[en] The main objectives of the present paper are to perform a study of the behaviour of the primary circuit during a LBLOCA 35% RIH event for a CANDU 6 reactor and analysis of the main important parameters regarding nuclear safety. FIREBIRD models for a PHTS circuit have been developed for a CANDU 6 reactor with standard fuel and a demonstration analysis for a 35% RIH large break LOCA has been done. This type of accident gives the highest peak clad temperature for CANDU fuel elements. This analysis uses the FIREBIRD code coupled with a point kinetics module. The point kinetics module includes the models for neutronic measurement instrumentation (the platinum detectors and the ion chambers) and the RRS (Reactor Regulating System) module. The main results in this paper are divided in two types: neutronic and thermalhydraulic. The neutronic results consist in the total relative reactor power, the void reactivity, the total reactivity and the neutronics trip times. The thermalhydraulic results represent system parameters such as flow, pressure, inventory and heat structure parameters such as cladding and fuel temperatures, cladding-to-coolant heat transfer coefficient. The paper concludes that the first two safety barriers (fuel and cladding) are not overridden. (authors)