An Analysis of Hot Leg Thermal Stratification Effect on Hot Leg Temperature Measurement for KSNP

摘要

For Korean Standard Nuclear Power Plant (KSNP), four Resistance Temperature Detectorsrn(RTDs) are installed at each hot leg - two in the upper region and two in the lower around thernpipe wall. It has been recognized that the degree of so-called thermal stratification in the hotrnleg grows as the non-uniformity of power distribution grows in the reactor core, usuallyrnhaving a peak in the center area. Since it is hard to quantify the stratification effect, anrnunrealistically large uncertainty has been assumed for conservatism, which has been thernlargest contributor to the hot leg temperature measurement uncertainty.rnThe present work is aimed at quantifying the correction offset and associated uncertaintyrnbased on the coolant velocity profiles and local temperatures in the hot leg pipe which havernbeen measured during System 80 reactor flow model test and KSNP operation, respectively. Arncircle temperature distribution model has been developed to conservatively estimate anrnenveloping value of the correction offset. The analysis results show that the correction offsetrnis minimal and, therefore 0.3℉ is bounding the uncertainty due to the hot leg thermalrnstratification observed in KSNPs. Comparing with the current uncertainty of 0.5℉, it isrnexpected that the excessive conservatism included in the measurement uncertainty of the hotrnleg temperature can be reduced.