Session: CS-01-02/03-01 Structural Integrity of Pressure Components & Environmental Fatigue Issues (Joint M&F, D&A)

Paper Number: 60433

Start Time: Tuesday, July 13, 2021, 09:00 AM

60433 - Review of Determining Transformed Temperature Over 325℃ Based on Monte Carlo Simulation for Environmental Fatigue Evaluation 

NUREG/CR-6909, which had provided the technical background for the environmental fatigue evaluation, was revised in May 2018 to incorporate the Japan Nuclear Energy Safety(JNES)’s extensive fatigue test results and to address interested stakeholder’s concerns. The revised NUREG/CR-6909 set limits of the temperature range to 325℃ for the application of the environmental fatigue correction factor(Fen). Many stakeholders inquired of the U.S. Nuclear Regulatory Commission(NRC) the reason for the temperature limit in the revised NUREG/CR-6909. And U.S.NRC responded that the limits of temperature range to 325℃ had been determined to cover the expected temperature in the most anticipated LWR operating condition and it is adequate for expected operating LWR conditions, especially when considering the use of average temperature.
The use of the average temperature for the environmental fatigue evaluation is only applicable to the Average Strain Rate Approach(ASRA), which determines the transformed temperature using the average temperature in a strain transient. Also, in accordance with Section 4.4 of the revised NUREG/CR-6909, the ASRA is only available to be used if it yields conservative estimate of the Fen compared to the Modified Strain Rate Approach(MSRA), which determines the transformed temperature using the average or maximum temperature of a small integration time step in a strain transient. However, the temperature over 325℃ is not applicable to the MSRA due to the transformed temperature limit in the revised NUREG/CR-6909. Consequently, no justification can be made for the use of Fen based on the ASRA application. Therefore, it is necessary to develop the transformed temperature over 325℃ for application of the revised NUREG/CR-6909.
In this paper, the Monte Carlo analysis was performed to determine the transformed temperature over 325℃ for carbon steels, low-alloy steels, Ni-Cr-Fe alloys, and austenitic stainless steels, which are not covered in the revised NUREG/CR-6909. To validate the Monte Carlo procedure used in this paper, the Monte Carlo simulations were re-performed to find out the adjustment factors, and then the re-calculated adjustment factors were compared to the values presented in the revised NUREG/CR-6909.
JNES had developed its own environmental fatigue life models using extensive fatigue test results, and published JNES-SS-1005 to evaluate the environmental effects on the fatigue lives. The transformed temperature over 325℃ is presented in JNES-SS-1005. Also, the major reason for revising NUREG/CR-6909 was to re-evaluate the environmental fatigue life models in the original NUREG/CR-6909 by incorporating much larger database from JNES. Therefore, in determining the appropriate transformed temperature over 325℃ for application of the revised NUREG/CR-6909, the environmental fatigue life models in the revised NUREG/CR-6909 and JNES-SS-1005 were used to compare the fatigue lives in LWR environments. The transformed parameters, which affect the fatigue lives in LWR environments such as temperature, strain rate, dissolved oxygen level, and sulfur content of the steel, were selected randomly to determine the population of fatigue lives in LWR environments. The random parameters were determined using the uniform distribution, and a total of 100,000 simulations were performed. And then the population of fatigue lives in LWR environments were fit to a lognormal curve to estimate the cumulative distribution with the 95/95 confidence level. The 95th percentile values of cumulative distributions using the environmental fatigue life models were compared with each other, and the appropriate transformed temperatures over 325℃ for carbon steels, low-alloy steels, Ni-Cr-Fe alloys, and austenitic stainless steels have been determined for application of the revised NUREG/CR-6909.

Presenting Author: Bonghee Lee KEPCO Engineering & Construction Company

Authors:

Bonghee Lee KEPCO Engineering & Construction Company
Ilkwun Nam KEPCO Engineering & Construction Company