Why was Chernobyl so much more of a disaster?

As the days progress, I’m learning to a limited degree of the design of–which–? plant in Japan. (Do they all follow the same design?)

What was up w/ Chernobyl that made it the “worst” ever accident? I’m particularly interested in its design features.

Three reasons: 1) Chernobyl had no “last ditch” super-strong external containment. In the worst case in Japan, control rods would not have inserted (they did, AFAIK), the reaction would have run away, the core would have melted… and the whole lot would have been caught in the “core catcher”, the poured concrete base under the reactor, and been entombed. That’s the last ditch defense, which won’t be needed in Japan but which wasn’t even present in Chernobyl.

  1. Chernobyl allowed heat-dissociated oxygen and hydrogen to build up within the reactor vessel, which then exploded. In the Japanese reactors, gases were vented into the space outside the containment, which meant the eventual hydrogen explosion did not affect the reactor.

  2. Chernobyl was a graphite-moderated design. Graphite is carbon, and while it is very hard to get graphite to burn, it’s not impossible if you get it hot enough. Hot graphite will also react with steam to produce a flammable mixture of hydrogen and carbon monoxide. The Japanese reactors are pressurised-water designs, so there is no graphite, hence no risk of hot graphite reactions.

The situation in the Japanese reactors is that reactors shut down and the emergency cooling systems (that deal with the decay heat) kicked in. However, the tsunami took out the diesel generators running the emergency cooling, so then battery-powered emergency-emergency cooling kicked in. However, for some reason they failed to restore the emergency cooling system before the batteries ran out and the reactor began to boil away the water immersing the fuel rods, which meant that steam had to be vented to control the pressure. The detection of traces of radioative caesium and iodine in the steam meant that one or more fuel rod had become heat-damaged and ruptured its zirconium casing, at which point they gave up on trying to save the reactor and flooded the whole thing with seawater.

That is pretty much game over for the reactor. The sheer volume of water present has enough heat capacity to absorb all the decay heat as the reactor winds down, but they can probably kiss that particular reactor goodbye now.

In addition to the difference in design, the nature of the accidents is different. The reactors in Japan are being destroyed by a failure to manage decay heat. This is the same thing that happened at Three Mile Island - the reactor was shut down, but the residual radioactive elements in the core were still generating enough heat to melt the fuel rods once the cooling water drained out.

Chernobyl wasn’t destroyed by decay heat. The reactor at Chernobyl was through a combination of terrible design and an unwise test procedure put into a state where the core was briefly generating far more power than it was ever designed to. It’s like if you took a car’s engine and removed the electronic governor and any other overspeed protection and then wired the throttle full open. By the time the operators realized what was happening it was too late for them to stop it, and their attempts to shut down the core may actually have made the situation worse.

Great explanations, guys! I can’t really think of much to add.

One aspect of Chernobyl’s terrible design is that the rate of reaction actually increased with increasing temperature (referred to as a positive temperature coefficient of reactivity), which made it more likely that a runaway power spike could occur.

Chernobyl also had a positive void coefficient of reactivity, so that when the coolant flashed to steam, the rate of reaction increased.

The pressurized water naval reactors I qualified on had negative temperature coefficients of reactivity and negative void coefficients, which contributed to their passive safety features. Even if the pressure vessel cracked open and all of the coolant was lost, because of the loss of neutron moderation, the nuclear reaction would at least halt. (Of course, you would then have to deal with the decay heat.)

This was not the case with Chernobyl, which continued to increase its rate of reaction until it blew itself apart, dispersing the fuel and halting the fission reaction.

More detail here:
http://en.wikipedia.org/wiki/Pressurized_water_reactor#Moderator
http://en.wikipedia.org/wiki/Void_coefficient
http://energyfromthorium.com/2006/04/25/chernobyl-nuclear-safety-and-the-central-role-of-the-temperature-coefficient/

Another contributing factor to the accident was the lack of training of the plant operators, who were grossly deficient in their knowledge of nuclear reactor physics and engineering.

You should also add that when the operators tried to put the control rods back in to stop the reaction a design quirk actually saw the energy from the reactor spike dramatically. This then damaged things badly enough the rods could not be inserted further and moments later the reactor blew apart.

We don’t know if Chernobyl was worse because Japan isn’t over yet. If a meltdown occurs in Japan, it could be as worse, or even worse, than Chernobyl.

According to the general opinion expressed on this board, Chernobyl was worse because it was the Soviets, and can never ever happen in capitalist countries. But that’s not a factual answer. If you wait a little and worst comes to worst (which I certainly don’t hope!), a meltdown with the wind in the wrong direction towards Tokyo will be much worse than Chernobyl.

Ummm, no, the general opinion on this board seems to be that Chernobyl was worse because the basic reactor design was much worse and is not used in capitalist countries.

No, Chernobyl being worse has nothing to do with it being Soviet, and I don’t think a single poster here has claimed that. Chernobyl was worse because the reactor was a vastly different design than the Japanese reactors, and the nature of the accident was completely different. Chernobyl had no containment vessel, and through a combination of terrible design and extremely unwise operator actions was manipulated into a state where it was producing about 100 times as much power as it was designed to handle. That massive power surge blew the core apart, and set the graphite moderator rods on fire, which spread a large plume of radioactive smoke. The reactors at the Fukushima site have containment vessels designed to contain the core even if it melts down. They were also shut down well before the tsunami hit, so there is only decay heat to deal with, rather than a massive reactivity surge. They use water for moderation rather than graphite, so even if the core is somehow opened to the environment there’s little possibility of a massive fire that spreads reactor debris into the air. A Chernobyl style disaster simply can’t happen with the type of reactors in use at Fukushima .

