I’ve tried to understand the Chernobyl disaster by reading the wikipedia page, but I have trouble getting it. In words of few syllables, can someone take a crack at explaining exactly how the explosion happened? Assume I know little more than “Big boom - Bad!”
Thanks.
It’s been a few decades but I read a book that went through the whole thing back when I was in high school.
The short version is, basically, that they were testing how hard they could ride the thing and once they saw that it was going wrong a giant game of pass the buck and refusing to admit that something had gone wrong took over.
As I recall, up until something like 30 minutes before it melted down, they could have still stopped it after like three days of watching it get progressively worse. But to do that would have been an admission of failure and no one was willing to take the hit.
The very simple explanation is that a test of the reactor’s emergency cooling system was performed badly, with the day, evening, and night shifts guilty of not adequately informing each other of what was going on. Then the amount of water flowing through the reactor was reduced to such a point where it couldn’t adequately cool the rods. The rods overheated and superheated the water that was still in there to highly pressurized steam. The steam exploded, rupturing the reactor, which in turn caused the graphite in the control rods to catch fire when exposed to air. Without the graphite shielding, the radioactive material in the reactor reached critical mass and started a chain reaction.
Imagine a pressure cooker. The first explosion was the safety cap on the cooker blowing off. The second explosion was the stuff in the cooker getting so hot so fast that the safety valve wasn’t able to vent the steam fast enough, so the entire cooker blew up.
I read This book and because he wrote it for the layman found it very accessible.
Would anyone in here be able to assess the accuracy of HBO’s Chernobyl miniseries? I am aware that some characters (Khomyuk, for example) are composites and not actual historical people and that some tweaking of timelines and personalities occurred for dramatic reasons.
If it is accurate, the lead character does a pretty thorough and easy to understand demonstration about how it happened with red and blue tile cards. Basically, during the test, the readings suggested that the technicians should do the opposite of what they did (e.g. lowering the power on the control knobs with insufficient cooling and the rods extracted will actually raise the power and keep raising it).
There’s a lot there. Are you looking for the technical nuclear-engineering details to be explained in simpler terms, or are you trying to understand the logistical mismanagement that led to the big boom? or something else?
A bit of both. The control rods somehow contributed to the accident by changing the neutron flux? And what was the experiment supposed to fo? Do you normally run experiments with a nuclear power plant?
Super simplified version (here is a very good video that describes it all better than I ever could: Why Chernobyl Exploded - The Real Physics Behind The Reactor - YouTube):
NOTE: Chernobyl is an RBMK reactor which means it has a positive void coefficient. As steam is made (a void) the reaction increases. Nuclear reactors in the West are made with a negative void coefficient meaning if the water boils away the reaction slows down.
Because a constant water flow through an RBMK is important there is worry about what would happen in case of a power loss (the reactor stopped producing power to make steam). They have backup generators but those take a minute to come up to power. What happens in that minute? The theory was that the turbines’ own momentum would spin long enough to bridge that gap and generate power to the pumps giving time for the backup generators to come online. This had never been tested successfully though so they were going to run another test.
In order for the test to be done the reactor power needed to be reduced to about 700MW from its normal 3000MW. However, Kiev had asked that they not reduce power as planned so they stayed running longer than they had meant to. When the time came they had to reduce power more quickly than planned for.
This allowed xenon to accumulate in the reactor rather than decaying away over time as it would have had they slowed down on the planned schedule. Xenon is a natural by-product of this reactor and it is extremely “poisonous” to the reaction (it is a very strong neutron poison…captures all the free neutrons flying about which are needed to sustain a reaction). When running at full power it is just burnt away, no biggie and part of how it is all supposed to work. When running at low power it is not burnt away and so has to decay naturally over the course of several hours.
When they reduced power xenon accumulated in the reactor and lowered power even more. They call this the “xenon pit” and once in this state the reactor needs to be slowly and carefully restarted over the course of 24 hours. But, the head manager in the control room insisted they continue with their test and ordered the power be brought back up. The only way to do this was to remove the control rods. Control rods absorb neutrons (much like the xenon is) and regulate the reaction. In order to get any semblance of power back (they were now down to 30MW when they wanted to be at 700MW) they removed almost every control rod in the reactor (like 211 out of 218).
Even then they only got the power back to 200MW. The test was ordered anyway and the pumps were stopped. Remember the positive void coefficient above? As water boiled it made voids which increased the reaction. As the reaction increased more water boiled away creating more voids. Since the pumps were now off no new water was being supplied to the reactor. Power on the reactor was rising at an alarming rate. As the power went up the remaining xenon, which had been limiting the reaction, burned off thus increasing the reaction rate even more. A vicious cycle had started spiraling out of control.
But, it gets worse. Realizing the reactor was spiraling out of control the control room tried to SCRAM the reactor. They press a button and all of the control rods are dropped back into place at once. Instantly stopping the reactor. But, they do not insert instantly, it takes time for them to move down. In Chernobyl the boron control rods were tipped with graphite. Graphite greatly enhances the reaction in the reactor. As the rods were inserted the graphite tips accelerated the already out of control reactor to unreal levels (hard to say but they think it went to 30,000MW or more, far, far beyond its usual 3,000MW).
As the graphite tips of the rods increased the reaction to incredible levels whatever water was left flash boiled into steam (so it’s all void now). The control rod channels were ruptured by the steam explosion so the control rods could not be inserted further and just stopped where they were, ever increasing the reaction. There were no brakes on the reaction at all anymore.
BOOM
I watched the series, found it incredibly compelling and well made, and read at the time that the producers paid great attention to detail and accuracy.
