Analysis of Transient Accident Characteristic in Supercritical Water Oxidation Reactor

Supercritical water oxidation (SCWO) is an emerging technology for treating organic waste, particularly suitable for the treatment of radioactive organic liquid waste. Utilizing supercritical water (temperature＞374.3 ℃, pressure＞22.1 MPa) as the working medium, SCWO enables the degradation of organic waste into small molecular compounds through oxidation reactions under high-temperature and high-pressure conditions. However, the core equipment in the reactor operates in a harsh environment and is susceptible to pipe blockage or material degradation due to long-term exposure to chemical corrosion and salt deposition. The operation involves multiple coupled physical phenomena such as exothermic chemical reactions and phase transitions across critical points. These complex factors may lead to flow instability in the reactor and its connected inlet and outlet piping, affecting normal operation and potentially causing non-design-basis accidents. Severe rupture incidents could result in the release of radioactive materials, which would compromise the operational reliability of the SCWO system and pose safety risks to personnel. This study focused on the dual-casing SCWO reactor designed by the China Institute of Atomic Energy for processing radioactive distillation residues. A risk identification analysis was conducted based on the reactor’s structural design, revealing that the inlet and outlet piping are highly vulnerable to breach incidents. Therefore, a rupture accident analysis was carried out on these pipelines. The transient variations of key parameters during sudden breaches under normal operating conditions were simulated using an enhanced version of the RELAP5 code. By comparing different sets of water property data and computational models, the capability of the RELAP5 code in capturing key transient parameters during rupture events was evaluated. Based on comprehensive analysis, more appropriate property tables and calculation models were selected. A sensitivity simulation matrix was designed considering various breach locations (top/bottom) and sizes (ranging from 0.1% to 1.5% of the cross-sectional area), covering the most extreme breach scenarios for the reactor’s inlet and outlet piping. Monitoring points were established at the breach positions to record the transient changes in pressure and leakage flow rate. The results reveal the quantitative influence of breach size and location on the evolution of accidents, demonstrating that different breach positions and dimensions significantly affect the progression of rupture events. The study quantifies the characteristics of rupture accidents under multi-physics coupling and proposes tiered emergency response strategies based on breach size and position for effective risk mitigation. This provides data support for optimizing reactor structural design and improving the activation logic of safety systems. Furthermore, the calculated parameters such as rupture leakage time and flow rate serve as a theoretical foundation for subsequent radiological risk assessments.