Image courtesy of http://www.solocomhouse.com/chernobyl.htm
While an earlier poster noted what happened after Chernobyl, I thought learning exactly what happened that day would be interesting and informative. I wrote this because of my deep interest in this disaster. If you have any questions or comments about this, please contact me and I will do my best to answer them. If you have any questions about some of the details of the day, please let me know. I used personal knowledge and wikipedia.org for most of my research. I tried to take what I learned and turn it into a more lemans version of what was on Wikipedia.
The following is some of what happened leading up to and on April 26, 1986:
During the daytime on April 25th, reactor 4 was shut down for maintenance. This was a decision made in order to test the reactor's ability to generate power, even if the reactor were to lose power. The point of the test was to determine whether the turbines in the rundown phase could power the pumps while generators were being turned on. The test had previously been tried on a different reactor, with negative results. However, there were some improvements made and the staff felt a new test was needed.
The disaster started to buildup during the day on the 25th. Unexpectedly, a power station went out of line and reduced the power to the nuclear reactor by 50%. Although, this was still enough power, much of the day was spent trying to keep the power station from reducing power further. Because too much time was spent keeping the power station up, the day shift was over by the time they were ready for testing. Instead, the night shift was placed in charge of the test. While this would not be a problem in most places, the night shift staff was not well educated on nuclear reactors-- most had come from coal reactors.
Throughout the night this uninformed crew worked on the tests. They continued to lower power to see if their theory was correct. However, because of their ineptness they were unable to see problems looming. The new crew was unaware of the prior postponement of the reactor slowdown, and followed the original test protocol, decreasing power too rapidly. When the operators commaded a small power reduction, the the reactor powered dropped to 5% of what was expected. The operators believed that the rapid fall in output was due to a malfunction in one of the automatic power regualtors. In order to increase power, automatic rods were pulled out of the reactor beyond a safe level. This pullout caused a buildup of Xe-135, a type of xenon found in nuclear reactors.
Not knowing how unsafe the conditions were, the plant workers began their experiment at 1:23:04 a.m. on April 26, 1986. Unfortunately, the unstable state of the reactor was not reflected in any way on the control panel, and it did not appear that anyone of the crew knew there was any immediate danger. The steam to the turbines was shut off and, as the momentum of the turbine generators drove the pumps, the flow of water decreased, thus decreasing the absorption of neutrons by the coolant. The turbine was disconnected from the reactor, increasing the steam level in the reactor core. As the coolant heated, pockets of steam formed, causing voids in the coolant lines.
Thirty Four seconds later, the operators ordered a shutdown of the reactor, inserting all control rods--including the manually inserted ones that had been withdrawn earlier. The slow insertion speed, along with the flawed rod design which initially reduces the amount of coolant present meant that the shutdown increased the reaction rate. This caused the reactor to jump to ten times the normal operational output. The rods began to melt and steam pressure rapidly increased. This increase cause a large steam explosion. This caused part of the roof to blow off, allowing oxygen to rush in. This combined with the extremely high temperature of the reactor fuel and graphite started a graphite fire. The fire greatly contributed to the spread of the radioactive material to the outlying areas.
While this is a detailed explanation of how the Chernobyl disaster occurred, it is definitely not the main story. The lack of information form the government and time delay in letting the citizens know is what is spoken about today. As mentioned earlier, a poster already went into great detail regarding these issues.
Again, most of this information came from Wikipedia, and I would be more than willing to help with any questions you may have. This is an edit from the previous post, so one of the commenter, "MarkR" may also be of some help.