In a high pressure and high temperature reaction system suitable for oxidative waste treatment, particularly a reaction system for supercritical water oxidation (SCWO), a method is disclosed for injecting a first fluid of a first temperature at a first flow rate into a second fluid of a second temperature at a second flow rate, mixing the first and the second fluids within a mixing length (115, 215), and wherein the first and second temperatures and the first and second flow rates are selected such that a temperature of the mixed fluids downstream of said mixing length (115, 215) is obtained, at which said first fluid being substantially non-corrosive.