Traditionally the analysis of the evolution of severe core disruptive accidents (CDA) is broken down into different phases. This is mainly done for a better focussing on the key phenomena of the accident phase and also allows the application of specific codes for the analysis. In the current paper we mainly deal with the initiating phase and the transition phase of an accident as the ULOF (unprotected loss of flow). The key phenomenon of the initiating phase is the start of boiling and the development of voiding; key phenomena of the transition phase are the progression of core melting and the occurence of recriticalities by fuel compaction. The first level of optimizing safety is oriented to the initiating phase by reducing the positive void worth in order to avoid that a ULOF accident would enter a severe development.

If accident prevention is not achieved the transition phase, characterized by a progressive core degradation leading to the occurrence of recriticalities, can be mitigated by dedicated features that enhance and guarantee a sufficient and timely fuel discharge - e.g. by a controlled material relocation (CMR) - and influence and `brake'; the recriticality path.

In the paper both phases are analyzed. The results presented are in agreement with the activities performed within the European Collaborative CP-ESFR project.