How Chernobyl Exploded - PART ONE: April 25th, 1986

The time is midnight, April 25th, 1986. A pleasant warm Friday in the northern region of the Kyiv oblast. And in a little over 25 hours, this scene will be changed forever. Tomorrow, the fourth reactor of the Chernobyl Nuclear Power Plant will explode, two dying due to the explosion, and another twenty-nine succumbing to radiation related illnesses. The total number of deaths can only be estimated in the thousands. But how did Chernobyl explode? Let us follow that story along.

With thanks to the following for creating the locations seen in this video:

Control Room 4 – Hydroproject.

Control Room 3 – Unit Three

Exterior – Gherkinbeans

With thanks to Bobby and Sredmash for reviewing the scripts.

[WhereIsTheCheese]

It’s a shame that the cgi didn’t capture the sheer awesomeness that is Akimov’s mustache.

[AskMrScience]

Rock em Sock em robots in the control room. There's your problem!

[TikhonT.]

1:56 I am summoned every time the red mercedes is mentioned

[bumblebeerror]

I stumbled upon your channel by accident - I’ve had a massive special interest in nuclear disasters for months now, Chernobyl in particular, and you’re the first person I’ve found who has a detailed rundown of the events and sources for them listed !! Excellent work, my guy.

[michaelmoser]

you didn't mention a known design issue: the RBMK had an unusually large positive void coefficient. RBMK doesn't keep water inside the reactor under pressure, now you end up with fewer water within the reactor, as the water evaporates. This creates temporary voids. Now water is also a neutron absorber, so that these voids are resulting in a stronger chain reaction and increasing the thermal output. This means you have a positive feedback loop built-in, by design. Eventually the whole thing exploded...

The book "Midnight in Chernobyl" by Adam Higginbotham mentions an accident at the first installation of the RBMK reactor in Leningrad (a partial meltdown, like at Three Misle Island!)

"On the night of November 30, 1975, just over a year after it had first reached full
operating capacity, Unit One of the Leningrad nuclear power plant was being
brought back online after scheduled maintenance when it began to run out of
control. The AZ-5 emergency protection system was tripped, but before the chain
reaction could be stopped, a partial meltdown occurred, destroying or damaging
thirty-two fuel assemblies and releasing radiation into the atmosphere over the Gulf
of Finland. It was the first major accident involving an RBMK reactor, and the
Ministry of Medium Machine Building set up a commission to investigate what had
gone wrong. Afterward, the official line was that a manufacturing defect had led to
the destruction of a single fuel channel. But the commission knew otherwise: the
accident was the result of the design faults inherent in the reactor and caused by an
uncontrollable increase in the steam void coefficient.
Sredmash suppressed the commission’s findings and covered up the accident. The
operators of other RBMK plants were never informed of its true causes. Nevertheless,
the commission made several important recommendations, to be applied to all
RBMK-1000 reactors: develop new safety regulations to protect them in the event of
coolant loss; analyze what would happen in the event of a sharp rise in steam in the
core; and devise a faster-acting emergency protection system. Despite their apparent
urgency, the reactor designers failed to act on a single one of these directives, and
Moscow promptly ordered more of the reactors to be built. The day after the
Leningrad meltdown, the Soviet Union’s Council of Ministers gave its final approval
to construct a second pair of RBMK-1000 units in Chernobyl, expanding the
station’s projected output to an impressive 4, 000 megawatts"

[Frankthetank-zrmc]

Thanks for posting. Don’t let one day mess it up.

[oscarr.g.]

One does not simply delay a turbine rundown experiment in an RBMK nuclear reactor 👌 I'm currently reading Midnight at Chernobyl (following your references), and having these videos this week is like the icing on the cake. THANK YOU !!!

[danielle]

Yoooo this is exactly the kind of timeline breakdown I've been wanting! Can't wait for part 2! This channel is so slept on you deserve all the views!

[willyengland]

Very detailed and clear! 👍👍
Thank you!

[ellenbryn]

Hearing about it in so much detail so many years later is surreal. I appreciate it very much.

