[en] As an approximate evaluation of pressurized water reactor (PWR)-blowdown flows (in particular for the HDR experiments) we consider incompressible one-phase, transient potential flow in flow ducts with rigid impermeable walls. For arbitrarily chaped ducts it is shown that the temporal variation of the velocity is described by the ordinary differential equation of Riccati which can be integrated analytically for a class of important cases. The velocity and pressure fields for time-independent boundary conditions are determined for all times by the solution of three Laplace equations only. If the potential flow is known for some individual ducts, it is possible under certain conditions to enumerate the temporial and spatial flow behavior in a complex duct composed of the single ducts without solving additional differential equations. This allows for the analysis of three-dimensional problems by piecewise analysis of one- and two-dimensional ducts. In particular, we consider the transient flow through the downcomer and the blowdown-pipe in a pressurized water reactor, as expected in the HDR-blowdown experiments. The analysis is one-dimensional (analytically) as well as two-dimensional (numerically) and three-dimensional (by coupling two two-dimensional ducts). The computations are done by means of finite differences (using cyclic reduction) as well as by the integral equation method which corresponds to the singularity method. Both methods show advantages and disadvantages. The programs (LAPL) used for the two-dimensional integral equation method are listed in an appendix. (orig./RW)