Turbine trip transient analysis in Almaraz

Almaraz Units 1 and 2 each of 930 MWe includes a three coolant loop pressurized light water reactor nuclear steam supply system and turbine generators furnished by Westinghouse. The aim of the work presented in this paper is to develop a technically sound Thermal-Hydraulic model, yet flexible and reliable enough, to be used in transient safety analysis. The first two phases of this work have been completed namely, establishing a dependent Steady-State (SS) medel and analyzing the Turbine Trip (TT) transient. The calculation tool used through this work is the Relap-5, Mod. 1, ver. 27 computer code, as received from INEL. Although in the preoperational tests carried out at the plant it was determined that the TTT was ended after a 12 minute period, in this particular study, the period analyzed was only 65 seconds, subdivided in two periods of 5 and 60 seconds, respectively, due to two reasons, computer costs and the fact that the more important transient effects take place during the early portion of the transient period. When the turbine stop valve closes it produces a pressure and flow, damped oscillation which wear off after approximately 4.6 sec. That oscillation produces a pressure increase both in the main steam line and in the SG, where it reaches a value of 80.9 E + 05 Nw/m2, thus no opening of the safety valve happens. The steam mass flow rate vs. time is examined. After the initial 5 seconds oscillations wear off it experiences a drastic decrease. After those 65 seconds into the transient, no important additional effects take place. (Nogami, K.)