A pump drive unit for conveying a process fluid includes a housing which surrounds a pump having an impeller, a drive for the pump, a shaft for driving the impeller which connects the drive to the pump, and a restrictor extending around the shaft and arranged between the impeller and the drive. The housing has a pump inlet and outlet for the process fluid, with an inlet for introduction of a barrier fluid into the drive and an outlet for draining the barrier fluid from the housing. A plurality of storage chambers for the barrier fluid are disposed at the shaft in the region between the restrictor and the drive. The storage chambers are arranged behind one another with respect to the axial direction, with a respective two adjacent storage chambers being in flow communication with one another.
