Microstructural Evolution of AGR Steel Cladding During Processing and Proton Irradiation

UoM administered thesis: Phd

  • Authors:
  • Chiara Barcellini

Abstract

The aim of this thesis is to assess the formation and evolution of grain boundaries and second phase particles in Nb-stabilised 20Cr-25Ni austenitic stainless steel during industrial processing, and subsequently their stability and role in the occurrence of local solute redistributions induced by proton irradiation. This steel is currently being used as cladding material in UK Advanced Gas-cooled Reactors (AGRs). Due to a recent change in fuel cycle policy, spent AGR fuel elements will be stored in caustic- dosed water ponds for >20 years. Intergranular corrosion attacks during wet storage are ascribed to local Cr depletions in the vicinity of grain boundaries occurring during reactor service operations, a phenomenon termed radiation-induced sensitisation (RIS). This phenomenon is reported for fuel claddings exposed to reactor core temperatures of about 320-550C, with a peak effect at 420C. Systematic corrosion studies rely on the availability of sensitised specimens, ideally generated under well-defined/reported environmental parameters. This thesis explores the possibility of producing damaged structures of 20Cr-25Ni Nb-stabilised stainless steels through proton irradiation, as a surrogate of the neutron damage caused during reactor service. Initially, the formation and evolution of grain boundaries and second phase particles prior to proton irradiation were investigated through two different processing routes: step-wise annealing up to 1100C, and isothermal annealing at 930C up to 4 h. The results revealed that after 1 h at 930C an optimised microstructure for radiation resistance is reached, consisting of small recrystallised grains of austenite, separated primarily by high angle boundaries and special coincidence-lattice-site Sigma-3 boundaries, together with a large number of finely dispersed nano-sized Nb(C,N) particles. An increase in the annealing time or temperature causes dissolution of those particles, triggering grain growth in the matrix. Fully recrystallised specimens were proton irradiated in-situ in a transmission electron microscope. The early stages of the lattice damage evolution caused by 40 keV proton bombardment were monitored at three different irradiation temperatures, i.e. 420, 460 and 500C, for incremental damage levels up to 0.8 dpa, whereas the local chemical instabilities induced by irradiation where assessed ex-situ using Energy Dispersive X-ray spectroscopy in a scanning transmission electron microscope. At 420C, the dam- aged microstructure is mainly characterised by black spots and faulted a/3 Frank loops. Defect saturation is reached at only 0.1 dpa. In contrast, at 460C and 500C proton bombardment induces the formation of a mixture of a/2 Frank loops and perfect a/2loops. These perfect loops evolve into dislocation lines that form dense network. The vacancy supersaturation in the matrix at 460C and 500C causes the additional formation of voids and stacking fault tetrahedra. Furthermore, proton irradiation induces the depletion of Cr, Fe and, to a lesser extent, Mn from grain boundaries, whereas Ni and Si become enriched. However, Sigma-3 grain boundaries proved to be resistant to proton-induced solute redistributions and therefore to RIS. High-angle grain boundaries with a misorientation angle greater than 40 become mobile at 460C and especially at 500C, and also experience a relatively large solute redistribution, with local Cr contents in a significant number of boundaries falling below 12 wt.% and profile widths >100 nm. Similar chemical profiles where found close to radiation-induced dislocation loops and Nb(C,N)/matrix interfaces.

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Original languageEnglish
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Award date1 Aug 2019

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