A method of hydrothermal processing of highly toxic substances, wastes and metal powders, comprises a batch processing cycle of at least following steps i. to viii: i. preparing a batch of highly toxic waste, substances and metal powders for processing; ii. applying precision dozing to the batch to obtain a mixture of the batch with water and at least one additional reagent in preset proportions; iii. feeding the mixture into a reactor; iv. spraying the mixture inside the reactor with a mechanical spray nozzle; v. supplying an additional oxidizing agent inside the reactor; vi. adjusting a temperature inside the reactor to above 374.2° C.; vii. adjusting a pressure inside the reactor to above 21.8 MPa, thereby driving the water into supercritical condition and initiating a supercritical hydrothermal oxidation process; and viii. keeping the mixture in the reactor for a preset amount of time, and at the same time. The method further comprises removing a produced steam and hydrogen mixture from the reactor via a steam-gas unit and droplet separator into a wastewater condenser unit and a gas drainage pipe, and venting suspended solids through a bottom part of the reactor into a solids collector unit. The method then provides the steps of repeating the batch processing cycle after it has terminated; and monitoring and controlling the temperature and pressure inside the reactor via an Automatic Process Control System (APCS) during all steps.