The Chernobyl Nuclear Power Plant three days after it experienced partial meltdown. (Image credit: SHONE via Getty Images)

QUICK FACTS

Milestone: Chernobyl partially melts down

Date: April 26, 1986

Where: Chernobyl, Ukraine

On April 26, 1986, operators at the Chernobyl Nuclear Power Plant were running a test to see what would happen to its nuclear reactors in a power outage — and they triggered the worst nuclear accident in human history.

Reactor 4 was scheduled to be shut down for regular maintenance, so the operators decided to test whether, during a power outage, the turbines could keep the coolant water circulating long enough for the emergency diesel generators to kick in.

The operators started reducing power to the reactor around 1 a.m. on April 25. However, a Kyiv-based operator that controlled the electricity grid wouldn't allow for a complete shutdown, as the grid needed power. So contrary to the prescribed test protocol, the reactor was kept at half-power levels from 2 p.m. to around 11 p.m.…