Three-Dimensional Versus Axisymmetric Finite-Element Analysis of a Cylindrical Vessel Inlet Nozzle Subject to Internal Pressure—A Comparative Study

This paper presents a comparative study between a three-dimensional and an axisymmetric finite-element analysis of a reactor pressure-vessel inlet nozzle subject to internal pressure. A quarter-symmetric section of the nozzle is modeled with a three-dimensional quadratic isoparametric finite element. This comparative study proves that the axisymmetric analysis is unconservative if based upon common axisymmetric modeling techniques. This inadequacy, for the PWR vessel inlet nozzle studied herein, can be offset by a modification of the modeling techniques, i.e., if the value of the radius of the equivalent spherical vessel is taken as 3.2 instead of, say, 2. The results of the three-dimensional finite-element analysis are also compared with those of a photo-elastic stress analysis and with the stress indices indicated by the ASME Section III Code. These additional comparisons, based upon a continuous distribution of hoop and tangential stress indices in both the transverse and longitudinal planes, shows good agreement between the three-dimensional finite-element and photoelastic analyses. The ASME Section III stress indices are found to be relatively conservative.