주급수/주증기 유량기반 原子爐 熱出力 測定에 관한 연구

Abstract

The Optimized Power Reactor (OPR1000) uses the secondary system calorimetric calculation method based on the feedwater flowrate as a Reactor Thermal Power (RTP) calculation method. However the fouling phenomena of Venturi meters increase the measured pressure drop across the Venturi meter, thereby causing the overestimation of the feedwater flowrate. As a result, RTP must be decreased to match the overestimated feedwater flowrate by Venturi meters to observe the operating nuclear power limit. To resolve this problem, the RTP calculation methodology based on the steam flowrate was additionally introduced at Yonggwang Nuclear Power Plant Unit 3&4 (YGN 3&4). However, it is not yet clear how accurate the methodology based on the steam flowrate could be and which methodology is more reliable because the methodology based on the steam flowrate was just introduced last year for the first time. For this reason, this thesis evaluated the reliability for these two methodologies and suggested a few approaches to improve the accuracy of the measurement.

In the steam flowrate calculation method, the steam flowrate measured by SG outlet nozzles is used instead of the feedwater flowrate measured by Venturies. Since the steam flow is measured by a differential pressure type instrument like a Venturi meter, the steam flowrate equation uses the same equation as the feedwater flowrate equation uses. However there are some differences compared with the feedwater flowrate calculation. For the calculations of area expansion factor, steam specific volume and compressibility in the steam flow equation, safety class SG pressure meters are used. And also, the steam flowrate should be calibrated by the feedwater flowrate at the beginning of the fuel cycle when Venturi meters are not influenced with the fouling phenomenon because there is no way to remove steam outlet nozzles for calibrations. Thus, in order to calculate the steam flowrate, the main steam calibration factor(MSCF) is needed.

According to the evaluation results at the full power level of YGN 3&4, the uncertainties based on the feedwater flowrate and the steam flowrate are 0.969% and 1.34%, respectively. Considering the results, these two methodologies are acceptable. However, as yet, there is no single method that is guaranteed to ensure perfect confidence in RTP measurement. If instruments have measurement errors, it leads to the false RTP calculation. Thus it is necessary to implement some sort of monitoring to confirm that measurement errors are not occurring. In order to support more reliable determination of RTP, operators should monitor related parameters and analyze their trends. Except for Venturi fouling, the methodology based on the feedwater flowrate is more accurate than the methodology based on the steam flowrate. Thus it is important to monitor Venturi fouling phenomena. For example, HP turbine first-stage pressure is not influenced by Venturi fouling. Through monitoring the HP turbine first-stage pressure, we can early detect the Venturi fouling. Including the HP turbine first-stage pressure, it is very important to monitor and analyze related parameters (primary ΔT, feedwater temperature, SG pressure, steam header pressure, gross electric power, SG blowdown flowrate etc). The efficiency of error detection is dependent on the monitoring and analyzing skills. The accurate estimation of the error is essential. Through continuous monitoring and analysis for related parameters, we can expect the high level reliability of RTP measurement.