Evaluation of surge line thermal stratification and implementation to piping design

Recent observations at operating plants and subsequent U.S.NRC requirements have identified flow stratification in surge lines as a phenomenon that must be considered in the design basis of surge lines. During YGN Unit 3 pre-core hot functional testing, surge line pipe wall temperatures and pipe displacements were measured to determine the degree of surge line flow stratification. Test data from YGN Unit 3 indicate that stratified flow exists in the surge line during both heatup and cooldown. The measured displacements and temperatures were extensively reviewed and evaluated in detail: to verify the adequacy of thermal hydraulic model used to predict the pipe top-to bottom temperature differentials; to analytically correlate measured surge line temperatures and displacements; and to confirm the validity of the stratified flow analysis procedure. The generic nature of the thermal loading on the surge line as well as the correlation between the measured data and plant operations observed during surge line stratified flow test are also discussed. (author). 6 refs., 13 figs., 2 tabs