Consideration of Uncertainty in the Strength Analysis of the VVER Pressure Vessel during Factory Hydraulic Tests

Abstract

The article addresses the problem of applying the uncertainty and sensitivity analysis methods to strength calculations. Information on the relevance of this problem for ensuring reliable operation of nuclear power facilities with pressurized water reactors is given. All computer codes contain uncertainties, the sources of which are measurement errors and empirical models. The following loading modes were analyzed: factory hydraulic tests and seismic load. The article presents an analysis of stressed states and distribution of plastic deformations for these loading modes. It is shown that along with the statistical distribution close in appearance to the normal one, there are also extreme values that differ significantly from it, with outliers having a low occurrence probability. It has been found that a number of resulting parameters can have a strong variability in the uncertainty analysis, and that high variability coincides with a significant relative maximum deviation from the reference values. A sensitivity analysis for these loading modes is carried out. The analysis results have shown that the Poisson ratio, Young modulus and yield strength have the strongest influence, and so is the tensile strength in the calculations for seismic loads. The obtained study results may be relevant for brittle fracture calculations for the reactor pressure vessel having been in operation for a long period of time in view of accumulation of flaws in the metal and changes in the vessel material properties under the long-term effect of neutron fluence and high temperature.