Prediction of a boiling transition location and heated surface temperature for upward flow in a vertical tube (Investigation and improvement of prediction method for fuel rod wall temperature)

Abstract

BWR fuels are covered with liquid water under normal operation conditions but in such events as pump trip, supply of water decreases and fuels may touch on vapor directly. Such condition is called dryout. An occurrence of dryout results in increase of fuel surface temperature, which may cause a severe accident. Therefore it is important from a view point of nuclear safety to make an accurate prediction of surface temperature. Though the final target of this study is to develop a prediction method of surface temperature for actual geometry and conditions of BWR fuel assembly, in this study we focused on the heat transfer phenomena for upward flow in a vertical tube as a first step. The prediction was carried out in order of (1) dryout point location, (2) heated surface temperature in pre-dryout region and (3) in post-dryout region. For prediction of dryout point location, a conventional model of liquid film dryout was used. We attempted to improve the prediction accuracy by adopting equations considering the traction at the vapor-liquid interface. In pre-dryout region, conventional equations for subcooling and nucleate boiling were applied. In post-dryout region,surface temperature prediction with a conventional method showed low accuracy at low flow rate. We specified the cause of low accuracy with statistical method and changed the prediction model. Major conclusions are as follows: (1) Prediction results were compared with various experimental data which cover the conditions of actual reactor to show that the developed method provided dryout location in good agreement but didn't bring a significant improvement compared with conventional model. (2) Developed model can accurately predict surface temperature including the range of high temperature (>650^◦C) where conventional model showed low accuracy. (3) Calculation results were compared with experimental data of several researchers to confirm that the dependence on experiment geometry is small enough.