Numerical study on flow inversion in UTSG under natural circulation

For natural circulation, it is shown that reverse flows may occur in some U-tubes of UTSG, which is harmful. In this study, the effects on reverse flow phenomenon of approaching natural circulation by changing the inlet mass flow of UTSG in different ways are compared. Numerical model of a given UTSG is established by best estimate code RELAP5/MOD3.3. Three kinds of circulation condition are simulated to validate this model with the experimental results from NPIC. Two conditions reaching a certain natural circulation by changing the UTSG inlet mass flow upwards and downwards are compared. Also, another case is simulated in order to compare the UTSG inlet mass flow of reverse flows occurring and that of the disappearance of existing reverse flows. Results show that approaching the natural circulation by decreasing the inlet mass flow will lead to less reverse flow U-tubes than increasing the inlet mass flux from zero does, and the UTSG mass flow of reverse occurring in the decreasing method is much smaller than that of existing reverse flow being eliminated the increasing method. The reasons are discussed based on the quantitative characteristic curves of steady-state pressure drop versus inlet mass flux of the parallel U-tubes. Two conclusions are drawn: (1) Approaching the natural circulation by decreasing the inlet mass flux lead to comparatively less reverse flow U-tubes and flow resistance than the increasing way. (2) It is difficult to eliminate the existing reverse flows in UTSG thoroughly by adding UTSG inlet mass flow.