Thermal transient test and strength evaluation of a tubesheet structure made of Mod.9Cr–1Mo steel. Part II: Creep-fatigue strength evaluation

doi:10.1016/j.nucengdes.2014.04.029

Nuclear Engineering and Design

Volume 275, August 2014, Pages 422-432

https://doi.org/10.1016/j.nucengdes.2014.04.029 Get rights and content

Highlights

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The strength of a tubesheet model subjected to cyclic thermal transients was evaluated.
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Heat transfer analysis and stress analysis were performed.
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The failure life was evaluated by the several methods using finite element analyses.
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Failure life could be predicted within a factor of 3 using the inelastic finite element analyses.

Abstract

The tubesheet structure is one of the components that suffer the most severe loading in fast reactors, and it is one of the most difficult components to design because of such severe operation conditions and its complex three-dimensional structure with an arrangement of numerous penetration holes. In this study, the strength of a tubesheet test model simulating a semispherical tubesheet structure subjected to cyclic thermal transients was evaluated using the finite element analysis (FEA). A test model made of Mod.9Cr–1Mo steel was subjected to 1873 cycles of severe thermal transient loading using a large-scale sodium loop, in which elevated-temperature sodium at 600 °C and 250 °C was flowed repeatedly and kept at the final temperature for 2 and 1 h, respectively. Heat transfer analysis and stress analysis were performed using the sodium temperature data measured during the test. The boundary conditions were adjusted to simulate the measured temperature distribution on the inner and outer surfaces of the test model in the heat transfer analysis, and the result was used for the stress analysis. Then, the elastic and inelastic stress analysis results were used to investigate the failure mechanism by creep-fatigue damage and evaluate the failure strength. The evaluation based on the results of inelastic analysis estimated the number of cycles to failure within a factor of 3 of the total number of thermal loading cycles 1873, which corresponds to the number of cycle at which the crack reached 2.59 mm.

Introduction

Mod.9Cr–1Mo steel is a candidate material for the primary and secondary heat transport system components of the Japan sodium-cooled fast reactor (JSFR) (Aoto et al., 2011). However, there is little hard evidence to support the structural integrity of components made of Mod.9Cr–1Mo steel under actual environments. Therefore, a thermal cyclic test was performed with a tubesheet model simulating the center-flattened spherical tubesheet (CFST) (Ando et al., 2013a). The test results were summarized in the associated paper (Ando et al., 2014), which includes the details of the tubesheet model design and the test procedure.

Since the CFST was an original design and no data validating its structural integrity are available, the main objectives of this study are to clarify the failure mode and mechanism and to validate the applicability of the strength evaluation methods for the tubesheet model. To achieve these objectives, finite element analysis (FEA) was performed using the temperature data measured during the test, and the calculated stress and/or strain data were compared to the test results to analyze the failure mode of the tubesheet model simulating the CFST structure. To validate the strength evaluation methods based on elastic and inelastic FEA, the experimentally obtained strength data reported in the associated paper (Ando et al., 2014) were used.

The CFST was designed as the tubesheet structure of steam generator (SG) to satisfy several requirements for the SG of the JSFR (Chikazawa et al., 2012, Kurome et al., 2010, Futagami et al., 2009). In this SG, the planned pressure of the outlet steam is 19.2 MPa with temperature reaching 497 °C under normal operation. Mod.9Cr–1Mo steel is planned to be adopted as the material of the SG, because this steel has both excellent thermal properties and high-temperature strength with good stress corrosion cracking resistance.

To validate the manner of failure in the originally developed CFST under cyclic thermal transients, a tubesheet model simulating the CFST was designed and a cyclic thermal loading test was performed. Since sodium has a large thermal capacity with a low pressure, the strength test was performed in a sodium environment, although the real tubesheet in the actual SG is used in a steam environment, where water flows from the lower plenum to the upper plenum through the heat exchanger tube. However, sodium flows from the upper plenum to the penetration holes of the tubesheet in this test, which makes the environment different. Nevertheless, the location of crack initiation, the distribution of cracks, and the direction of crack propagation were supposed to be simulated in the test. In fact, the stress inducement mechanism in the test model was comparable to that of the CFST, because the mechanism in the CFST under the thermal transition was analyzed and considered in the design of the test model (Ando et al., 2013a).

In the test, hot and cold sodium were supplied from the inlet nozzle and then flowed into the upper plenum with a constant rate of 100 l/min. During the hot transient, sodium heated to 600 °C was flowed into the test model, and a constant sodium flow was maintained for 2 h. During the cold transient, sodium heated to 250 °C was flowed into the test model, and a constant sodium flow was maintained for 1 h. The 2 h hold at 600 °C after the hot transient was chosen to generate creep damage due to stress relaxation. The 1 h hold at 250 °C after the cold transient was chosen to eliminate the temperature distributions in the test model and the TTS components for the following cycle. The electromagnetic pumps installed in each circuit enabled the temperature change rate of the flowing sodium to be controlled at 5 °C/s. A total of 1873 thermal transient cycles were applied to the test model. Afterward, the test model was removed from the TTS. Then, the test model was inspected by performing liquid penetrant testing (PT) on the outer surface and cut to perform PT on the inner surface. No cracks were observed on the inner or outer side of the test model, except for on the hole edges of the tubesheet. In fact, many cracks were observed by PT on the upper surface of the tubesheet and on the inner surfaces of the penetration holes. Therefore, only the tubesheet structure was examined in this study. Details of the test model, experimental procedure and results of the PT and hardness test were summarized in the associated paper (Ando et al., 2014).

