They imply if Chernobyl had melted down into the groundwater it would’ve exploded, causing the other 3 cores to explode. It would’ve made Europe uninhabitable (something like that).
Is that a realistic outcome? If the USSR had just tried to pretend nothing was happening and ignored it, whats the worst that could have happened?
If the worst case scenario had happened (whatever that is) how many would have died and how much land would’ve become uninhabitable?
It was the water tanks that were going to explode, not the groundwater. Instant transition to stream due to sudden nuclear pile arrival. After they drained the tanks they started worrying about the groundwater contamination.
Yeah, I am very curious about this as well. I believe the show said that the steam explosion would have been multiple megatons, which seemed implausible to me, but random intuition isn’t very good.
The physics folks over at Stack Exchange agree with you. It’s extremely implausible. They ended up with a numbers ranging from significantly less than 1 kiloton of TNT to 4 kilotons, which is a few orders of magnitude off from the megaton numbers thrown around in the show.
One of the posters there also pointed out that if all it took to make a megaton-level explosion was dropping some molten corium into some water, someone would have tried to do it intentionally and possibly weaponize it long before then. It actually took a lot of effort to create megaton-level explosions.
Some of lower voted answers suggest that the water could have acted as a moderator and initiated a nuclear explosion, not just flashing the water to steam. seems likely though.
Ok, secondary question. Lets say there is a steam explosion, would that have left Kiev, Minsk, and all the area between uninhabitable? What if the nuclear explosion was possible?
Why are people positing a steam explosion? A genuine explosion would require one of two things:
Option A: a slower transfer of large amounts of heat to a body of liquid water contained in a pressure vessel, raising its temperature to well above the boiling point before the vessel ruptures, resulting in a BLEVE.
Option B: a near-instantaneous transfer of large amounts of energy to a body of liquid water, raising it all to a temperature well above the boiling point before it (the liquid water) can get out of its own way.
Option A? There was mention of water tanks, but if the arriving corium is energetic enough to raise the threat of a megaton BLEVE, wouldn’t it melt or otherwise damage whatever part of the tank it was in contact with? Wouldn’t there have been blowoff valves/plates to vent these tanks before a BLEVE is threatened?
Option B is right out. You can’t heat such a large mass of water so quickly.
An unlisted option is an actual nuclear criticality explosion somehow happening when the corium reaches the water. like Oklos. Except there’s no mention of explosions associated with Oklos: it goes critical, heats up, boils the water away, and cools down, and the water returns, and the cycle starts again. ISTM something similar could be possible during a major nuclear incident like Chernobyl. It might generate a whole lot of radioactive steam over time, but not any sort of mechanically destructive explosion.
There’s also the effect whose name escapes me, that explains why beads of water dance around on a hot frying pan. The heat creates a steam layer which actually insulates between the extreme heat and the liquid water, slowing the evaporation process. I imagine the same thing happening here.
The initial failure would be “China Syndrome”, where the overheating core melts down into the earth. (all the way to China, hence the name). The flaw in this is that the suggestion would be that the more material encountered and melted, the more material melted into and diluted into the core melt. At a certain point, the core would be so dilute the reaction would slow and stop (concentration of uranium gets less and less pure, less reaction). If it did reach groundwater first, would it reach as a flat melt the size of a football field and flash it all into steam at once or a more irregular shape where a few fingers of it hit water first and create a layer of steam to push past the rest of the melt, creating a steam layer between?
Regardless, we are talking steam explosion not nuclear. Giant steam clouds of water contaminated with nuclear byproducts, not a megaton explosion.
Off topic, but a fun fact: The fictional movie “The China Syndrome” premiered 12 days before the nuclear accident on Three Mile Island.
And as I understand the accident at Chernobyl, the use of graphite in their reactors (in order to save money) was a major contributor to the disaster. They have since retrofitted all the nuclear power plants in the former USSR.
It is true that graphite can catch fire, but does that in and of itself make them unsafe? Note that there are still loads of graphite-moderated reactors in use in the UK, not to mention all the RBMK reactors in Russia.
