Monte Carlo model of annular flow in boiling water reactors

Annular flow regime consists of a liquid film layer, a wave layer attached on liquid film layer and droplets non-uniformly dispersed within vapor core. Each has a great importance from the viewpoint of nuclear reactor safety calculations as annular flow occupies about two-thirds of total length of a BWR fuel rod. For this purpose, the annular flow in unit cell geometry was modeled by Monte Carlo method for various void distributions. Effect of each distribution was investigated to calculate the magnitude of the deviation in infinite multiplication factor due to modeling. Since the phase structures present in annular flow are added step-by-step (starting from homogeneous model approach (HEM)) in this work, δk is calculated by taking the difference from the previous case. The results show that k∞ is strongly dependent on liquid film thickness, existence of wave, shape of wave and droplets. Liquid film thickness leads to increase k∞ whereas wave and droplets have a negative impact on k∞. Wave shape has double-sided effect on k∞. Although maximum deviation is about 400 pcm for liquid film layer, 300 pcm for wave layer, 150 pcm for wave shape and 300 pcm for droplets, the combined effect is at most 400 pcm.