NOTE: This was adapted from another question that I asked recently.

Apparently, a 30km radius around Chernobyl will not be safe for humans for another 20,000 years. The more recent 2011 Fukushima disaster is still causing a growing number of problems worldwide (Fukushima Radiation). There are currently 433 nuclear reactors in the world:

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If the majority of these were destroyed (either by a terrorist act, nuclear war, or natural disaster) how would the radiation affect the entire world? Would there be any areas safe from radiation? How long would it be before the earth was safe to live on again? Would there be any adverse affects of the radiation?

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    $\begingroup$ If you are interested in the math behind fallout numbers, This article is a good place to start! $\endgroup$ – Anoplexian Dec 19 '16 at 20:19
  • $\begingroup$ Also, if you can determine kiloton meltdown sizes, you might find Nukemap a good resource. $\endgroup$ – Anoplexian Dec 19 '16 at 20:20
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    $\begingroup$ Incorrect premise, as people - mostly ones who refused to leave after the accident, or chose to return - are living quite well in that 30 km 'unsave' zone. Apparently they are healthier than their neighbors who stayed in the cities they were evacuated to, though of course that could be a selection effect. Also, there is no actual evidence of problems caused by Fukushima, just fearmongering. $\endgroup$ – jamesqf Dec 20 '16 at 3:23
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    $\begingroup$ No radiation, or at the most minimal radiation, has leaked from ten mile island. Honestly, if it was a simple meltdown, there would be few problems. Blowing it up on the other hand, that would be a very big problem indeed. $\endgroup$ – Xandar The Zenon Dec 20 '16 at 16:47
  • $\begingroup$ Worth keeping in mind that both Chernobyl and Fukushima were relatively mild in terms of how much of the radioactive inventory was released. Only an estimated 13-30% of Chernobyls uranium and fission products were released. $\endgroup$ – Innovine Jul 9 '17 at 12:44

Nuclear weapons are meant to inflict massive damage due to the energies they release. They are not really meant to permanently irradiate an area. Hence, older bombs are considered "dirty", whereas new ones are presumably much "cleaner".

The difference here is nuclear reactors are not ever meant to explode. That might sound like a silly statement, so allow to explain: all the contingencies around a nuclear reactor focus on stopping it from failing catastrophically (exploding - no, not in the same way as a nuclear bomb, they wouldn't do that). However, if one were to fail catastrophically (and there are many, many safeguards in place to make sure that it won't), there are no features in place to make the resulting explosion "clean".

In fact, that (most likely explosive) failure would simply cause the nuclear fuel to be thrown up into the atmosphere, causing huge clouds of deadly radioactive debris to spread for hundreds of kilometers around.

If many of the 400+ nuclear reactors in the world were to all fail simultaneously (or within a very narrow window of time), an enormous amount of radiation would be released into our atmosphere. This radiation would travel with the weather patterns, and spread along much of world, killing off most surface dwelling life.

Would any place be safe? I don't think anyone can say for sure, but I don't think any place would be safe in the long run. That much radiation would stay up in the air for a very long time, and would eventually make its way to pretty much every corner of the planet. Might some places be very minimally affected? It would depend entirely on the weather patterns.

How long would it be before the earth was safe to live on again? Safe for who? For the radiation to fade to current levels it would probably take tens of thousands of years, and I don't think humanity would last that long, except maybe off-planet, somewhere.

Would there be any adverse affects of the radiation? Of course. A majority of surface dwelling species would die off. Horrible mutations would likely become common place for those unlikely to survive the initial cataclysm. Over time the Earth would be reclaimed by other species, but not a for a long time.

