A serious event no one had ever remember happening. Conflicting and uncertain information on how serious of a problem existed. Stores and shops nearly empty and people worried about their jobs. On March 28, 1979, Three Mile Island Unit 2, a pressurized water reactor (PWR) in Pennsylvania experienced a partial core meltdown. TMI-2 is the worst disaster experienced at a US commercial nuclear plant. A small release of radiation resulted in a voluntary evacuation of the surrounding area. Ultimately, a combination of design errors, equipment malfunctions and operator errors combined to result in the meltdown.
It was a pretty routine night shift, with operators monitoring the plant running at 97% power, when, around 4 AM, the pumps feeding water to the steam generators tripped off line due to a minor malfunction in their circuitry. Auxiliary feedwater pumps, a backup for the main system, started but did not pump water into the steam generators since the outlet valves were shut for maintenance. This was one of the first errors leading to the accident since Nuclear Regulatory Commission (NRC) regulations require the unit to be shutdown if all auxiliary pumps are unavailable. Since the steam generators where no longer cooling the reactor’s cooling water, reactor pressure and temperature increased, causing the reactor to scram (shutdown) as designed. Even though it is shutdown, a reactor still generates decay heat for a while and must be removed. As a result, reactor pressure increased and a safety relief valve in the reactor cooling system opened, as designed, to relieve pressure. This ultimately set into action the chain of events that resulted in core meltdown
High pressure pumps (HPCI) began pumping cooling water into the reactor as designed. At this point, the system was operating as designed, except for the pressure relief valve which had stuck open, resulting in cooling water being released from the cooling system via the pressurizer, a component designed to maintain reactor coolant pressure. The valve’s position indicator, however, showed it was closed since it only measured power to the valve and not actual position. Operators even verified it was closed by checking temperatures downstream, not realizing the low temperature did not indicate the valve closed but was exactly what it should be for steam rapidly expanding. Since the reactor cooling system is under high pressure, the drop in pressure resulted in water expanding and giving operators the impression, since they believed the relief valve was closed, that the system was too full of water and in danger of over pressurizing. This resulted in operators shutting off the HPCI pumps, removing the only source of water to replace that being lost. This was the beginning of the end for TMI-2.
Operators were trained not to allow the pressurizer to fill and over pressurize the plant and risk rupturing the reactor vessel or piping. As the cooling water boiled away, the reactor’s core began to uncover. A PWR requires the fuel rods to be covered with water to adequately cool them, and as the uncovered the overheated and began to melt, releasing radioactive material into the coolant, along with hydrogen, and eventually into the reactor containment. The hydrogen release resulted in a hydrogen “burn” in containment, as evidenced by a sudden sharp pressure increase. The containment held, although later efforts to move the gases resulted in a release to the environment, triggering a voluntary evacuation. The release material was short lived, biological inert noble gases that resulted in no measurable health effects. Nevertheless, the release caused concern and an estimated150,000 people left the immediate area.
Operators eventually diagnosed what was happening, but the damage was already done. TMI 2 would never operate again and become a symbol of the nuclear industry. The event was study and changes made to prevent reoccurrence. Movies such as The China Syndrome were hits, although a core meltdown would wind up in the South Pacific if it made its way through the Earth, not China since the US and China are in the same hemisphere. Saturday Night Live did a classic skit as well. TMI, along with the severe cost overruns when building plants, ended nuclear industry construction in the US. Only recently are any new plants being built, and the costs have soared to the point where several were cancelled, and the remaining ones may not be economically viable.
https://www.world-nuclear.org/information-library/safety-and-security/sa...
https://www.nrc.gov/reading-rm/doc-collections/fact-sheets/3mile-isle.html
