- Inoculate humanity with the plague, something that incubates for 5-8 weeks and then kills painfully. You want chaos, you want people to try to travel to see sick family members.
- At 5.5 weeks, reveal yourself, claim you’re extradimensional tracking down this extradimensional plague, and offer vaccines to important people.
- Nuclear workers who help to shut down factories get the vaccine. They’re important people!
- Once all nuclear plants are safely dismantled, release phase two, the counter-vaccine.
Or let the plants melt down because in reality the problem is very localized versus the global scale, and within some decades without any real care (it is also I think inaccurate the claim about the plants automatically melting down, as this is specific to the designs- but even melt-down does not mean Chernobyle bis).
I would assume that the reason the aliens are doing this in the first place is that they want our stuff. So it’s not extra work for them to take over our infrastructure; it’s the whole point.
I think I’d start by engineering some sort of vectored plague. It can infect humans and several species of birds, but nothing else. In the birds, it causes only minor symptoms, but in humans, it causes death within, say, a couple of months. Then, at the same time as I release that plague, I take out critical but easily-replaced pieces of the electrical infrastructure (say, the transformer substations) with rod-from-god attacks. Any time a substation is repaired by humans, it gets struck again. So now we humans have only a couple of months to try to come up with a solution, and we have to do it using almost none of our current technology.
Wait! There’s some dust on it! It’s actually “To Serve for Man”
Are you sure it’s not “Four Men”?
Also “Manna from Heaven” in George R.R. Martin’s excellent sf short-story collection Tuf Voyaging.
Provide every male on the planet* with a VIP, all access pass to the inter-galactic championship/con of their favorite sport/activity. Load them into pilotless luxury drones which merge just out of our atmosphere to form a giant, planet size space ship populated with sex bots of all manner. Send the giant ship on a one way trip to interstellar space. The guys will never even know they were duped.
*Every male except me because:
- I can’t foil the plan as my repopulation plumbing was disconnected several years ago, and
- I could see which ladies were actually serious about the whole “not if you were the last man on earth” stuff.
Are we talking about the same aliens? I’m referring to the ones who carried their own power source across hundreds of light years before infecting us with a nano-plague.
If you mean Syrian refugees, then I fully agree. They are poorly equipped to manage nuclear power plants and tall buildings, but even Trump would have to admit they’re not trying to wipe out *all *human life. 
Even today the Chernobyl hot zone is over 1,000 square miles. That was a case where humans were working furiously to limit the damage. Even if the 437 current reactors were no worse, that would still be 437,000 square miles which is double the size of Texas, spread across the earth.
It does also not consider the possibly worse contamination from uncontrolled fires in highly radioactive spent fuel pools, deprived of cooling water and human oversight. Some studies have shown a single spent fuel pool fire could dangerously contaminate 25,000 square miles. There is at least one spent fuel pool at each major nuclear reactor so that would be 10 million square miles, or roughly the land area of North America, but distributed over the earth.
http://www.rense.com/general20/sabotagingspent.htm
The OP scenario is all humans would die suddenly – not gradually. Also none would be alive as slave workers to manage the complex industrial systems suddenly abandoned.
In this fanciful scenario, the space invaders could dispatch tens of thousands of alien technicians across the planet to manage each nuclear plant, each oil refinery, each supertanker, etc, then safely decommission them.
For the nuclear plants it would require time-critical emergency dispatch of previously trained alien teams and previously-developed compatible equipment. It would require a huge amount of coordinated work over a long period.
The aliens wouldn’t be sitting on satin thrones in their flying saucers, rubbing their chins and idly surveying their new conquest. If they didn’t immediately roll their sleeves up and get to work, large portions of their invaded planet would be ruined.
Post 41 has a scenario in which the aliens need, say, six months to eliminate humanity without fallout–assuming it takes about four months to shut down a power plant, which I suspect is a lot longer than it’d actually take.
You really need to start reading real data.
In many respects, the area around Chernobyl is turning into quite the wildlife preserve. Yes, many animals die young with tumors or deformities. It’s ideal, sure but they are not only surviving, but growing their populations. Us humans are more picky about that whole dying from cancer thing, and since we stay away, the animals are pretty happy overall.
Here’s a start: Effects of the Chernobyl disaster - Wikipedia
In any event, I would question any information on websites like the one you cited, which has a home page here that starts by selling you survival rations, magnetic sleep pads, homemade radiation remedies, vitamins to cure home-diagnosed illnesses, and wild legal ramblings about the case that might crash the entire US judicial system. You’re simply not working from sources that value reason very highly.
You mean the exclusion zone which wildlife is thriving in?
You mean capping it and then doing nothing further. Of course it was a horrible soviet design that made it explode.
so even taking the exaggerated numbers, perhaps you might think about the size of the Texas related to the size of the planet earth.
The human industrial plants degrading creates some local problems. On the planetary scale it is nothing.
(why it is fantastic to have aliens who travelled interstellar distances managing eventual local contamination is something I have a hard time understanding…)
Aha voila, that says it all.
Steal an idea from Pinky and the Brain.
Thousands of ladders leading up into their ships with signs at the bottom pointing up, saying “Free T-Shirts”.
What wildlife are doing now has nothing whatsoever to do with the OP scenario. Entire forests and untold wildlife were killed by the radiation: Red Forest - Wikipedia
The issue is not what wildlife or humans are doing now, it’s what happened during the event – and what would happen on a 1000x larger scale in the OP scenario of all humans suddenly dying.
I don’t recollect the OP scenario saying anything about aliens having to wait several decades after they foolishly unleashed an environmental catastrophe by killing all humans.
Of course he literally did not state the aliens want to do anything with the earth or even land on the earth. He just said kill humans and don’t harm the animals. So it’s conceivable they were just going from one star system to another killing higher order life.
That is only for reactors themselves, not the much more fragile spent fuel pools. They have essentially no protective containment and would likely cause devastation across much wider areas.
The OP scenario is the animals are not harmed. Did you ever see the wildlife devastated by a single oil spill? With Exxon Valdez only about 1/5th of the oil in the tanker was spilled. This was because of major, immediate human intervention.
With all humans suddenly gone, every supertanker on earth would eventually break up. It would be like Exxon Valdez on an approximately 10,000 times larger scale. Maybe the aliens don’t care. Or maybe they would send jumpsuit-clad alien sailors and roughnecks who had been previously trained in the myriad industrial systems on earth to manage them all.
First, they might not have to manage anything. The OP scenario did not state anything about occupying earth. Maybe they are just marching across the galaxy leaving a trail of ruin except the animals which for some reason they like.
However it would be a reasonable extrapolation that they have some continuing interest in earth residency.
It is not impossible to believe that advanced star-faring aliens could simultaneously mitigate many thousands of industrial disasters caused by their own actions. Maybe they would put a permanent impenetrable force field around each one, or just teleport each facility to the phantom zone. But if they have those powers, why are they messing around trying to kill humans with a virus, which in turn causes a vast cleanup problem? Why not just dematerialize them or teleport to deep space?
Thus we see the problem of uneven application of hypothetical technology. The same situation exists in depictions of alien UFOs. They are commonly described as joyriding above farmer brown’s back pasture at low altitude, with lights flashing. Why would a star-faring civilization have to do that? Today we have reconnaissance satellites that can minutely inspect earth from orbit. We have tiny robotic nano drones that look like birds and insects. Why would an alien civilization not use far more advanced methods? The answer is simple: the teller of those tales didn’t think about it, and framed the scenario only in areas he was familiar with. There is a lesson there.
It would be easier just to market Star Trek style holo decks or Futurama style sex bots (“Don’t Date Robots!”).