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The effect of Nuclear Fallout comes up a lot the immediate survival post-apocalyptic type questions.

In these situations the power grids are all off, and there is usually a drastic reduction in the human population - we can expect that there will not be many nuclear specialists left around to maintain the power plants.

My research indicates that:

Nuclear powerplants storing spent fuel rods outside of the core will explode if they cannot be kept sufficiently cool. This cool documentary says this will probably happen about a week after the apocalyptic event.

http://topdocumentaryfilms.com/aftermath-population-zero/

There are many hundreds of Nuclear Powerplants in the Northern Hemisphere especially North America, Europe and Japan.

World Nuclear plants

All these explosions will cause a lot of smoke containing highly radioactive fission products. This smoke will then circle the entire Northern Hemisphere for a few weeks, and then precipitate out in the rain.

Things would would not be as bad in the Southern Hemisphere because there are a lot less nuclear powerplants, and apparently the winds don't really cross the equator.

Questions

What is the impact of losing power/operators to all the nuclear power plants in the world?

Will the Nuclear Powerplants definitely all fail?

Should all future post apocalyptic movies/tv shows be set in Australia because people in the Northern Hemisphere will be dead?

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    $\begingroup$ They will all definitely fail. However safeties are in place that prevent Chernobyl scale incidents. Fukushima is probably the worst that can happen. All the smoke would probably offset the massive greenhouse effect that 100 years of using cars has done to our planet. $\endgroup$ – the_OTHER_DJMethaneMan Apr 22 '15 at 14:32
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    $\begingroup$ @DJMethaneMan The asker is clearly referring to SFP (spent fuel pool) incident. These are water pools in which recently spent fuel is being stored. Most of them require active cooling (water pumps) and thus energy to operate. Even if reactor cores could be semi-safely shut down and contained by staff (even at the cost of their lives), there are very few pools that can operate without maintenance or power, and they most certainly release large amounts of irradiated water vapor. There would be no explosion, but a lot of radioactive contamination. $\endgroup$ – eimyr Apr 22 '15 at 15:04
  • $\begingroup$ If the situation was not started by a nuclear war, there would be no effects. $\endgroup$ – Oldcat Apr 22 '15 at 18:12
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I spent several years in the Engine Room of a Nuclear Submarine. I am not familiar with the spent fuel ponds in specific, but I can make some educated guesses.

After shut-down there are fission products in the core that undergo radioactive decay. This decay generates heat, and this heat is a threat to core integrity: at a certain temperature, the fuel elements will rupture, and the fuel matrix will be exposed to the primary coolant (operating reactor) or cooling pond (spent fuel rods).

If there is sufficient decay heat available, the coolant itself will also heat up beyond its safe, design temperature. At some point the coolant would boil - this could eventually lead to a steam-explosion, meaning a sudden release of pressurized steam. Since this steam has fuel and fission products mixed in, it could result in contamination.

There would likely not be any fire, certainly no nuclear explosion: generally no dramatic effects (beyond the steam explosion, would could be impressive... it's what happened at Chernobyl after all.)

Likely the facilities in question are designed with layers of containment defenses to keep fission products in. Overall contamination to the environment would be low, though over time leakage could result in the local area experiencing moderate levels of radioactivity.

Severity would also be impacted by how sudden your catastrophe is. Decay heat does just what the name implies - it decays away. Given even a couple of hours warning, its likely the operators could place the plant in a safe condition.

So: its entirely likely the entering a nuclear facility in the wake of the apocalypse would expose you to dangerous levels of contamination. Its possible that the areas immediately surrounding them could have elevated levels of background radiation. It's unlikely that any effect would be noticed more than a few miles away.

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    $\begingroup$ +1 for Navy nuke school. The SFP is going to become wildly radioactive, melty and unapproachable but it will not burn unless something else happens. $\endgroup$ – Sean Boddy Apr 22 '15 at 18:29
  • $\begingroup$ My understanding is that Chernobyl was the reactor going prompt critical as the graphite-tipped control rods came down--a wimpy atomic blast, not merely a steam explosion. $\endgroup$ – Loren Pechtel Apr 22 '15 at 21:40
  • $\begingroup$ @ Loren Pechtel The reactor went prompt critical - which resulting in rapid, bulk boiling of primary coolant. Primary pressure exceeded the ultimate strength of the limiting component in the system, and... steam explosion. en.wikipedia.org/wiki/… $\endgroup$ – codeMonkey Apr 22 '15 at 23:53
  • $\begingroup$ But note that storage ponds are not confined, so there would be no explosion. Instead, the water will boil away and the exposed rods melt down. I'm pretty sure there will be some (or perhaps a lot) of combustion as the hot rods react with the air, but at some point the molten mass will accumulate at the bottom of the now-dry enclosure. The conglomeration will likely be accompanied by a few steam explosions. $\endgroup$ – WhatRoughBeast Oct 8 '15 at 17:20
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The nuclear powerplants will fail.

However, with modern designs, most of them would contain the resulting meltdown, assuming that all the staff didn't just immediately drop dead. You would expect a degree of contamination around the plant for sure, but as long as there is some degree of control, the reactor core would not explode - rather it would melt or partially melt and contain itself in the concrete sarcophagus. However, spent fuel pools would invariably evaporate without power and this could contaminate state-sized areas around the power plant.

Nuclear Winter is not a go-to scenario

As most of the plants would not explode (as in transform into atom bomb scale craters), but only contaminate the atmosphere. As the type of radioactive material is different, you would expect much less instant deaths but similar level of long-term (>3 years) effects on population. Also, there would not be a large level of dust particles ejected into the atmosphere so you can rule out the nuclear winter scenario unless there is an other significant contributing factor. That being said, areas near the fallout plume would still be depopulated and a you can expect a worldwide irradiation. Unfortunately I do not know how lethal would the radiation dose be on average or how long would it take for the radiation to be present in the southern hemisphere (yes, it will travel there too, albeit after a delay period and with lesser force)

Post-apocalyptic story can be placed anywhere

Because after a time in this case the radiation goes down and people who survived in shelters can and will go out. Long-term radiation effect such as misscarriages, infertility and mutations will be present, but generally you can assume that after a certain period of time measured in decades people can re-inhabit northern hemisphere. There will be limitations though - looking at areas near Chernobyl you can come to a conclusion that there are certain no-go zones, where radiation is still very high but also there are more benign, safer locations where life might be possible.

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The question makes one big mistake in terminology: "Fallout". An atomic weapon produces a huge amount of hot air that carries the fission products high into the atmosphere where they spread and then fall back.

When the cooling ponds boil dry there's no huge circulatory pattern to lift the crud into the sky, thus "fallout" is not a reasonable description of what happens. There will be radioactivity scattered about but with nowhere near the spread that war would bring and with nowhere near the threat level, either.

Also, note that in case of war the usual advice is to stay under cover for a few weeks. In the case of a cooling pool that clock was already running long before the rods went in the pool--in other words, it's already run out. Stay away from the pools themselves, beyond that the slight cancer risk is small compared to all the other threats the survivors face.

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