For the creep-fatigue-life evaluation, the environmental effect of sodium was not considered in this study. According to several reports regarding Mod.9Cr–1Mo steel, its fatigue life is significantly improved in a sodium environment (Asayama et al., 2001, Kannan et al., 2009). In contrast, the creep-fatigue life is not improved in a sodium environment (Asayama et al., 2001), and the same results were obtained under high-vacuum conditions (Riou, 2008). Overall, these results suggest that internal creep damage becomes predominant in creep-fatigue tests. In particular, crack initiation on the surface is retarded in a sodium environment because oxidation was suppressed under an oxygen density of less than 6 ppm.

Section snippets

Model for the finite element analysis

Both elastic and inelastic FEAs were performed using the FINAS code (Japan Atomic Energy Agency, 2008). Here, inelastic FEA means an elastic–plastic–creep FEA. The FEA model is shown in Fig. 1, and the dimensions were based on those of the test model (Ando et al., 2014). A three-dimensional (3D) FE model with 30° sectors was used to evaluate the peak stress generated by the stress concentration around the hole edges. The 8-node quadrilateral axisymmetric elements HQAX8/QAX8 of the FINAS code

Characterization of the failure

In the associated paper, it was observed that the failure mode was crack initiation at the surface of the penetration hole due to cyclic loading and propagation of these cracks in the creep-damaged area near the upper surface of the tubesheet (Ando et al., 2014). This failure mode was supported by the fracture surface observation and hardness test. To confirm this failure mode, the FEA results were further analyzed. In particular, the relationship between the crack initiation/propagation region

Conclusion

The result of a cyclic thermal transient test of a tubesheet model made of Mod.9Cr–1Mo steel was evaluated using FEA. To simulate the stress and strain in the complex 3D structure of the CFST, a 3D FEA model was used in the evaluation. Based on the measured temperature on the surface of the test structure and the measured sodium temperature, the thermal distribution during the test was simulated using adjusted thermal coefficients. Elastic and inelastic stress analyses were conducted using

Acknowledgements

This paper includes results of the “Technical development program on a commercialized FBR plant” entrusted to the Japan Atomic Energy Agency (JAEA) by the Ministry of Economy, Trade and Industry of Japan (METI). The authors wish to express their gratitude for the enthusiastic effort in the design and fabrication of the test model by Mr. Usui and co-workers of Mitsubishi Heavy Industry, Ltd. The authors are also grateful to Mr. Osamu Inoue of IX Knowledge, Inc., for performing the FEA,

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¹: Tel.: +81 29 267 4141; fax: +81 29 267 2397.

²: Present address: The University of Tokyo/Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. Tel.: +81 3 5841 0247; fax: +81 3 5841 0247.

³: Tel.: +81 78 672 5099; fax: +81 78 672 3435.

⁴: Tel.: +81 78 672 3418; fax: +81 78 672 3435.

⁵: Present address: Japan Atomic Energy Agency, Oarai-cho, Higashi-ibaraki, Ibaraki 311-1393, Japan. Tel.: +81 29 267 4141; fax: +81 29 267 1676.

View full text

Thermal transient test and strength evaluation of a tubesheet structure made of Mod.9Cr–1Mo steel. Part II: Creep-fatigue strength evaluation

Highlights

Abstract

Introduction

Section snippets

Model for the finite element analysis

Characterization of the failure

Conclusion

Acknowledgements

J. Nucl. Eng. Des.

J. Nucl. Mater.

An International Code 2010 ASME Boiler & Pressure Vessel Code 2011a Addenda, Section II, Part D (Customary), Materials

Comparison of creep-fatigue evaluation methods with notched specimens made of Mod.9Cr–1Mo steel

Verification of the prediction methods of strain concentration in notched specimens made of Mod.9Cr–1Mo steel

J. Press. Vessel Technol.

A study on fatigue and creep-fatigue life assessment using cyclic thermal tests with Mod.9Cr–1Mo steel structures

Stress mitigation design of tubesheets with consideration of thermal stress inducement mechanism

J. Press. Vessel Technol.

Development of 2012 Edition of JSME Code for Design and Construction of Fast Reactors (6) Design margin assessment for the new materials to the rules

Thermal transient test and strength evaluation of a tubesheet structure made of Mod.9Cr–1Mo steel. Part I: Test model design and experimental results

J. Nucl. Eng. Des.

Design study and R&D progress on Japan sodium-cooled fast reactor

J. Nucl. Sci. Technol.