Fatigue is an important ageing effect to manage for long term operation (LTO) of nuclear power plants (NPPs). One of the steps in the process of LTO assessment is the revalidation of TLAAs (time limited ageing analyses). The safety margins of NPP Borssele with respect to fatigue were demonstrated by projecting the fatigue analyses to the end of the intended period of LTO. Besides this, it has to be ensured that the analyses remain valid during the entire period of LTO. Periodic verification of the load assumptions that are made in the analyses is therefore an important aspect of adequate fatigue management. In this paper, an integrated fatigue management approach is presented, coupling load monitoring, transient counting and fatigue assessment.

The approach for periodic verification of the load assumptions in the fatigue analyses consists of two parts. First, it is verified whether the occurred numbers of cycles of the different load cases remain smaller than the numbers of cycles assumed in the fatigue analyses. Secondly, it is verified whether the thermal transients defined for the load cases in the fatigue analyses conservatively represent the occurred thermal transients. For the verification, the application LEAF (Load Evaluation Application for Fatigue) was developed. At NPP Borssele, thermal transients occurring during different load cases (e.g. start-up, shutdown, reactor trip) are registered by a temperature monitoring system. The load evaluation application processes the measurement data and verifies the conservatism of the load conditions in the fatigue analysis of the fatigue relevant components in the system. This paper explains the steps that are followed for the load evaluation and gives a demonstration of the results. The presented procedure is an essential part of adequate fatigue management during LTO.

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