Would Not Pressing AZ-5 SAVE Chernobyl?

The final chain of events that ultimately led to the explosion of Chernobyl Unit Four in 1986 were set in motion when Leonid Toptunov pressed the AZ-5 button. However, let’s envision a scenario where Akimov doesn’t order Toptunov to shut down the reactor, and instead tells him to wait for some unknown reason. That doesn’t matter. Let’s instead look at this scenario, because there are actually two diverging opinions here.
Sources
INSAG-7
The Chornobyl Accident Revisited
Multidimensional Analysis of the Chernobyl Accident
INSAG-1
Turbine Rundown Programme
Accidont.ru

[@AiOinc1]

So what you're saying is that the only way unit 4 wouldn't have blown up is if they weren't goofing off with their nuclear reactor at 2am

[@thing_under_the_stairs]

Why did this make me laugh so much?

[@markusw7833]

@@thing_under_the_stairs Considering they weren't goofing off, I don't know.

[@tonamg53]

It will blowup eventually. The AZ5 button was a fatal flawed design. Any scenario that triggered them to emergency scram the reactor will cause it to detonate.

[@RainbowManification]

They weren't goofing off. They were running a safety test that was supposed to be done prior to the reactor being certified but the Soviets rushed the reactor into service before it was ready. This safety test unknowingly triggered a critical design flaw in the reactor.

[@AidanMcCaleb]

@@RainbowManificationit had already failed the tests 4 or 5 times they may as well have been goofing off they were just hoping it would work this time

[@adamw.8579]

In my opinion, the reactor was lost when the control rods were raised too high to break the xenon poisoning in the core. The use of AZ-5 only made the situation worse. Failure was already inevitable. Interesting and good video.

[@markusw7833]

?

[@erikziak1249]

Exactly. The reactor was in a state that was unrecoverable by any means they had at hand a minute or so before the power surge and explosion. Or maybe, and only really maybe IF they knew the real situation of the reactor with its design flaws, taking into consideration the critical reactivity margin they had as well as the violation of operational procedures they made in order to do the test, they could prevent an explosion by "flooding" the core with "cold" feed-water before the start of the test (thus invalidating it and shutting down the reactor for a longer time) and only after the reactivity dropped below critical inserted the control rods and eventually the scram rods, would they save the reactor from exploding. But this is pure hypothetical situation and they had absolutely no reasons to do what I just described. They wanted to do the test and did everything they could to run it. The test should not have been made. Once the power dropped practically to zero a few minutes before the test, it was clear that it made no sense anymore, especially since they managed to rise power only to about 200 MWt. Which itself is quite impressive considering the Xenon poisoning and burn-up of the fuel rods. The reactor running at 200 MWt was in a very unpredictable state. Maybe my speculation would have not worked anyway and the reactor would experience a power surge. Maybe it would not blow up, but it would certainly cause massive damage to the core, maybe even so as making the reactor inoperable after that. Like what happened at Bohunice A-1 in 1979. The incident was caused by different reasons, but the result might have been similar - melted fuel assemblies, contaminated primary and secondary loop (the RBMK has no secondary loop) and a "bricked" core that only can be written off and eventually dismantled (after decades). Chernobyl would have remained an INES-4 or maybe INES-5 incident, if judged by the IAEA scale introcued in 1991. The question is, if a far worse accident would have not happened on another RBMK design, since it would have been covered up by the soviets and no lessons would have been learned.

[@markusw7833]

@@erikziak1249 In other words you don't know what you're writing about.

[@MinSredMash]

Great job completely ignoring the entire video

[@HistoryChef]

Well even if we consider not pressing AZ5, then as well as mentioned would not have made much of a difference. The reactor was much lost when toptunov withdrew almost all rods from the rector. The root of the problem is AZ5 and the reactivity of low grade uranium. Toptunov shld have rather scrapped the test and rather shut down.

[@robertgaines-tulsa]

Hindsight is 20/20, but without having such knowledge, the operators might have been in big trouble if they didn't press the AZ-5 button. It's like Titanic would have been saved if they had crashed the ship head first into the iceberg, but it would have looked like incompetent operation, and the crew probably would never sail a ship ever again. Society is funny like that. Instead of doing the safe thing and crash the ship head first into the iceberg, we do all we can to not cause any damage to the ship gambling that it will pay off even if the odds are stacked against you.

[@nehorlavazapalka]

you don't know the under-water shape of the berg

[@franknachname730]

I get your point, if you cause something bad in order to maybe prevent something worse, you have the problem to prove that something worse would have happened. Concerning titanic there was a risk to damage too many compartments anyway by unterwater ice right in front of it and/or by causing the iceberg to rotate and cause unforeseeable patterns of damage. Maybe it would have saved the ship, maybe it would have sunk way faster with up to 100% casualties. However an interesting tradeoff.

[@andy99ish]

@@franknachname730 That was hardly the trade-off they thought about: Firstly judging distances at night is very hard, so they had the right to assume that they would pass it (which they nearly did). Secondly the side extension of the submerged part of a berg at night cannot be estimated, even less so when its waterline cannot be seen, as no waves were breaking at its base in that very calm night. Thirdly a head - on collision would cause massive damage (they were at 21 knots), like the chimneys coming down, machinery being torn out of its sockets etc. In reality they hit the berg so gently, that they did not bounce off, nor was their forward momentum impaired. As a result 6 main compartments were sliced open. If it was 1 compartment less the ship might have been saved, or at least kept afloat for many more hours in that very calm night. So any other impact - a harder one and even a head-on one would be preferable to that smooth and long slicing, which given the information they had, was extremely unlikely. Hence in my opinion they made the right decision.

