 On March 28, 1979, the United States suffered its most serious commercial nuclear power plant accident at the Three Mile Island facility near Harrisburg, Pennsylvania. The accident released very small amounts of radioactivity and had negligible effects on public health and the environment. But it brought about sweeping changes at the Nuclear Regulatory Commission. The NRC upgraded its regulations related to everything from operator training, plant design and operations, and emergency response planning. The lessons of Three Mile Island still resonate today. Hi, I'm Tom Wallach, the Historian for the U.S. Nuclear Regulatory Commission. I study nuclear power's history for its relevance to the NRC today and to capture important insights for future generations. Three Mile Island, which we usually just call TMI, taught the NRC a lot. The accident started around four o'clock in the morning when pump failures and improperly closed valves prevented the steam generators from removing heat produced by the chain reaction in the nuclear reactor. Removing this heat is essential to prevent reactor damage and cooling water is pumped through the reactor and the steam generators to do so. When the cooling water stopped flowing to the steam generators, the reactor automatically shut down just as it was designed to do in such a situation. Unfortunately, relief valves that helped lower pressure in the reactor didn't close properly. Pressure continued to drop and cooling water escaped from the reactor. Instruments that should have helped the reactor operators understand the situation provided confusing information, including false indications that the relief valves were closed. As a result, the operators did not understand why pressure was falling and some of their subsequent actions made matters worse when they mistakenly turned off the reactor coolant pumps. The fuel began to overheat. Eventually, the nuclear fuel in the reactor overheated so much the zirconium cladding, long metal tubes that hold the nuclear fuel pellets disintegrated and reacted with the steam to create hydrogen gas. The fuel pellets inside tumbled out and began to melt at over 5,000 degrees Fahrenheit. It was later determined that about half the uranium pellets melted during the early stages of the accident. The NRC dispatched a team of experts as soon as news of the accident reached authorities. Other agencies, such as the Department of Energy and the Environmental Protection Agency also mobilized response teams. By midday, TMI's owner and the Department of Energy sampled radioactivity in the atmosphere above the plant. By evening, the operators had restored cooling to the core and the reactor appeared stable. But within a short time, new worries emerged about the hydrogen created when the zirconium cladding reacted with the steam. A large bubble of hydrogen was lodged at the top of the pressure vessel. The steel container that holds the reactor fuel. Experts worried that the hydrogen bubble might explode, rupturing the pressure vessel and the containment building allowing radiation to escape. The state's governor advised pregnant women and young children within a 5-mile radius of the plant to leave the area. The crisis ended when experts determined on Sunday, April 1st, that the bubble could not burn or explode because of the absence of oxygen in the pressure vessel. Today, the TMI Unit 2 reactor is permanently shut down. The reactor cooling system is drained, the radioactive water has been decontaminated and evaporated, the radioactive waste, reactor fuel and debris from the core have been shipped off site. In the months after the accident, thousands of environmental samples of air, water, vegetation, soil and food were collected. Thorough assessments by respected organizations concluded the actual release of radiation was small and had negligible effects on the physical health of individuals and the environment. Nevertheless, the NRC made extensive regulatory changes. New requirements aimed at reducing human errors affected plant design, human performance standards and fitness for duty programs. The NRC also expanded incident response staffing, emergency planning and its resident inspector program. The vigilance of the NRC today can be directly tied to the events of 33 years ago. That's what makes history so important. It's not just about events of the past, but also how it affects the present and the future. Thanks for watching.