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    $\begingroup$ I think you're confusing meltdowns and explosions. the worst a meltdown gets, to my knowledge, is releasing a massive cloud of radioactive stream when the containment vessel is breached; this is different from an outright nuclear explosion. Reactors simply can't explode in the same manner as bombs- although it's still not pretty $\endgroup$ – Marky Dec 19 '16 at 20:58
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    $\begingroup$ @AndreiROM Even if every single reactor in the US 'failed' today, I doubt there would be significant radiation leakage, just as there wasn't on Three Mile Island. Fukushima was only so bad because of the catastrophic damage to various backup systems (emergency diesels flooded and wouldn't start, portable pumps couldn't be brought in because a gaping chasm opened in the access road to the reactors and the surrounding terrain turned to mud, not making this up). Without a 40 foot tsunami, I think this is more like Three Mile Island. $\endgroup$ – kingledion Dec 19 '16 at 21:12
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    $\begingroup$ @kingledion - the OP mentions Chernobyl, so I was assuming that the failures would be on par with that. Yes, I agree, safety features today are significantly more advanced than they used to be, and nothing short of incredibly bad luck, or very deliberate tampering (and dropping a nuke on it counts as "tampering") would cause that sort of disaster. Conflate it by 400 times, and people would wish they'd stayed in bed that morning. $\endgroup$ – AndreiROM Dec 19 '16 at 21:14
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    $\begingroup$ @kingledion: Perhaps, but that's why they evacuate people, not just for nuclear reactor meltdowns, but for fires, floods, earthquakes, &c. The problem is that there is a great deal of exaggeration in the public's idea of radiation - the "Omigawd, it's radioactive! We're all gonna DIE!" effect. But contrast the idea that the Chernobyl exclusion zone (according to the OP) "will not be safe for humans for another 20,000 years" with the fact that people are living there now, and have been since shortly after the accident. $\endgroup$ – jamesqf Dec 21 '16 at 5:13
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    $\begingroup$ Potentially a damaged reactor could do more damage than a nuclear explosion, for the simple reason that a single reactor contains hundreds of tons of uranium. If the reactor core catches fire (as happened with Chernobyl, and WIndscale) then the rising column of hot air can carry radioactive particles hundreds of kilometers $\endgroup$ – Innovine Jul 9 '17 at 12:47

Short answer not much would happen. There just is not enough of them and even in the worst case breakdown they don't release that much radiation. most reactors aren't designed to be be giant pressure bombs/cannons which spread radioactive material over a wide area like the early russian designs. Many reactors have a negative void coefficient of reactivity and thus lose reactivity without coolant unlike the Chernobyl design. Others like breeders and gas cooled reactors don't have the issue to begin with.

You would see death in the immediate area for some sure but on a global scale all you might see a slight global uptick in cancer rates and that's about it. The sudden failure of global power grids and evacuation effects would be a bigger problem.

Lets try the impossible hypothetical scenario, all 433 reactors pull a chernobyl, Most can't but let's ignore that. if you pretend none of their exclusions zones overlap and even include some addition area for secondary zone overlap. you end up making an area roughly the size of Alaska unlivable. That is a a lot of land, but it is nowhere near close to making the planet unlivable. .
people just don't understand radiation, or how much you are already dosed with https://www-tc.pbs.org/wnet/need-to-know/files/2011/03/radiation.png

  • $\begingroup$ All reactors will go Fukushima if abandoned. It's not a matter of going boom, it's a matter of simply melting their containment if they lose their cooling water. $\endgroup$ – Loren Pechtel Dec 20 '16 at 5:41
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    $\begingroup$ 1 not all reactors use water, 2 many reactors will actually slow down if you take out the water. Water is a moderator as well as a coolant without water these reactors can't maintain a chain reaction. $\endgroup$ – John Dec 20 '16 at 11:11
  • $\begingroup$ Also how many people died from radiation at Fukushima, zero, expected deaths from secondary effects (cancer) less than a thousand. It was a horrible failure of the system but just did not do all that much. A few hundred Fukushima would do not much of anything on the global scale, you might see an uptick in cancer rates. $\endgroup$ – John Dec 20 '16 at 11:22
  • $\begingroup$ It's not a matter of what the moderator is. The problem is that reactors do not have a true off switch. If the operators hit the SCRAM button the chain reaction ceases but the power output simply drops to 3%, not to 0%. Thus you must continue to provide cooling. Fukushima is what happened when they were unable to provide that cooling. $\endgroup$ – Loren Pechtel Dec 20 '16 at 22:19
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    $\begingroup$ @John: IIRC, one of Chernobyl's many problems (common to Soviet reactors) is that it didn't actually have anything we'd consider to be a containment vessel. $\endgroup$ – jamesqf Dec 22 '16 at 19:08

Really bad things

A nuclear reactor creates about 20,000 kg of waste per year for a total of 8.8M kg per year. Assuming that stockpiled waste, which is an even greater quantity of material, isn't released as well as the material in the reactor, that's still a lot of waste to put in the environment.