[@upthebracket26]

@@nehorlavazapalka you dont know about how ships are built. The front would crumple & maybe the 1st water tight compartment floods, but it gets to New York. This happens to lots of ships.

[@franknachname730]

@@andy99ish I don't think they considered that either and i don't blame them for it because as you say, they couldn't have known. But theoretically I find the question interesting. I'm not completely sure about the damage behind the bow you mentioned since the ship would slow down a bit until impact and the forward compartments would have acted as a crumple zone to spread the impact over several seconds. Did other ships that hit icebergs head on with similar speed suffer that damage? (assume similar mass relations) On the other hand the iceberg more likely might have turned

[@anatolydyatlov963]

It doesn't need saving, IT'S FINE. Just get that water flowing, for the love of god, how many more years will I have to wait? The reactor NEEDS WATER

[@9WEAVER9]

Needs some LaCroix, more like it.

[@9WEAVER9]

water + CULTURE = LaCroix!!!!! Oh yea

[@jbutler8585]

Wasn't all of the reserve water already hot? IIRC that's why so much of it started boiling in the first place, they had activated all the extra tanks when trying to reduce power output for the test. And because there was so much water in the loop it was getting pushed back to the reactor after barely falling below boiling.

[@James-nf2kg]

IT BLEW UP

[@jannespor8178]

I thought that the reactor needs graphite.

[@swokatsamsiyu3590]

Thank you for another well-done video. There may be one (little known) reason why the guys in Canada did their own report quite quickly after the accident; their own CANDU reactors. The CANDU, like the RBMK, is a pressure tube reactor. It even shares a special characteristic with the RBMK, the positive void coefficient. After the accident, they may have wanted to find out if they would run the risk of a possible similar accident in certain scenarios. However, the CANDU is very well-built, with full containment, a lot of redundancy and two separate safety systems that can independently, _and_ without power or operator intervention, shut the reactor down within 2 seconds flat.

[@markusw7833]

I'm guessing it didn't have a positive power coefficient.

[@swokatsamsiyu3590]

@@markusw7833 Oh, but it does. Look it up. It is the main reason CANDUs have a hard time being licensed in the USA because the US does not allow any reactor design with a positive void coefficient to operate in the States.

[@markusw7833]

@@swokatsamsiyu3590 Not void, power.

[@swokatsamsiyu3590]

@@markusw7833 You're correct. I misread. But the US still is heavily biased against allowing the CANDU because of the positive void coefficient. *Needs new reading glasses.

[@HistoryChef]

Candu and IPHWRs are actually well built heavy water reactors. BWRs are also good as they use water as a moderator and an absorber. But it's not as correct to say that the RBMK was not as good. The power output is well enough and it's still being used at kursk and smolensk.

[@killman369547]

After learning everything i could about this disaster over 10 years. I think there was only one possible way to save the reactor. They would've had to resist the urge to press AZ-5 and instead slowly start reintroducing control rods a dozen or so at a time. Because at that point in the evening the reactor was at its limit. You can't make huge changes to a system under that much strain, it will fail. You have to slowly and carefully walk it back from the edge of disaster first.

[@fraisertinko]

They did everything right, the problem was AZ-5 had a "surprise". In any normal system "oh shit" button should stop everything, instead of "stop everything except for accelerate everything in some situation".

[@notsureyou]

Or they could not have broken numerous rules and as a result not ended up in the situation in the first place. Without knowing the issue with AZ-5 there was no reason for them to try anything else.

[@SukSukulent]

The problem with this is that the button was sold exactly for this kind of "oh shit" situation - I guess no one would permit nuclear reactor operation without "yes we have a button which will save everyone"

[@noahbarton9723]

If they hadn’t pressed AZ-5 then likely the worst that would’ve happened would’ve been a reactor meltdown, not an explosion. The main reason the reactor exploded was the immense pressure created by the steam, so if they had kept the valves closed then the reactor likely wouldn’t have exploded.. at least not in a nuclear explosion because the Chernobyl reactor had a positive coefficient.

[@midelro97]

No, it would only take more time to explode. The xenon was already in the reactor and the reactor could not be switched off as lowering the control rods was the same as pressing AZ5, therefore it would only take more time but have the same effect, the error was rising the control rods before. But a engineer or someone with more knowledge can explain this better probably.

[@justindyer8911]

@@midelro97Xenon doesn’t raise reactivity, it lowers it. The control rods could have been inserted a few at a time, because they are graphite tipped, which raises reactivity, but made of boron higher up, which lowers reactivity. The few control rods being inserted would have raised reactivity initially, due to the graphite, but would work against the reactivity as soon as they were fully inserted. This could continue going on, inserting a few at a time, until as many could be inserted as possible. The point the speaker in the video was making that the reactor was already going to sustain some damage, as the reactivity was on the rise and would have risen to the point of causing damage to the fuel and some control rod channels. However, the cause of the explosion was hitting AZ-5, or inserting all the control rods at once. All those graphite tips entering the reactor at once skyrockets the reactivity, ruptures the fuel and control rod channels and locks the control rods in place, preventing them from being fully inserted so that the boron portion of the control rods could work to reduce the reactivity and temperature. The other point the speaker made is that there were systems present that would have SCRAMed the reactor even if AZ-5 had not been pressed, and so the effect and result would have been the same. I’m not sure if it would have even been possible for the operators to have overridden these systems, but if they had been able to insert the control rods a few at a time, the reactor would have escaped with damage and ruined fuel, not the catastrophic explosion that occurred.

