fromkentucky

Damn right we do.

Unless you're fighting one.

Or consume child porn or hard drugs...

"The victim shot himself in the back 9 times and then sawed his own head off... Worst case of suicide I've ever seen."

Chernobyl was much more than just the positive void coefficient. The entire accident started with a planned emergency shutdown test to see if the rotational inertia of the turbine alone could power the cooling pumps for the 45-50 seconds needed to get the diesel backup generators up to full output.

To conduct the test, they had to greatly reduce power output, but slowly enough that the surrounding power plants could pick up the load safely. The Day Shift operators slowly reduced reactor output from its nominal 3200MWt to about half, and bypassed several automated safety and cooling systems.

Night Shift took over around midnight and further reduced power to ~700MWt, the minimum output deemed safe for the test. At such low reactivity levels, the core suffered severe Xe135 poisoning, which dropped output to less than 30MWt. This was less than 5% of the output needed to maintain enough pressure to prevent steam voids, but the Xe135 prevented power from rising.

They started manually withdrawing control rods to get the power level back up, and then manually increased the feed water rate to control the rising power level. That new, cold water reduced both steam pressure and thermal power output again due to its natural tendency to absorb neutrons, contributing to further Xe135 poisoning. The operators pulled more control rods in an effort to raise output.

When they finally reached 200MWt they activated additional circulation pumps (in preparation for the test), but the increased rate of coolant flow reduced output yet again, prompting operators to pull even more control rods. The high flow rate also prevented the water from adequately shedding heat in the turbine and condensers, raising the coolant temperature near its boiling point.

The core was designed to operate with a minimum of 28 control rods, but the operators continued removing control rods until power output stabilized around 200MWt, leaving only 18 out of 211 control rods in place, and dangerously high coolant temperatures.

In this configuration, they decided to perform the test. The turbine spun on its own inertia, at first providing enough power to keep circulating the coolant, but toward the end of the test the turbine had slowed enough that the coolant circulation pumps couldn't maintain adequate coolant flow, allowing steam voids to form. With so few control rods in place, the positive void effect created a positive feedback loop that began quickly raising power levels. The last automated control rod system was able to keep up for a bit, but only managed 12 control rods and thus could not keep the rising power levels from accelerating. Someone in the control room pushed the AZ-5 button initiating a SCRAM but the control rods had neutron-moderating graphite tips that inserted first.

All remaining 181 control rods inserted at the same time, and as the neutron-moderating graphite tips displaced the neutron-absorbing water, power spiked to over 530MWt in just 3 second. The stress of that sudden heat load fractured the fuel rods, jamming the control rods in their partially-inserted position.

Thermal output skyrocketed to over 30,000MWt over the next few seconds, rupturing fuel channels (allowing nuclear fuel to mix with coolant) and ultimately leading to a steam explosion that ripped off the 4 Million pound reactor-casing cover, ruptured additional fuel channels and severed all coolant lines. The remaining coolant flashed to steam and escaped, causing a Nuclear Transient explosion a few seconds after the first.

tl;dr- Graphite-tipped control rods and a positive void coefficient all certainly contributed, but ultimately Chernobyl was due to human stupidity.