Early morning on March 28, 1979, Unit 2 of the 900-MW reactor at the TMI-2 plant at Three Mile Island in Pennsylvania experienced a partial core meltdown. As a result, between 13 and 43 million curies of radioactive krypton gases were released, 90% of the fuel rod cladding was destroyed and half the core was melted. Radiation given off by this disaster reached 83 millirem (cancer inducing levels), but was luckily limited to the offsite and did not spill into the community at large doses.
The operator’s workers on an upstream demineralization caused 1 or more HCV-1 valves to close by accidentally bringing water into the air system. Bad design was also a fault here, but the operators failed to realize that the valves had been closed. This created a domino effect starting with the main feed water pumps stopping which further lead to the temperature and pressure in the reactor to rise. This chain reaction ultimately lead to a discharge of steam and water to the quench tank that ruptured the relief diaphragm of the radioactive coolant. At this point, the time was 4:11 AM, nearly 10 minutes since the fatal error that caused all of this, yet it was too late to repair and radiation was emitted. While there were no direct deaths that could be traced to the incident like Chernobyl or Fukushima, Three Mile Island will forever live in infamy and negligible health effects were seen by the surrounding populace.
In order to control complex processes such as a nuclear plant, it must be fully understood and instrumentation reliability must be monitored. Through the use of multiple of sensors and computerized control and relegation, security can be assured in order to withstand severe accidents.
If SCADA system software was put in place at Three Mile Island such an event would never have occurred as the SCADA system software would have been alerted and alarms would have sounded. All levels would be monitored by the SCADA system software, and the security of the operation sound. Knowing what we know today, if Three Mile Island enacted automated alarm software such as SCADA system software, history could be rewritten.