[@justindyer8911]

@@midelro97 Xenon lowers reactivity, it does not raise it. If the operators were able to lower control rods into the reactor a few at a time, the graphite tips would have raised reactivity briefly, but the boron higher up in the control rods would have been able to enter the reactor and work to lower the reactivity as a whole. Over time, the boron control rods entering the reactor would have prevented the total meltdown that occurred. For sure, damage was inevitable, but nothing like what happened that day.

[@v3es473]

​@@midelro97inserting a few rods at a time wouldn't have the same effect as all at once

[@dez1989]

Thank you again for doing our homework for us! This is a great channel. It amazes me how many people are so fascinated with the Chernobyl accident. It amazes me more that we have a few people who are covering it today. 3 channels that cover different aspects of the accident. All are fantastic in their own way. You are the one who covers all of the technical aspects of what happened that night. Personally, I am very thankful for you and your work. I stand amazed at your understanding of the accident and how you help some of us to understand that night a little better. The "what if's" are even more important here. Knowing that the explosion would have most likely happened anyway no matter what. Thank you for your work! You are appreciated.

[@oscarr.g.509]

And once more, another great video. Thank you Sir !

[@ThomasBriard]

This channel is a gem I have been looking for for years!

[@joez.2794]

My understanding is if the control rods were designed to "slam home" when AZ-5 was pressed (vs. operating at "servo speed"), an explosion _might_ have been averted. As far as the explosion being caused by the graphite tips (which is what you're asking), I think they passed the point of no return a while back.

[@prismpyre7653]

maaaaybe but idk my understanding is the 'drop' or 'slam home' (which they did implement after this) gets the insertion time down to 11 or 12 seconds vs 18-20, but that still seems too long when the graphite is starting to molt and shafts are getting distorted or splitting open... I don't understand why they didn't start the water flowing again before that

[@markusw7833]

@@prismpyre7653 They literally had all main circulation pumps going and the reactor had a very high total flow rate, which is supposed to have contributed to the incident.

[@swokatsamsiyu3590]

@@prismpyre7653 After the 1986 accident, they have given the remaining RBMKs an additional, fast-acting SCRAM system called BAZ (Быстродействующая = Fast-acting, Аварийная = Emergency, Защита = Protection) that can insert 24 special control rods fully into the core in under 2.4 seconds or 7 seconds, depending on the signal sent. These control rods are gas-driven, their channels are cooled by a waterfilm, and they're Boron only. No graphite displacers allowed. They will pull reactivity down by at least -2 β-effective. When an RBMK is scrammed today, this system will go first. After that, the rest of the normal system follows.

[@robertgardner8680]

@@markusw7833they had 4 of 8 pumps going not all 8 hero.

[@orangejuche]

When you're talking about a prompt critical excursion, as happened at Chernobyl Unit 4, one second, ten seconds, eighteen seconds doesn't really matter, the reactivity spiked so hard and so fast that even if you "slammed home" the rods, the fuel channels would have still ruptured and jammed a bunch of control rods with graphite moderator still stuck in the lower half of the core. I had a bunch of other stuff below here, but a nuclear fission event happens in 1 "shake" (it is a real term in nuclear physics, it's about 10 nanoseconds, or 10^-18 seconds. An entire chain reaction that blows apart a nuclear bomb is completed in 50 shakes, or 500 billionths of one second. As soon as the reactor reached one dollar of reactivity, or the point where there's enough free neutrons to self-sustain the reaction, that was it, it's over, *boom*. For another fun little tidbit, the amount of uranium-235 fissioned in the Little Boy bomb that blew up over Hiroshima amounted to about 7/10ths of a gram of U-235. The energy from that tiny amount of fissile material was enough to level the city, and the entire reaction happened 500,000 times in the amount of time it took you to realize there's a period at the end of this sentence. Chernobyl was not a nuclear explosion, or anything like it, but the chain reaction and the steam explosion of the reactor happened on the same timescale.

[@SamwiseOutdoors]

This is actually a really interesting topic to explore. I've often wondered whether the die was cast long before the SCRAM function was initiated, or if there could have been a way to gradually reduce reactivity and slow down to a safe stop.

[@georgewilson7432]

Excellent video as always.

[@adamc2451]

Can you do a video about them not having chefs run the nuclear reactors

[@Chase1297]

Underrated comment

[@nicklockard]

Irony: a good chef would have run the reactor according to the SOP recipe, and nothing bad would have happened. You'd have never heard of an RBMK.

[@Edaros]

Great video! I remember reading a translation of the Legasov tapes, where it was stated that the whole „test“ was allegedly just a cheap way to save money, as faster diesel generators were readily available. But the responsibles did not want to invest any additional money, do you know if that is really true? And do you by any chance know if better containment could have prevented the worst case scenario of an open reactor? From the tapes I only know that europeans insisted on additional safety precautions but unfortunately there was no detailed information on these precautions. Thanks a lot and good job!