Specifically, it was the use of graphite tips on the control rods.
The main cause of the accident was getting the reactor into a state where they had almost no control over it. They were attempting to do a safety test, simulating a loss of grid power. The idea was that if the grid power fails, the cooling pumps would stop, and the reactor would melt down. The point behind the safety system was to use the momentum in the generator turbine to power the pumps for the minute or so that it took for the backup generators to come online and take over powering the pumps. They had never successfully tested this system.
When they went to do the test, it was at the end of the month when everyone needed to meet their production quotas, so the folks in charge of the grid complained that they wouldn’t have enough power. So instead of doing the test, they postponed it, but they kept the generator in a low power state. This produces xenon gas, which poisons the reactor. At higher power, the xenon gets blown off and isn’t a problem, but because they ran the reactor for so long at a lower power state, the xenon built up. They ended up pulling most of the control rods out to compensate for the xenon poisoning, which left them with very little control over the reactor.
So up until this point, it’s all human error screwing the pooch. Where the graphite comes into play is that in this type of reactor, even if you totally screw things up, there is a safety button. If you press it, all of the control rods go back into the reactor and stop it. So it’s fairly idiot proof. And these guys were basically being idiots. The problem with the graphite tips though is that when you first insert the rods, the graphite increases the reaction. Then, when the control rods are more fully inserted, the control rods do their job and slow the reaction down. The engineers apparently didn’t know about the graphite tips causing a momentary surge when the rods are inserted, so when they realized they had not only totally hosed their test but had also totally hosed the reactor, they pressed the safety button, the graphite tips went in first, and the reactor power spiked and blew the whole thing up.
While there is definitely a point to be made that the graphite tips were a bad thing (and they have since been replaced in similar reactors), if they hadn’t operated the reactor in such a way as to completely poison it and get it into a fragile state with barely any control rods in it, they wouldn’t have needed to rely on the safety button to try to get them out of it.
It’s kinda like racing down a hill, stomping on the gas, disconnecting your brakes, then finding out the hard way that your emergency brake actually hits the accelerator before applying the brakes. Yeah, that emergency brake is a problem, but you shouldn’t have gone racing down the hill and disconnecting the brakes in the first place.
I wouldn’t call the graphite tips so much a major contributor to the accident as I would call them the final nail in the coffin. If they had done the test as designed and hadn’t poisoned the reactor, they would have never gotten the reactor into a state where they needed to press the safety button. It’s a long chain of events that ended with the graphite tips spiking the reactor’s power output and causing the accident. A break anywhere in that chain would have prevented the explosion.
Graphite is still used as a moderator. It’s only the graphite on the tips of the control rods that was changed out so that the safety shutdown wouldn’t blow up the reactor if they managed to get the reactor into a screwed-up state like they did at Chernobyl.
Chernobyl was basically fizzled Hiroshima type event. Still it yielded 0,5 KT, but it most of that energy was not a blast. Remember that glowy thing in E1 going straight up? That was no laser show.
It could reach similar critical mass when that hot radioactive blob reached basement and then underground water. But this time, the rest of the plant would probably gone into the air too. Or at least into way serious ruins. So yes, much worse.
If by “changed out” you mean “lowered so it hangs 125 cm lower”, then yes. The “new” manual control rods have the same graphite tips of the same length, but the water column at the bottom of the core when the rod was withdrawn has been eliminated.
I have not done even back-of-the-envelope calculations, but I have read that such criticality excursions are much less destructive than however many pounds or tons of actual TNT, since most of the fission energy is deposited as heat rather than as kinetic energy. I understand that an additional steam explosion was, at least initially, taken into account as a potential risk and therefore personnel risked their lives to drain all the water below the core, cool down the soil, and related activities.
“Glowy thing going straight up” sounds like some radiation-related phenomenon, but even a ridiculously lethal radiation field is not the same as a “blast”. I mean, you’ll die quickly after being bombarded by subatomic particles, but you wouldn’t feel even a gust of wind.
Why would anyone ever use graphite as a moderator, considering that water is an even better moderator, and just about the cheapest substance on the planet?