THREE-DIMENSIONAL PROBABILISTIC NONLINEAR RESPONSE ANALYSIS OF THE MUHLEBERG NUCLEAR POWER PLANT REACTOR BUILDING DRYWELL INTERNAL STRUCTURE

摘要

The Muehleberg Nuclear Power Plant (KKM) in Switzerland is a single-unit General Electric Mark I boiling water reactor owned and operated by BKW Energie AG. BKW has recently updated the seismic probabilistic safety assessment (SPSA) of KKM, which included fragility evaluation of the Reactor Building internal structure within the drywell. Analysis of the internal structure for the previous SPSA was performed using a probabilistic simulation approach with two-dimensional (2D) nonlinear response analyses. To more realistically capture the response and load redistribution after initiation of various nonlinear events, evaluation of the internal structure for the updated SPSA was done by probabilistic nonlinear response analysis using a refined three-dimensional (3D) finite element model. Nonlinearities modeled in both analyses included disengagement of welds that connect a stabilizer truss to the top of the biological shield wall and uplift at the base of the internal structure, which in combination allow the internal structure to rock. The Latin Hypercube Sampling method was used to generate random variables incorporated in thirty response simulations, which were performed at multiple input motion levels. Variables included the input acceleration time histories, structural stiffness, structural damping, and the weld capacities. Both analyses uncovered valuable insight into the internal structure seismic response. Notable differences between the 2D and 3D analyses were mainly due to the unique sensitivity of the rocking and rolling response to phasing of the horizontal input motions.