Localized deformation in spent nuclear fuel disposal canisters

Research output: ThesisDoctoral ThesisCollection of Articles

Abstract

Spent nuclear fuel disposal in copper canisters in a deep geologic repository will be the main disposal method of spent nuclear fuel in Finland and Sweden. The long timespans involved in the disposal bring certain uncertainties, for example, when considering localized deformations and environmental factors, such as corrosion and hydrogen effects on the mechanical properties. The canisters are sealed by friction stir welding (FSW), which results in strong welds, but the variation in cross-weld microstructure may lead to localized deformations during the long disposal of the canisters. Thus, localization of plastic deformation in the canister welds was studied by digital image correlation (DIC). The FSW welding method has recently been modified by argon shielding gas to reduce oxidation during welding. When compared to the old weld, which was welded without shielding gas, the new weld exhibits considerably less oxide particles, and the weld microstructure is more uniform. However, DIC testing indicates more pronounced localization of plastic deformation on the weld boundary on the lid side of the canister. This is attributed to the combined effect of softer lid material and stronger weld material, which leads to worse mechanical performance in terms of strain localization. To minimize this effect, the strength mismatch between different parts of the copper shell should be minimized. In addition to localized deformations, studies were conducted on hydrogen effects on mechanical properties of the copper welds and the cast iron insert. Hydrogen charging of the copper FSW welds indicates that oxide particles trap considerable amounts of hydrogen, but it did not compromise the ductility of the copper welds. However, the cast iron insert of the canisters exhibits a significant sensitivity to hydrogen. In the context of detecting localized deformations by DIC, the trade-off between spatial resolution and noise level of the measurement is critical. Thus, methods for quantifying that trade-off, as well as for improving the DIC data quality were developed. Patterning of copper, by utilizing copper oxide and a photolithographic method, enables the measurement of large deformations in copper, as well as detection of small deformations due to improved data quality. The improvements were quantified by extracting the noise level and autocorrelation length of the noise, which allows the evaluation of precision and spatial resolution of the DIC measurement based on the DIC data itself, and does not require imposing known displacements beforehand.
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
Supervisors/Advisors
  • Bossuyt, Sven, Supervisor
  • Hänninen, Hannu, Supervisor
Publisher
Print ISBNs978-952-60-8699-6
Electronic ISBNs978-952-60-8700-9
Publication statusPublished - 2019
MoE publication typeG5 Doctoral dissertation (article)

Keywords

  • digital image correlation
  • DIC
  • spatial resolution
  • patterning
  • localized deformation
  • spent nuclear fuel disposal
  • copper canister
  • cast iron insert
  • hydrogen embrittlement

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