Modification of boron transport model of coupled nutronic and thermal-hydraulic system dynamics code for fuel debri recriticality analysis

In this paper, the boron transport and critical behavior of the fuel debris was studied by the three-dimensional coupled nutronic and thermal-hydraulic system dynamics code since the boron transport model is originally built in it. In this model, the effects of diffusion and boron sinking or sedimentation due to the larger specific gravity of boron are omitted, so that only the advection of boron in the liquid phase is calculated. The estimation of the boron transport conducted by the system dynamics code was compared with the results obtained from computational fluid dynamics (CFD) code, STAR-CD^® v4.20. It was found that the effect of boron sedimentation is dominant, concerning that of the diffusion, which plays only a small or negligible role. The sedimentation of boron solution, however, is not calculated in the system dynamics code. In order to consider the sedimentation in that code, the boron transport model was modified in this study. By this modification, the boron transport behavior can be evaluated by system dynamics code with a precision equivalent to that of CFD. Then, the new boron transport model was applied to the recriticality analysis of the fuel debris located on the pedestal sump, which shows that the injection of boron suppresses the criticality of the debris soon after boron reaches there. The results of this study conclude that the method of injecting boric acid into RPV has an effect on controlling the criticality of the debris. (author)