Severe accident simulation of Kuosheng NPP in reactor shutdown mode with MAAP5

The Modular Accident Analysis Program version 5 (MAAP5) is a code developed by Fauske & Associates, Inc. for simulating severe accident progression in light water reactors. Kuosheng Nuclear Power Plant (KSNPP) is currently in the early stage of decommissioning, during which the reactor pressure vessel (RPV) remains open with fuel still present in the core. This study presents the development, steady-state verification, and severe accident simulations of a MAAP5.06 model specifically configured for shutdown open-vessel conditions. The model was constructed using actual plant parameters and verified against both normal operation and shutdown steady-state conditions. Three severe accident scenarios were investigated: Station Blackout (SBO), Loss-of-Coolant Accident (LOCA), and LOCA with recovery injection (LPCI/LPCS). Results demonstrate that the upgraded MAAP5.06 model successfully reproduces thermal-hydraulic behavior, water level progression, cladding temperature evolution, and hydrogen generation. The developed model provides a technical basis for emergency planning, risk-informed decision-making, and safety evaluations during decommissioning.