[@saschakrause2374]

Hello Sir. Again an example for how good your videos are and how well you’ve made your investigation into the incident. Please go on doing such great work. Greetings from Germany 🇩🇪

[@davidbaca7853]

Great video, Thank you

[@bobb941]

When the button was pushed, the reactor was already highly unstable and experiencing a runaway power excursion. It takes several seconds to insert the control rods (20 seconds to be fully inserted), and it may have been headed to an uncontrollable explosion without regard to pushing the AZ-5 button.

[@thereal757_ap]

Another banger of a video. Thank you!

[@thatchernobylguy2915]

Thank you!

[@Robin-nm1is]

The void stuff was good explanation 🎉! Ty

[@EoineyMTX125]

Another great video. Can we get a half lives story on Akimov

[@MultiBUSFAHRER]

Thank you very much for the great video. I would also like to thank the many commenters for being sophisticated and very scientific. Both the video and the comment section were very entertaining and informative. Thanks.

[@danielle3064]

Can you do a video that shows which things they did that was part of the test procedures and what was improvised? I'm still a little confused on what actually happened. I blame HBO and Medvedev's book. Once I heard you say that was inaccurate I ordered Chernobyl: A Documentary Story on your recommendedation. Thanks as always for the great video, this answered the question I had about if it really was the AZ-5 button that was the final nail in the coffin.

[@danielle3064]

I'm also reading INSAG-7. I've fallen down the rabbit hole haha

[@paulamore1630]

What I want to know is, when the reactor crashed before the test, if, rather than withdrawing the control rods entirely, the technicians had removed them partially to put the graphite tips in place, could they have brought the reactor up to power in a stable enough way to perform the test?

[@anikamaurer4970]

8:56 Vyacheslav Akinfiyev was replaced with Nickolay Fomin not Anatoly Dyatlov

[@AimlessSavant]

Chernobyl was a parable of failures. Perhaps it could, perhaps it wouldn't.

[@gamer07208]

Good video's you make! Keep it up!

[@prismpyre7653]

This is always what I've wondered- did they have a button, like an AZ-3, that would insert *some* of the rods all over the reactor? I would think the thing to do if you are aware of possible xenon poisoning and also that once you cut the water obvi reactivity will spike... I would think the thing to do is as soon as you release the turbine for the run-down test, you IMMEDIATELY switch water back on and start lowering SOME control rods, 25% at a time maybe.. (but I realize that they hadn't been made aware of the issue with the water-displacing tips causing a spike at the bottom of the reactor on insertion). I always wondered if they could have saved it if they didn't panic-- sometimes, accepting that you are going to be in *A* accident instead of trying to avoid it, lets you avert the worst-case scenario.

[@markusw7833]

"I always wondered if they could have saved it if they didn't panic" What in the world?

[@saierali4588]

Good video Keep on

[@stevenclarke5606]

Wasn’t the official operating procedure, that in the event of a low power situation, is that the reactor should be shut down, and then later restarted, with a very gentle and steady rise in power, until the reactor was stable?

[@grimmig13]

Could they have maintained control of the reactor if they started fully inserting some of the control rods as soon as power started rising?

[@thatchernobylguy2915]

The automatic control rods did insert, and they consequently brought reactivity back down, without the manual rods. Power only started skyrocketing after AZ-5 was pressed. :)

[@hvnterblack]

Xenon, I tried simulation, it was perfect timing for test. Few minutes, maybe seconds before, there was not enough xenon to stall reactor. Few momments later and it would be stalled, no way to power surge to occure.

[@adrianmlridgewayarcmlramll1965]

Utterly outstanding - I never knew anything of us, which all makes perfect sense - seems obvious now that the Soviet version of events would be lies. The various temperature coefficients and voids in the cooling water say it all. I get it it might have scrammed anyway and I see now how AZ5 might have saved it - you’ve taught me much tonight, am so grateful to you. Look fwd to more of your vids!!! Thanks again!! Bravo!! Adrian in Bermuda 😊😊😊😊😊

[@JoeHynes284]

two things i had read form IAEA report...it is unknown what the avg power density was with all rods out and some channels may have already been promt crit prior to AZ5, also, oscillations in reactor coolant flow were observed via the computer and these may/may not have caused the same thing...in other words, it'll be impossible to ever know, but there is evidence showing that it may have been unavoidable by the time the decision to SCRAM was made..

[@sonicnarcotic.]

Akimov & Toptunov completely withdrew 205 control rods, they were obviously ordered to do everything possible to get the power back up.

[@thatchernobylguy2915]

A significant majority of those control rods were already withdrawn before the power drop; it was quite normal for RBMKs to operate in that position :)

[@sonicnarcotic.]

@@thatchernobylguy2915 I am sure someone if not everyone in the room that night would've been well aware of the Xenon poisoning that pretty much doomed the test and the reactor from the start.

[@thatchernobylguy2915]

@@sonicnarcotic. Actually, the person to first start raising the power, Yuri Tregub, was well aware of the xenon pit and did not believe that it mattered. It didn't anyway, because the test could be conducted at any level of power so long as the turbine ran at full speed - 700MW was just an arbitrary number in case other experiments had to be done.

