Loss of coolant accident and uncertainty analyses in VVER-1200 reactor

The objective of this thesis is to perform the thermal-hydraulic analysis of VVER-1200 reactor under Loss of Coolant Accident (LOCA) conditions including uncertainty calculations for the Peak Cladding Temperature (PCT) prediction using RELAP5/SCDAPSIM MOD 3.5 (RS MOD 3.5) best-estimate computer code. In this work the emphasis has been given to the analysis of the performance of the Emergency Core Cooling System (ECCS) under large to intermediate break conditions. Two specific break sizes were studied: the Double-Ended Guillotine Break (DEBG), and the Transition Break Size (TBS). The selection of these break sizes emerged from the current 10.CFR.50.46 acceptance criteria, where DEGB is considered as a design basis accident (DBA), and particularly from the "proposed amendments to 10.CFR.50.46 providing risk-informed alternative LOCA break size". Within the scope of this thesis, an RS input model was generated based on the "Moscow NPP" design of VVER-1200 type reactor. The model was then used to observe the system behavior in the specified LOCA conditions namely for a 200% (double-ended, guillotine) cold leg break and for a 40.7% (single-ended, TBS) cold leg break. Furthermore, to account for the uncertainty in the PCT predictions, uncertainty calculations were carried out utilizing the integrated uncertainty package available in the RS MOD 3.5 code version. The simulations performed in this study show that the ECCS performance is satisfactory in both accident scenarios. Therefore, we conclude that both hypothetical accidents can be tolerated in the VVER-1200 reactor. In the DEGB scenario the PCT reached a maximum of 948.1 K degree. In the TBS scenario, the system pressure loss rate is inferior to that in the DEGB case, resulting in a delay in the ECCS initiation. Nevertheless, smaller break results in a decreased mass of coolant loss rate and as a consequence decay heat removal were accomplished in a successful manner. The calculated PCT value never exceeded 627.7 K which is the cladding temperature during normal operation. Upon performing the uncertainty calculations, the upper limit for PCT was determined to be 1006.0 K under the DEGB scenario and 630.1 K for TBS. It is also concluded that the oxidation of the cladding is negligible and no remarkable hydrogen generation will result in both accidents.