Thermal and structural analysis of light-water reactor vessel in-place annealing

Abstract

A major concern for a localized annealing of a reactor pressure vessel is the dimensional stability both during and after the annealing cycle. The effects of residual stresses in the vessel after such a localized heat treatment also need to be assessed. A two-dimensional, axisymmetric finite element study has been conducted using a typical vessel design, and the results indicate that there is no problem with the vessel itself. There is, however, a problem with the attached primary piping in that during the heating operation the nozzle region sees a thermal gradient which produces a small bending, rotation which can plastically deform the piping. Further analytical studies were conducted using an expanded heating zone in an attempt to reduce this axial gradient. Even though the overall temperature gradient was minimized, the actual gradient acting at the nozzle was only slightly reduced and plastic bending can still occur. This temperature gradient problem needs further analytical study but can be resolved with proper heating conditions which are well within the industrial heat treating state-of-the-art for temperature control and monitoring.