What /Actually/ Happened at Chernobyl 

Corrections! 1. Not all of the uranium in a nuclear bomb...fizzes...actually only pretty small amount does before the core blows up and the atoms get too far apart. And yet...it's still a pretty big deal. 2. It's not "fizzes." Fission is both a noun and verb, so uranium "fissions" 3. I imply in this video a little bit that you can't have a light (regular) water reactor with a negative void coefficient...you can, you just need a different fuel / moderator realtionship. There are lots of light water reactors that are much safer than RBMK designs....they tend to require more fuel enrichment. 4. An earlier version of this video got the date wrong and said October 26th instead of April 26. I just removed that line, which is something RU-vid lets you do now. Let me know if you've got anything else I should add here!!

After watching Chernobyl. You know I am something of Nuclear Physicist myself.

Dyatlov: Hank is clearly in shock. Get him to the infirmary.

"Graphite tipped rods" Pencils?

0:03 : no it didn't. This man is in shock. Toptunov, take him to the infirmary.

Wait, did you just explain how an RBMK reactor explodes? This man is in shock, take him to the infirmary.

*Reactor explodes* Dylatov: “Not great, not terrible”

In defense of the Soviets regulations for running the reactor stated the exact amount of control rods that were to be in the reactor at all times. Dyatlov broke that rule the computer suggested they shut down the reactor , Dyatlov didn't listen. The biggest mistake is they did not make the reactor idiot proof.

Correction, there was no graphite, BECAUSE IT'S NOT THERE!

Hank, this is a great explanation! I can clarify the moderator confusion though. It's not as complex as you think. All you need to understand is that *no atom "smashing" occurs during nuclear fission*. Quite the opposite. The actual reaction is that atoms absorb the slow moving neutrons like magnets snapping into each other. The additional atomic weight makes the atom too heavy to maintain its cohesiveness, resulting in the atom falling apart. That's what we mean by "splitting the atom". Once you understand this, it's easy to understand why fast-moving neutrons are a problem. They have too much energy to get captured by nearby atoms. It's like the difference between hand-tossing a magnet past another magnet versus firing the magnet out of a high-powered rifle. In the former case the magnetic field pulls the magnets together and halts the throw. In the second case the magnet moves out of the magnetic field too fast for the bond to be established. Now replace magnetic fields with strong nuclear forces and you've got it. :)

John: Good! I know how a nuclear reactor works. Now I don't need you.

Well the "thing that broke" during the SCRAM of the reactor was actually two things: The control rods did move very slowly in an RBMK reactor because of tight fits between control rods and their channels and the reactor is very huge, so it has to travel very long distances, while even a very slight inbalance in neutron generation per fission is a rapid increase of power: 1.01 neutrons per fission on average are a doubling of power every 7 milliseconds. That lead to a thing, the reactor was never designed for: A rupture of more than one fuel channel. This resulted in the reactor vessel turning into a giant steam piston and shooting out it's upper lid through the Ceiling of it's hot box, while pulling out all control rods and evaporating all the coolant at once because of the pressure release, it creates. For the following burning of the core: Graphite basically is the highest grade hard coal.

Had to listen at .75 speed to make my brain work faster. Much like slowing down neutrons to speed up the reactor :-o

Hank saying why does anything exist is how I like to get pumped up for my day.

I majored in Physics at university, and you've done a really good job of explaining each term and each process step in a coherent way for the general audience - well done Hank :)

I don't know much about nuclear physics and reactors, but I think the words "CHEAP" and "NUCLEAR REACTOR" in a sentence is not the best idea in most cases Edit: I cant believe that i need to to mention it explicitly, but here we go... THIS IS A JOKE!

Since HBO’s Chernobyl, most of us have become nuclear ‘experts’ ☺️

Today's XKCD alt-text gives a pretty clever analogy to explain Chernobyl: "You know when you can't hear your speakers, and you keep turning various volume controls up higher and higher in confusion, and then someone hits the mute button and there's a deafening blast of sound? That's basically what happened at Chernobyl."

“It’s not 3.26 röntgen......it’s 15,000” Dyatlov: not great, not terrible

That was a really great description of why the control rods were "graphite tipped". Because the oversimplified explanation just makes you go "WHY WOULD THEY DO THAT?" But seeing your diagram and what actually happened made it make sense. And it WOULD have been safe if they followed their own safety procedures.