[@abloogywoogywoo]

Don't press AZ-5: explosion happens later Press AZ-5: explosion happens sooner

[@Yazovheimer]

Great video! I need to ask something, who do you think had the worst fate from the night of 26th april?? Like injury or radiation dose

[@thatchernobylguy2915]

Hmmm... in terms of injury, probably Kurguz, who lost most of his skin in the explosion (worse than Degtyarenko) and still had to crawl through corridors flooded with radioactive steam, or Shashenok, whose injuries were absolutely horriffic. In terms of radiation dose, likely one of the firefighters or the electricians. In terms of overall worst fate, I'd say Ivan Orlov. It's hard to describe in just a comment how truly tragic his death was.

[@Yazovheimer]

​​​@@thatchernobylguy2915 thanks for your opinion, its very interesting

[@FuzzyCollieDoggo]

What is the name of the Sim at the start of the video?

[@allgrainbrewer10]

MS flight, steam edition. Just watch out for the Mach 3 ballons

[@mixsmasher]

Interestingly the Canadian CANDU reactor is not too dissimilar to an RBMK which makes the Canadians arguably themost knowledgeable in the west on RBMK operation. Of course the reactors are different but they both feature a separated moderator from the coolant system resulting in the positive void coefficient (CANDU uses heavy water RBMK uses graphite) and a couple others things as well. But they are also in many ways different lol!

[@ga3680]

At around 1:06 you mentioned that water enters the reactor at a temperature of around 265 degrees Celsius. I don't understand how this can be. I learned at school that water boils to steam at 100 degrees Celsius. Is there a mistake with the units; or am I missing something? Thanks very much for the interesting video.

[@biggiedickson]

Pressure.

[@madarab]

Water has different boiling points depening on atmospheric pressure. The higher the pressure, the higher the boiling point and vice versa. 100C is the boiling point at 0 sea level.

[@ga3680]

@@madarab I see. Thankyou very much for taking the time to answer.

[@GWNorth-db8vn]

By the time they pressed AZ5 it was already too late. They might have saved it shortly before that by inserting the shorter control rods from the bottom, where the reaction was running out of hand. They didn't have the graphite displacers and could conceivably have slowed things down enough for the main control rods to be lowered a few at time. Of course, shortly before that they could have just not pulled them all out in the first place.

[@TheTransporter007]

Answer: *OH HELL NO*

[@skylineXpert]

a very interesting & educational perspective. But what has happened has happened.

[@mr.pilgrim1241]

I've heard it said that in order for a modern nuclear reactor to blow up like Chernobyl's did, someone would have to be in the control room *actively trying* to make it explode. It sounds to me like, in the case of Chernobyl, to have kept the reactor from exploding, someone would have had to have been in the control room actively trying to make it *not* explode.

[@kasel1979krettnach]

the reactor was flooded with water. I am 100% with diatlovs opinion that the reactor was doomed due to the reactivity insertion via the graphite ends and not via the voiding. the voidiing can only have contributed when it was too late anyhow. he called it " two reactor effect "

[@kensai7]

What application/simulation are you using for the 3D rendering of the reactor control room? 😊

[@thatchernobylguy2915]

It's ROBLOX, ha ha.

[@robinwells8879]

The two scenarios are moot really. The reactor should never ever have been in the state it was. That it was driven purposely and with determination into this state was the primary failure mode. The rest is just consequences and symptoms of the failure. Any piece of plant is delivered with operating instructions by legal decree in fact. They’re a formal contractual requirement. Equipment is mixed scrap without them. Deliberately disregarding those operating instructions voids warranty and is usually taken as gross misconduct and would rightly lead to criminal charges against the senior management in the event of accident or injury.

[@tommysamojlowitsch7028]

Tell me how a RMBK reactor core explodes not a meltdown an explosion Id love to know?

[@thefamilydogs3213]

What I want to know is if there is a way to reproduce two more elephant's feet and see if shooting it really accelerated decomposition or not. I really want to know if that is why it happened. I like to think it did but I'm doubtful we will ever conduct that test to confirm.

[@crusher9z9]

Im certain you could build a 4 billion dollar test pit for all that.

[@margraveofgadsden8997]

Stupid question, but how do they keep water liquid at 165-265 Celsius?

[@thatchernobylguy2915]

High pressures force gases to become liquids again. If you, for example, took the lid off of the reactor while it was running, all that pressure would be released and the liquid would become gas :)

[@lsq7833]

Pressure.

[@StephanAhonen]

Ever heard cooking instructions that say you need to boil things longer at high altitude to cook them? It's because there is less air pressure at high altitude, and water boils at a lower temperature at a lower air pressure. The opposite happens at high pressure, so you can keep water from boiling by keeping it under high pressure.

[@Aspire198]

So in summary, if the crew hadnt pressed AZ5 could the situation have been saved? No, because the reactivity was already on such an upwards climb that the automatic scram system wouldve been triggered, causing the same issue. Like everyone has been commenting, the issue was the dangerous condition the reactor was in, creating very rapid and uncontrollable reactivity changes.

[@isbestlizard]

How does water go missing requiring it to be topped up? Like... it's being cycled directly through the core in a closed loop?

[@jackradzelovage6961]

probably through steam, but in a manner that seemed to imply accident or emergency i have no idea how they did that

[@helene4397]

What if when someone who realised xenonpoisoning, had forced some person/s out of the room for about 24 hours in order to get that thing BACK under control?

