Investigating the effectiveness of outfitting nuclear power plant with automatic seismic trip system based on the early detection of earthquake

Abstract

Safety analysis indicates the significant effect of seismic induced accidents in overall risk of nuclear power plants located in those quake-prone regions of faults vicinity. In other words, since earthquake impacts the reliability of safety functions, seismic induced Core Damage Frequency is of considerable importance in total CDF of NPPs. One of the main concerns in seismic safety is the ability of plant safety systems to immediately shutdown the reactor and reach a safe shutdown state, upon earthquakes of predefined accelerations. Albeit, consensus on whether to implement an automatic trip signal upon earthquake of predefined acceleration, or not, is a controversial issue, with ambivalent interpretations of nuclear safety and electric power dispatching bodies. This article intends to perform a so-called risk-informed-decision-making study to decide on provisioning of Reactor Protection System of an, with automatic digitized seismic trip signal, in addition to the existing trip signals. In this regard, the two most severe accident sequences of Loss of Offsite Power and Loss of Coolant Accident are chosen to investigate and compare the advantageous and disadvantageous of early trip of the reactor on non-destructive primary wave before arrival of the potentially damaging secondary wave. Both deterministic behavior, from MELCOR model of typical PWR plant, and the probabilistic measures, from Risk Spectrum model of Level 1 Probabilistic Safety Assessment, are taken into consideration to analyze the accident progression for defined sensitivity cases on peak ground acceleration (pga) level of actuating the seismic trip setpoints.The results are useful to reach an agreement for pros and cons of outfitting NPPs with Automatic Seismic Trip System. Based on the results, an optimized actuation level of pga for seismic trip setpoint can be determined from the risk point of view. Investigating the late and early effects of the proposed change on the response of the plant will be useful in accident management decision making.