Current state of in-cabinet response spectra for seismic qualification of equipment in nuclear power plants

Many nuclear power plants in the south eastern region of South Korea are undertaking seismic qualification studies due to the recent earthquake in Gyeongju region. The records from this earthquake contain high frequency content compared to the frequency exhibited in most of the design basis Safe Shutdown Earthquakes used in the design of nuclear power plants. While the high frequency ground motions are not detrimental to the structures due to relatively smaller displacements, such motions can lead to functionality related failures in electrical equipment and systems such as control relays. The seismic qualification of relays and other such instruments require characterization of in-cabinet response spectra (ICRS) in the cabinet or control panels where they are mounted. The current practice of generating ICRS, in many cases, uses constant amplification factors which are based on studies containing low frequency ground motions. Cabinets can often have higher frequency modes which can amplify the motion if it contains high frequency pulses. Finite element analysis to evaluate the dynamic characteristics of cabinet is highly impractical because each cabinet is vastly different from other cabinets and a given plant contains hundreds of such cabinets and control panels. Therefore, a method is needed to evaluate accurate dynamic characteristics of cabinets and control panels in a relatively simple manner. This paper presents the details of such a method called Ritz vector approach and also compares the ICRS of an actual cabinet for a high frequency input motion with that for a low frequency input motion.