Abstract The VR-1 training reactor with 500W maximum power operates at the Czech Technical University in Prague. Its control system was approaching obsolescence. It was therefore decided to upgrade the control system using up-to-date hardware and software using a new industrial PC with Windows 10 and the RTX real time system. The control system provides control and safety functions. Regarding the control function, the control system involves reactor I&C tests, reactor startup, reactor operation, and automatic power control. The control system receives operational data from the reactor safety (protection) system, computes average reactor power and power rate safety variables, and calculates a third safety variable that provides a percentage deviation of the reactor real power from the given (wanted) power. The control system also provides safety functions; it can shut down the reactor when safety limits are exceeded. The development of the control system software was accompanied by a quality assurance process, and verifications of the software development phases were carried out. After the control system installation, thorough non-active tests of the control system with removal of two fuel elements (because of nuclear safety) from the reactor core were carried out. Input signals for the reactor I&C (neutron chamber pulses and current) were simulated by computer controlled pulse and current generators. All safety functions, reactor start up, and operation were carefully tested. After successful non-active tests, active tests with a standard reactor core configuration were successfully conducted.