Editing Three Mile Island accident (section)

===Background===
[[File:Tmi-2 schematic.svg|thumb|upright=2.2|A simplified schematic diagram of the TMI-2 plant<ref name="FactSheet">{{cite web |url=https://www.nrc.gov/reading-rm/doc-collections/fact-sheets/3mile-isle.html |title=Fact Sheet on the Three Mile Island Accident |publisher=U.S. Nuclear Regulatory Commission |access-date=November 25, 2008}}</ref>]]
In the night hours before the incident, the TMI-2 reactor was running at 97% power while the companion TMI-1 reactor was shut down for refueling.<ref>{{cite book |last=Walker |first=J. Samuel |url=https://archive.org/details/threemileislandn00walk/page/71/mode/1up |title=Three Mile Island: A Nuclear Crisis in Historical Perspective |date=2004 |publisher=University of California Press |isbn=0-520-23940-7 |location=Berkeley, California |page=71 |quote=TMI1 was not operating because it had been shut down for routine refueling. |access-date=October 18, 2021}}</ref> The main chain of events leading to the partial [[Nuclear meltdown|core meltdown]] on Wednesday, March 28, 1979, began at 4:00:36&nbsp;a.m. EST in TMI-2's secondary loop, one of the three main water/steam loops in a [[pressurized water reactor]].<ref>{{cite book |last=Walker |first=J. Samuel |url=https://archive.org/details/threemileislandn00walk/page/72/mode/1up |title=Three Mile Island: A Nuclear Crisis in Historical Perspective |date=2004 |publisher=University of California Press |isbn=0-520-23940-7 |location=Berkeley, California |pages=72 |quote=The chain of events that set off the severe accident at TMI-2 and melted a substantial portion of its core began innocently enough at 4:00 A.M. |access-date=October 23, 2021}}</ref>

The initial cause of the accident happened 11 hours earlier, during an attempt by operators to fix a blockage in one of the eight [[condensate polisher]]s, the sophisticated filters cleaning the secondary loop water. These filters are designed to stop minerals and other impurities in the water from accumulating in the steam generators and to decrease corrosion rates on the secondary side.

Blockages are common with these resin filters and are usually fixed easily, but in this case, the usual method of forcing the stuck resin out with compressed air did not succeed. The operators decided to blow compressed air into the water and let the force of the water clear the resin. When they forced the resin out, a small amount of water forced its way past a stuck-open check valve and found its way into an instrument [[air line]]. This would eventually cause the [[feedwater pump]]s, condensate booster pumps, and condensate pumps to turn off around 4:00&nbsp;a.m., which would, in turn, cause a [[turbine trip]].