A Fatigue Evaluation Approach for Graphite Core Components Using a Reduced Dataset

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Abstract Components manufactured from graphite are commonly used in nuclear fission reactor cores, where they are subjected to cyclic stresses, e.g., due to temperature changes during reactor start-up, shut-down, and load-following operations. Fatigue from cyclic stresses may result in a reduced load-bearing capacity and initial cracking of Graphite Core Components (GCCs). Therefore, fatigue must be evaluated to qualify GCCs. Paragraph HHA-3144 of the ASME Boiler and Pressure Vessel Code Section III Division 5 states that the graphite fatigue rules are in course of preparation. As such, there are neither formal requirements for testing nor a universal approach for the evaluation of GCCs. While a universal method to assess fatigue in GCCs may be difficult to define, this paper proposes an approach to evaluate a graphite grade used in High Temperature Gas-cooled Reactor designs. By exclusively using the low cycle fatigue data from multiple stress ratios, a Modified Goodman Diagram is constructed, to evaluate the fatigue life to the full range of cycles (up to 106 cycles). This paper demonstrates that the approach is conservative and sufficient. Compared to previous approaches, this method requires a reduced fatigue test dataset, which is advantageous for designers as it is more time and cost-effective.

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