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In the somewhat near future (50-200 years) the government wants to repopulate the city of Pripyat. To limit radiation, they decide to send the remains of reactor 4 into space (which is possible due to the brand new handwavium engines they use). But due to unexplained reasons, the rocket explodes in the atmosphere.

What are the consequences to earth depending on the altitude it explodes at?

The rocket body is made of a handwavium alloy completely radiation and heat-proof but not neccesarily super strong. When one of the engines explodes, it causes a chain reaction pulverizing the whole rocket, creating a cloud of radioactive dust in the skies.

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    $\begingroup$ (Thank you for correcting the spelling on Pripyat) $\endgroup$ – ThePainfull Jul 12 '19 at 15:05
  • $\begingroup$ I'm not sure I corrected it so much as I anglicised it. $\endgroup$ – Ash Jul 12 '19 at 15:07
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    $\begingroup$ The lump of handwavium stabilized radioactive lava falls back to Earth. Probably into an ocean. What are you expecting to happen? Need to know just exactly how they managed to load up such hot things into a rocket that is immune to radiation to figure out just what exploded, where, and if the explosion was powerful enough to re-disperse anything radioactive. $\endgroup$ – Trevor Jul 12 '19 at 15:18
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    $\begingroup$ How... explodey do you intend the explosion to be? If the rocket simply breaks up, standard containment measures can prevent the release of radioactive material even from reentry; they've done so before. (Note especially the Apollo 13 RTGs, which reentered on a particularly fast and violent course.) If there's foul play involved of course such containment protocols might not be sufficient. $\endgroup$ – Cadence Jul 12 '19 at 15:21
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    $\begingroup$ "The act of exploding will reduce large quantities of radioactive debris to fine particles": you don't have much faith in the ability of engineers to engineer, do you? Why would the explosion of the rocket breach the containment of the nuclear material, despite said container being calculated so that it won't be breached? Or else why do you think that the nuclear material would be loaded uncontained? $\endgroup$ – AlexP Jul 12 '19 at 15:28
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The result would be [drum roll] not very much.

At present the material remaining in the reactor building ruins has been decaying for 33 years. In another 50 years it will have been decaying fully 80 years.

This is trinity crater, the site of the first atom bomb. I've been there. It was exploded in 1945, so is now just over 80 years old. The radiation is round about double normal background, as measured by a guy wandering around with a Geiger counter. A guy in shorts and sandals. No special radiation protection is required. It's a tourist site with kids and old people wandering around.

enter image description here

The only real protection needed is to prevent people taking chunks of Trinitite as souvenirs. I bent down, picked up a chunk, looked around at the guy in army gear and carrying a machine gun. And he was looking at me. So I carefully put the chunk back, and he relaxed.

enter image description here

By the time of the beginning of your scenario, the material from Chernobyl will be marginally more radioactive than the Uranium that was mined to produce the fuel. By the end date of your scenario it will be round about the same. If you dumped roughly 100 tonnes of Uranium out of a rocket the result would be much more worrisome from the standpoint of heavy things falling from the sky than it would from radiation.

The real question is, why would anybody bother shooting this stuff into space? The energy costs would be huge, and the benefit is minimal.

Edit for comments.

Only a small fraction of the Uranium in a nuclear weapon fissions, typically a few percent.

The fission products from a weapon are similar to that in a reactor. There are minor differences in the concentration of different isotopes. The Uranium that does not fission goes through very similar reactions as in a reactor.

The danger of an isotope depends on the half life and the amount, but also the kind of radiation it emits, and how well absorbed it is by living things.

If the half life is very short then the isotope is dangerous but decays quickly, so after several half lives it becomes much less dangerous.

If the half life is very long then it produces very little radioactivity per hour.

Neptunium has isotopes with half lives of 396 days or less, or over $1.54\times 10^5$ years. So at 80 years pretty much all the dangerous stuff is gone.

Plutonium is primarily an alpha emitter. Unless you eat it, it's not really a problem. Similarly, Uranium is primarily an alpha emitter.

