Coolant flow blocking in WWER-440 fuel rod bundle

By means of subchannel analysis the major heat transfer and hydrodynamic characteristics of WWER-440 fuel rod bundle have been determined for the case when the coolant flow is blocked. The calculations were performed on COBGOF code taking into consideration both turbulent and convective mixing between the interacting subchannels. The code allows the problem solution for a considerable blocking. The effects of blocking extent and location on the channel thermal-hydraulics have been studied. The fuel rod bundle safety was analyzed from the viewpoint of heat transfer crisis due to flow cross-section decrease, hydraulic resistance growth and subsequent reduction of mass flow rate both through the entier rod bundle and within the blocked cross-section. It has been shown that WWER-440 reactor at the stationary regime has a considerable margine over the heat transfer crisis even if a substantial coolant flow blocking takes place. At 60% blocking of the fuel rod bundle there is no crisis even at 10% power rise. Though such regimes are already dangerous since an insignificant further blocking results in an abrupt hydraulic resistance rise with all undesirable consequencies