[en] Highlights: • A passive cavity injection system is suggested for Chinese improved 1000 MWe PWR. • The system is assessed with Risk Oriented Accident Analysis Methodology. • Three factors are discussed as sensitivity study for the assessment. • Assessment result shows an acceptable high successful probability. - Abstract: After the Fukushima Daiichi accident, the Chinese National Nuclear Safety Administration requires enhancements of safety and mitigation capability under severe accidents for nuclear power plants (NPPs). In-vessel retention (IVR) strategy is proposed to mitigate the consequences of severe accidents for pressurized water reactors (PWR) to satisfy the requirements. Therefore, a proposal with passive cavity injection system (PCIS) is designed for Chinese improved 1000 MWe PWR for implementation of IVR strategy. Besides, to condense the steam generated in the cavity head, a passive heat exchange system (PHES) is also designed for passive containment cooling. To evaluate the coolability of the PCIS, a computer code is developed to assess the effectiveness of IVR strategy based on the integrated risk oriented accident analysis methodology (ROAAM) approach. In order to obtain input parameters for assessment, seven severe accident sequences are screened based on Level 1 PRA results and analyzed with integral accident analysis computer code. With the characteristics of the molten pool and key parameters gained from accident analysis, the IVR assessment program is developed with MATLAB code considering parameters of decay power, zirconium oxide fraction, the mass of stainless steel (SS). The influence of thermophysical parameters, critical heat flux (CHF) correlation and mass of metal layer on IVR effectiveness are discussed as sensitivity study for the assessment. Thermophysical parameters have the major effect. Combining the sensitivity study, assessment results show that the IVR strategy for Chinese improved 1000 MWe PWR has an acceptable high success probability. However, for engineering practice, detail system design and analysis should be evaluated in the near future