Thanks for the shout out!

Akimov: The reactor is about to blow we have to shut it down right now AZ-5: Ima bout to end this man's whole career

You're wrong with your definition of Neutrons. It's *B U L L E T S*

Hank, I took the NYS chemistry regents two days ago and there was a question on “heavy water.” All of my friends were quite confused, but I calmly answered the question thanks to my interest in this video! This, combined with crash course chemistry, were very helpful in my exam taking! Thank you as always!

I missed explanation videos! These are the best! +4 minutes of good content right here!

As a Nuclear Engineer (by training), that was perfectly succinct for the depth required to accurately describe reactor design. You never said the buzzwords though! "Positive Void Coefficient of Reactivity." We talk about things in cents and use symbols like β (no, not that β, a different one), its so much fun! Still, the thing both your video and Scott's made me aware of was that it didn't *always* have a positive void coefficient, only when operating with all the control rods out. One nitpick: neutron flux is actually neutron flux density. Neutrons aren't stationary, they've moving all the time; talking about a static population density doesn't make sense, so we think about "flux density" which is the Number of Neutrons * Velocity / Volume, so n/cm^2/s, and even though its defined with a 2d perspective (1/cm^2), most reactors have isotropic scattering so its the same in all directions and you can just think of it as "neutron density".

Gosh, this brought back warm fuzzy memories of me in Nuclear Power School thinking I was going to fail for sure. Unfortunately, I passed and realized how much it sucks to work in the engine room of a submarine.

Cpt. Mcmillan - “50,000 people used to live here, now it’s a ghost town” Est. 2007

This explains why for the last two weeks all of my RU-vid recommended videos have been either about the marblelympics or chernobyl. p.s. great video. I learned so much! -John

Thank you, this was the first time I learned to understand what has happened. My dad was on the medical team at cleanup. Luckily, got no lasting ill effects.

A note on explosions. They absolutely do not require a phase change. You can get an explosive by pressurizing hydrogen and oxygen gas, which react to form steam/water vapour, also a gas. An explosion is primarily about pressure and energy.

The HBO show be like: Power Plant Worker: "So wtf do we do with the crossed out instructions? Do we skip them or what?" Control: "follow the crossed out instructions, ignore the rest" Power Plan Worker: "for real? Are you sure? Why..." Control: "Yeh, trust me bro" *Reactor explodes*

Hey there! So i actually work in nuclear power, in fact I'm currently sitting inside a nuclear power plant watching this... I just wanted to pop in and say thank you for not painting the nuclear industry big mean and scary! Chernobyl was unprecedented and because of the safeguards in place today unrepeatable, I really appreciate that you took the time to explain how everything went down! DFTBA

Dyatlov: "How does an RBMK reactor explode!?" Google: Here's a vlogbrothers video for you, comrade.

Its ironic and borderline Kismet that i watched that Scott Manley video FIRST, then found this video. Now, having watched Scott's in depth video first, and then this slightly watered down version I feel like I understand Chernobyl better than if I only watched one or the other. Thanks guys!

Damn, Chernobyl memes are too high down here in the comments.. I think we need higher capacity of dosimeter..

At 11:45 “and so... and so, and so...”. My favorite line reading of any video. More Vonnegut than Vonnegut could ever Vonnegut.

You DIDN'T see the reference on the video because it's not there! *projectile vomits*

I think you are mistaken comrade. RBMK reactors don’t explode.

Three Mile Island: I'm going down in history as the worst nuclear disaster in human history Chernobyl: hold my vodka

i love that hank went so deep into what was the problem with the reactor where i was like : there was much heat things went wrong and caboom

I'm a physics grad student and one of my professors worked in a lab near chernobyl during this time. he's told us a lot of stories from living in the soviet union but by far the scariest ones are from chernobyl. he said the number of friends that he had die from the disaster was in the double digits. he remembers working in his lab and the kgb going door to door taking any equipment that could have been used to test how bad it really was, but he got to his office, put his coat on, stuck a radiometer under his arm, and snuck it out of there. he said he measured radiation levels that were 100 times higher than what the government was claiming they were. and once he got over to america, he wouldn't take any assistance from the government, even though i think he was essentially a refugee, because he didn't trust any government at that point. on a slightly lighter note, he said he made a satellite dish out of a metal sled, like one of those disk shaped ones, so he could watch tv. also he has a very strong accent so sometimes during lecture he has difficulty saying some words, but one time last fall he said "yknow i dont think our president could say that word so its fine that i cant either."

