A reactor in the form of a cylinder or prism for the thermohydrolysis of urea to ammonia, comprising a reaction chamber in the form of a cylinder or a prism with (i) a feed device for feeding a urea-water mixture into the reaction chamber, wherein the feed device is connected to the reaction chamber via a connecting element, and wherein the feed device is arranged to prevent contact of the urea-water mixture with the connecting element when the urea-water mixture is fed into the reaction chamber; wherein the connecting element is arranged on one end of the reaction chamber and the other end of the reaction chamber opposite this end is closed in a gas-tight manner; wherein the connecting element is a flange; (ii) a tubular discharge device for discharging an ammonia-water mixture from the reaction chamber, wherein the tubular member is disposed in a region of the reaction chamber closer to the end comprising the flange than to the end which is closed in a gas-tight manner; a device for heating the inner walls of the reaction chamber which is arranged to generate sufficient heat to subject the urea-water mixture in the reaction chamber during operation of the reactor to thermohydrolysis of urea to ammonia; characterized in that the feed device comprises a solenoid valve which is provided with a perforated disk on the outlet side, and wherein the reactor does not comprise a catalyst which catalyzes the thermohydrolysis of urea to ammonia. a reaction chamber in the form of a cylinder or a prism with (i) a feed device for feeding a urea-water mixture into the reaction chamber, wherein the feed device is connected to the reaction chamber via a connecting element, and wherein the feed device is arranged to prevent contact of the urea-water mixture with the connecting element when the urea-water mixture is fed into the reaction chamber; wherein the connecting element is arranged on one end of the reaction chamber and the other end of the reaction chamber opposite this end is closed in a gas-tight manner; wherein the connecting element is a flange; (ii) a tubular discharge device for discharging an ammonia-water mixture from the reaction chamber, wherein the tubular member is disposed in a region of the reaction chamber closer to the end comprising the flange than to the end which is closed in a gas-tight manner; (i) a feed device for feeding a urea-water mixture into the reaction chamber, wherein the feed device is connected to the reaction chamber via a connecting element, and wherein the feed device is arranged to prevent contact of the urea-water mixture with the connecting element when the urea-water mixture is fed into the reaction chamber; wherein the connecting element is arranged on one end of the reaction chamber and the other end of the reaction chamber opposite this end is closed in a gas-tight manner; wherein the connecting element is a flange; (ii) a tubular discharge device for discharging an ammonia-water mixture from the reaction chamber, wherein the tubular member is disposed in a region of the reaction chamber closer to the end comprising the flange than to the end which is closed in a gas-tight manner; a device for heating the inner walls of the reaction chamber which is arranged to generate sufficient heat to subject the urea-water mixture in the reaction chamber during operation of the reactor to thermohydrolysis of urea to ammonia; the feed device comprises a solenoid valve which is provided with a perforated disk on the outlet side, and wherein the reactor does not comprise a catalyst which catalyzes the thermohydrolysis of urea to ammonia.