Discussion of the concept of safety indicators from the point of view of TfUX2 accident sequence for Forsmark-3

This paper contains general considerations on the safety indicators, with details at the system level and for the operator actions. For the system analysis, a modular analysis at a low detailed level is proposed (Nodule System Approach) in order to emphasize the safety related aspects at the subsystem (module) level.

The operator actions are divided in "active actions" (actions in the control room during incident/accident situations) and "passive actions" (actions during tests, maintenance, repairs, etc.) and are analyzed separately. In the second part, a discussion of a possible way to apply some SI to the TfUX2 accident sequence for FORSMARK-3, is done.

For the analysis of the Auxiliary Feedwater Systems (AFWS) an equation is proposed to derive target values for the failure probability on demand at the train level, given the target value at the system level, including the common cause failures between the redundant trains.