thinking about or understand very well. However, that is not an excuse to include egregiously-wrong mistakes (apparently intended for their sensational impact) in a program covering the events of the Chernobyl disaster--spending even a few moments in the hour-plus program on the physics would have been a wise investment of time and programming. Perhaps the segment would not have been as sensationalistic, but it would have been more accurate, and instead of being scary--Chernobyl was scary enough as it was!--would have been informative.
To that point, I hope my overview was clear, and apologize in advance for any errors or omissions I made in attempting to explain the basics above!
Personally, I'm ambivalent on nuclear power. As an environmentalist, I see the need to move away from using stored ancient sunlight (in the form of oil and coal and natural gas) as our primary sources of energy. I also see the downsides and impacts of many "green" energy solutions: For example, hydropower dams destroy habitat, potentially killing many species who rely on rivers that freely run, and they pose risks to communities downstream should they fail. Wind turbines can kill migratory birds and impact the behavior (and thus lives) of many animals living near them. Solar panels require polluting mining and manufacturing, both for the panels and batteries needed to store their power, and their installation in large-scale "farms" can disrupt habitat.
Likewise, nuclear power runs the risk of large disasters that contaminate broad areas for hundreds of years, and reactor operation generates waste that is dangerous for thousands of years. Nuclear reactors are complex pieces of technology which like any complex systems can fail in unexpected ways--the least of which being by human error.
Yet coal mining destroys huge amounts of habitat and pollutes even more, and operation of coal power plants dumps toxins like mercury into the air and leaves behind radioactive ash. Natural gas is a potent greenhouse gas--methane is far more so than carbon dioxide, so any that leaks during production and transport, and any that burns incompletely during power generation, is even worse for global warming than the carbon dioxide generated by burning coal, gas, or oil.
I'm not one of the techno fan bois or "nuclear bros" who shout that nuclear power is far safer than any other power source, nor would I be comfortable with a nuclear power plant right next door (just like I don't want to live right next door to a coal-fired power plant). There are design problems with our current nuclear reactors--designs which haven't changed much in nearly 70 years--and as we saw at Chernobyl and Fukushima, disasters involving nuclear power are hard to control and clean up. The Chernobyl disaster was one of the straws that broke the Soviet Union's back, with the economic cost alone nearly bankrupting the once-superpower. But I am certain there is a place--indeed, a necessity--for nuclear power in our modern society.
Whether it involves "breeder reactors" that create more fuel than they consume (transforming common Uranium-238 or even-more-common Thorium-232 into fissile fuel to continue to power reactors), spent fuel reprocessing (only a small percentage of fuel is "burned" during normal reactor operations today, with the rest contaminated by highly-radioactive waste and stored), or whether it will require a new generation of reactor designs is yet to be seen. The engineering needed to scale up these technologies is not trivial, and statements that a given design is "fail safe" ring hollow given the truism that complex systems will experience unexpected failure modes. But we need energy for our society, need it to maintain every part of our daily lives.
To that end, an accurate understanding, even at a layperson's level, of what actually occurred at Chernobyl is something that serves us all well.
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