#1
So in a few days (April 26th) it will have been 25 years since the nuclear disaster at Chernobyl's commercial power-plant in North-East Ukraine. The incident killed dozens instantly and lead to the deaths and health problems of thousands years after.

I thought to pay respects, and in light of recent comparisons to Japan's nuclear troubles I would give a rough guide to what actually happened at the plant. I'll show what main factors of human and design error lead to the total meltdown of Reactor 4 at Chernobyl.

Bear in mind there is still confusion about the exact chain of events so what I'm saying is what I've found to be consistent from various sources. Please be aware I apologize if some details are inaccurate.

Here's some information you may need to know first off:
- Nuclear fission is the process of splitting unstable radioactive nuclei into smaller nuclei to release energy as heat. This is done by giving a nucleus (eg. Uranium 235) an extra neutron, thus making it very unstable so it split into smaller radioactive nuclei. 3 more neutrons are released and are absorbed by the next Uranium nucleus, therefore the process is self perpetuating and can be an explosive chain reaction.

- Chernobyl Reactor no. 4 was water cooled and graphite moderated. A moderator is a substance used to slow down the emitted high-speed neutrons so they are at a suitable speed to be 'caught' by the next Uranium nucleus. The moderator is blocks of graphite surrounding the rods of uranium fuel.

- To stop the chain-reaction getting out of control (like a nuclear bomb) control rods are used to absorb free neutrons and keep the chain-reaction manageable. They are literally long rods inserted in-between the nuclear fuel rods. A material that does this is called a nuclear poison.

- The RMBK reactor at Chernobyl was poorly designed in that it had a positive void-coefficient , meaning the hotter the reactor gets, the higher the level of radioactivity (due to the bubbles in the cooling system)



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In the early hours of April 26th 1986, tests were being undergone at Chernobyl Nuclear Power Plant. It was one of the largest of the former Soviet Union, with four 1000 mega-watt Soviet designed reactors. This is a high output even for today's modern power-plants.
The test was being undertaken at Reactor 4, the purpose being to see how long the reactor could be cooled if the cooling-water turbines were to loose power. This involved turning off electricity to the turbines and letting them run down.
This is generally considered a rather unsafe test, as nuclear reactors by nature create a lot of heat very quickly if not cooled. If a reactor gets too hot the uranium fuel can melt (a meltdown) or reactivity can spike (positive void coefficient).

To do the test the reactor had to be running at a low power, and the junior engineers were delayed in performing the test as they were waiting for power demand to drop low enough. This delay meant an excess of nuclear poison had built up in the reactor thus reducing it's output power too low. It is unsafe for the reactor to be running at too low, so an engineer decided to increase the water flow in the reactor in an attempt to increase steam pressure. Crucially though, too much water was added which meant too little steam was being sent to the turbines, power dropped even more.

At this point the test should have been aborted, but the chief engineer was concious that his already delayed test must be completed. He ordered that the dangerously under-powered reactor must have its control rods retracted to increase radioactivity. This was strictly against protocol, but the engineers were ordered to ignore warning alarms and retracted almost all of the 250 control rods from the reactor.
The power creeped up to a 'safe' operating level, and the test resumed. The reactor was however in an extremely vulnerable state, if heat or radioactivity were to increase with no control rods in place the situation could escalate dramatically... now the cooling system was about to be turned off.

Finally, the order went through to shut off the cooling turbines. Now cool water flow was beginning to slow, and the reactor began to get warmer.
Remember Chernobyl's reactor has a positive void coefficient, so as the reactor got hotter, the radioactivity started to climb steeply, the reactor was now dangerously unstable. Engineers saw the rapid rise in power output and in a panic decided to put an emergency stop to the reaction. This involves inserting all 250 control rods to stop the chain-reaction.

