he accident began about 4 a.m. on Wednesday, March 28, 1979, when the plant experienced a failure in the secondary, non-nuclear section of the plant (one of two reactors on the site). Either a mechanical or electrical failure prevented the main feedwater pumps—component (1) in the animated diagram)—from sending water to the steam generators (2) that remove heat from the reactor core (3). This caused the plant's turbine-generator (4) and then the reactor itself to automatically shut down. Immediately, the pressure in the primary system (the nuclear piping portion of the plant shown in orange) began to increase. In order to control that pressure, the pilot-operated relief valve (5) opened. It was located at the top of the pressurizer (6). The valve should have 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. As alarms rang and warning lights flashed, the operators did not realize that the plant was experiencing a loss-of-coolant accident.
Other instruments available to plant staff provided inadequate or misleading information. During normal operations, the large pressure vessel (7) that held the reactor core was always filled to the top with water. So there was no need for a water-level instrument to show whether water in the vessel covered the core. As a result, plant staff assumed that as long instruments showed that the pressurizer water level was high enough, the core was properly covered with water too. That wasn't the case.
Unaware of the stuck-open relief valve and unable to tell if the core was covered with cooling water, the staff took a series of actions that uncovered the core. The stuck valve reduced primary system pressure so much that the reactor coolant pumps (8) started to vibrate and were turned off. The emergency cooling water being pumped into the primary system threatened to fill up the pressurizer completely—an undesirable condition—and they cut back on the flow of water. 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.