[@TheXenProject]

Im more curious if the test "result" was the worst possible result, or if a significantly greater nuclear event would have occurred if this was done under an overload.

[@MultiBUSFAHRER]

According to calculations by some scientists, the reactor output increased to 100 times its nominal output during the disaster. At the time of the explosion, the entire Soviet Union could have been supplied with electricity using reactor 4 alone. That was already extremely violent. But I can't tell you what would have happened at full load. Perhaps at full load the AZ-5 button would have prevented a catastrophe. During partial load there was a high proportion of xenon gas in the reactor. Without xenon gas in the reactor, nothing might have been unstable.

[@EJDERMALO-qk7kh]

Yes it does,it just melted like reactor 1 or three miles incident and probably not gonna be useful for future. RBMK already dangerous design and main purpose is generating power and veapon grade plutonium from vver wasted uranium .In theory ,its win win situation but non homogenous fuel of rbmk cause control problems like using gasoline alcohol maybe tinner mix for your car,and lack of safety and soviet style management lead to this unfortunate accident.In soviet doctrine,meltdown must be avoidable in any expense due to high construction cost of a rbmk ,they were thinking to change/clean damaged units ,refurbish it and use again.In. meltdowns this is impossible.I am not even mentioning soviet factor of safety ratios ❤Great content by the way, you rock❤

[@AiOinc1]

For the record, an alcohol-gasoline mix for your car is what all gasoline has been mixed with gasoline since the 1920s and mixtures today can be as high as 85% in either direction.

[@EJDERMALO-qk7kh]

@@AiOinc1 but you got the point :)

[@MinSredMash]

RBMK doesn't have 'non-homogeneous fuel'. Where did you get that idea? It runs on perfectly ordinary uranium oxide. EVERY reactor creates plutonium as a byproduct.

[@EJDERMALO-qk7kh]

@@MinSredMash ideally it should.today rbmk s run on high quality Uranium.However this doesn't make them adventurous like in 80s.Currently not very financially efficient than expensive vver s .

[@MinSredMash]

@@EJDERMALO-qk7khModern RBMKs run on the same uranium, just enriched 0.4% more.

[@Maebbie]

According to the video, not pressing the AZ-5 button would not have saved the Chernobyl nuclear disaster. The video explains that the AZ-5 button was designed as an emergency shutdown for the reactor, but pressing it during the experiment at Chernobyl actually caused a rapid power surge that led to the explosion and meltdown. The video argues that the root cause of the disaster was the flawed design of the RBMK-1000 reactor, which had an inherent positive void coefficient that made the reactor unstable and prone to power surges. Pressing the AZ-5 button exacerbated this issue, but the disaster was ultimately inevitable given the reactor's design flaws. Therefore, the video concludes that not pressing the AZ-5 button would not have prevented the Chernobyl accident, as the reactor was already on a path to catastrophic failure.

[@ThugShakers4Christ]

Mea culpa. I spilled borscht on the control panel

[@rohitgoyal7258]

But it's like reliance, As told in films. The truth was hidden from them, They were in the dark. For any operator of any kind the "Emergency Stop" is the last thing. It's the last ditch effort in a hope that the misery stops. It's like if the train hadn't pulled the emergency brake it wouldn't have derailed. But how can you predict that when a situation is going out of control rapidly the only thing that is meant to stop it will turn out disastrous. They were believed that "this" button will stop it. But that's the thing, the last button should've been made for the safest output.

[@GWNorth-db8vn]

AZ5 was actually the way they usually shut it down.

[@fuciledisedano8625]

Where did you find the 3d model of the control room?

[@darylkemp1257]

the reactor would have blown weather the az 5 was pressed or not the fatal damage was done already by removing too many control rods yes the graphite tipped rods going back in sped the chain reaction up

[@joyadeceren]

what explosion?

[@BigT5]

Yes it would, they they knew how to fix xenon built up in core.

[@bygonewonders]

Nothing could have saved that reactor but hitting the Az5 button which lowed the borax control rod made of borax mite has dampened it but sense the soviet cut cost and tipped it graphite the reactors power output skyrocketed befor the borax could neutralize the reaction

[@dukeofurl01]

The problem happened in Chernobyl 4, so I was just thinking, were the other 3 unattended when everybody was scrambling around because of the issues with #4?

[@thatchernobylguy2915]

They were all fully staffed, but basically pretended in part nothing was going wrong :)

[@red-cc4xp]

Everyone else's nuclear safety standard from 1970 onward: 1+1=100 The Soviet Union: 1+1=Три Пойнт Один Шесть Рентген

[@fouzaialaa7962]

maybe if they activated the water pumps at full capacity it would melt and not explode , but then again ,that is a very bad day

[@highconfessor7222]

Is the HBO chyrnoble TV show not considered good? I liked it but don't know how accurate it was.

[@RaphaelFrancaisAnglais1506Z2N]

Can you do half lives Alexander Akimov please

[@thatchernobylguy2915]

It is planned...

[@RaphaelFrancaisAnglais1506Z2N]

​@@thatchernobylguy2915oh cool !

[@onwardsdown1294]

I still don't understand how lowering the control rods increases the reactivity or how the voids increase the reactivity doesn't the water act as a moderator slowing down the neutron increasing the reactivity and the voids make the reactor less reactive.

