Editing Three Mile Island accident (section)

===Depressurization of primary reactor cooling system===
Less than a minute after the beginning of the event, the water level in the pressurizer began to rise, even though RCS pressure was falling. With the PORV stuck open, coolant was being lost from the RCS, a [[loss-of-coolant accident]] (LOCA). Expected symptoms for a LOCA were drops in both RCS pressure and pressurizer level. The operators' training and plant procedures did not cover a situation where the two parameters went in opposite directions. The water level in the pressurizer was rising because the steam in the space at the top of the pressurizer was being vented through the stuck-open PORV, lowering the pressure in the pressurizer because of the lost inventory. The lowering of pressure in the pressurizer made water from the coolant loop surge in and created a steam bubble in the reactor pressure vessel head, aided by the decay heat from the fuel.<ref>Kemeny, p. 94.</ref> 

This steam bubble was invisible for the operators, and this mechanism had not been trained. Indications of high water levels in the pressurizer contributed to confusion, as operators were concerned about the primary loop "going solid", (i.e., no steam pocket buffer existing in the pressurizer) which in training they had been instructed to never allow. This confusion was a key contributor to the initial failure to recognize the accident as a LOCA<ref>{{cite book |last=Rogovin |first=Mitchell |title=Three Mile Island: a report to the commissioners and to the public. Volume I |date=January 1980 |publisher=U.S. Nuclear Regulatory Commission |location=Washington, D.C. |url=https://www.osti.gov/servlets/purl/5395798 |access-date=October 26, 2021 |page=16 |doi=10.2172/5395798 |osti=5395798 |quote=A more important factor contributing to the operators' failure to recognize that a LOCA is in progress is the pressurizer water level indicator. Their training on this particular equipment has taught the operators that the only credible check on the amount of coolant in the system is the indicator showing water level in the pressurizer. (In this Babcock & Wilcox reactor, there is no instrument for measuring, as a gas gauge does in an automobile, the amount of fluid in the reactor core portion of the coolant loop—or, stated more simply, the depth of water around the fuel rods.) If the pressurizer level remains high, the operators are not trained to anticipate that coolant water may be leaking out of the primary system. Indeed, the operator training at Met Ed, at B&W, even back in the navy, tells these men that the condition to avoid at all costs is 'going solid'—permitting the pressurizer to fill with water and thus losing the ability to regulate system pressure through the control of the pressurizer steam bubble. The training and the written emergency procedures of the operators never postulated a loss-of-coolant accident through the top of the pressurizer itself, as is happening now. With the relief valve stuck open, the steam bubble vanishes like a jinni out through the valve, and the coolant water right after it. The system pressure continues to be low—a sign of a loss-of-coolant accident. But the pressurizer water level indicator keeps getting higher. Why is this?}}</ref> and led operators to turn off the emergency core cooling pumps, which had automatically started after the PORV stuck and core coolant loss began, due to fears the system was being overfilled.<ref>{{cite book |last=Rogovin |first=Mitchell |title=Three Mile Island: a report to the commissioners and to the public. Volume I |date=January 1980 |publisher=U.S. Nuclear Regulatory Commission |location=Washington, D.C. |url=https://www.osti.gov/servlets/purl/5395798 |access-date=October 26, 2021 |pages=16, 17 |doi=10.2172/5395798 |osti=5395798 |quote=Going by the book as it was taught to them, however, the operators continue to read the pressurizer indicator in the old mode: The coolant level is rising; the system is going solid, for heaven's sake. Convinced by this logic that the system, indeed, is overloaded with coolant water, the operators override the emergency system and sharply reduce flow from the HPI pumps. It is a human intervention in the automatic chain of events not inconsistent with the operators' training, but it will have awesome consequences. At Zewe's direction, Operator Ed Frederick shuts down one HPI pump and throttles back the other one from a maximum of 400 gallons per minute (gpm) to about half that flow. Not only does he throttle HPI, Frederick also lifts the plug at the bottom of the reactor coolant system to maximize "letdown" through the normal "makeup and letdown system" that, like a swimming pool filtration system, constantly works to purify primary reactor coolant water. The effect of these two actions is to reduce to a trickle the amount of water being added to the system. This miserly flow rate, perhaps 25 gpm, will continue for the better part of the next 3 hours and is more than offset by the amount of coolant lost every minute through the stuck-open PORV.}}</ref><ref>{{cite book |last=Walker |first=J. Samuel |url=https://archive.org/details/threemileislandn00walk/page/76/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=76–77 |quote= |access-date=October 24, 2021}}</ref><ref name=upi>{{cite news |url=https://www.upi.com/Top_News/US/2019/03/28/Three-Mile-Island-clings-to-survival-40-years-after-1979-meltdown/1121553642219/?ls=1 |title=TMI clings to survival 40 years after 1979 meltdown |work=[[United Press International]] |date=March 28, 2019 |access-date=March 16, 2022}}</ref>

With the PORV still open, the pressurizer relief tank that collected the discharge from the PORV overfilled, causing the containment building [[sump]] to fill and sound an alarm at 4:11&nbsp;a.m. This alarm, along with higher than normal temperatures on the PORV discharge line and unusually high containment building temperatures and pressures, were clear indications that there was an ongoing LOCA, but these indications were initially ignored by operators.<ref>{{cite book |last=Rogovin |first=Mitchell |title=Three Mile Island: a report to the commissioners and to the public. Volume I |date=January 1980 |publisher=U.S. Nuclear Regulatory Commission |location=Washington, D.C. |url=https://www.osti.gov/servlets/purl/5395798 |access-date=October 26, 2021 |pages=17, 18 |doi=10.2172/5395798 |osti=5395798 |quote=(1) (Operators)... will dismiss two warnings from a temperature instrument showing relief valve discharge temperatures about 100 degrees above normal range. (These are incorrectly reported to Zewe to be about 50 degrees lower than they really are.) He attributes the discrepancy to the fact that the PORV had been leaking anyway, and to residual heat from the early discharge of steam from the PORV when it opened (supposedly) for just a few seconds. (2) At 4:14{{nbs}}a.m., with the accident sequence barely settling in, there are other conspicuous clues that the relief valve is still open. Continued discharge of coolant into the reactor coolant drain tank from the stuck-open relief valve causes the tank's pressure to increase. (3) When the pressure reaches 192 psi, the rupture disc at the top of the tank bursts. Zewe takes note of this around 4:20... With the rupture disc open, coolant from the stuck-open valve running into the reactor coolant drain tank overflows the tank onto the reactor containment building floor. (4) At 4:38{{nbs}}a.m. an auxiliary building operator reports that the automatic containment sump (floor drain) pumps are pumping this water into the next-door auxiliary building... (5) By 5:00{{nbs}}a.m., the temperature inside the containment building is up from 120°F to 170°F, and building pressure has increased from 0 to 2.5 psi—still another sign that the PORV is stuck open.}}</ref><ref name="Kemeny, p. 96">Kemeny, p. 96.</ref> At 4:15&nbsp;a.m., the relief diaphragm of the pressurizer relief tank ruptured, and radioactive coolant began to leak into the general [[containment building]]. This radioactive coolant was pumped from the containment building sump to an auxiliary building, outside the main containment, until the [[sump pump]]s were stopped at 4:39&nbsp;a.m.<ref name="Kemeny, p. 96"/>