Elemental Characterization of Neutron-Irradiated Tungsten Using the GD-OES Technique

Abstract

In reactor-relevant fusion divertor conditions, tungsten (W) will be used as an armor material due to its excellent thermal properties. It will be exposed to impurities from numerous sources, including ion implantation and mixing, neutron transmutation, low-Z plasma-facing-component (PFC) redeposition and codeposition of deuterium and tritium fuel, and trapped helium bubbles. The impurity plasma material–interaction effects are a concern because they can cause gradual degradation of the material and of plasma performance due to dust formation, fuel retention, and even changes to the thermal and mechanical properties of the W armor. It is crucial to measure the amount of impurities in W, and the glow discharge–optical emission spectroscopy (GD-OES) technique is exceptionally well suited for analysis of irradiated samples. GD-OES can measure a sample’s elemental composition by sputtering the surface of the sample, ionizing the eroded material, and measuring the optical emission of the excited atoms. In order for the GD-OES technique to be applied to neutron-irradiated tungsten samples, a mounting system for miniature samples was designed. The sample mounting and centering procedure was successful in measuring the depth distribution of control W and W alloy sample elemental concentrations. These control depth spectra will be used as elemental references for postirradiated samples. The residence time of surface layers was measured, a comparison of signals from different anodes was completed, and the influence of initial surface roughness or nonuniformity was understood. The depth distribution of an arc-welded W-0.4% rhenium (Re) alloy was measured to have a stable Re signal that was distributed evenly in the W matrix. The methods developed here will allow for quantification of impurities and transmutation amounts in neutron-irradiated W. GD-OES is a powerful tool but requires calibration and careful optimization of the parameters to obtain meaningful results.

Keywords:

• Plasma-material interactions
• depth profile
• transmutation
• neutron
• glow discharge–optical emission spectroscopy

Acknowledgments

The work was performed as a part of the U.S.-Japan PHENIX Cooperation Project on Technological Assessment of Plasma Facing Components for DEMO Reactors, supported by the U.S. Department of Energy (DOE), Office of Science, Fusion Energy Sciences and Ministry of Education, Culture, Sports, Science and Technology, Japan. This manuscript has been authored by UT-Battelle, LLC, under contract number DE-AC05-00OR22725 with the DOE. This research used resources at the HFIR, a DOE Office of Science User Facility operated by ORNL. This work was supported in part by the Oak Ridge Institute for Science and Education under the Nuclear Engineering Science Laboratory Synthesis program.

The authors would like to thank Michael Lance, Xunxiang Hu, Peter Mouche, and Walter Koncinski for assistance with running the GD-OES experiments and preparing the manuscript and Eric Lang at the University of Illinois for providing the W-ZrC sample.

Notes

^a The number of photons that are emitted is directly proportional to the number of excitations. (Fewer photons are emitted as the order of emission gets larger.) Quantification of the light that is detected by each element-dedicated PMT can be converted from a voltage signal to an elemental concentration.