Study on rod surface temperature behavior of BWR 8x8 high burn-up fuel assembly under post boiling transition condition

A study of thermal-hydraulics in a BWR 8x8 high burn-up fuel assembly under post boiling transition (BT) conditions has been carried out using two full-scale heater rod bundles. Rod surface temperature under post-BT conditions was measured with different axial power distributions at different pressures and mass fluxes for steady and transient conditions. Test results under steady conditions indicate that post-BT rod surface temperature excursions are rather slow and temperature oscillations are not excessive. A comparison with several existing heat transfer correlations for the post-BT region, as recommended by Dougall-Rohsenow, Groeneveld, Condie-Bengston, Koizumi, and Sugawara indicates that the predictions tended to be conservative for the transition boiling region before the onset of fully developed film boiling. Further, the results of transient BT tests indicate that rod surface temperature excursions are suppressed by re-wetting. An analytical method based on RELAP4 with the GEXL correlation for critical power prediction, the Dougall-Rohsenow heat transfer correlation, and re-wetting correlations (the Iloeje and Groenveld correlations) gives good agreement as regards rod surface temperature behavior. (author)