Traditionally, the deterministic safety analyses are performed at the NPP Gösgen by the vendor AREVA (now Framatome),
which developed an own version of the RELAP5/Mod 3.3 code (S-RELAP5). In the framework of building in-house
expertise, the NPP Gösgen – together with the GCR Ltd. team – have been developing an additional thermal-hydraulic model
of the plant using the US NRC code TRACE. This paper is focused on the main changes in the latest TRACE model, which
extended the model usage over MSLB and MB LOCA analyses. These changes are related to the update of the reactivity
feedback model using void curves, which is important for LOCA analyses, and the containment model. Entirely new containment
model was developed based on the MELSIM data and has been validated in a stand-alone mode against MELSIM
results. Based on the obtained results, it is concluded that the current TRACE model is appropriate for simulation of LOCA
scenarios with break size up to 380 cm2 and for double ended MSLB scenarios with and without containment.
Keywords: TRACE, MSLB, LOCA, containment, reactivity feedback.