Response Analysis on Electrical Pulses under Severe Nuclear Accident Temperature Conditions Using an Abnormal Signal Simulation Analysis Module

Unlike design basis accidents, some inherent uncertainties of the reliability of instrumentations are expected while subjected to harsh environments (e.g., high temperature and pressure, high humidity, and high radioactivity) occurring in severe nuclear accident conditions. Even under such conditions, an electrical signal should be within its expected range so that some mitigating actions can be taken based on the signal in the control room. For example, an industrial process control standard requires that the normal signal level for pressure, flow, and resistance temperature detector sensors be in the range of 4~20？mA for most instruments. Whereas, in the case that an abnormal signal is expected from an instrument, such a signal should be refined through a signal validation process so that the refined signal could be available in the control room. For some abnormal signals expected under severe accident conditions, to date, diagnostics and response analysis have been evaluated with an equivalent circuit model of real instruments, which is regarded as the best method. The main objective of this paper is to introduce a program designed to implement a diagnostic and response analysis for equivalent circuit modeling. The program links signal analysis tool code to abnormal signal simulation engine code not only as a one body order system, but also as a part of functions of a PC-based ASSA (abnormal signal simulation analysis) module developed to obtain a varying range of the R-C circuit elements in high temperature conditions. As a result, a special function for abnormal pulse signal patterns can be obtained through the program, which in turn makes it possible to analyze the abnormal output pulse signals through a response characteristic of a 4~20？mA circuit model and a range of the elements changing with temperature under an accident condition. 1. Introduction To diagnose a severe nuclear accident, it is essential to determine a plant’s status and to continuously monitor a plant’s responses, expected from the actions taken by operators to mitigate a given accident [1]. In addition, a correct interpretation of a plant’s conditions during such an accident is of significant importance for a successful accident management [2]. According to SECY 89-012, the United State Nuclear Regulatory Commission (U.S. NRC) requires that in a reasonable way instrumentation is identified as one of the key elements in utility accident management plants [3]. Moreover, a review of a plant’s accident management capabilities is regarded as a crucial element in achieving a