800℃ 에서 Alloy 800H의 크리프 균열 성장 속도의 확률론적 평가

Abstract

A very high temperature reactor (VHTR) is a fourth-generation reactor that uses helium as a coolant and graphite as a moderator. It could generate mass production of hydrogen and high-efficiency electricity using high thermal energy at 950oC without CO2 emissions. Alloy 800H is the primary candidate for use a control rod system (CRS), core supports, a core barrel, a HGD, and a shutdown cooling system (SCS) in VHTR system. Alloy 800H, which is a modification of Alloy 800, was developed for applications in which require additional creep resistance. Alloy 800H is approved for use up to 760oC under ASME Code Section III Subsection NH for nuclear applications. Many studies for its creep behavior were sufficiently done, but the study for the creep crack growth (CCG) behavior was rare and the CCG data were not also available in currently elevated temperature design (ETD) codes; such as, the ASME or RCC-MRx. Therefore, the CCG behavior for Alloy 800H should be fairly evaluated for a design consideration, and also, the CCG data should be provided for design use of Alloy 800H. In this study, a probability for evaluating the creep crack growth rate (CCGR) of Alloy 800H was investigated through a series of the tension, creep, and CCG tests at 800oC. In the evaluation of the CCGR behavior, the C* fracture parameter was used, and the material constants of B and q used in the CCGR equation, da/dt =B(C*)q were statistically analyzed and compared in terms of three methods: probabilistic distribution method (PDM), a mean value method (MVM), and the commonly used a least square fitting method (LSFM). From the results of the CCGR evaluation, it was found that the LSFM was similar when compared with the PDM and MVM. It was clarified that the B and q constants followed the PDM using a log-normal distribution method, because it has higher value of correlation factor. The PDM using the log-normal distribution was more favorable than the LSFM, because it could assess the CCGR lines from the probabilistic viewpoints. Using the PDM method, a huge number of random values of the B and q were successfully generated by Monte Carlo Simulation (MCS) technique. The CCGR lines from 10% to 90% in the ranges were drawn from the probabilistic viewpoints. Fracture micrograph exhibited the ductile dimple fracture on the micrograph of crack propagation, and crack propagation occurred from the interconnection of the micro voids along grain boundary. It is expected that the results of this study can be utilized for evaluating the CCGR in various heat resistant steels. In addition, it is useful to achieve the ergonomic design because a probability technique in the CCGR assessment.