[@jmpeters499]

The control rods increased reactivity because they were tipped with graphite. This was designed to help with reactor power output when the rods were fully withdrawn (normal operations). That being said, pushing the rods in caused the displacement of the water below it. The water was acting as an absorber and was replaced with that of a moderator (the graphite tips). As far as the voids go, it’s also all relative too. Steam has a lower neutron absorption rate than water and with more and more water boiling to steam, the positive feedback loop is formed.

[@pkelly6618]

When you withdraw the control rods, what you want is an increase in reactivity. But if they're fully withdrawn they are replaced in their channels by water which is a shielding material, and this produces the opposite of the desired effect. The rods therefore need to be tipped with something that is left in the core when withdrawn to displace the water. This could just be stainless steel, but if the tips are graphite then this boosts reactivity more, because graphite is a moderator and more moderator usually increases reactivity. Where the design flaw occurs is that the tips didn't extend to the bottom of the core. When the rods are fully withdrawn they sit centrally in the core, boosting reactivity there. But as the tips don't extend to the bottom of the core, the reactivity *at the bottom* is relatively depressed when the rods are withdrawn. Then when the rods are inserted, the reactivity at the bottom of the core goes up. Water overheated and boils, voids form, reactivity goes up further, BOOM.

[@thebigint-hw1on]

If they had pressed tab they could have at least got a drink during it

[@SurnaturalM]

I can't imagine being in charge of a nuclear power station, and just, asking the employees do stuff just to see what happens. You just don't. That's the only way to prevent the catastrophe from happening.

[@andyb1653]

The best way to prevent the Chernobyl disaster would be to go all the way back to the late 60s and design a better/safer reactor

[@DaleDix]

Back then it should have been Ctrl+G

[@argilaga]

We could try to simulate it...

[@shingshongshamalama]

The meltdown was inevitable from the very formation of the management structure that led to the people in command making the decisions they made. Hell, the meltdown was inevitable as soon as the Soviet state decided to use the RBMK instead of a pressurized water vessel reactor.

[@am74343]

Engineers giving the workers a tour: "...And over here, we've installed a SAFETY STOP button which does absolutely nothing! But don't worry... Our reactor design is so safe, you'll probably never need to use it... Have a nice day, kids! See ya later! Time for lunch! Gotta go eat my borscht before it gets cold!"

[@TheKarlton93]

I think the problem was getting chefs to run a nuclear reactor

[@shawnstover917]

Should have pressed spacebar

[@s1p0]

Nature does not like rapid changes. I think it would be enough to slow down gradually by one at time.

[@GWNorth-db8vn]

Hot machines like changes even less than nature does.

[@andrewsmithty]

Celsius or Fahrenheit? water is boiling at 100°C unless presurized

[@GWNorth-db8vn]

It was pressurized. It was built like an old-timey locomotive boiler, with thousands of individual pipes each with a fuel or control rod in it.

[@beeftec5862]

why did their operators wear those white hats?

[@christebbitt1967]

Nuclear steam valve?...

[@NuclearSavety]

4:25 ... no, prompt criticality is not the same as an atomic bomb, a reactor still needs moderation of neutrons, which slows down the power escalalion... a bomb is prompt-fast critical ....

[@thatchernobylguy2915]

Prompt criticality in a reactor is fission sustained by prompt neutrons alone. It's the exact same thing. That happened at Chernobyl. :)

[@NuclearSavety]

@@thatchernobylguy2915 chernobyl blew up by prompt thermal(!) neutrons, but a bomb runs on promp fast(!) neutrons .... chernobyl never would have reached criticality with fast neutrons allone ....

[@thatchernobylguy2915]

​@@NuclearSavety Fast neutrons have been modeled in the explosion since 1998, and is largely used as evidence in support of a nuclear-jet style of explosion.

[@NuclearSavety]

@@thatchernobylguy2915 the capture cross section of fast neutrons is way too high in u238 and way to small for u235 for sustaining a chain reaction in 5% enriched uranium. ... for that reason, in nuclear reactors using 5% enrichment you only get criticality when you moderate the neutrons, making them from "fast" to "thermal". When you enrich fuel further like in atomic bombs, you remove more and more U238, making fast unmoderated neutron capture by U235 more and more likely. But with reactor-type fuel, a fast criticality is physically impossible. There is the concept of fast reactors, these would explode like an atomic weapons in case of reaching prompt criticality, but with thermal nuclear reactors like the RBMK thats not possible. You can get a power excursion to 500GW, destroying the reactor, yes, but thats still not a weapon-type explosion. Latter occurs on a time scale 1000 time shorter than a reactor excursion. For that reason your comparison of Chernobyl to an atomic weapon unfotunately stinks.

[@NuclearSavety]

@@thatchernobylguy2915 fast criticality is impossible in reactor-grade low-enriched fuel. Thats the reason you need high enrichment for weapons....

[@honestinsincerity2270]

You're delirious, RBMK reactors can't explode. Someone take this man to the infirmary

[@geronimo5537]

So doing your job causes crisis. And when everything tells you "this is the off switch". It actually became the trigger itself. There was no winning in this chain of failures and events. Just people stuck in it to play it out.

[@warthunder223]

What if the test was conducted within the reccomended levels

[@thatchernobylguy2915]

Assuming the power didn't fall, then it would have been a successful experiment with no explosion.

