Development and application of the modular severe accident analysis program MOSAP

The integrated severe accident analysis program constitutes a cornerstone of nuclear reactor safety, functioning as a pivotal tool for predicting accident sequences, diagnosing critical failure nodes, and formulating evidence-based management strategies throughout the accident progression. It enables the simulation of extreme scenarios—ranging from core overheating to containment breach—without exposing operational facilities to risk, thereby playing an irreplaceable role in optimizing safety protocols and enhancing emergency response capabilities. However, the landscape of global nuclear technology collaboration shifted profoundly with the imposition of U.S.-China trade restrictions, which explicitly prohibited the transfer of advanced nuclear safety analysis software to China. This move disrupted existing technological dependencies and posed a critical challenge to China’s nuclear sector, as access to such software is indispensable for conducting independent safety assessments, validating novel reactor designs, and ensuring compliance with international safety benchmarks. The restriction underscored the urgency of technological self-reliance, highlighting that reliance on foreign tools could compromise both the development trajectory and security autonomy of China’s nuclear industry. To address this gap, the Nuclear Thermal-hydraulic Laboratory at Xi’an Jiaotong University leveraged nearly two decades of cumulative expertise in nuclear thermal-hydraulics, severe accident phenomenology, and computational modeling to develop a domestically tailored solution. The research team overcame formidable challenges in simulating complex physical phenomena—including core melting, molten material interactions with structural components, and radioactive material release—each requiring rigorous mathematical modeling and validation against empirical data. The outcome of this endeavor is MOSAP (MOdular Severe Accident Analysis Program), a groundbreaking software specifically engineered for pressurized water reactors (PWRs), which dominate China’s operational nuclear fleet. Endowed with complete intellectual property rights, MOSAP’s modular architecture enables high-fidelity simulation of full severe accident sequences, encompassing initiating events such as Station Blackout (SBO) and Small-Break Loss-of-Coolant Accidents (SBLOCA), as well as subsequent phases: core degradation and melting, redistribution of molten corium within the reactor pressure vessel’s lower head, vessel wall melt-through, and containment-level phenomena (e.g., hydrogen combustion, radioactive fission product transport). MOSAP has ushered in a new era of technological autonomy for China’s nuclear industry. It empowers nuclear power plants to conduct quantitative assessments of accident probabilities and consequences, facilitating targeted preventive measures. For instance, operators can simulate the efficacy of emergency core cooling systems under extreme conditions or evaluate containment structural integrity during postulated melt-through scenarios, thereby optimizing safety protocols. Additionally, the program supports the development of nuclear simulators for operator training, ensuring proficiency in responding to emergency scenarios. Beyond its immediate applications, MOSAP represents a milestone in China’s pursuit of self-reliance in high-end nuclear technology. It eliminates dependence on foreign software, positions China as a competitive player in global nuclear safety research, and strengthens its capacity to contribute to international collaborative initiatives—all while safeguarding the sustainable and secure development of its nuclear energy sector, a critical pillar of the global low-carbon transition.