A pump drive unit is proposed for conveying a process fluid having a common housing (4) which surrounds a pump (2) having an impeller (21) for rotation about an axial direction (A) and a drive (3) for the pump (2), having a shaft (5) for driving the impeller (21) which connects the drive (3) to the pump (2), and having a restrictor (13) which extends around the shaft (5) and is arranged between the impeller (21) and the drive (3), with the housing (4) having a pump inlet (22) and a pump outlet (23) for the process fluid, with an inlet (43) being provided for a barrier fluid through which the barrier fluid can be introduced into the drive (3) and with an outlet (44) being provided for the barrier fluid through which the barrier fluid can be drained from the housing (4), and wherein a plurality of storage chambers (11) for the barrier fluid are provided at the shaft (5) in the region between the restrictor (13) and the drive (3), said storage chambers (11) are arranged behind one another with respect to the axi