This was the right course of action; the reactor was spinning out of control, despite orders the engineers stopped the reactor.
But reactor no. 4 had a fatal design flaw. The tips of the control rods were made of graphite. Remember what graphite is? It's a nuclear moderator, so for the brief moment when 250 graphite tips were dropped into the reactor, the reaction rate shot up massively and there was a freak power-surge.
Reactor 4 exploded, blowing the 1200 tonne cap right off the top. The bowels of the reactor was a wreck, and now tons of burning Uranium was exposed to the atmosphere, spewing out incredibly high levels of radiation.

Dozens of fire fighters arrived at the explosion scene unknowing of the radioactive nightmare. Levels of radiation near the reactor were tens of thousands higher than a safe level. They felt the effects immediately, vomiting and loosing skin. It would only be a matter of days before their organ systems failed.
The Soviet communist party denied a nuclear disaster for 2 days. Only on the 3rd day did the near-by town of 50,000 Pripyat evacuate, never to return.

To try and control the massive amounts of spewing radiation, soldiers were drafted to dump sand and boron on top of the gaping reactor from helicopters in order to reduce the updraft. They all were exposed to deadly amount of radiation.
As a more comprehensive block, the government drafted in thousands of 'Liquidators', soldiers in charge of dealing with the radioactive debris and building the protective sarcophagus. Thousands slaved for weeks, being exposed to dangerous levels of radiation, working only 40 seconds at a time.
Some liquidators were in charge of preventing molten nuclear fuel from seeping into the water table below, as if they came into contact an explosion 100 times the size of Hiroshima would have been set off, making much of Europe uninhabitable. These miners were all exposed to deadly amounts of radiation.

Thousands of civilians in the Ukraine and Belarus were exposed to a lot of radioactive fallout, causing a huge spike in cancer cases (especially thyroid). People are still suffering from the effects, in the form of cancers and mutations, including almost all the thousands of liquidators.

Amazingly the plant continued to operate it's remaining 3 reactors for years after the disaster, although no workers lived near the plant as radiation levels are still far to high to safely live in. The radiation is likely to remain dangerous for hundreds if not thousands of years to come.




Let us remember the heroic actions of the liquidators who compromised and lost their lives to avoid a nuclear catastrophe far greater than the meltdown.

The disaster was down to many unfortunate errors in judgement and reactor design. If any one of them didn't happen then the disaster would have been avoided. If Fukushima's 40 year old reactor had similar design problems, it could have ended in the same way. This was horrific, but I myself am a supporter of safe nuclear power in the future. If we don't keep using old rushed built reactors, we won't have to worry about a disaster like Chernobyl again.

Thank you if you read that! I just felt it were only right if people knew what happened, and how we were actually lucky how much less of a disaster Fukushima has been.
Last edited by vitchb at Apr 22, 2011,
#2
tl;dr
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#3
Quote by Wuai Bel Ree
tl;dr
You're cool and your post was obviously needed. /s

Great read, thanks.

And yea, I can read that and reply in 3 minutes.
#4
Interesting read, I knew the general idea of how it had occurred from studying nuclear fission in A-level physics, but didn't know the specifics about why the reaction went out of control.

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#6
Brilliant read, thanks man, helped my understanding of the whole thing!
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#7
Why do you wanna know about my knob?

The news always exaggerates serious stuff. I'm surprised they haven't gotten involved in all the superstitious 2012 junk.
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#8
Quote by Caleb B
You're cool and your post was obviously needed. /s

Great read, thanks.

And yea, I can read that and reply in 3 minutes.



I got to "obviously" then tl;dr.
OBEY THE MIGHTY SHITKICKER
#9
I think they got it on wiki.

