An adjustable speed drive system for a primary loop recirculation pump and method of control in which a first breaker is connected to an auxiliary power supply system, an inverter is connected to the first breaker, a second breaker is connected to an output of the inverter, a gate driver supplies a gate pulse to the inverter, and an inverter control portion drives the gate driver. The adjustable speed drive system is combined with a backup adjustable speed drive system. When a malfunction in a main adjustable speed drive circuit portion including the inverter and the gate driver in the adjustable speed drive system used in a normal operation is detected, the main adjustable speed drive circuit portion is switched to the backup adjustable speed drive system, and an operation of a recirculation pump motor for driving a recirculation pump that controls a flow of a reactor core is continued.