Background: The Core Cooling and Monitoring System(CCMS) of CPR1000 nuclear power station supplies monitoring means for two primary state functions defined in State Oriented Procedure(SOP). Purpose: We aim to deal with the errors caused by L VSL measurement and discuss the influence on SOP accident treatment. Methods: The calculation formula of natural recirculation flow rate was estimated. The maximum flow rates of single phase natural recirculation(at the time of reactor trip) and two phase natural recirculation(the void fraction in hot leg was 0.2) were calculated. The impact on SOP due to the measurement error of L VSL was analyzed. Results: Firstly

in the condition of single phase recirculation

the estimated error of L VSL measurement induced by the natural recirculation was less than 1.6%. Secondly

in the condition of two phase natural recirculation

the natural recirculation induced an over-estimated error about 3.6% to L VSL measurement when the void fraction in hot leg was 0.2. Conclusion: As multi-parameters are adopted for SOP to diagnose the core cooling state

the implementation of the safety related operations is guaranteed.