Assessment of the Real Fatigue Usage of the Volume Control System

As a major task of aging management, the quality status of every safety related system has to be assessed regularly. Thus the safety margin can be verified (and demonstrated) at every stage of operation and — if necessary — it can be reacted in time. One of the main aspects regarding the assessment of the quality status is the fatigue usage. Regarding the example of the volume control system of the nuclear power plant KKP 2 (PWR), the procedure to determine the actual fatigue usage is shown. There is a comprehensive monitoring of the global and the local loads of this system. The global monitoring is done using the plant instrumentation, the local monitoring is performed on the basis of individually installed transducers like thermocouples, displacement transducers, etc. Using the results of the monitoring the load cases are identified first and then their partial fatigue usage factors are summarized using the number of cycles determined by monitoring, usually. Thus it is demonstrated that all fatigue usage factors are below limits. However, if there are relevant new (i.e. not specified) transients these have to be analyzed and assessed. Regarding the volume control system the local monitoring showed “new” transients (thermal stratification) near a recuperative heat exchanger that were due to bypass operation. Consequently, the real actual fatigue usage was evaluated. In a first step, the relevant load cases (that were identified during the “local” monitoring history from 1991) were re-analyzed and correlated to the global signals. On this basis, the relevant transients and the number of cycles of the operation time without local monitoring (first startup of the plant in 1984) have been determined using the global signal records (digital and paper charts stored in archive). A conservative fatigue analysis showed that the maximum usage is related to the region of the inlet nozzles of the volume control system into the main coolant loops. With the proven history of transients the maximum fatigue usage D of this system was extrapolated to D = 0.27 (60 years of operation). The comprehensive global and local monitoring will be continued. On this basis the fatigue usage will be reviewed, regularly, and updated if necessary in future operation.Copyright © 2008 by ASME