It was 20 years ago today that the world's worst nuclear accident occurred at the Chernobyl nuclear power plant in the Ukraine, and these past few months the event has been continued source of morbid fascination for me.

The BBC has a very good summary of the events that caused the disaster. But the fact is that the whole event was a monumental edifice of human error that it would be impossible to explain the entire scenario in any way which would qualify as succinct. Operation, design, and disaster management all bear the earmarks of Soviet halfassedness.

• Positive void coefficient: In a nuclear reactor which uses water as a coolant, steam bubbles can form in the system. When this happens, less cooling takes places. Also, the nuclear reaction speeds up because water absorbs neutrons better than steam. Both these effects raise the temperature inside the reactor, which creates more steam. A feedback loop rapidly occurs which can lead to a steam explosion, which happened in the Chernobyl accident. (read more about void coefficients)
   
• Graphite tipped control rods: In nuclear reactors, control rods (which absorb neutrons) are inserted into the reactor to slow down the reaction, and removed to accelerate it. The boron carbide control rods in Chernobyl's RBMK reactor were tipped with graphite, which is transparent to neutrons. In an emergency situation, a nuclear operator will likely order the full reinsertion of all control rods to halt nuclear reaction. In the case of the original RBMK, performing such a procedure would initially speed up the reaction.
   
• No containment: The majority of nuclear reactors worldwide are incased within steel and concrete containment buildings which are several feet thick. Not only can they protect the reactor from external attack, but in the event of catastrophic events, such as meltdown, the reactor building is meant to protect the outside world from any contamination. Original Soviet reactors had no containment structures. When the steam explosion occurred at Chernobyl, the roof was blown off. The fresh oxygen and intense heat sparked a fire, and smoke lofted nuclear materials high into the atmosphere.
   
• Underqualified personnel: The plant manager and chief engineer of the plant were only familiar with conventional plants. The deputy chief engineer had some experience operating the reactors used on nuclear subs, and claimed that instability of the RBMK reactor designed was never explained to them.
   
• Violation of safety procedures: The plant operators engaged in a ridiculous experiment to operate the reactor's safety systems on minimal power. Several safety regulations were violated.

It goes on, too. Wikipedia has an excellent article on the subject, as usual. The world nuclear association has an explanation of the RBMK reactor's deficiencies. There's also the accounts of journalist Mary Mycio's travels throughout the 30KM exclusion zone around the site. Some of the photos are pretty amazing. I picked up National Geographic's April issue on the subject. I encourage anyone with an inkling for science to have a read through some of these links. It's important to understand the events. Nuclear energy is no toy, and failure to handle it with the utmost seriousness can have dire consequences which effect both present and future generations.