This paper focuses on reliability assessment of creep rupture life under the service conditions at high temperatures and stresses for Gr. 91 steel which is considered as one of the prime structural materials for next-generation nuclear reactors. An interference model based on Z parameter, which is considered for the fluctuations of both service temperature and stress besides the scattering of rupture data, was analyzed and used to assess the reliability on creep rupture life of Gr. 91 steel. The scattering distribution of the creep rupture data of the Gr. 91 steel was investigated by using the Z parameter. It appeared that the Z parameter of creep rupture data for Gr.91 steel exhibited normal distribution. Using the normal distribution, a Monte-Carlo simulation (MCS) was carried out to generate a number of random variables for Zs and Zcr, and the reliability of the creep rupture life under the fluctuations of service temperature and stress conditions was estimated by using the interference model. It showed that the value of reliability decreased with increasing service time. Higher temperature caused the trend of faster deterioration. The value of reliability decreased rapidly at higher temperature fluctuation amplitude, and the reliability decreased as the scattering of the creep rupture data became serious.

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