Guidelines for situation recognition The operators were not aware of the valve being stuck open. In an maintenance procedure, they blew compressed air to clean the water. The water found its way through the open valve into an instrument air line.
The control of the system did not function properly, due to the contamination, resulting in turning off the primary water pumps.
With the steam generators no longer receiving feedwater, heat and pressure increased in the reactor coolant system, Within eight seconds, control rods were inserted into the core to halt the nuclear chain reaction.
The reactor continued to generate decay heat and, because steam was no longer being used by the turbine, heat was no longer being removed from the reactor's primary water loop
Once the secondary feedwater pumps stopped, three auxiliary pumps activated automatically. However, because the valves had been closed for routine maintenance, the system was unable to pump any water.
The loss of heat removal from the primary loop and the failure of the auxiliary system to activate caused the primary loop pressure to increase, triggering the pilot-operated relief valve at the top of the pressurizer – a pressure active-regulator tank – to open automatically. The relief valve should have closed when the excess pressure had been released, and electric power to the solenoid of the pilot was automatically cut, but the relief valve stuck open because of a mechanical fault. The open valve permitted coolant water to escape from the primary system, and was the principal mechanical cause of the primary coolant system depressurization and partial core melt that followed.
Because of the lack of a dedicated instrument to measure the level of water in the core, operators judged the level of water in the core solely by the level in the pressurizer.
The exceptional situation was indicated by exceptional values of many parameter, each generating an alarm. The operators were not able to identify the single most important of them, to focus on the proper recovery procedures.
Guidelines for situation recognition
Updated on 18 May 2016.