In case of depressurization of a SCWR, the steam pressure can be used to pump coolant back into the reactor, either by using a turbine driven pump or by a steam injector. The latter option is cheaper, more robust and requires less periodic inspections. The numerical simulation of a steam injector, however, has still been a challenge. In principle, a steam injector consists of a steam nozzle, a mixing chamber and a diffuser. This paper describes a one-dimensional two-phase flow model simulating the flow and condensation phenomena occurring in the steam nozzle, the structure of shock fronts at the nozzle outlet, the direct contact condensation of steam with cold water in the mixing chamber, and the sudden pressure increase in the final condensation shock front. The model is based only on first principles and on existing correlations. Correction factors adapting the model to experimental results have not been used yet. For validation, the model has been applied both to separate effect tests and to steam injector tests.