The dangerous items are the fission products with intermediate half lives, that are gamma emitters, and that are also absorbed by living things. The big ones are usually Strontium and Cesium. Scroll down this article to Health Concerns.

From the same article comes this graph. 10 thousand days, has already passed, and the total gamma dose is already less than 1% of what it was shortly after the accident. The decay continues, and in another 50 years will be much smaller than that, probably about .01 percent of the first day dose.

enter image description here

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    $\begingroup$ Except the material at Trinity started life as pure enriched Uranium which underwent near total fission in the blast. Chernobyl on the other hand contained a lot of other radionuclides including the medium half-life Neptunium and Plutonium produced by Uranium decay during operation and still contains multiple tons of raw Uranium with a half-life of between quarter of a million and 4 and a half billion years. Uranium which is still leaking into the surrounding environment. Which is why it's worth cleaning up and way more dangerous than you're suggesting. $\endgroup$ – Ash Jul 12 '19 at 16:25
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    $\begingroup$ The amount of radioactive material at Chernobyl site is orders of magnitude higher than it was at Trinity test. And most of it is still there. $\endgroup$ – Alexander Jul 12 '19 at 16:39
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    $\begingroup$ The surface material at Trinity test has been scraped away, too $\endgroup$ – L.Dutch - Reinstate Monica♦ Jul 13 '19 at 4:06
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    $\begingroup$ This may be true for the 200-year end of the timespan, but at the 50-year end, there'll still be about 15% of the original caesium-137 and strontium-90 around. Releasing that in the atmosphere will cause a re-run of the original disaster. $\endgroup$ – Mark Jul 15 '19 at 6:50
  • $\begingroup$ @L.Dutch The material at Trinity has not been scraped away. There has been some drifting of sand. But you still see lots of trinitite just laying on the ground. The original crater surface is still there, disturbed by lots of tourist feet and some wind, but not removed. The photo is not mine, but might as well be since I have one almost identical. $\endgroup$ – puppetsock Jul 15 '19 at 14:43
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Nothing good at any altitude. Regardless of where it explodes on the way up the act of exploding will reduce large quantities of radioactive debris to fine particles that wind will be capable of moving around. The original blast sent about 15 tons of radioactive debris, 5% of the total fuel at the plant, (about 400 times what the Hiroshima bomb put up) and a small percentage of that debris traveled around the world on the wind. This explosion would do the same, only much more so and that's if it goes off on the launch pad. Europe got it worst last time and probably will again since the prevailing winds are East-West, but everyone will be badly effected, depending how much of the material gets into the air it could kill a large cross-section of all life in the northern hemisphere.

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    $\begingroup$ "The act of exploding will reduce large quantities of radioactive debris to fine particles": you don't have much faith in the ability of engineers to engineer, do you? Why would the explosion of the rocket breach the containment of the nuclear material, despite said container being calculated so that it won't be breached? Or else why do you think that the nuclear material would be loaded uncontained? $\endgroup$ – AlexP Jul 12 '19 at 15:26
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    $\begingroup$ @AlexP I have more faith in people to rush things and/or do them cheap resulting in the corners that have been cut causing horrible carnage that shouldn't have happened. It's not the engineers I lack faith in its the directors. $\endgroup$ – Ash Jul 12 '19 at 15:29
  • $\begingroup$ Why is anyone rushing anything? Prepping the rocket itself will take months if not years from mission preplanning to the day of the launch. There's no "rush" to be obtained no matter what you do. $\endgroup$ – Cadence Jul 12 '19 at 15:50
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    $\begingroup$ @Cadence People always rush things, regardless of whether they need to be, or should be, rushed or not. In this case they want the site cleared so they move people back into the area, there will be time pressure, a schedule to keep and things will get rushed. $\endgroup$ – Ash Jul 12 '19 at 15:55
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    $\begingroup$ Nope. There's already less than 1% the radioactivity present as there was at the time of the accident. So if 5% was distributed before it could not possibly be as bad as the original. $\endgroup$ – puppetsock Jul 12 '19 at 21:03

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