I read through a book on Chernobyl when I was in high school, so my memory is a bit foggy, but effectively:

  1. The Russians pretty much did everything in their power to make it blow up over the course of something like 12 hours.
  2. Up to something like 30 seconds before it blew up, they still could have simply dropped the graphite sheaths over the rods to turn the thing off.
  3. Because they were afraid of their bosses more than they were afraid of radiation, they continued through many warnings where they should have stopped and shut the thing down.

It was effectively a concerted effort to make it go boom. And as the others have noted, the design of the plant was such that it actually could go boom if you wanted it to.

Do you have a cite for any poster on this board saying that Chernobyl was worse because it was built by the Soviets?? Because not only is this not what’s being said in this very thread (have you read any of the responses??) but I’ve never seen anyone say something like that in all the threads on nuclear power or disasters before. So, since this is GQ could you provide a link to someone saying that? From there you can work your way up to proving that this is the ‘general opinion expressed on this board’.

Do you have a cite to back up this incredible claim?

-XT

Der Trihs has made the claim in other threads that it was worse because it was run by the Soviets. And indirectly, that’s true: The direct cause was because it was a terrible design that was having stupid things done to it, but the reason such terrible design and stupid operation was tolerated was the Soviet system and its emphasis on politics over practicality.

I’ve seen him talk about the poor design in the past though, so he was probably trying to make a specific point, or was just being lazy. If you have a link though I’d appreciate it…though, really constanze should provide since he made the assertion. Regardless, this would hardly constitute ‘general opinion expressed on this board’, let alone the answers (which have been very good) in this thread alone.

-XT

Uh…look, I’m not saying the doomsaying is going to happen, but you’re taking “debunking” to ridiculous extremes and it’s making the pro-nuclear side look bad. You actually want a cite that IF Tokyo gets doused with radiation it will be worse than when Chernobyl got doused? Fine, that’s easy.

http://en.wikipedia.org/wiki/Tokyo

Bolding mine.

http://en.wikipedia.org/wiki/Chernobyl

Using the average figures, that puts 2,642 times as many people at risk, not to mention the world’s leading economic center. Before the accident, how much of the world’s economy depended on Chernobyl?

I was looking for a cite showing that it’s even theoretically possible for the reactors under discussion COULD melt down and discharge radiation on par with the Chernobyl disaster. That seemed to be the meat of constanze’s claim there.

-XT

Well, Japanese officials are saying a meltdown may in fact be underway, and the US Navy confirms detecting airborne radiation. That’s the two foremost technical authorities onsite, one surmising meltdown and the other proving airborne release. What burden of proof are you looking for? I’d say that “the realm of the possible” has already been entered.

That it will melt down? Sure. That it will be as bad as Chernobyl EVEN IF IT MELTS DOWN? Um no…no proofs have thus far been offered. Were this GD I’d just disagree, but this is GQ. An assertion has been made. No proofs have been offered. I’m pointing that out.

-XT

Kiev, is roughly 100 km from Chernobyl as the fallout flies. It has roughly 3.6 million people in the metro area. Minsk, is about 350 km away, with a metro population of 2 million.

Tokyo is about 240 km from Fukushima, if I’m reading the map right. Its getting irradiated, which is incredibly unlikely for the reasons stated, would be horrifying, but the population hazards were much, much more serious for Chernobyl, due to the core being exposed and burning in the open air. Anyone have any good epidemiological data for cancer deaths in/around Kiev/Belarus from Chernobyl? I thought statisticians had come back and found it was much lower than projected?

This is roughly correct.

It is not that the Soviets were incapable of building and operating a nuclear power plant safely. They certainly had the smarts and the industrial know-how.

However, the Soviet system contributed to many issues that made Chernobyl so dangerous. No containment dome. No physical barriers between reactors. The roof, which was supposed to be fire proof, was built with materials that were flammable (contractor cutting corners). The reactor design was stupid but allowed to go forward in the Soviet system where such a reactor would never be built in the West. The way the politics in the control room worked no one would challenge the lead engineer even they knew dumb things were happening (apparently they did try at one point but were overruled…these were good jobs in the Soviet system and the workers had “good” apartments and food and luxuries…they did not want to risk their jobs so continued the ill-fated test).

In short, the Soviet system enabled such a thing to be built when it never should have (not in the form it was).

Well, we’re being asked to take claims that it’s “safer” at face value too, to some degree. This is partly a question of whether one trusts the technical and political powers-that-be, despite a somewhat dubious history of misunderstanding nuclear reactions and deliberately risking public exposure in nuclear releases. Here’s one cite for both those issues.

The reactor design is fundamentally different than Chernobyl’s was.

The literal physics of the thing will not allow for a Chernobyl-type event.

That is not to say the thing cannot make a mess. It can but it has in-built features that steer it away from being like Chernobyl.

Not to mention the one in Japan has a containment dome whose purpose is exactly what the name says. Chernobyl did not have one.