Fuel mixed in water is combusted in a reactor having an internal operating pressure and temperature greater than 3200 psi and greater than 374°C, where the combustion of the fuel is exothermic. Air and fuel are pressurized for introduction into the reactor to a pressure greater than the internal operating pressure using energy generated from the combustion of the fuel, and the pressurized air and the pressurized fuel are injected into the reactor. Pressurized water from the reactor is injected into a drive water column that is partially filled with water to increase a pressure of the drive water column, and water at a temperature less than 100°C is injected into the reactor to replace water from the reactor that is injected into the drive water column. Pressurized water from the drive water column is used to drive a hydroelectric drive system to produce electrical power.