[@markusw7833]

The recommended levels of what?

[@pedrohenriquemachadocaldas1881]

@@markusw7833 power

[@markusw7833]

@@pedrohenriquemachadocaldas1881 The power level they were on was permitted. The power figure in the testing program was misrepresented by Soviet experts, intentionally. Had they done the test at midnight at >700 MW and more importantly at an ORM >20 they would have been fine, but they had another task left to do that presented a conflicting situation.

[@82dorrin]

Dyatlov was on the toilet... 😂

[@shevek5934]

Please get a sibilant filter for your microphone.

[@tehnarelhok718]

So it seems that not pressing the AZ-5 button might have delayed the inevitable, but wouldnt have prevented it.

[@bygonewonders]

Nothing at that point could have stopped that reactor from going critical but it was the pressing of the Az5 button that ultimately made the disaster so much worse because the soviet being rhe soviet cut cost on the control rods which Az5 woulf lower all at once they were supposed to be made of borax which would was supposed to negate the reaction but they got tipped with graphite which entered first and instantly increased the reactors power

[@scratchpad7954]

What would have happened if the water were fed into the system at 1°C?

[@allgrainbrewer10]

Heat shock would have shattered everything it touched

[@causehavoc3107]

Not bad, not terrible.

[@ridhobaihaqi144]

Saturday night entertainment 😊😊

[@jfan4reva]

More than one nuclear power plant disaster could have been averted if the operators had just kept their hands in their pockets and not tried to 'save the reactor' (chernobly, TMI)

[@kalkuttadrop6371]

TL:DR, the explosion likely would have been prevented, but meltdown was going to happen to some degree or another. Also who the heck would make that choice in context? It's like saying the Titanic captain should have rammed the iceberg head on, like sure, but imagine if he actually did steer into it instead of dodging?

[@alex_zetsu]

To be fair to the operators, the AZ-5 was supposed to be their emergency shutdown switch, the idea of it making things worse is completely counterintuitive. I can't fault them for that action. I can fault them for starting the test late and doing it without the nuclear physicist who was supposed to oversee the test.

[@vynxie]

Girls at 1:23:45 a.m: watching Instagram reels trying not to laugh Boys at 1:23:45 a.m:

[@Darkness-tk1hx]

A zed 5

[@oleksiirublov8298]

It is ChOrnobyl!!!

[@dmbeaster]

The reactor was rapidly heading to explosion without regard to the AZ5 button being pushed. It was experiencing a runaway reaction and power was increasing exponentially. That was why they pushed the AZ5 button. It did not trigger the explosion. But it is believed that it added more reactivity. The reactor was already so out of control that the control rods jammed after only partial insertion because the channels were ruptured. There is a lack of data because the explosion destroyed the evidence of exactly what happened. You can compute the impact of the graphite tips on reactivity. I have not seen the computation, but given the other factors that had caused the runaway power spike, the tips do not seem to have added that much. They did increase reactivity, but were they the difference between a problem and an explosion?

[@thatchernobylguy2915]

This is not true; this is the version falsely presented in Vienna in 1986 and completely debunked by 1991, with the publication of INSAG-7 the year after sealing the deal. Equally, there is a lot of surviving data from the reactor, which is how we can form these videos. The power surge did not begin until AZ-5 was pressed. Also, the control rods did not jam; they're inserted a third of the way in, which takes eight seconds, the same time between AZ-5 being pressed and the explosion. I'd suggest my "How Chernobyl Exploded" video series, especially part 3 as it covers this specific topic.kzbin.info/aero/PLDYm-CcwBBdFBWIpf5vfbJf_DjNf3H2Nb

[@dmbeaster]

@@thatchernobylguy2915 well. Your analysis is wrong per the MIT lecture on this subject by their nuclear expert. The power excursion was well underway when the AZ5 button was pushed. You can compute the boost to reactivity from the carbide tips. It is not the difference that then caused the explosion. It provided a boost to something that was already blowing up. Had it not been pushed, it still blows up. Every account notes that the control rods jammed because the ongoing explosions ruptured the channels. Why do you claim otherwise?

[@thatchernobylguy2915]

@@dmbeaster The MIT lecture is wrong; it has been well established as such in the Chernobyl research community for years. It's in complete contrast to the IAEA's official report, a previous MIT report by a former professor who went to Chernobyl, and pretty much every scientific study out there. If you watch Part 3 of my video series, the excursion comes from the displacement of water which was already close to boiling, causing the a positive insertion of reactivity that flashed the rest to steam. This is the same version of events repeated in the aforementioned article. I do have a script going over every mistake in the MIT video in the future; to say he's an expert, he gets quite a lot wrong. The easiest way to prove the control rods did not jam is as follows: The control rods only inserted a third of the way. The time between AZ-5 being pressed and the destruction of the reactor is 6 seconds. The time is takes the control rods to fully insert from a withdrawn position is 17.5 seconds (they moved at 0.4 m/s). 17.5/3 = 5.83. The only way they could have time to jam is if they were travelling faster than they actually could.

[@markusw7833]

@@dmbeaster Read INSAG-7 written by actual experts on the topic. In fact, read just pages 65 and 66. You're either misunderstanding the MIT lecture or perhaps just as likely the MIT lecturer doesn't know what they're talking about.

[@adamf663]

Pressing it after the reactor crashed would have saved it.