re: If humans suddenly disappeared, would nuke/chemical plants explode/meltdown?

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Would the nuke power plants meltdown without crews and power?

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Most nuclear disasters have resulted from people deciding they know better what the plant is doing than the plant does. Chernobyl literally was caused by an engineer removing every single safety trip the plant had to put it into a condition to run a meaningless test and even then still had an opportunity to prevent it or mitigate it towards the end.

If you stop posting, will everyone's IQ go up 10 points?

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If humans suddenly disappeared, would nuke/chemical plants explode/meltdown? by Tigris

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No - lots of safety trips and shutdown sequences are built in.

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This.

Here is reasoning from someone that works in a nuclear reator facility.

Scenario: Power Grid goes away. Everyone at facility drops dead.

The Main Turbine trips/ Reactor trip.
Emergency Diesel Generators(EDG's) auto start, and in within a minute pumps start feeding water to the steam generators, cooling down the Reactor. The S/G's bleed steam through their safety valves, which helps remove the heat from the Reactor. Within an hour, Reactor pressure lowers enough for Safety Injection to actuate. However, pressure is still high enough that the Safety Injection pumps do not inject. The Charging pumps will inject water, and make up for shrinkage due to primary coolant cooling down ( the water in the Reactor). Within a day, the water going into the Steam Generators is greater than the steam being bled off, and the S/G's start to fill up. This cools the Reactor, even more, and eventually the Safety Injection pumps will start injecting water, and stabilize pressure. Eventually, the S/G's are completely full, and water starts coming out of their safety valves, until the water tank that supplies water to them goes dry. At that point, They are still heat sinks, and continue to cool down the Reactor, until temperature equalizes. Slowly, the Charging Pumps fill the Reactor up, and raise pressure until the Safety Valves for the primary system open which will bleed water out, and dump the water into Containment. Due to the low flowrate of water through the core, the fuel heats up, until boiling occurs. This is because the Charging pumps keep pressure way too high for the Safety Injection pumps to inject.

Within about a week, the EDG's run out of fuel, and all power is lost. The fuel will slowly heat up, until it melts. once it does that, it should melt through the vessel. At that point it will fall into the water that is in Containment. Pressure spikes, and slowly lowers as the Containment building looses heat to the outside air. It will be raining in Containment. If Containment holds, 99.9% of the radioactivity is contained. Now, these two scenarios are just based on "back of the napkin" math, and 18 years of running this reactor.

Now, let's say you walk away from the Spent Fuel Pool. Cooling is lost. boiling starts to occur in about 72 hours. If the pool was full of fuel, you would start to uncover fuel in about a week, at the earliest. Depending on the age and power history of the fuel bundles, some may rupture and catch fire. This will be bad news. There will be an external release of radioactive particles and gasses. The building will trap most of it, and it will slowly leak out. It is not going to destroy all life, though. Nature will take care of it.

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member that time you took a journey on a train but you lied about it and got called out? I member, we all member.

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Oh boy, I 'member.

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If humans suddenly disappeared

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Well, it's a good thing we'll never have to worry about that. Don't care what the rest of this post says...

I would imagine large fires would consume most of the land areas of the Earth and smoke inhalation would be a problem for any survivors.