26. Chernobyl — How It Happened

MIT 22.01 Introduction to Nuclear Engineering and Ionizing Radiation, Fall 2016
Instructor: Michael Short

Using all the information from the course thus far, we explain how the Chernobyl accident happened from a technical point of view (and briefly explain the failings of Soviet culture which led to the cascading human errors). The RBMK design is shown to have positive feedback coefficients, a physically dangerous situation, which along with lack of operator knowledge about long-term neutron poison transients (xenon buildup and decay) led to the 600x increase in power in four seconds, which itself led directly to the explosion, fire, and scattering of radiation around Europe.

License: Creative Commons BY-NC-SA

[davedavids]

Am absolutely not on the level of these students, but can still understand and follow this lecture. This professor is superb in making a difficult subject understandable. Thanks for posting this video.

[kgucmen]

I am thrilled by the simplicity and understandability of this class..
This teacher made me think about returning to school, with my age of 50...
Hats off to you. Mr. Michael Short... If I only had 1 or 2 teachers like you, I would have been the best student of the class..

[christopherkurylo]

I'm an engineer and am very impressed and humbled by how well spoken and clear this professor is with such a difficult subject.

[normaneustice]

I would like to thank MIT for this superb presentation. It has brought closure to my wife's passing nearly a year ago from thyroid & bowel cancer. She was living in Sofia Bulgaria at the time of the disaster. I remember her telling me that she accidentally ingested rain water that tasted metallic & bitter. First her thyroid failed no matter how much iodine she took to correct the issue it got worse, then a few years ago, no matter how much food she ate she lost weight & strength, now I now why.. Again Thank you for bringing closure to a painful situation.

[johnwaldeck]

This guy is excellent. The students taking his class are really lucky to have such a responsive, alert, intelligent person to present the issue - and answer the questions coming from it.

[avincombat]

I watched the mini-series about Chernobyl, found this, and ended up going through the whole course. Thank you for uploading!

[chinruiz]

Knowing the science is one thing, but teaching it well is a completely different beast. The professor tamed both

[Tocsin-Bang]

I'm 73 years old. I spent most of my working life teaching science. This class makes me want to go back to school. I used to explode hydrogen and oxygen when I taught chemical reactivity, I used to always warn my colleagues when I was going to do it, one time the Head of Department didn't listen and the bang made her fall of the steps she was using to put up a poster!

[owenwilliams]

A really good explanation of the Chernobyl event. I was working at a nuclear site in the UK, at the time. We had just started our evening shift at 14:00 on a Friday i think, one of the guys had some work to do in the Reactor equipment building, he entered the building through and then realised that he had forgotten some tools, he decided to go straight out, and had to go through the exit radiation monitors, which alarmed to high haven one they started to monitor him. The radiation protection engineer was called, and he explained that he had just come in from outside and had not been in any contamination areas within the building . We had just had a shower of rain and so decided to use portable radiation detectors outside, to our horror, we were having reading 200 - 300 counts/sec... this was baffling to us at the time as to where i the contamination had come from, we eventually found out the following day, i think, what had occurred at Chernobyl. Reactor Safety systems are designed to keep us all safe, shame they by-passed them, but they guys who tried so valiantly to contain the contamination spread at the time were real heroes.

[TheGrimravager]

Graduated in nuclear physics, and not once had the pleasure of having a lecture dedicated to chernobyl. Guess this makes up for it, thank you very much!

[Matisyahuwu]

I love when a teacher says “let’s just wind down a bit since your other classes are going full throttle”

[JetPackDino]

This professor is a rock star, and I wish I could have been in the front row. Nicely done.  

And I loved one of the last things he said. "The data isn't out yet. Hopefully it never will be." I kind of think that people who are very intelligent in these STEM areas have a reputation for being uncaring or emotionless but what he said, to me, is the evidence of real understanding and humanity. It was just phrased in his way. I thought it was kind of poetic.

[PaulLundgren]

I have no background in nuclear (or any other) physics, just wanted to say I appreciate a person with extensive knowledge speaking from a position of expertise and legitimacy. Very well done, professor.

[timandshannon]

Great lecture. I am a simple blue collar guy who is fascinated with subject matters that are usually well above my pay grade. As a craver of knowledge, this lecture was interesting, and he keeps your attention. I wish I would have had a few teachers like this.

[wreckim]

That any human can get to this level of understanding is quite impressive, but that you can teach and entertain the mind with your talents as an instructor is quite rare and motivating to me as a 4th grade teacher. I dreamed of getting to this kind of level of knowledge, and failed. But this lecture motivates me to have my students accomplish what I could not. What a great post, thanks for the OCW, MIT.

[gfsm]

This is one of the videos from MIT that makes me think about how bad my lectures really are... Concretions MIT and the professor for sharing this great content with the humanity

[matthewgrasso]

I’m impressed that at 20 minutes in to the student’s question he had the confidence to answer “I’m not sure.”

[BachNBack]

I'd give this lecture a rating of 3.6 roentgen. Not great, not terrible..

[sarahgondos]

I've been searching for a WHILE to find someone who could explain this too me in a way that I understood. If I'm being honest, MIT was the last place I thought I'd be able to learn this because I'm not on that level of education. This teacher was BORN to teach. He breaks it down in such a way that anyone could learn. I feel confident that I could explain this to someone else.

[GiGaHarrySfotter]

Graphite tips is not entirely correct description.

In RBMK reactor a control rod consisted of two connected parts - 7 meter absorber part at the top and 4.5 meters of graphite displacer part at the bottom. When a control rod is withdrawn to the top position and the top absorber part is completely outside of the core, the bottom graphite part remains in the core and displaces water to prevent this water from absorbing neutrons and slowing down the reaction. (note: to improve the yield of fissile material)

But the bottom part of a control rod is only 4.5 meters long so at the bottom of the core there is still 1.25 meter of water which absorbs neutrons (the same at the top) and when a control rod goes down this water (absorber) is replaced by graphite (moderator) for 14 seconds until absorber part gets down. Why the bottom part is only 4.5 meters? There is only 5 meter space below the core for the bottom part when a control rod is fully inserted.

Before AZ-5 was pressed there were about 60 control rods out of 211 still partially inserted for about 1 meter on average. The core was split into three parts - the bottom part with about 10 special control rods which were inserted from below and with about 1.25 meter of water (acting as absorber) present in about 140 channels, the middle part without absorber rods but poisoned by xenon and the most active top part which had dozens control rods. So when all control rods went down these rods pushed out 1.25 meter of water (absorber) from 140 channels in the bottom part and replaced it with graphite (moderator), increasing reactivity at the bottom significantly (about 3-4 times compared to the level before control rod insertion and about twice if compared to the most active top part). Such significant increase in the reactivity in the bottom part caused a runway there and explosion.


Alex Demidov