On Tuesday the 25th of July 2006 at around 13:15, Forsmark-1 nuclear power plant experienced a loss of external power event, initiated by a short circuit in the offsite 400 kV switchyard. Due to voltage and frequency fluctuations that followed, together with additional component failures, two of the four auxiliary diesel generators did not start, causing loss of power in 2 of four redundant trains existing in the power plant. The loss of power in trains A&B resulted in reactor shutdown and abnormal intervention of safety systems. After 20 minutes, the water level inside the Reactor Pressure Vessel (RPV) decreased to 1,9 m above the reactor core, and the pressure inside the RPV decreased to 1,5 MPa. The aim of the present study is to evaluate the capabilities of U.S. NRC codes RELAPS and MELCOR to simulate the Forsmark-1 event, and then to reconstruct the sequence of the event based on the known behavior of the plant systems, such as activation of depressurization valves. To examine the safety margin, it is of interest to address "what if" questions related to this event, such as i) what if the operator would delay the recovery of the two failing diesel generators, and ii) what if all four diesel generators would fail. The results show that both RELAP5 and MELCOR codes are able to reproduce the system thermal-hydraulic behavior during such an event. The intervention of emergency cooling systems and effort of operators to start the remaining two auxiliary generators have prevented the core from becoming uncovered. The analysis also shows that even in case of failure of all four auxiliary generators, the timely action of the plan operator, as demonstrated in the action during the event, would prevent a core damage from occurring.