This is the text is an adapted and shortened version of the NRC report (see reference for full text and pictures, video). It describes the evolution of the accident before hydrogen was formed: The plant experienced a failure in the secondary, non-nuclear section of the plant (one of two reactors on the site), which prevented the main feed water pumpscomponent from sending water to the steam generators that remove heat from the reactor core. This caused the plant's turbine-generator and then the reactor itself to automatically shut down. The pressure in the primary system began to increase. In order to control that pressure, the pilot-operated relief valve opened, located at the top of the pressurizer. The valve should have been closed when the pressure fell to proper levels, but it became stuck open. Instruments in the control room, however, indicated to the plant staff that the valve was closed. As a result, the plant staff was unaware that cooling water in the form of steam was pouring out of the stuck-open valve. When the alarms want off flashing warning lights, operators did not realize that the plant was experiencing a loss-of-coolant accident. Because of other misleading or inadequate information, they concluded that there pressurizer water level was high enough and that the core was properly covered with water too. That wasnt the case and took a series of actions that uncovered the core, such as turning off the reactor coolant pumps which had started to vibrate. They also reduced the flow of water from the emergency cooling water being pumped into the primary system. Without the reactor coolant pumps circulating water and with the primary system starved of emergency cooling water, the water level in the pressure vessel dropped and the core overheated, creating the condition for the Zircaloy steam reaction.