While most people were still asleep just before 4 a.m. Wednesday, March 28, 1979 — 46 years ago today — a malfunctioning valve set off what would become the most serious accident in U.S. commercial nuclear power plant history. 

The incident at the Three Mile Island nuclear power station in Londonderry Township, Dauphin County, according to the Nuclear Regulatory Commission, happened when “a combination of equipment malfunctions, design-related problems and worker errors led to TMI unit 2’s partial meltdown and very small off-site releases of radioactivity.” 

TMI was de-activated in 2019 , but after 45 years is on track to reopen by 2028 with a new name Intent on fueling artificial intelligence (AI) data centers.