VOLUME 19 (Supplement)

Evaluation of reactor safety response to multiple steam generator tube rupture scenarios in a generic PWR simulator

A Multiple Steam Generator Tube Rupture (MSGTR) constitutes a Design Extension Condition (DEC) in pressurized water reactors (PWRs), introducing challenges related to primary-to-secondary mass transport and potential radiological risks. While thermal-hydraulic behavior under single-tube failures is extensively studied, a critical gap remains in the literature concerning transient plant response during prolonged, unmitigated multi-tube ruptures. This study addresses this gap by performing transient simulations using a 1400 MWe Generic PWR (GPWR) simulator. Three MSGTR scenarios of increasing severity were evaluated in a single affected steam generator over a four-hour window: 5 tubes, 10 tubes, and 131 tubes, assuming zero operator intervention. The results indicate that the automatic safety protection systems effectively moderate the accident sequences. The volume control tank provides an initial coolant inventory buffering effect prior to automatic safety injection from the refueling water storage tank. This highly borated water injection introduces negative reactivity, initiating a steady reactor power coastdown. At the time of reactor trip, thermal power had decreased to approximately 3% of nominal power in all three cases, the majority of the remaining power was generated by decay heat. The maximum fuel centerline temperature decreases in strong correlation with this power coastdown, stabilizing at approximately 300°C. However, unmitigated auxiliary feedwater injection leads to complete flooding of the steam generators. To enhance control room diagnostics, a differential level indicator was proposed, demonstrating its effectiveness as an operator aid for rapid identification of the leaking steam generator.