Transient overpressures in Cernavoda NPP cooling water system

Ansaldo Nuclear Division methodology to evaluate overpressure peaks induced by water column separation phenomenon consists of a two-step computer code simulation. The procedure has been used successfully to analyse the Cernavoda CCW system. Two main results have been obtained. As far as cavitation phenomenon in the condenser water box is concerned, an upper bound for condenser transient pressure consistent with maximum allowable condenser pressure has been evaluated. As far as cavitation phenomenon in network piping is concerned a lower bound for the butterfly valve closure time has been defined, such that piping cavitation can be avoided. The simulation of the cavitation in the downstream condenser water box has allowed the evaluation of maximum overpressures in the condenser device to be used in the structural design activity. The simulation of the cavitation in the pipe network, in particular at pump outlet ducts, allowed the evaluation of the minimum butterfly valve closure time in order to avoid unacceptable overpressure induced by vapor cavity collapse. Such results have been useful to design the system to cope water-hammer events induced by water column separation. (author)