The flow characteristics of natural circulation are relying on the heated fluid density difference as a driving force. Based on density gradient theory, a characteristic factor of natural circulation was proposed. The forced circulation models of Onset of Nucleate Boiling (ONB) of Bergles&Rohsenow and SU Shunyu were modified through the characteristic factor. The calculation results showed that the modified forced circulation models could calculate the occurrence of ONB in the condition of natural circulation better. Meanwhile, the calculating results and differences of heat flux at ONB between the models of natural and forced circulation were given by changing the thermal parameters. The results revealed that, most of the calculated values of natural circulation model were lower than those of forced circulation models when inlet subcooling and pressure varied, which meant the occurrence of ONB in natural circulation preceded that in forced circulation. The calculated results of forced circulation models could close to those of the natural circulation model through modification, which indicated the characteristic factor of natural circulation proposed in this paper was one of the respects which could reflect the inherent mechanistic difference between natural and forced circulation.

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