Statistical aspects of best estimate method—I

In former years, thermal hydraulics phenomena have been analyzed mostly by means of conservative models. Recently there is a tendency to revert to best estimate safety analysis of thermal hydraulics phenomena in a nuclear power plant. In a best estimate analysis we have to know the uncertainty of the estimated values. We investigate two aspects of the error analysis. Firstly, we define a black box model of the thermal hydraulic calculation and derive the precise number of sample calculations needed for a given tolerance level even for several statistically dependent output variables. Then we point out possible chaotic behavior of calculation along with suggesting methods for recognizing the appearance of chaos.