You're the second "how it happened" vodeo after I fell into this rabitt hole. Your explanation of the 2 seconds of rods and how they were used is by far the clearest I have seen so far. Also read one appendix to an IAEA document that has translated Russian report from way back then. One component they mention is that when they turned off the turbine and only ran the pumps (as part of the test), there was less cooling happening and the water being pumped into reactor was getting closer and closer to boiling point at that pressure (about 1000psi as I recall), and eventually flash boiled.

In layman's terms, the reactor is like a diesel engine. Rather than using the accelerator pedal to slowly bring the engine up to speed, they took the governor off and set it to wide-open-throttle. It started to overspeed and their "solution" was to try to slam the intake closed. And not only did it completely destroy the intake, it blew off the cylinder head in dramatic fashion and caught on fire. (Obviously an oversimplification, but I think it fits).

In addition to all the harm done to the environment and human life, Chernobyl made the world scared of nuclear power. I theorize that if the Chernobyl disaster didn't happen we'd have more nuclear power plants today and a smaller carbon footprint as a result.

Understatement of the century: “... the top popped off the reactor...”

Best explanation I saw thus far. Chernobyl is a bit of a passion of mine. As it led me to prove for my physics exam piece that nuclear power is dangerous and how we should avoid it. Only to find out the opposite. And people are wrongfully scared. These situations like Chernobyl and Fukushima are rare and impossible even with modern fast breeders. To keep fission happening there it’s like riding a unicycle, whilst rubbing your stomach and juggling. What stunned me the most, is that for the most excessive deaths estimate of all 3 nuclear reactor disasters. In the worst case 8000 people died. Whilst 8000 people alone die in The Netherlands of fine dust alone. Often from coal and oil burning. And nuclear reactors safe about the same amount of lives almost weekly. Due to its isotopes being used in medicine to cure people from cancer.

Thank you, thank you, thank you! Have been reading/watching/listening to so much material to try to understand what happened in Unit 4 at Chernobyl, and this is by far the most succinct and approachable explanation I've come across. Again, thank you!

Great video! I have studied Chernobyl since ‘97 and this is one of the best ways I have heard how to explain the cause and effect of this disaster to someone who may not understand the exact physics

I always disliked physics in school, but now I realize that it was just because I didn't have great teachers.

I literally enjoy learning ANYTHING 10x more if you or John are teaching it, Hank. God bless you

Actually, an "explosion" is just a "relatively rapid" variation in volume. it does not have anything to do with phase change. I can assure you that a oxy+hydrogen tank can explode as well and they are both gasses. The rapid expansion is usually caused by combustion which add a lot of thermal energy to the mix, and not by phase change. Also, to be extremely precise, "detonation" is the world you were looking for. deflagrations are explosions as well, but expansion occurs slower than the speed of sound.

Saw the Scott Manley video first. I really think the neutron flux diagram added to my understanding.

Hey John can you explain the amendment of extradition bill requested by Hong Kong Government, which caused 1 million citizens to protest at 9th Jun last week. It means a lot to Hong Kong people!Thanks.

Thank you!! I loved the HBO show and as a completely non-science person, I still tried to do as much research on this as possible to understand, at least in layman's terms, what really happened. You are the first person that has actually helped me to understand the graphite tips thing. The only explanation they gave on the show was that they used them because it was cheaper. That never made sense to me and I never understood why the had to have "tips" on the control rods in the first place. Thanks, again for helping clarify this.

You’ve explained this simply, much better than anyone to date, even Scott Manly, and better then the scientist portrayed in HBO show. Gas and brake analogy is excellent. Thank you.

Thanks for including a link to Scott Manley's video, nice work on keeping it simple,still early hopefully this thread doesn't get filled with the anti nuclear hype.

Please tell me how an RBMK Reactor Core Ex- Wait nvm

Thanks for posting this! I was curious about the science behind the explosion but when I tried to read about it, it completely flew over my head, i have a little grasp on it now!

Great vid. Your enthusiasm makes me wish it was about 30 minutes long

Damn, at about the middle point of the video I was sure I was lost and that I would either have to rewatch this video a few times or just outright give up on understanding what happened, but at the end there somehow everything you talked about fell into place and I experienced a moment of clarity and understanding I have not experienced since I was a kid. Props to everyone who contributed, in any way, in writing this video. I am truly thankful!