I just wonder if the current generation can imagine what it was like pre-internet and, mostly, pre cable TV, when we might get as much news in a week as most of us can view now in half an hour by following links and watching full color videos, sometimes live, from all over the world.

back then we just had a few news shows a day on TV; more in-depth info of what happened yesterday was in the morning paper. or we could listen to news radio which often had more current breaking news, but ssldom reported anything outside one's local area.

[adamf]

One thing you needed to emphasize is that xenon-135 is generated with a 4 hour delay through iodine-135 decay. That's why the spike appears. When the power output crashed, the xenon was still being generated at the rate from when the reactor was running at 1600mw. Once XE135 absorbs a neutron, it becomes XE136 which isn't a neutron poison. With most of the control rods out, and only xenon and water holding back the reactor, it was like a stretched slingshot. When the reactor was scramed, the graphite portions caused a runaway positive void feedback loop at the bottom of the reactor. XE135 vanished, water flashed to steam, and the static graphite moderator caused power levels to reach 200x maximum in a fraction of a second.

[ExploreGamesAndMore]

Nice video. Looking forward to the rest of the parts, but please take your time. I really appreciate all the detail you put in.

[markusw]

As xenon is mostly a product of iodine-135 decaying which is a product of nuclear fission when power is lowered xenon's concentration increases while the higher power iodine-135 is decaying and the lower power xenon let's call it utilization is less. Then, after about 4-6 hours, xenon concentration begins to be steeply reduced as its iodine-135 precursor is reduced and excess xenon is decaying. Ultimately with a power reduction the concentration of xenon is reduced as well, which is why the Kiev delay actually resulted in a depoisoning of the reactor.

I think it's really important to know what the plan was for finishing the turbine vibration measurements. If the idea was to complete them at alternating loads between the two turbines then this is the virtually unknown event that got the ball rolling toward disaster. The Kiev delay depoisoned the reactor so the xenon narrative in the HBO mini-series is complete nonsense, but it's extremely likely the power level was intentionally reduced under 700 MW so that the remaining turbine could be disconnected with less steam in the system to complete these measurements as well the rundown test without shutting the reactor down. In addition to there being a video on this (in the Unanswered Questions series) we have recently found what appears to be an operating log confirming an intentional decision accompanying a KOM switch from Chernobyl. This separate program being left over is what caused a deviation from the power figure in the rundown testing program. The confusing thing is with turbogenerator no. 7 being disconnected at 13:05 their intentions for whether they originally intended to complete these measurements with both turbines available at 1500-1600 MW or if they intended to complete them at a lower power with one turbogenerator already being disconnected is unclear.

[Dream_]

Another great video. I was thinking a month or so back how I would like to put together as detailed a timeline as possible for at least the week building up to the 26th, either to catalog in social media posts or just for my own personal reference, and I have a feeling that's pretty much what you've got planned for this week, so I'm really excited.

[Just.A.T-Rex]

Almost as good as the Stanford lecture series on nuclear physics and power and the classes specifically on Chernobyl. Great rehash of everything that professor went over with some more info on the background of the choices made.

[swokatsamsiyu]

Too bad I can only give one "like" because this is just awesome. What a well-done video with these animations laying it out in such detail. There is one little oopsy pertaining to the Xenon poisoning. When you lower reactor power, initially Xenon levels will actually go UP because it will be produced as if you still were at full power, and the now lower neutron flux thus less burn-out. There's another thing. Every time you lower reactor power, you start that whole Xenon song-and-dance all over again. But apart from that, the video is great. Can't wait to see the next video in this series!

[isaowater]

11:54 PLEASE have mercy on my guy's name lol, it'd be pronounced as "deek"

be weary of lip-smacking too, stay hydrated

[NorceCodine]

My guess is that Toptunov and Akimov had turned off the auto-correct function of the reactor already at the beginning of the test, and tried to manually run the core during the entire test. This was probably not admitted or swept under the rug during the trial. It is the only way to explain the drop of power to 30 MW - they didn't have enough experience how to control the reactor manually, and had overdone the power reduction. Then they tried to raise the power, but again, did not do it correctly. Finally they decided to push AZ-5 when they realized that they have no effective control over the core, as it was yoyoing up and down, way lower and then way higher than they wanted.

[richardgadberry]

I'm giving this a score of 3.6 roentgen.

[spitfire]

Why am i so hooked on everything surrounding old reactors and especially the history and happenings of old Russian reactors?