The accident occurred during a test that really should have already been preformed. In the event of the plant losing external power it would take backup generators about a minute to get up to speed and start coolant circulating again. This was dangerous and the engineers believed that they could use the inertia of the already spinning turbines to generate enough power to run the plant for about 45 seconds. The details are well documented, but in the end the power spiked, control rods jammed, and the reactor went out of control.
These RMBK reactors are inherently unstable at low power levels, and the operators did not understand reactor physics well enough to understand what the instrumentation was telling them. The control rods only reached just over 2m into the reactor before they seized. The reactor was 7m deep.
There were two explosions, a steam explosion, followed closely by an explosion from nuclear excursion. All of this is well explained elsewhere, but at the time the causes were unclear. In fact, several of the operators thought a tank of hydrogen had exploded, even though they could see chunks of graphite on the ground outside. Almost immediately radioactive dust was showering from vents in the plant and many crew received lethal doses and a 'nuclear tan' very quickly. They were simply replaced with workers from the other reactors. Some were sent i to lower the control rods manually, only to find that none of that existed anymore. They instead saw fire hoses dangling unmanned, the firefighters had already fallen after standing right above the core of the unit.
The government lied in many cases to the workers and public. One community was given masks and iodine but didn't distribute them to prevent a panic. The supplies went to waste. After that liquidation crews were brought in. The robots were unreliable and failed quickly. The government sent soldiers onto the reactor with little protection, they were called 'bio-robots'. The human side of the story is excellently captured in the book Voices from Chernobyl: The Oral History of a Nuclear Disaster, available here. The book is also available in many libraries and I highly recommend it, though some of the stories are graphic. Pictured above is the reactor after the explosion, it was enclosed in a hastily built sarcophagus, which is now unstable and must be rebuilt. The condition of the tunnels dug under the plant by 'bio-robots' is also unknown. The next picture is of a robot bulldozer being tested. These were used to bury the cities, but failed to work well and were often replaced by humans. Some photos from the zone also show spots, recording the massive radiation release. So what can be learned from this accident? The obvious results are more training, containment domes, and better reactor design. The other lessons are about government and public education. Let's focus on the less controversial public education issue. Many people refused to leave the zone because nothing was wrong. They couldn't understand the power of the atoms, the sky was blue, and there was a fire in the distance; why should it mean anything to them? This is why the public needs a basic science education, but now I fear the US has gone too far and induced a fear of the atom. Nuclear power is a technology to be respected, not feared. With proper construction, safety measures, and current technology there is no reason to fear an accident in the US. Waste products are another issue entirely.
In closing I encourage you to do some research and learn about this disaster. Also remember all the brave 'bio-robots' that were sent to the station to die while attempting to contain the situation. You can watch this short video for some photographs from the area. A documentary about the accident is also available on youtube.
(None of the photographs are mine, all sourced from Wikipedia)