I needed 13 minutes of vlogbrothers today even if I learned all about this in one of my classes. Also, Fun Fact: Fission is a noun and a verb.

A more understandable Analogy for that "Bouncing Off" would be a Stone you make ditch over water. If he is not fast enough going or does not spin enough, it drops in - but when fast, it bounces off.

By far the best explanation of the Chernobyl reactor control rods, their construction, and their role in the nuclear disaster. Long overdue.

"Something isn't possible" - Said by someone, right before something happens.

Everyone after watching Chernobyl: You know, I'm something of a scientist myself.

Dyatlov:I am inevitable. Button:And i am AZ-5.

Explained very very very well :) hours and hours of research of this, and u actually still managed to tell things I didnt know. :)

"Good. I know how a nuclear reactor works. Now I don't need you" No really though. This explanation was much appreciated. I just watched Chernobyl yesterday and that graphite tip thing was bothering me because it seemed too stupid to be the whole story and I hadn't gotten around to looking it up yet.

The biggest issue with Chernobyl was that they actually knew about this very specific lost of things or at least suspected but suppressed the information.

Although I got an M.Sc. in chemistry I had still been able to understand the physics involved prior to your video. It took some time.... well done! Great job you have done! Btw: In my country we had light-water reactors as well... 😉

Totally loved this one. Thanks so much for making it. :)

"Whyyy? Forces? I guess?" Same, Hann, same. And I'm studying physics 😂😂😂

I failed honors chemistry this year and this made sense to me. Thanks Hank, for making extremely hard concepts suddenly so beyond simple.

Hank this is a seriously AWESOME video. I learned so much and I've watched the show! Thank you!!!

Great breakdown/explanation. I do recommend that you watch at .75x playback speed though. Lots to process!

RBMK reactors are perfectly balanced, as all things should be. Dyatlov: Hold my vodka.

Me: _watches Chernobyl_ All my gamer tags 1 day later: *RBMK_Reactor*

I am watching the Chernobyl series on HBO. Your explanation of the dynamics of the event were invaluable to me and is making my enjoyment of the series way more intense. Mad respect from Virginia 👍✌️

"Why? Forces, I guess???" This is a valid answer to any "why" question.

Is this the guy that did "crash course" the biology channel? If so thank you so much I got a distinction in every assignment because of your videos! Love how you teach 💓💓💓

Watched Chernobyl. Came to RU-vid for more info. Bam! Hank does a video on it.

I’ve made this point several times, but if you truly know a subject, then you should be able to explain that thing to any person with any level of knowledge. This is a great dumbed down video for people who want the gist of what happened without having to strain too much. I think you could probably explain this to a fourth grader in a way they could comprehend. Thank you very much. This explained much more than Chernobyl HBO series, which explained much more than any video up to that point that was comprehensible to a lay man. Thank you

A big reason for accident of the Russian reactor at Chernobyl is that it did not have a containment vessel like western designs do. An even better video of what happened and why, on youTube, is the MIT OpenCourseWare video: "26. Chernobyl - How It Happened". This video is a university lecture for nuclear physicists and the instructor uses Chernobyl as a lesson for the class on what not to do. The MIT OpenCourseWare series is very good.

We actually just learned ab fission, fusion, radiocative decay, isotopes, and some related terms in physics 1 so i actually understood some of this!

I usually find physics very difficult, but you explained it very well. Thank you for posting this video.

