Uncertainties of creep model in stress analysis and life prediction of graphite component

The irradiation-induced creep is a key factor in stress analysis and life prediction of nuclear graphite in high temperature gas-cooled reactors (HTRs). Numerous creep models have been established and good agreements have been observed with uni-axial creep experiments. However, the effect of creep strain ratio has not been fully addressed, and the primary creep strain is considered in some cases less important in comparison with the secondary one. These uncertainties in creep model might result in large discrepancies in the evaluation of stresses and service lives of graphite components. In this paper, the variation of creep strain ratio and the impact of the primary creep strain are studied numerically and the corresponding discrepancies in stresses and life prediction of graphite components in HTRs are discussed. Two implicit formulations of the incremental finite element solution for the parameter variations of creep models are presented and integrated into a finite element code developed by INET. The numerical results show that both increase of the creep strain ratio and absence of the primary creep strain will lead to an increase of stress levels and decrease of service life dramatically, suggesting that uncertainties of creep models have to be taken into account in the design of graphite components in HTRs.