Analysis of irradiation induced creep of bolts of a PWR core baffle

The core baffle of a PWR is loaded by the pressure difference between bypass and core and by temperature profiles developing from gamma heating and heat transfer into the coolant. Strain, deformation and gaps between the sheets resulting from this load are determined considering the effect of neutron irradiation induced creep of the core baffle bolts. The finite element code ANSYS is applied for the thermal and mechanical analyses. The FE-model comprises a complete 45° sector of the core baffle structure including the core barrel, the formers, the core baffle sheets and about 230 bolt connections with non-linear contact between the single components and the effect of friction.
The analysis shows that some of the bolts loose a significant part of their pre-stress during operation, especially those bolts that are located in the neutron flux maximum. However, the load is taken over by other bolts located at lower flux positions. This global stress relocation is a self regulating process, which keeps the mechanical and geometrical stability of the core baffle structure till end of life.