Reflooding heat transfer in zirconium alloy rod bundle of VVER reactor

An experimental study has been conducted to investigate the heat transfer process in reflooding in seven rod bundles of Zr+1%Nb alloy with water injection from the bottom only, from the top only and from both. The results of the experimental research are several empirical correlation for (i) rewetting velocity, (ii) heat transfer coefficient downstream of the quench front and, (iii) heat transfer enhancement due to grid spacers for the bottom reflooding condition. Comparing the bottom and the top reflooding conditions, it is found that the first case is preferable because the heater rod peak temperature is 100 K lower and the full rewetting time is three times less than for the second case. Flow regimes in bundles differ from those in tubes. The inverted annular flow boiling regime in the rod bundles is very short and changes to dispersed flow film boiling rapidly. (orig.)