STRESS-ANALYSIS OF A 900-MW PRESSURIZER SURGE LINE INCLUDING STRATIFICATION EFFECTS

Following temperature monitoring programmes performed on 900 MW pressu rized water reactor pressurizer surge lines, it has been reported that those lines are stratified in steady state, owing to their geometry. The highest temperature difference occurs during reactor heat-up and c ool-down, reaching 110 degrees C. Obviously, this phenomenon was not c onsidered in nuclear steam supply system (NSSS) design transients and stress reports. Based on Electricite de France and FRAMATOME experienc es, such as temperature measurements on site and mock-up, and thermal hydraulic computations, NSSS transients are updated. Stratification co nditions are defined in different cross-sections of the line, using pr essurizer temperature, hot leg temperature and flow rate, through the Froude number. A complete stress analysis of surge lines is performed including the updated transients and bending moment increase due to st ratification. First of all different sensibility studies are carried o ut in order to simplify assumptions. Using a two-dimensional-one-dimen sional method developed by FRAMATOME, the usage factor is then compute d in different cross-sections, distinguishing upper and lower parts. I n the presence of stratification, the surge line is subjected to therm al stresses following thermal shocks and to bending moment variation. These two load types are studied vs. time in order to reduce conservat ism present in usual analyses.