The CANDU system design has specific features which are important to severe accidents progression and require selective consideration of models, methods and techniques of severe accident evaluation. Moreover, it should be noted that the mechanistic models for severe accident in CANDU system are largely less validated and, as a consequence, the level of uncertainty remains high in many instances. Unlike the light water reactors, for which several different computer codes to analyze severe accidents exist, for CANDU severe accidents analysis only two codes were developed: MAAP4-CANDU and ISAAC. However, both codes started by using MAAP4/PWR as reference code and implemented CANDU 6 specific models. Thus, the two codes have many common features. Recently, a joint project involving Romanian nuclear organizations and coordinated by Politehnica University of Bucharest has been started. The purpose of the project is the assessment and adaptation of the SCDAP/RELAP5 code to CANDU 6 severe accidents analysis. The present work investigates the progression of a severe accident in CANDU 6 reactor starting from a LBLOCA and a subsequent loss of all heat sinks. The paper provides details concerning the methodology and nodalisation used, and interprets the results obtained. Comparisons of the SCDAP/RELAP5 simulations with the MAAP4-CANDU and the ISAAC codes reported results are presented. Also, some insights are given on possible reasons for the discrepancies between the SCDAP/RELAP5, MAAP4-CANDU and ISAAC codes predictions.

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