[dreadgeek]

So, the two nuclear events that occurred that most people remember are TMI and Chernobyl. These were VERY different kinds of events caused by VERY different causes and by the design of both plants.

TMI was a less severe event than Chernobyl for two reasons:

1) TMI-2 (the reactor that had the problem) was in a containment building.
2) TMI-2 was a pressurized water reactor which are safer.

So what happened at TMI-2 was that a pump failed, then the main pumps feeding water into the system failed. This caused the reactor to SCRAM, as it was designed to. However, just as a pot stays hot for a little while after you remove it from flame, a reactor core stays hot for a while after the control rods are inserted into the pile. This is called decay heat. So the reactor still needed to be covered. TMI-2's operators violated an operating rule and had all three auxiliary pumps shut down for maintenance (the NRC requires that if you're doing this, you shut down the core). Because the pumps couldn't get water into the reactor, steam began to build up in the containment vessel. A valve got stuck open and this allowed coolant (water) to escape which caused the core to be partially uncovered.

To make matters worse, the human operators misinterpreted an indicator light on the stuck valve. The design of the light only recorded if power was going to the solenoid for the valve, NOT the actual valve position. So the operators interpreted the light being out as meaning that the valve was closed.

At this point, pressure in the reactor was dropping. The higher the pressure, the higher the boiling point of water. With the pressure reducing the coolant started to boil off, causing a buildup of steam. A hydrogen bubble formed but fortunately there was no oxygen in the containment vessel which meant that the hydrogen couldn't ignite or explode. To bring the reactor under control, the operators released steam (which was controversial at the time) and used other mechanisms which I won't belabor to shrink the hydrogen bubble.

As it turns out, even though people treat TMI as a truly horrific event, more things went right than went wrong. Even with a cascade of mechanical failures, exacerbated by human errors, there was no significant release of radiation to the atmosphere. Several epidemiological studies have been done in the last nearly 40 years and found no higher incidence of cancers in the area surrounding the TMI facility than would be otherwise expected.

Chernobyl was a very different kind of event:

During a test, one of the reactors experienced a power spike. This caused lots of bubbles to form and steam pressure to build up in the reactor core. This prevented water from getting into the system to cool it down. There were then two explosions which damaged the fissile pile, preventing the control rods from being fully inserted, which led to a second explosion. This explosion tore the roof off the building, set fire to an adjacent reactor building and ruptured the containment vessel. At this point the whole core was aflame.

Part of why Chernobyl went SO bad is that the containment vessel (which is metal) was not in a containment building. It was in a normal, industrial building, built to Soviet building standards at the time.