Xie, Kelvin. Determination of spatial and chemical relationship of hydrogen and helium in 316 SS PWR flux thimble tube at 76 dpa for extrapolation into beyond-40-year operation

Principal Investigator
First Name:
Kelvin
Last Name:
Xie
Institution:
Texas A&M University
Title:
Assistant Professor
Team Members:
Name: Institution: Expertise: Status:
Frank Garner Radiation Effects Consulting Radiation effects in structural materials Other
Digvijay Yadav Texas A&M University focused ion beam and transmission electron microscopy Graduate Student
Cheng Sun Idaho National Laboratory mechanical properties, Irradiation Effects, characterization, nanostructure, advanced manufacturing, metallic fuel, nickel alloys, 304 stainless steel Other
Aaron French Texas A&M University Nuclear engineering Graduate Student
Experiment Details:
Experiment Title:
Determination of spatial and chemical relationship of hydrogen and helium in 316 SS PWR flux thimble tube at 76 dpa for extrapolation into beyond-40-year operation
Describe the work that you are proposing in detail. Please include as many specifics as possible (e.g., dose, dose rate, ion energy, types of ions, beam line x-ray energy, irradiation temperature, analysis temperature, atmosphere, etc.):
It is first required to use the FIB capability to produce specimens thin enough to allow high-resolution imaging and EELS measurements from currently available, already-prepared “bulk” specimens. The FIB-produced lamella will be examined using high-resolution TEM and STEM capability at IMCL to clearly image the nano-bubbles and then especially using EELS to separate the distinct contributions of the helium and hydrogen edge signals and the bulk plasmon signal from the matrix. It is anticipated that a maximum of three days of FIB time and four days of TEM/STEM/EELS is required for this project.
Technical Abstract
The project will use the FIB and TEM/STEM “hot” facilities at IMCL to prepare and examine exceptionally thin lamella for determination of the location and physical relationship of hydrogen and helium distributions in neutron-irradiated 316 stainless steel that served as a flux-thimble tube in a commercial PWR to ~76 dpa at ~320°C. These specimens are currently available at Texas A&M University in the form of mechanically and electro-polished quarter-ring tube segments supplied by Paula Freyer of Westinghouse and Frank Garner of Radiation Effects Consulting. The specimens were cut from a tube of 7.7 mm diameter with 1.2 mm thickness, with the quartering cuts yielding ~65° segments instead of 90° segments. After final polishing the specimen thickness is ~0.4 mm, with a gamma activity reading ~17 mR/hr at 6” and ~5 mR/hr at 12 inches, primarily arising from Co-60. Microscopy measurements conducted in 2009 at PNNL on essentially identical specimens from the same reactor at ~70 dpa/315°C shows nano-bubbles 1-3 nm in diameter at a density of 1.6E23 m-3, resulting from measured helium and hydrogen concentrations of ~600 and ~2400 appm, respectively. Various recent atomistic modeling studies suggest that bubbles formed in various metals during neutron irradiation have a high-density helium core, surrounded by a hydrogen-enriched metal matrix interface or “halo” surrounding helium bubbles. High-resolution electron energy loss spectroscopy (EELS) technique coupled with scanning transmission electron microscopy (STEM) will be employed as the major tool. There are two major challenges to overcome. First, multiple very-thin lamellae must be produced to allow clear images of many single, non-overlapping cavities in a minimum matrix, requiring significant effort in specimen preparation. Second, both the helium and hydrogen edge occurs within the bulk plasmon signal of the matrix, making background subtraction rather complex. However, recent soon-to-be published EELS studies conducted at Canadian Nuclear Laboratories on Inconel X750 from CANDU garter springs indicate that the halo effect appears to be real, but the neutron-flux-spectra and non-aqueous environment of the CANDU X750 produce a much higher, overlapping bubble density and a very low retained H/He ratio of ~0.1, leading to some remaining doubt that the hydrogen signal was not a plasmon-artifact. The 316 flux-thimble tube specimens examined in earlier studies had a much higher H/He ratio of ~4 and a lower density of more easily-separated bubbles at 70 dpa/315°C. Somewhat larger values expected at 76 dpa/320°C, allowing a much more conclusive determination of the halo presence and its characteristics compared to that derived in the X750 effort.