Stress analysis of a 900 MW pressurizer surge line including stratification effects

Following temperature monitoring programmes performed on 900MW pressurized 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 cool-down, reaching 110 C. Obviously, this phenomenon was not considered in nuclear steam supply system (NSSS) design transients and stress reports.Based on Electricite de France and FRAMATOME experiences, such as temperature measurements on site and mock-up, and thermal hydraulic computations, NSSS transients are updated. Stratification conditions are defined in different cross-sections of the line, using pressurizer 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 stratification. First of all different sensibility studies are carried out in order to simplify assumptions.Using a two-dimensional-one-dimensional method developed by FRAMATOME, the usage factor is then computed in different cross-sections, distinguishing upper and lower parts. In the presence of stratification, the surge line is subjected to thermal stresses following thermal shocks and to bending moment variation. These two load types are studied vs. time in order to reduce conservatism present in usual analyses. ((orig.))