Chernobyl 4 released about 8 tons or 5% of its radioactive fuel and machinery into the atmosphere from an initial fuel load of 192 tons. Reports indicate that the heavier dust returned to earth fairly quickly but the lighter radioactive material stayed aloft and spread over a huge area covering much of Europe and Russia.

Let's assume that all 443 reactors failed at the same time and with the same results as Chernobyl. While this perhaps isn't very realistic because of the considerable differences in reactor designs, the Chernobyl incident does offer a more complete picture of the manpower required to address such an incident.

Its a reasonable assumption that containment failures at every nuclear reactor in the world would vastly exceed the ability of any nuclear regulation agency to handle. The scale of failure is just too big. Trained technicians are too few and knowledge about radiation and nuclear materials too slow to teach to quickly train more technicians. Managing one containment failure is nightmare fuel. Managing all failures is impossible.

Economic Effects

This failure event causes an immediate depression for lots of reasons. First, much of the electricity required to run a modern economy comes from those reactors. While it may be possible to run coal and natural gas plants at full capacity to help meet the demand, this could not be done long enough for additional capacity to be built.

Second, a casual glance shows those nuclear plants are located in very close proximity to three of the most productive economic zones on the planet, the US East Coast, the US West Coast, Europe and Japan. Losing these areas would be a fantastic blow to the world economy. And while the people in those areas may not die, the spectre of nuclear death would cause panic and monstrous disruptions to the economy. If casualties are light, they may be able to return to work fairly quickly, depending on local conditions. If casualties are heavy, the economy may take decades to recover.

Third, the spread of so much radioactivite material is a huge area denial on some of the best farmland in the world. World food production will go down with some "interesting" effects on politics. Formerly weak but uncontaminated food suppliers will suddenly be in a much stronger position with their stronger neighbors.

Fourth, whatever power source humanity continues to use after this event, it absolutely will not include nuclear. No one, anywhere would permit reactors to be built. This may preclude fusion power too since it contains the world 'nuclear' in it, which is a bit of a shame since fusion unlocks so much energy.

  • $\begingroup$ Just so you know, the Chernobyl disaster happened because of astounding only-in-the-USSR level of technical and mangerial incompetance, and a safety-last design of the core. The RBMK core had a positive coefficient of reactivity, which means that as it got hotter the reactor gave off more energy. Also, its control rods (that absorb neutrons and stop the reaction) were made of a low melting point metal, so they fell out of the core as the disaster unfolded. Never say never, but I do not believe that ANY western reactor could have a disaster as bad a Chernobyl. $\endgroup$ – kingledion Dec 22 '16 at 16:28

It could be very, very bad indeed.

The majority of radioactive releases from Chernobyl did not come during the actual explosion. After the explosion, the graphite in the reactor core caught fire, creating a huge rising column of hot air which carried uraniumn and fission products out of the core and high up into the atmosphere.

This reactor fire burned for 10 days. Attempts were made to seed the radioactive clouds and trigger rainfall, to bring down as much particles as possible before the clouds passed over Kiev. Cesium was spread all over Europe. Eventually the fire was put out, ending the major source of contamination.

During this time, just a small percentage of its radioactive inventory was released. To prevent further leaks, a giant concrete sarcophagus was rapidly constructed over the disaster site. This cleanup operation took 600,000 people, and more or less bankrupted and ended the Soviet Union.

Now consider what would happen in the case that several (or 433) of these types of disasters occurred simultaneously. There simply isn't enough resources (people, machinery and economic) available to stop the disaster. So we could very roughly imagine each reactor burning off most of its inventory producing a disaster an order of magnitude worse than Chernobyl.

However, on the plus side, Chernobyl, being a rather old and crappy design, didn't have any major containment systems designed to prevent massive release. They simply never considered it at the time. But reactors constructed since have been severely regulated and safety systems improved.

