[en] The dynamics of pipe circuits containing flowing liquids, and in particular the case of an accident, present a problem for the safety of nuclear power plants. The rupture of the pipe involves a depressurization in the circuit and this is followed by a whipping action of the broken pipe. More generally the situations which involve the rapid speeding up of fluids in circuits, with or without fluid-structure coupling, may be treated by PLEXUS. The results for the rigid-pipe case, incorporating the most characteristic situations, will be shown first. The second part will present some preliminary calculation results for the complete system with deformable pipe circuits and taking into account the fluid-structure coupling