You guys don’t know what you’re talking about! RBMK reactors can’t explode! 🤦‍♂️

The reason that the stablest isotopes tend to have roughly the same number of protons and neutrons (bending over a bit toward more neutrons as the nuclei get heavier) is actually pretty easy to understand. You may remember from chemistry class that no two electrons can occupy the same quantum state (the Exclusion Principle), and as more electrons get added to an atom, they pile up into more and more energetic quantum states. But you can put two of them in the same "orbital", with a certain shape, because they can have opposed spins, and there are two spin states. Well, protons and neutrons are like that too. Only being a proton vs. being a neutron is another state difference. So you can stuff two protons AND two neutrons into the same nuclear orbital. (And the attracting force keeping it all together comes from collective strong nuclear force attraction, instead of electrostatic attraction.) But there's another force, the weak nuclear force, that can *transform* a neutron into a proton or vice versa. That involves creating an electron or a positron, and a neutrino or antineutrino. All that takes energy, and it's not going to happen unless the energy difference between the initial and final state is large enough. But if the energy difference is there, it's going to happen eventually, after a somewhat randomized delay. That's called beta decay, because the electron or positron emitted in the reaction was called a "beta ray" before people were quite sure what they were. A neutron has a slightly larger mass than a proton, for... reasons, and the difference is just barely enough to allow beta decay to happen, so a neutron minding its own business in free space will eventually turn into a proton (emitting an electron and an antineutrino), but not the other way around. But in a nucleus, if you pile up too many protons, they'll occupy higher energy states than they COULD occupy as neutrons, since there are lower-energy slots available for neutrons. So now there's an energy difference sufficient to go the other way and they can do "positive beta decay" and become neutrons. Or if there are too many neutrons, they'll turn into protons: "negative beta decay". When this happens, there's often extra energy left over that can be emitted as high-energy photons ("gamma rays") or cause something else to happen. The mass difference between a neutron and a proton is small enough that the equilibrium for light elements is generally close to equal numbers of neutrons and protons. But protons have a positive electric charge so they repel each other electrostatically. That means it takes more energy to squish them together. As the number of protons increases, their common electrostatic repulsion means that the equilibrium has a bit more neutrons than protons, to keep the protons a little more separated. Fission, and alpha decay (where the nucleus spits out a helium-4 nucleus, two protons and two neutrons), are a bit harder to understand because that gets into the details of the strong nuclear force. But beta decay is relatively straightforward.

"Reactor design is all about creating Balance" Sounds like Thanos but ok.

Saw Scott's video first. As informative as it is, and as much as I was able to understand the sequence of events that led to the explosion (I'm a historian, not a scientist, so the crucial bits went over my head), your explanation of the neutron spike at the very end made it much more understandable. I went back and rewatched Scott's video and was able to better understand his explanation of the neutron spike. Between the two of you I now have a clearer picture of what happened. I was 13 years old when it happened, and watched as it all played out on the news back then. But only now, 33 years later, thanks to HBO, and thanks especially to you and Scott, do I know the full story about what happened then and, more importantly, how it happened.

This is the stuff I subscribe to you for. Thanks.

What an excellent video - not least the explanatory introduction. Thanks!

I think the reason I’m so early is because notifications haven’t/aren’t showing up for this vid. Love the vids, shame that some people will have to wait a while or seek this out bc it’s really interesting.

Hank thank you for always making learning interesting!

There is a big mistake in the video: Water does NOT absorb neutrons, but it moderates them (That's how most of the western nuclear reactors work. You don't need heavy water for it... you only need heavy water when you want to use natural uranium as fuel). And when the water gets hotter or boils the moderation is reduced because the water is less dense. Thats called a "negative void coefficient". Graphite, which is used in the RBMK as a moderator, does not reduce it's moderating properties but increase it when it gets hotter, and the increase of moderation by the graphite is more than the decrease of moderation due to the evaporating cooling water. And if you replace the bit of water or steam which was left inside the reactor with the graphite tips you pulled out before by pressing the AZ-5 button you press the final trigger of that dangerous mix.

Best explanation of the accident I’ve ever seen! NOW I understand why the tips were made of graphite. It was completely unclear, but now I see the light. Thank you so much!

This is the best explanation I have heard on the how and why of the explosion at Reactor 4 for us non-physicists. Thank you!

Love seeing one fantastic YTer (that’s you, Hank) call out another fantastic YTer (Scott effing Manley). Thank you both.

Tremendous explanation of the disaster. Especially the graphite-tip clarification.

"So hot that it, in fact, dissociated into hydrogen and oxygen..." This description of the development of H2 and O2 in the steam-pressurized reactor strikes me as a little too simplified. My understanding--and I've seen the HBO series twice, mind you--is that the zirconium jacket around the fuel reacts with the steam to form hydrogen. No one knows for sure exactly what sequence of (chemical) events led to the two explosions, but I've always imagined that the first, smaller explosion was related to the compromise of the reactor top due to pressure, while the second explosion was the bulk of the hydrogen that had been formed inside the reactor finding the needed oxygen when the top popped. In any case, the "dissociation" of H2 and O2 is, I believe, the result of a chemical reaction between steam and zirconium at high temp. I only bring this up because your description makes it sound like the water molecules simply break apart.