It is designed to ensure that when there is a LOCA the water in the core remains at a safe level. Operators observe that the water level in the Primary System is rising while the pressure is decreasing. When they observe that the water level in the core is rising the operators shut off the EIW system. Water level in the core still appears to be rising. In actuality the water level in the core is dropping and turning off the EIW increases the amount of steam being produced by the reactor core.
Operator notices that the valves for the secondary loop backup pumps are off.
8 minutes into the accident the closed valve is noticed by an operator. The instrument checking the radiation has not registered an alarm, and the gauges in the control room are wrongly indicating that the water level is up. Primary loop pumps start to shake violently. Steam produced by the lack of cooling water in the core passes through the primary loop pumps and causes them to shake. Remaining two pumps in the primary loop turn off.