We conducted boiling flow experiments in a 3 × 3 rod bundle containing a spacer grid with split type mixing vanes where the central rod was heated. Void fraction around the spacer grid was measured with high-resolution X-ray computed tomography. The study focusses on the effects of mass and heat flux on the void fraction downstream of the spacer for mass flux and heat flux ranging from 535 to 1950 kg/m² s and 42.8 to 107.2 kW/m², respectively. As the working fluid, refrigerant Octafluorocyclobutane (RC 318) was used and fluid scaling was applied. We found that the void fraction increases as the flow passes through the vanes and then decreases downstream until Z≈4D_h, and then increases again. In addition, we found that the mixing vanes cause a local increase in void fraction even at low heat flux or high mass flux, and that the arrangement of the vanes influences the size and location of the high and low void content regions. We also found that the effect of heat flux on the relative void fraction is more noticeable at high mass flux than at low mass flux. In the second part of the study, which will be presented in a separate paper, we investigated the effect of different vane angles on the void fraction.