Large pipe break opening time in pressurized heavy water reactors.

• Break opening time assumption in LBLOCA Large Break Loss of Coolant Accidents Analysis. • Demonstrating the efficiency of the second shutdown fast boron injection system during the accident analysis. • Dynamic Break Opening development for the analysis. In the frame of the licensing process of the Atucha II PHWR (pressurized heavy water reactor) the BEPU (Best-Estimate Plus Uncertainty) approach has been selected for issuing Chapter 15 of FSAR (Final Safety Analysis Report) dealing with accident analysis. In case of a LOCA (Loss of Coolant Accident) the assumption of instantaneous opening time is too conservative, especially for this kind of reactor with a positive coolant void reactivity coefficient. Analysis was performed to assess the ability of Atucha II nuclear power plant's fast boron-injection system to prevent overheating of the fuel elements in the event of a Beyond Design Basis Accident (BDBA). The goal of the Analysis was to develop a Dynamic Break Opening which was used as the input for the LOCA analyses. The results show that the fast boron injection can effectively shut down the reactor, thereby preventing the overheating of the fuel elements. [ABSTRACT FROM AUTHOR]