Investigations on effect of primary mass flow change rate on flow instability among parallel U-tubes of steam generator

Current investigations reveal that reverse flow phenomena in parallel steam generator U-tubes under natural circulation significantly reduce circulation capacity, compromising operational safety of marine nuclear power plant . During transitional phases from forced to natural circulation, the steam generator's primary mass flow rate experiences an abrupt decline, inducing complex reverse flow phenomena. To solve the above problems, this paper used experimental methods to measure the effect of the steam generator primary mass flow change rate on the reverse flow based on the steam generator experimental loop. The results demonstrated that with an increase in the primary mass flow change rate, the critical mass flow rate of parallel U-tubes decreased obviously, while the critical pressure drop remained nearly unchanged. In addition, the mechanism was analyzed using the RELAP5 code. The results can facilitate the technical support necessary for the operation safety analysis of marine nuclear power plant.