An apparatus for cleaning inlet mixers while they remain in their operating position within a nuclear reactor. A cleaning tool is inserted by remote control into the inlet mixer via a secondary inlet opening. Following insertion of the cleaning tool, the internal surfaces of the inlet mixer are cleaned with a waterjet created from an ultra-high-pressure source and directed by controlled positioning of a cleaning head having a nozzle which scans the cleaning waterjet across the surface to be cleaned. The cleaning system includes a nozzle cleaning tool, a throat/barrel/flare cleaning tool, pumping systems for supplying ultra-high-pressure equal to at least 20,000 psi and low-pressure water to an installed cleaning tool, a launching system located at the top of the opened reactor vessel for feeding the water conduits (power), control cables, monitoring cables (instrument) and cleaning tool into and out of the inlet mixer, and a computerized process monitoring and control system.