Stress and Fatigue Analysis Considering Thermal Stratification With the THERMAXS Code

This paper describes the stress and fatigue analysis of the feed water nozzles of the replacement steam generators of the Doel 2 NPP in Belgium. In the framework of the steam generators replacement, thermal stratification transients were considered in the stress and fatigue analyses of the feed water system components such as the feed water lines, their reactor building penetrations and the steam generator feed water nozzles. To do so, long term external wall temperature measurement have been performed on one feed water line of Doel 2 between the steam generator replacement in 2004 and December 2007. From those measurements a number of typical stratification phenomena were identified. For each phenomenon, a design stratification transient and its number of occurrences were derived from the measurements. A fatigue analysis and primary+secondary stress intensity analysis of the feed water nozzles of the Doel 2 NPP was performed taking into account the presence of the design thermal stratification transients in the feed water lines and feed water nozzles. The fatigue analysis was performed according to the rules of the ASME Boiler & Pressure Vessel Code, Section III, Division 1 – Subsection NB-3200 using the in-house developed computer code THERMAXS that is capable of dealing with measured stratification transients. For the analysis, one may finally conclude that the fatigue, P+Q stress intensity range and thermal ratcheting criteria are respected throughout the 40 years of plant life.