On March 28, 1979, Three Mile Island (TMI) had a reactor malfunction. In a Fission-type nuclear reactor, water that washes the nuclear fuel elements is heated by energy making reactions which take place in the fuel core. This transfers heat to a separate water loop known as the second watery, which works the turbines that create electricity.

In the early morning, the secondary water pump stopped flowing. As a result, because the primary water could not transfer heat, the primary heated up. Along with that, the backup cooling system failed. It is believed that one of the workers turned off the system after it had activated. An overflow of hot water and steam was released from the cement- enclosed reactor vessel, to the floor of the building.

The fission reaction splits atoms into 2 or more other atoms. This releases neutrons. They bombard the primary and turn it into "tritiated" water. This means the hydrogen atoms collect extra neutrons. Also, small amounts of metal ions can come off the pipes. Neutron bombardment of these ions make them radio-active.
Another thing is that if any of the fuel elements get damaged, fission products might be released into the water system, which steams up into the atmosphere.

After the accident, no law was enforced, however increased awareness in citizens rose. There has not been an application to build a new Nuclear Power Plant since the accident.

The nuclear Regulatory Commission (NRC) makes sure that citizens are safe from the affects of nuclear power. The accident caused the NRC to tighten and heighten its regulatory oversight.

Pawelski, Lisa A. "TMI Defects Spelled out." Editorial. TMI Defects Spelled out [Carlisle] 12 Apr. 1979: 6+. Three Mile Island. Dickinson College. Web. 15 Feb. 2011. <http://www.threemileisland.org/resource/item_detail.php?item_id=00000015>.

"NRC: Backgrounder on the Three Mile Island Accident." NRC: Home Page. Web. 16 Feb. 2011. <http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/3mile-isle.html>.