Thermal stratification can occur in pressurizer surge lines when flow rates are low and large temperature differences between the pressurizer fluid and reactor coolant loop fluid exist. NRC Bulletin 88-11 requires that utilities address this issue with respect to the licensing basis. Additionally, significant fatigue usage can develop due to insurges and outsurges from the pressurizer that occur when the temperature difference (ΔT) is large. An important aspect of the surge line fatigue analysis is determination of global pipe moment loads due to insurges, outsurges, and related stratification effects.
The Westinghouse AP1000® plant surge line is designed to limit the global effects of stratification using a surge line geometry that is inclined more than that of the previous generation plants. However, this design provides unique challenges to the analytical solution in comparison to the mostly horizontal geometries in previous PWR designs. The main challenge is due to the location of the stratified fluid hot/cold interface. The horizontal geometry has historically allowed conservative (based on analytical testing) treatment of the stratified interface profile. The inclined geometry requires consideration of positional changes, as the stratified interface moves through the line due to flow changes. Because of this, iterative calculations were required to predict global piping moment loads through the transient histories based on the movement of the stratified interface. Software was developed to solve a series of correlative equations representing moment loads due to pipe temperatures and stratification ΔT within discretized portions of the surge line throughout each transient. The correlative equations, which are based on the matrix solution of piping analysis load cases, predict the global pipe moment loads to a high degree of precision.
The surge line is also a significant location in the online fatigue monitoring program implemented at the sites. The complexity of the design analysis that is reflected in the monitoring model requires a more detailed accounting for the location of the stratification interface, the effect on prediction of moment loadings for controlling locations, and the effect on local stresses at the monitoring locations. This paper describes the approach used to address all of these factors in the design evaluation and subsequent fatigue monitoring application.