  The purpose of this study is the representation of Common Cause Failures
(CCF) in large digital systems. The system under study is representative of a
control system of a nuclear plant. The model for CCF is the generalized Atwood
model. It can represent independent failures, CCF non-lethal for some system
elements and CCF lethal to all. The Atwood model was modified to "direct"
non-lethal DCC on certain parts of the system and take into account the
different possible origins of DCC. Maintenance and repairs are taken into
account in the model that is thus dynamic. The main evaluation results are
probabilistic, the considered indicator is the probability of failure on demand
(PFD). A comparison is made between the estimator of the PFD taking into
account all the failures and the estimator taking into account only the
detected failures.
