[en] This paper is a preliminary report about the methods and the models applied to a computer code named THYDE-P which is concerned with thermal-hydraulic transients of a PWR plant following a large or small area break of a primary coolant system pipe, generally referred to as a loss-of-coolant accident (LOCA). The THYDE-P code deals not only with blowdown phase, but also with reflooding phase. What characterizes the THYDE-P code is its entirely new model for the primary loop network. The code user information and the programming detail are not included in this report, but in a future documentation. (auth.)