• combination of passive and active features
• defense in depth principle
• licensing
• pre-operational test

This PhD thesis is on Pre-operational Tests and Design Basis Accidents Analyses for a generation III+ Nuclear Power Plant. The very first milestone of the present work has been the development of a fully detailed Relap5 model of the selected Nuclear Power Plant design, which can be used to simulate a variety of steady state and transient conditions. The developed Relap5 model has been extensively used in this work, which can be thought as divided in two main parts.
In the first part, pre-operational tests procedures have been investigated and pre-tests calculations have been performed for three selected pre-operational tests which need to be successfully executed as one of the steps for the licensing of the selected generation III+ Nuclear Power Plant.
In the second part of the work, a combination among passive systems and an active one has been analyzed in order to evaluate the impact on transients progression (two Design Basis Accidents have been considered). Furthermore failure of the active feature has been taken into account to investigate whether a safety issue is raised upon active system failure once it has been put into operation, it has interacted with the passive features and has modified the plant conditions. This second part is towards a full application and exploitation of the defense in depth principle.
Pre-tests calculations give good results, i. e. the goals of the tests are achieved. Design Basis Accidents simulations results demonstrate that a combination among passive features and the selected active one provides a better response in comparison to an accident management based exclusively on passive systems, and that no safety issue is raised upon failure of the active feature.
Even if a particular generation III+ Nuclear Power Plant has been considered, the backbone of this work is absolutely general and can be extended to other generation III+ and generation IV Nuclear Power Plants.