Thermal-Hydraulic Transient and Containment Response Analysis of a VVER-1200 Reactor Under Beyond-Design-Basis ATWS Concurrent with Dual LBLOCA and Station Blackout
Year: 2026
Type: preprint
Abstract: Background: An Anticipated Transient Without Scram (ATWS) represents one of the most severe conditions in nuclear reactor operations, escalating any concurrent malfunction into a Beyond Design Basis Accident (BDBA). When coupled with dual Large-Break Loss of Coolant Accidents (LBLOCA), station blackout, and initial fuel failure, the thermal-hydraulic stability and containment integrity of the reactor are severely compromised. Materials and Methods: In this study, the transient behavior and safety response of a Russian Gen-III+ VVER-1200 reactor were simulated using the IAEA-recommended Personal Computer Transient Analyzer (PCTran). An unmitigated accident scenario featuring simultaneous 1000cm2 breaks in both the hot and cold legs, failure of the SCRAM mechanism, a total loss of AC power, and 5% fuel failure at power was evaluated over a 300-second simulation window. Model performance was benchmarked and validated against Final Safety Analysis Report (FSAR) core flow data. Results: The simulation demonstrates that the Departure from Nucleate Boiling Ratio (DNBR) dropped below 1.0 within 30 seconds, triggering an immediate boiling crisis due to sustained core power. Complete steam voiding across the reactor core occurred in 80 seconds. Consequently, reactor building pressure exceeded the 5-bar design threshold in only 90 seconds and reached 14.93 bar at 300 seconds, while containment temperatures peaked at 193.7 °C. Although peak fuel and cladding temperatures remained within design limits to prevent immediate core meltdown, the rapid containment over-pressurization severely restricts available operator response windows, posing a critical containment failure hazard. Conclusion: Simultaneous ATWS, dual-leg LBLOCA with fuel failure, and station blackout in a VVER-1200 induce a critical boiling crisis within 30 seconds and full core voiding in 80 seconds due to unmitigated core power. Although passive systems prevent immediate fuel and cladding meltdown, containment pressure breaches the 5-bar design threshold in just 90 seconds, highlighting that containment over-pressurization rather than core degradation is the most immediate threat requiring enhanced passive mitigation.
Source: Preprints.org
Language: English
Subfield: Nuclear Energy and Engineering
Field: Energy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7204807741