Understanding the Mechanisms of Pellet Cladding Interaction in Zr Alloys and their Influence on the Degradation of Light Water Reactor Fuel Assemblies

  • Conor Gillen

Student thesis: Phd

Abstract

Due to an increased global contribution to energy from renewable energy sources, nuclear power is required to be more reactive to other sources than has historically been the case. Power manoeuvrability is severely limited in water cooled nuclear reactors by Pellet Cladding Interaction (PCI), a fuel failure phenomenon that causes unexpected failure of zirconium cladding particularly when power is increased following time spent at low power. Iodine-Stress Corrosion Cracking (I-SCC) is thought the responsible failure mechanism, and for large improvements in power manoeuvrability a much better mechanistic understanding of I-SCC is required. I-SCC was recreated in the laboratory, using iodine dissolved in ethanol and zirconium c-rings cut from cladding tubes of different alloys and processing conditions stressed using compressive loading. As well as producing failed samples for fractographic analysis, careful observation allows cracking to be observed in real time, and interrupted when desired for incipient cracks to be studied using advanced characterisation techniques. Quantitative fractography was applied to failed I-SCC specimens subject to a variety of stresses and iodine concentrations, and the resulting proportions of the different cracking modes; intergranular (IG), transgranular and ductile tearing recorded. The role of stress and iodine concentration in determining the responsible crack mechanism was demonstrated. X-ray computed tomography identified the 3-D crack morphology in its entirety for the first time. The furthest progressing crack finger was subject to repeated focused ion beam milling and electron backscatter diffraction (EBSD) mapping producing a 3-D EBSD map, including the 3-D crack location. Crack propagation in cold worked material was observed to progress in a non-perpendicular direction to the applied stress. Nanoscale secondary ion mass spectrometry (NanoSIMS) detected iodine at the tip of an I-SCC crack for the first time. Neutron irradiation appeared to activate TG crack progression in non-basal crystallographic planes, something not otherwise observed. NanoSIMS and Scanning-Transmission Electron Microscopy Energy Dispersive X-ray (STEM-EDX) analysis identified iodine in active crack tips. Zirconium cladding exhibited faster failure when zirconium-hydrides were present within with EBSD and fractography both demonstrating the favourability of I-SCC to occur at these hydrides. This had not satisfactorily been observed in literature, and is expected to be important as fuel burn-up increases, thereby increasing the hydrogen content in the cladding.
Date of Award1 Aug 2020
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorDirk Engelberg (Supervisor), Philipp Frankel (Supervisor) & Michael Preuss (Supervisor)

Keywords

  • EBSD
  • X-ray Computed Tomography
  • SCC
  • Serial Sectioning
  • NanoSIMS
  • Zirconium
  • Iodine Stress Corrosion Cracking
  • Pellet Cladding Interaction
  • Fratography

Cite this

'