|Fig. 1: Three Mile Island prior the accident. (Source: Wikimedia Commons)|
The Three Mile Island accident took place in the early morning of March 28, 1979 on the Susquehanan River, Pennsylvania (see Fig. 1).  There was only very small radioactive downfall detected from the nuclear power plant accident, thus there were no health consequences. The 2 million civilians living near the plant received a harmless average radiation dose of only 1 millirem over the natural level.  After the incident, approximately 50 000 people were evacuated from their homes.  Today, the power plant is permanently shut down and its fuel has been removed.
At approximately 4 a.m., the power plant experienced a failure in its non-nuclear secondary system. First, either an electrical or mechanical malfunction prevented the feedwater pumps from pumping water to the steam generators, which are responsible for removing heat from the reactor core.  As a result, the turbine generator shut down, causing the pressure in the primary to increase. This pressure triggered the pilot-operated relief valve to open. The power plant operators were unaware of these failures due to malfunctioning instruments in the control room. In fact, there was no instrument that indicated the cores water coverage, thus the operators had no way of knowing of what was occurring in the power plant. As a result, the staff made numerous poor decisions that led to even worse conditions.  Eventually, they lowered the amount of emergency cooling water being pumped into the primary system, causing the reactor core to overheat at 2400 K and ultimately melt. 
The Three Mile Island Accident has become a symbol for the anti-nuclear movement.  The accident solidified the nuclear power plant safety concerns of the general public, making it clear that new regulations were needed for the nuclear industry. Analysis of the accident identified serious flaws in the system, which lead to crucial changes in how the Nuclear Regulatory Commission regulates nuclear power plants. Since the accident, they've upgraded and strengthened the plant design and equipment requirements. Specifically, they've included fire protection, containment building isolation, and an automatic shutdown option in the new design. The reliability of equipment such as pressure relief valves and electrical circuit breakers, has bee improved as well.  They have also enhanced the emergency protocol in case of an emergency.  These are only two of the many changes that were made. These adjustments have diminished the risk to public health and safety. 
© Marc Joshua. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 M. Stencel, "A Nuclear Nightmare in Pennsylvania," Washington Post, 27 mar 99.
 "Three Mile Island Accident," U.S. Nuclear Regulatory Commission, February 2013.
 C. Hopkins, "Three Mile Island," Physics 241, Stanford University, Winter 2015.
 S. Sherman, "The Accident at Three Mile Island," Physics 241, Stanford University, Winter 2016.