Early phase behavior of steam line break accidents in the VVER-1200 reactor: Effect of break location, AC power loss, and break size

This study investigates the early-phase (0–300 s) behavior of steam line break (SLB) accidents in a VVER-1200 pressurized water reactor, with and without AC power availability, using the PCTRAN simulation code. Break location and power supply status are treated as the primary governing parameters. Eight comparative accident scenarios are developed to represent different break configurations. Reactor building pressure and peak fuel cladding temperature (PCT), which are critical safety indicators, are evaluated as time-dependent responses. The results demonstrate that break location and AC power availability exert the dominant influence on containment pressure evolution. SLB events occurring inside the reactor building lead to pronounced pressure increases, particularly under AC power-loss conditions, whereas breaks outside the building maintain pressure close to atmospheric levels. Across all investigated scenarios, peak fuel cladding temperatures remain below safety limits, indicating that natural circulation and passive safety systems effectively sustain core cooling even during power loss. Overall, the findings confirm the high early-phase safety margin of the VVER-1200 design against SLB accidents and highlight the decisive role of system configuration in transient safety assessment.