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Flow numerical simulations in a low specific speed radial pump-turbine scale model are performed to investigate off-design operating conditions in generating mode. The Best Efficiency Point (BEP) and the runaway operating conditions at 10° guide vanes opening are addressed. The computational domain includes the full reduced scale model water passage from the spiral casing inlet to the draft tube outlet. The numerical simulation is performed using the Ansys CFX code, solving the incompressible unsteady Reynolds-Averaged Navier-Stokes equations. Wall pressure measurements in the stator are used to validate the numerical results. Then, detailed analysis is focused on the onset of the flow instabilities when the machine is brought from BEP to runaway. In these severe operating conditions, one single stall cell is found to rotate with the impeller at subsynchronous speed in the vaneless gap between the impeller and the guide vanes. It is found to be the effect of flow separation developed at the inlet of several consecutive impeller channels which lead to their blockage.