During reflooding after a hypothetical loss of coolant accident (LOCA) in a pressurized water reactor (PWR), a flow of steam and dispersed droplets forms and propagates in the assemblies. During the accident, rods can be deformed and ballooned, thus reducing the hydraulic diameter of some sub-channels. In this scenario, the rods' cooling may be affected since the steam will be diverted to less blocked sub-channels. For security reasons, the characterization of heat transfers in these areas is important to know in order to well evaluate the consequences of such an accident (meltdown of the core). This study is dedicated to the estimation of heat and mass transfers in these blocked zones