Development of a nondestructive inspection method for irradiation-induced microstructural evolution of thick 304 stainless steel blocks J. Etoh a,⇑ , M. Sagisaka a , T. Matsunaga a , Y. Isobe a , F.A. Garner b , P.D. Freyer c , Y. Huang d , J.M.K. Wiezorek e , T. Okita f a Nuclear Fuel Industries, Ltd., Osaka, Japan b Radiation Effects Consulting, Richland, WA, USA c Westinghouse Electric Company LLC, Pittsburgh, PA, USA d University of Wisconsin, Madison, WI, USA e University of Pittsburgh, Pittsburgh, PA, USA f The University of Tokyo, Tokyo, Japan article info Article history: Available online xxxx abstract Ultrasonic testing was conducted on two long, Type 304 stainless steel blocks with a hexagonal cross-sec- tion that were removed from the reflector region of the decommissioned EBR-II reactor. One block had a dose range of 17–33 displacements per atom (dpa) and based on dimensional measurements exhibited a maximum of 2% average density decrease across its thickness. The second block had a dose range of 0.3–4 dpa, and exhibited smaller but positive range of density changes. Comparison of the ultrasonic measurements and the spatial variations in density change, as well as local swelling arising from voids and precipitates as determined by electron microscopy illustrate excel- lent agreement. Furthermore, this study clearly revealed that radiation-induced microstructural features produce measurable changes in elastic modulus and ultrasonic velocity. These results clearly demon- strate that ultrasonic techniques can be used to nondestructively measure the average swelling across a thick component. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Austenitic stainless steels have been and will continue to be used as core components for both light water cooled reactors and liquid metal cooled fast reactors. Austenitic stainless steels are de- graded by neutron irradiation, leading to hardening and ductility loss and also to potentially significant distortion arising from void swelling, irradiation creep and phase transformations. The objec- tive of the research described here is the development of a nonde- structive inspection method for measuring irradiation-induced microstructure changes in austenitic stainless steels, with a spe- cific focus on nondestructive measurement of void swelling. Ultrasonic testing is one of the popular nondestructive inspec- tion techniques for characterization of material properties, espe- cially for detecting structural micron-scale defects such as cracks. However, until recently, it has been difficult to detect microstruc- tural changes caused by irradiation using either ultrasonic or other conventional nondestructive techniques. Ultrasonic testing parameters, such as ultrasonic velocity or ultrasonic attenuation, are significantly affected by irradiation-in- duced microstructural changes. With recent advancements in elec- tronics, these ultrasonic parameters can now be measured very accurately, allowing for reasonably precise correlation with a vari- ety of microstructural features. We have chosen to examine thick blocks of steel known to have significant gradients in dose, dose rate and temperature. Note that temperature gradients within the blocks are due not only to increasing coolant temperature with axial height, but also to axial and radial gradients in gamma heating that are reasonably propor- tional to gradients in dpa rate. In this paper, we concentrate only on a method involving ultrasonic velocity and not ultrasonic atten- uation, the latter to be covered in subsequent publications. 2. Experimental procedure 2.1. Test specimens Assembly U9807 was irradiated in Row 8 of the reflector of the EBR-II fast reactor. The assembly was comprised of a thin (1 mm) hexagonal duct constructed from 304 stainless steel. Inside the hex-duct was a stack of long (218–245 mm), thick (50 mm face-to-face thickness) hexagonal-shaped blocks that were also 0022-3115/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2013.02.036 ⇑ Corresponding author. Address: Engineering Services Division, Nuclear Fuel Industries, Ltd., 950 Asashiro-nishi, 1-chome Kumatori-cho, Sennan-gun, Osaka 590-0481, Japan. Tel.: +81 72 452 7221; fax: +81 72 452 7225. E-mail address: junji-etoh@nfi.co.jp (J. Etoh). Journal of Nuclear Materials xxx (2013) xxx–xxx Contents lists available at SciVerse ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat Please cite this article in press as: J. Etoh et al., J. Nucl. Mater. (2013), http://dx.doi.org/10.1016/j.jnucmat.2013.02.036