A misleading alarm is an alarm that guides the operators to take a wrong action.
On March 28, 1979, following overheat due to loss of coolant, the PORV of the TMI reactor opened automatically, to release the high pressure, but was stuck open, due to a mechanical failure. The open valve permitted coolant water to escape from the primary system, and the pressure in the pressurizer reduced more than desired.
At this stage, the operators were expected to notice the indication of the PORV state. Unfortunately, the indication was misleading. Due to a design mistake, it indicated that a control command was sent to PORV, but not the real state of the PORV. Therefore, according to the procedures, they disconnected the backup pump of cooling water, which increased the overheating. Unfortunately, in this case, the operators relied on the indication, and did not check the actual state of the PORV, ending up in the most celebrated nuclear incidence in the USA.
Updated on 13 Mar 2017.