Thermo-Mechanical Distortion of Tungsten-Coated Steel During High Heat Flux Testing Using Plasma Arc Lamps

Abstract

An experimental setup and a test section were designed and fabricated for high heat flux testing (HHFT) of neutron-irradiated specimens using water-wall plasma arc lamps. Because of the radiological considerations and limitations of reactor irradiation, the size of the test articles was limited to disks less than 10 mm in diameter. The specimen was clamped onto an actively cooled block, and clamping allowed the insertion of several thermocouples on the back surface of the specimen through a copper (Cu) block. Five vacuum plasma sprayed tungsten (W)–coated F82H steel specimens were subjected to HHFT. Surface profilometry measurements, which were conducted after HHFT, revealed central bowing of the top W surface. This type of residual distortion occurred for all of the specimens, and the larger the specimens were, the larger was the distortion.

In an attempt to understand specimen distortion and address the science questions related to the testing of subsize specimens during HHFT, a simplified thermo-mechanical model was developed. By using a measured temperature in the Cu as an isothermal boundary condition, the model eliminated the need for coupling cooling fluid flow models with stress models, greatly simplifying the analysis. The main variable in the proposed model is h_C, i.e., the thermal contact conductance between the F82H and the Cu washer. Inelastic properties, including hardening properties, were considered for F82H steel and Cu. Numerical simulation results demonstrated a buildup of residual deformation during HHFT and a very complex state of stress and deformation during typical heat flux (HF) cycling. Hoop stress evolution during a high heat flux cycle reveals that F82H at an interface with W would be mainly in compression during HF application and experienced a transition to a tension state during cooldown. Also, specimen distortion evolves during each HF cycle, as the specimen bows downward during HF application and upward during the cooldown period between HF cycles. The final specimen distortion, i.e., upward bowing of the specimen center, was qualitatively predicted for h_C values of 4000 to 5000 W/(m²·K). This h_C range of values, for which bulging is obtained, is at the lower spectrum of the range of values for h_C, consistent with the low thermal contact conductance expected from the unpolished F82H surface.

Keywords:

• High heat flux testing
• infrared
• divertor
• thermo-mechanical
• stress

Acknowledgments

This work was supported by the U.S. Department of Energy Office of Fusion Energy Sciences under contract DE-C05-00OR22725 with UT-Battelle, LLC, and the “Technological Assessment of Plasma Facing Components for DEMO Reactors” Japan/U.S. Fusion Research joint project. The authors would like to thank E. Ohriner for selecting clamping material-based strength, oxidation, and fabrication considerations; Charles Ross (Chuck) Schaich for creating the drawings and coordinating the fabrication of the Cu rod; David Harper for operating the PAL; John J. Henry for profilometry; and Jim O. Kiggans for vacuum operation.

Disclosure Statement

No potential conflict of interest was reported by the author(s).

Correction Statement

This article has been corrected with minor changes. These changes do not impact the academic content of the article.