Thermal stratification in nuclear reactor piping system

Thermal stratification, cycling, and striping (TASCS) issue has drawn attention recently because of the incidents at several nuclear plants relative to thermal fatigue in piping systems connected to the main coolant piping. U.S. nuclear utilities are addressing the issue in response to the concerns. In particular, the Electric Power Research Institute (EPRI) initiated a major research program to resolve the TASCS issue. In Phase 1 research, a methodology and program have been developed to conduct detailed research into mechanisms which lead to fatigue in nominally stagnant piping systems near the reactor coolant piping. Three key efforts from the Phase 1 program are described in this paper. First, the line evaluation methodology is described, which also leads to requirements for the Phase 2 program. Second, tests to investigate interaction between main coolant piping and stagnant attached lines by turbulence penetration are described. Turbulence penetration into unisolable lines, or the transport of turbulence into stagnant piping from the reactor coolant system (RCS) line, represents a mechanism for carrying hot RCS water into regions filled with colder water. The possibility of stratification of the two fluids (and the resultant stresses) are the reason for developing an understanding of the turbulence penetration process. Lastly, results of an evaluation to develop a loading definition for thermal striping are included. Future plans and prospective results are also discussed. (author)