Editing Three Mile Island accident (section)

===Partial meltdown and further release of radioactive substances===
At about 5:20{{nbsp}}a.m., after almost 80&nbsp;minutes with a growing steam bubble in the reactor pressure vessel head, the primary loop's four main reactor coolant pumps began to [[cavitation|cavitate]] as a steam bubble/water mixture, rather than water, passed through them. The pumps were shut down, and it was believed that natural circulation would continue the water movement. Steam in the system prevented flow through the core, and as the water stopped circulating it was converted to steam in increasing amounts. Soon after 6:00{{nbsp}}a.m., the top of the reactor core was exposed, and the intense heat caused a reaction to occur between the steam forming in the reactor core and the [[Zirconium alloys|zircaloy]] nuclear [[Nuclear fuel|fuel rod]] cladding, yielding [[zirconium dioxide]], [[hydrogen]], and additional heat. This reaction melted the nuclear fuel rod cladding and damaged the fuel pellets, which released radioactive isotopes to the reactor coolant and produced hydrogen gas that is believed to have caused a small explosion in the containment building later that afternoon.<ref>Kemeny, p. 99.</ref>

[[File:Graphic TMI-2 Core End-State Configuration.png|thumb|upright=1.4|A NRC graphic of the TMI-2 core end-state configuration.
{{Ordered list | 2B inlet | 1A inlet | cavity | loose core debris | crust | previously molten material | lower plenum debris | possible region depleted in uranium
| ablated incore instrument guide | hole in baffle plate
| coating of previously-molten material on bypass region interior surfaces
| upper grid damage }}  ]]

At 6:00&nbsp;a.m. there was a shift change in the control room. A new arrival noticed that the temperatures in the PORV tail pipe and the holding tanks were excessive, and used a backup—called a block valve—to shut off the coolant venting via the PORV, but around {{convert|32000|USgal|L|abbr=on}} of coolant had already leaked from the primary loop.<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=19 |doi=10.2172/5395798 |osti=5395798 |quote=... checks over the reactor coolant instruments and quickly concludes there is a steam bubble in the "hot legs"—the pipes leading from the reactor to the steam generators—of the reactor coolant loop. With the coolant system pressure so low, there must be a bubble somewhere else, expanding and forcing water into the pressurizer. "I went to the computer," he will later testify, "and punched out the temperatures on both the [safety] valves and the electromatic (''another term for the PORV'') relief valves." Based on readings showing the relief valve discharge line some 30 degrees hotter than the safety valve discharge lines, Mehler dismisses the pressurizer level reading and moves to a fresh conclusion: The PORV is leaking. Mehler orders the PORV block valve closed... has arrived at exactly the right decision just 20 minutes after coming on the scene fresh from the outside.}}</ref><ref>{{cite book |last=Walker |first=J. Samuel |url=https://archive.org/details/threemileislandn00walk/page/78/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=78 |quote=Finally..., a shift supervisor who had just arrived at the plant..., concluded from the pressure and temperature readings in the primary loop that the PORV was at least partially open. He was not certain of what was happening in the core, but he reasoned that no harm and perhaps some benefit could be achieved by shutting the offending relief valve. At 6:22{{nbs}}a.m., he ordered that a backup for the PORV, called a block valve, be closed. By that time, about thirty-two thousand gallons of coolant, more than one-third of the volume in the primary system, had flowed out of the stuck-open PORV. None of the staff in the control room took action to determine how long the PORV had been open or to replace the coolant that had escaped. Closing the block valve was a sound decision but insufficient in itself to prevent the severe damage to the core that leaving the PORV open for about two hours and twenty minutes had caused. |access-date=October 24, 2021}}</ref> It was not until 6:45{{nbs}}a.m., 165 minutes after the start of the problem, that radiation alarms activated when the contaminated water reached detectors; by that time, the radiation levels in the primary coolant water were around 300 times expected levels, and the general containment building was seriously contaminated with radiation levels of 800&nbsp;[[rem (unit)|rem]]/[[hour|h]].