Sensitivity studies for the main steam line break in the Loviisa NPP with the Hextran-Smabre

The best-estimate modelling of nuclear power plants is part of the modern safety analysis. Because with three-dimensional neutron kinetics codes conservatism of all assumptions cannot be guaranteed, present trend is to use best-estimate analysis and to complete them with sensitivity and uncertainty methods. Especially statistical methods demand large amount of transient calculations. For that reason new sensitivity tool was programmed. The sensitivity analysis tool makes it possible to easily perform large amount of transient calculations with varied values of input parameters without remarkable amount of extra work. The main steam line break is one of the basic cases to be analysed as a part of the safety analysis of a pressurized water reactor. Its accurate modelling is very demanding task, because various complicated processes such as asymmetric power generation, flow mixing in the reactor vessel and various protection and conventional automation signals, contribute to the scenario. In this paper, sensitivity analysis tool has been utilised in the analysis of the main steam line break of the Loviisa VVER-440 power plant. Previous analysis has shown that the worst case might be reached with a smaller break size than double-ended guillotine break. In this paper, worst case has been searched by using several break sizes, by assuming failure of some protection systems and also starting from different initial power levels.