Many years ago I was involved in a research project about the Manhattan Project for the Smithsonian, so although I’m no nuclear engineer, I know perhaps a little more than the average man on the street about nuclear reactors. I saw nothing in the series that made me say, “That’s not right,” which set it apart from most popular movies and TV shows when they deal with highly technical issues.
What it also showed quite clearly was how disaster was made much, much worse because the Soviet power structure made it politically impossible for the plant’s managers to admit that something had gone catastrophically wrong and take actions that might have mitigated the damage.
The lead character, based on a real person, but partly a composite, has many haunting lines, but I found this one striking, particularly in the wake of the past six years:
“Every lie we tell incurs a debt to the truth. Sooner or later, that debt is paid.”
Needless to say, I highly recommend watching the series. It will give you a pretty good picture not only of the technical, but also the political failures that led to the disaster.
Sometimes when I’m reading a complex article on Wikipedia, I check under the available languages for that article for “Simple English”. That’s helped me a lot.
It is worth noting that “tipped” in this case was about 4 meters of graphite at the end of each rod. The rods served a dual purpose as a means to accelerate the reaction (graphite) and decelerate the reaction (boron). This allowed them to more finely manage the reactor during normal operations.
All I know about it (beyond simply that is happened) is what I just read in the posts above. So it sounds like this was a multi-clusterfuck of failure points that all failed, ALL of them failing UNsafe.
Is it typical of nuclear reactors to fail unsafe, instead of failing in safe ways? Or is that mainly just Soviet-style? Are American (or other non-Soviet) reactors substantially different in these ways? Would negative-void-coefficient reactors tend to fail safe instead?
From Whack-a-Mole’s discussion above, it looks like, at every failure point, the operators’ options were to make it worse, or to make it worse.
The HBO miniseries “Chernobyl” is very good and, as mentioned above, they supposedly worked hard to make it accurate. They fudged some but it is probably about as good as you will get without having actually been there.
In the series the “villain” is the chief engineer in the control room who insists the test be done no matter what. The other men in the control room object but he overrules them more than once (browbeats them).
I guess the other “villain” is the soviet system which allowed one man to bully everyone else under him into complying when they knew they shouldn’t. The jobs they had as nuclear engineers were extremely good and came with a lot of perks. Getting fired because you pissed off the boss was just something you didn’t do. It wasn’t just you who would would suffer, it was your whole family. Not to mention the chief engineer who felt there was some reason the test MUST be done (presumably someone higher than him putting the pressure on).
Also, apparently control room engineers were were not aware of the danger the SCRAM posed. They were well trained, intelligent people who knew a lot about their reactor but they were not taught everything they should know.
Yes. For one thing, American-type pressurized water reactors are designed such that coolant water is the moderator necessary to keep the reaction going (whereas the Chernobyl-type reactor used graphite blocks). If the coolant water is gone, an American-type reactor goes subcritical and shuts down.
(Which is not to say that there couldn’t still be a meltdown in a loss of coolant accident [as in Three Mile Island and Fukushima] due to residual decay heat, but at least the nuclear reaction is no longer continuing.)
American-type pressurized water reactors are also designed to have a negative temperature coefficient of reactivity, so that as coolant temperature increases, the nuclear reaction is inhibited. This is a negative feedback loop that makes the reactor inherently stable.
(If this is a hijack please let me know and delete or move)
What I do not get is the issue for the test they wanted to do at Chernobyl.
If they turn off the reactor (presumably a SCRAM) then what is the problem waiting a minute to get pumps working again?
I get decay heat is an issue (as @robby mentioned Fukushima and Three Mile Island) but decay heat is what…10% or less of max output (5%?)?
If stopped on the spot I assume the reactor only has 10% (or less) heat it had at maximum power. I doubt the pumps stop instantly and water will be flowing back in a minute or so.
Is that minute without the pumps being powered really that critical? The answer seems to be yes but I am not understanding why.
I remember an old article about immigrants who had trouble finding jobs in North America with their training. The article mentioned that what the East Bloc in the day called “engineers” was actually somewhere less than an accredited engineer here, more a technician. So presumably “nuclear engineers” over there were not all the rocket scientists we have working in power plants here. (Thinks… “they can’t all be Homer Simpson”).
Another fun story - a chemical engineer I knew here went over to consult for a refinery in Albania while it was still communist. He said there was one process where a pump moves one reactant that was a slurry. The propeller blades would wear away, and eventually the pump would stop being effective. However, nobody wanted to stop the process - if you stopped the process to replace the pump, it was your fault the production was stopped. If you let the pump fail and production was compromised and a whole day or two of product had to be redone - then that wasn’t your fault. Nobody was empowered, nobody stuck their neck out. Things just “happened”. I imagine the Chernobyl engineers were in the same situation.
Fun act, in case you all missed the news. The first place the Russians withdrew troops from in recent days was the Chernobyl area. Apparently they’d had the troops digging trenches in the area and many of them had to be sent home with radiation sickness. Maybe stirring up the soil in the area was a not-clever idea.
Could a situation arise where the reactor is at high power but power is lost to the turbines and therefore the pumps? I don’t know anything about this stuff but on the face of it, it seems like that might be possible.
I wouldn’t characterise it that way entirely. There was a point - before the explosion - at which they were completely screwed whatever they did. But there was a long downhill slide before that where they could have saved the situation. But they did not because of extreme bureaucratic pressure to do the test that night regardless.
Very likely, in the back of the minds of the operators was the thought that if it all went wrong, the SCRAM procedure could save their asses, which probably contributed to their recklessness. At least according to the HBO series, it was known but kept secret from the operators that the SCRAM procedure was flawed in certain circumstances. It was kept secret because it was embarrassing.
