system
Given the following post, respond as if you're commenting on Reddit:
human
Why did the Chernobyl nuclear power plant disaster happen, in terms of the physical processes, and what did nucleate boiling have to do with it?

I know that a test to see if the turbine could bridge the gap between a power outage and the startup of energency generators had been scheduled, and that the power output was way below what it should have been to safely run the test, but I don't quite understand what exactly happened. Why was the low power output an issue, why were the control rods taken out, and what generally went on?
gpt
The design of reactor at Chernobyl required actively pumped water to cool the reactor, even when "shutdown", as the fissile core continued to produce heat (normal for that generation of reactor). This meant using external diesel generators, but those took almost a minute to come online. So, they wanted to use the remnant energy in the turbines produced by the reactor to power the pumps.

But the reactor was too low power when the test started (due to poisoning from radioactive elements produced during the power-down, which normally would decay, but they shut it down to fast). That meant they needed to remove control rods to increase reaction speed. 

The low power itself was not a cause of the problem, but they had removed almost all the control rods that slowed the reactor down to increase power output. That meant when they tried an emergency shutdown, all the rods got inserted at once. A design flaw meant that *increased* reaction speed at the bottom of the core (since the rods took a few seconds to fully insert, and they displaced coolant while doing so). That meant the bottom of the core could overheat, since again they'd removed most of the control rods, and that lead to the rods getting stuck only part way in, which resulted in the explosion.

Boiling was an issue with the water-based coolant because steam doesn't absorb neutrons as well as liquid water does, and neutron absorption is one of the major factors in slowing nuclear reactions (thats what the control rods do). The boiling happened because they had too few control rods in, which resulted in rising temperatures, which wouldn't have been an issue had they steadily inserted new control rods, but since they inserted them all at once (why is unknown), displacing a bunch of coolant, the reactor spiked and overheated.

At that point, the reactor went into a runaway reaction, there was a steam explosion, and things went very very badly.