Insufficient thermal-hydraulic knowledge for analysis of a reactivity-initiated accident demands experiments of fast-transient flow boiling heat transfer from moderate- to high-pressure conditions. In this study, those experiments are conducted for vertical upward tube flows of pressurized water. The tube wall is joule heated by stepwise electric pulse power to achieve an abrupt wall heating condition. The applied pulse power is varied from 4.68 to 13.59 GW/m3, which is beyond the power required for steady-state critical heat flux (CHF) to occur. Rapid evolution of the boiling wall temperature is extracted from outer wall temperature data by solving an inverse heat conduction problem. As a result, with increasing the applied pulse power, the time to occurrence of departure from nucleate boiling gets shorter, and the corresponding peak heat flux increases over the steady-state CHF, which is evaluated at the same flow condition. A logarithmic relation between the wall heating rate and the CHF increment ratio is also demonstrated. The effects of pressure, inlet subcooling, and mass flux on the transient peak heat flux are also investigated. As the pressure increases, the nucleate boiling duration gets shorter with decreasing peak heat flux. On the other hand, as the inlet subcooling increases, the nucleate boiling duration gets longer, and the peak heat flux increases. Contrarily, the mass flux does not show any noticeable effects on the transient heat transfer evolution.