Still not fail-proof, obviously, since Fukushima suffered simultaneous meltdowns. The amount of radiation released is highly debated, with estimates from no radiation related deaths, just a few cancers, to the entire north pacific being annihilated. What is known, is that enough radiation was released to merit widescale topsoil removal, and that the reactors are continuing to leak contaminants into the sea even today.

It's important to note that Fukushima is an on-going disaster, and should be treated as such. Six years after the initial events it is still very unclear what has happened, where the fuel is, what condition it is in, and how much of it has escaped. No efforts to actually fix the meltdowns has taken place yet. Japan, supposedly a world leader in robotics, have sent a handful of probes in there, most of which malfunctioned and/or returned untrustworthy data.

The Japanese government have done very well to recover from the associated tsunami, which killed 150,000, destroyed so many homes and infrastructure, and was obviously a much worse disaster. It is very interesting to note that the same government has not been able to clean up the Fukushima situation in any meaningful way. The site still leaks, they don't know what happened to the fuel, and they have taken no real plan to fix the cores. For reasons both technical and political, the cleanup is likely to drag out for decades, so the total radiation release may yet be counted higher than Chernobyl.

If they cannot make any progress on cleaning up one disaster site, despite 6 years and tens of thousands of workers, I think its reasonable to assume that there will be an even poorer response to a multiple-site disaster scenario. A modern malfunctioning reactor might not violently burn off all its radiation to the atmosphere, but it may just continue to slowly leak and pollute indefinitely, with even wealthy and high-tech governments like Japan being totally unable to stop it.

Should many reactors be damaged and pushed into disaster situations, I think the governments will be totally overwhelmed and unable to respond, and the reactors will simply burn and melt uncontrollably until they eventually burn themselves out, producing far worse radiological disasters than we've seen with Chernobyl and Fukushima.

The released radiation will contaminate crops, soil and water pretty much everywhere, entering the foodchain, and hanging around for tens of thousands of years. It's likely civilization will continue in some form, but I expect a large degree of mutations, non-viable births, massive increases in cancer and leukemia, with a worst-case scenario involving the total collapse of the food chain due to some critical component dying out, and triggering a mass extinction event.

  • $\begingroup$ Few of the reactors now in operation are of the graphite-cored type that burned at Chernobyl (and Windscale). Of those, the UK AGCRs are meltdown-proof, being gas-cooled. $\endgroup$ – Andrew Dodds Jul 10 '17 at 14:08
  • $\begingroup$ Good points. Fukushima was considered meltdown proof and leakproof too. As was Chernobyl. $\endgroup$ – Innovine Jul 10 '17 at 14:47
  • $\begingroup$ No, Fukushima, as with all reactors of the BWR/PWR type, was subject to loss-of-cooling accidents. Most of these will suffer a loss of coolant followed by hydrogen explosion in extreme circumstances. Chernobyl was about as far from meltdown-proof as is possible to get, having a positive void coefficient and no real containment. $\endgroup$ – Andrew Dodds Jul 10 '17 at 15:12
  • $\begingroup$ As you probably know, BWR Like Fukushima have/had emergency core venting burst disks to prevent high overpressure and explosive buildup of hydrogen. The production of hydrogen from zircaloy during a meltdown scenario was well understood, yet was not prevented despite the implemented safety features. Chernobyl had no containment, precisely because they didn't even consider such a catastrophic failure was even possible. So in one, the unthinkable happened, and in the other, the prepared safety responses failed. "meltdown-proof" is maybe hyperbole, but studies show a 1 in 200,000 chance per ... $\endgroup$ – Innovine Jul 11 '17 at 17:05
  • $\begingroup$ @AndrewDodds .. plant per year of an "uncontrolled event sequence". No doubt modern reactors are less likely to undergo spectacular failure, but I do not completely eliminate the possibility of highly unplanned event sequences occurring, since history has shown this to occur over and over. Some research even suggests that these sequences may be a systemic, emergent property of very highly complex systems, and so unpreventable. $\endgroup$ – Innovine Jul 11 '17 at 17:09

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