Modal seismic analysis of reactor internals considering the initial stress at normal operating conditions: A study on Atucha-I NPP control rods

Abstract

Modal seismic analysis evaluates the seismic response of nuclear plants by combining dominant vibration modes under a prescribed pseudo-acceleration spectrum. Such analyses typically consider only the thermal and kinematic boundary conditions for normal operation, but initial stresses in this state significantly affect vibration modes and modal seismic response. Simply considering all boundary conditions is insufficient to account for initial stresses; a prior thermo-mechanical analysis is mandatory to compute the geometric stiffness and redistribution of internal forces under initial stresses. This study investigates the modal seismic response of the guide tubes and control rods of the Atucha-1 Pressurized Heavy Water Reactor, comparing configurations with and without initial stresses, given their oblique arrangement makes them susceptible to developing such initial stresses. Boundary conditions were provided by Nucleoeléctrica Argentina S.A. (NA-SA) and the 3-D finite element model was built and analyzed with the open-source suite Salome$-$Meca v2022. Results indicate that initial stresses alter the effective stiffness of the piece by redistributing internal forces, increasing the natural frequencies by about 50%, shifting away from the seismic excitation frequency range. This reduces the modal seismic displacement by one order of magnitude, leading to more realistic responses that may extend operational limits within safety margins.