It's something along the lines of the head engineer or whatever basically didnt follow the correct protocols because he thought he knew what he was doing. They had to shut the reactor for a bit to do some readings and checks and he thought the cooling system could still be run for a bit by the residual momentum of the turbine. He was wrong. It didn't run for as long as he thought and the cooling system failed which means you now have a reactor making endless amounts of heat (it don't stop emitting radiation for a loooooong time) and nowhere for it to go. I think they tried to put the moderator rods (made of graphite) but it took them a while because they had no power, I don't know I'm really trying to jog my memory here. But essentially it went into a meltdown and the massive steel and concrete containment lid blew off like it was nothing and that resulted in the explosion. They were really playing with fire, I really went through it all a while ago, it had all of the steps of what happened but i forget where. There's also a documentary that I think discovery does. They had witnesses and stuff, one guy said he saw some guy just instantly turn red, like he had the worst sunburn of his life but times 500 and also inside of him. And essentially that's what it does. You can't see it or feel the heat from it (well at some point you probably do if you're exposed to alot of it) but it pretty much cooks you.

I don't know, I could go on but really you should check it on youtube i think it's there.
#10
Wow. I never really new the specifics of what happened, just that there were some errors in judgement and design that lead to a catastrophic failure. Really fascinating read.

RIP to all the people that died containing the nuclear material.
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#11
Quote by metalblaster
I think they got it on wiki.

It's something along the lines of the head engineer or whatever basically didnt follow the correct protocols because he thought he knew what he was doing. They had to shut the reactor for a bit to do some readings and checks and he thought the cooling system could still be run for a bit by the residual momentum of the turbine. He was wrong. It didn't run for as long as he thought and the cooling system failed which means you now have a reactor making endless amounts of heat (it don't stop emitting radiation for a loooooong time) and nowhere for it to go. I think they tried to put the moderator rods (made of graphite) but it took them a while because they had no power, I don't know I'm really trying to jog my memory here. But essentially it went into a meltdown and the massive steel and concrete containment lid blew off like it was nothing and that resulted in the explosion. They were really playing with fire, I really went through it all a while ago, it had all of the steps of what happened but i forget where. There's also a documentary that I think discovery does. They had witnesses and stuff, one guy said he saw some guy just instantly turn red, like he had the worst sunburn of his life but times 500 and also inside of him. And essentially that's what it does. You can't see it or feel the heat from it (well at some point you probably do if you're exposed to alot of it) but it pretty much cooks you.

I don't know, I could go on but really you should check it on youtube i think it's there.


SOMEBODY didn't read the OP.
#12
I applaud you TS
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#14
t blew up wwell bam

radiocactivyty
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#16
it was a piece of shit.

thats what happened.
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#18
That was actually a brilliant read OP!. I had heard of the Chernobyl disaster before (as i am sure most people have) but only ever skimmed over it or got info from wikipedia.

You laid it out nicely which made it easy to read and having studied physics at a-levels learning a bit about nuclear reactors i felt i understood it quite well.
#19
Kudos, TS. This was an intriguing read and I'm happy I know more about Chernobyl than "some Russian nuclear reactor that blew up".
#20
I already knew of Chernobyl but always wondered what actually happened. Thanks TS, now I finally know

EDIT: I got ninja'd on this thread?

Last edited by TimTheWizard at Apr 22, 2011,
#22
Good read, thanks!

Didn't know about the liquidators, and seeing as I live in Croatia, big thanks to them.

But can you elaborate more on why it would've blown up if the fuel touched the water?
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#24
Thought I'd bump, as of now the incident happened more or less exactly 25 years ago!
#25
Quote by NoLaurelTree000
it was a piece of shit.

thats what happened.


This. The whole plant was shoddily and hastily put togather. It was a disaster waiting to happen.
#26
Quote by Moggan13
This. The whole plant was shoddily and hastily put togather. It was a disaster waiting to happen.


Unfortunately so, the Soviets had the science and the technology to make much more advanced and safe plants... just the wrong attitude.
#27
Great read! I already had plenty of knowledge of what led to the catastrophe, but not so much the aftermath and what the Soviets did to combat the effects. Thanks for that, TS
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#28
Quote by OneHappyCamper
Good read, thanks!

Didn't know about the liquidators, and seeing as I live in Croatia, big thanks to them.

But can you elaborate more on why it would've blown up if the fuel touched the water?


I think it's to do with the Uranium being super-heated at that point. Nearly 1000 celcius, if it hit water directly it would have been biblical.