Thermal creep modeling of HT9 steel for fast reactor applications
Introduction
Ferritic martensitic steels (FMS) composed of 9–12% Cr such as HT9 are often used as high temperature structural materials in coal power plants and gas power plants. FMS have a higher thermal conductivity and a lower void swelling under irradiation than austenitic stainless steels [1], [2]. Thus, HT9 is a primary candidate material for the fuel cladding material of advanced nuclear reactors such as liquid–metal-cooled fast reactors (LMFRs) owing to its excellent irradiation stability.
The nominal alloy composition of HT9 is given in Table 1. The development history of FMS with 9–12% Cr is listed in Table 2; it shows their rupture strength and maximum use temperature have been improved. Although HT9 belongs to the first generation FMS, which were developed as early as the 1960s, it is still a viable cladding material for LMFRs, small modular fast reactors (SMFRs), and advanced burner reactors (ABRs), mainly because of its proven performance records [3], [4].
The creep deformation of the cladding is not a critical performance issue for a typical LMFR fuel design provided with a smeared density of about 75% up to a burnup of about 20% [4]. However, for fuel designs that require a higher burnup, long life (e.g., 15–30 years of fuel life), or higher temperature, creep deformation may be a life-limiting factor. Hence, an improved creep correlation that is applicable to such design conditions is desired.
Although the thermal creep of HT9 has been studied for decades for both nuclear and non-nuclear applications [5], [6], [7], [8], [9], inconsistencies still exist in the correlations and the experimental data. In the present study, the test data available for the thermal creep tests of HT9 were assessed. A new generalized correlation, applicable to both typical- and long-life fuel designs, was developed for the performance evaluation of LMFR fuel cladding.
Section snippets
Existing correlations for creep strain
There are a few thermal creep correlations of HT9 steel that are available in the literature. The most widely accepted are the correlations developed by Amodeo and Ghoniem [5], [6] and by Lewis and Chuang [7].
The correlation by Amodeo and Ghoniem [5] employs the minimum commitment method (MCM) and uses time dependent creep strain curves at 873 K, as reported by Sandvik Steels [7]. The MCM is based on the observation that the 1% strain occurs during either the primary creep regime or at the
Creep strain and rupture stress correlations
A new creep correlation was developed in this study to overcome the limitations of the existing creep correlations for HT9 (i.e., MCM and TPM correlations). There are two significant aspects of the new correlation. Firstly, it should be noted that the phenomenological background of the existing correlations is insufficient to describe the creep deformation behavior of HT9 steels. Although the steady-state creep rates can easily be obtained from the literature, using typically available
Discussion
Thus far, our paper has focused on HT9 thermal creep correlations. However, since nuclear fuel cladding is commonly used in high energy neutron irradiation environments, it is important to consider the effect of irradiation. Irradiation displaces atoms from their equilibrium position to form vacancies and interstitials. These form voids and dislocation loops, thereby affecting creep deformation. The question arises as to what extent the irradiation enhancement influences the thermal creep.
Conclusions
The thermal creep strains of HT9 predicted using the correlations available in the literature have been found to be inconsistent. In some cases, they are also inconsistent with the measured data. In this work, a thermal creep correlation based on the Garofalo equation has been developed, and it has been shown to be applicable to both the primary and steady-state creep regimes. For the tertiary creep regime, an implicit model has been proposed by using the modified Monkman–Grant equation. The
Acknowledgments
This study was supported in part by Ministry of Education Science and Technology (MEST) of Korea, and in part by the UChicago Argonne, LCC as Operator of Argonne National Laboratory under Contract No. DE-AC-02-06CH11357 between the UChicago Argonne, LLC and the US Department of Energy.
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