HTGR Knowledge Base

Conference Article: The effect of creep-fatigue damage relationships upon HTGR heat exchanger design

Kozina, M.M.; King, J.H. (GA Technologies Inc., San Diego, CA (United States)); Basol, M. (Combustion Engineering Inc., Tennessee (United States))

Abstract

Materials for heat exchangers in the high temperature gas-cooled reactor (HTGR) are subjected to cyclic loading, extending the necessity to design against fatigue failure into the temperature region where creep processes become significant. Therefore, the fatigue life must be considered in terms of creep-fatigue interaction. In addition, since HTGR heat exchangers are subjected to holds at constant strain levels or constant stress levels in high-temperature environments, the cyclic life is substantially reduced. Of major concern in the design and analysis of HTGR heat exchangers is the accounting for the interaction of creep and fatigue. The accounting is done in conformance to the American Society of Mechanical Engineers Boiler and Pressure Vessel Code, Code Case N-47, which allows the use of the linear damage criterion for interaction of creep and fatigue. This method separates the damage incurred in the material into two parts: one due to fatigue and one due to creep. The summation of the creep-fatigue damage must be less than 1.0. Recent material test data have indicated that the assumption of creep and fatigue damage equals unity at failure may not always be valid for materials like Alloy 800H, which is used in the higher temperature sections of HTGR steam generators. Therefore, a more conservative creep-fatigue damage relationship was postulated for Alloy 800H. This more conservative bilinear damage relationship consists of a design locus drawn from DF=1.0, DC=0 to DF=0.1, DC=0.1 to DF=0, DC=1.0. DF is the fatigue damage and DC is the creep damage. A more conservative damage relationship for 2-1/4 Cr-1 Mo material consisted of including factors that degrade the fatigue curves. These revised relationships were used in a structural evaluation of the HTGR steam cycle/cogeneration (SC/C) steam generator design. The HTGR-SC/C steam generator, a once-through type, is comprised of an economizer-evaporator-superheater (ESS) helical bundle of 2-1/4 Cr-1 Mo tubes followed by a superheater of straight tubes of Alloy 800H in the central zone of the steam generator. The high-temperature components affected by creep-fatigue interaction are the tubing and the superheated steam tubesheet of Alloy 800H. The effects of the revised creep-fatigue damage relationships were evaluated by: (1) calculating the temperature-dependent allowable strain range with the assumed bilinear damage relationship for Alloy 800H. (2) calculating the temperature-dependent allowable strain range with reduced fatigue allowables for 2-1/4 Cr-1 Mo. And (3) predicting the strain range in the critical parts by extrapolation of finite element calculations for the second or last cycle analyzed for each transient to the expected number of cycles and hold times. The preliminary analyses indicate that the Alloy 800H tubing and tubesheets have predicted strains substantially under the allowables based upon the bilinear damage relationship but that the 2-1/4 Cr-1 Mo tubing at the hot end inner radius portion of the EES tube bundle presents a slightly overstressed situation. It is believed that there is sufficient design latitude to resolve this problem in the continuing preliminary design. It is concluded that the HTGR-SC/C steam generator design has sufficient margin to accommodate the more conservative creep-fatigue damage relationships.

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key words: Gas Cooled Reactor, Nuclear Technology
Reference:
Specialists' meeting on heat exchanging components of gas-cooled reactors Duesseldorf (Germany) 16-19 Apr 1984
International Atomic Energy Agency, International Working Group on Gas-Cooled Reactors, Vienna (Austria)
IWGGCR--9, pp:377-388