Radiation damage and its impact on corrosion in Zirconium-Niobium alloys

Zirconium alloys have been successfully used as fuel cladding and structural materials in fission reactors for decades, but the industry’s desire to employ high burnup fuels requires an improvement of resistance to in-reactor corrosion. It is therefore essential to understand the mechanisms of radiation-enhanced corrosion of zirconium alloys, and especially why Zr-Nb alloys show improved properties compared to Zircaloys. In order to achieve this goal, the MUZIC-3 (Mechanistic Understanding of Zirconium Corrosion) program was established; a wide collaboration between universities and industrial partners. As part of this program, this project used advanced electron microscopy techniques to investigate in-reactor corroded samples from the Canada Nuclear Laboratory and autoclave corroded samples from Westinghouse. The characterisation work used the (S)TEM as a primary tool, together with the analytical tools EDX and EELS. The in-reactor samples were Zr – 2.5 Nb samples corroded at 325 °C and 250 °C in the reactor core for 190 days and 2750 days. For comparison, samples corroded in the same primary loop but out of the reactor core, and hence without neutron radiation, have also been studied. In order to further study the mechanism of radiation damage in the metal matrix, in-situ heavy ion radiation in a TEM was carried out on the recrystallised Zr – 1.0Nb and Zr – 2.5Nb alloys provided by Westinghouse^TM, and proton irradiation was carried out at Dalton Cumbrian Facility (DCF) on as-received Zr – 2.5Nb alloys from CNL. All the TEM samples in this project were made by Focused Ion Beam (FIB) at either the University of Oxford or the Materials Research Facility (MRF) at the Culham Science Park. The first section of results presented in this thesis is the characterisation of the metal matrix of the CNL Zr – 2.5Nb alloy. The shape, orientation and number density of radiation-induced Nb precipitates in the α-Zr grains in samples corroded under neutron radiation at 325 °C was analysed. The direct observation of fully coherent Nb nano-precipitates in the α-Zr metal matrix is reported for the first time. A model describing the formation and evolution of the Nb precipitates under neutron irradiation is suggested. The impact on the evolution of the β-Zr phase from the in-reactor conditions is also presented, and it is shown that decomposition from β-Zr to β-Nb is encouraged by neutron irradiation and in-reactor heating. The second section of the results discusses the mechanisms of radiation damage to the β-Nb SPPs in the metal matrix, as studied by in-situ heavy ion radiation. It was found that due to the combined effect of radiation-induced mixing and local diffusion of solute Nb atoms, the β-Nb SPPs stay very stable under irradiation up to 40 dpa and show very limited Nb redistribution between the SPPs and α-Zr metal matrix. Reasons for the remarkable stability of these Nb-rich phases under radiation are discussed. The characterisation of proton irradiated Zr – 2.5Nb samples is also presented. The results help to understand the mechanisms of radiation-induced Nb precipitation under a variety of conditions. The third section presents a study of the corrosion of the decomposed β-Zr regions in the oxide. This is a systematic study of the corrosion behaviour, including morphology changes, oxidation rates of the Nb in the β regions, redistribution of the Nb, and the relation with the formation of other microscopic features such as micro cracks. It is shown that the longer these β regions stay in the metal under neutron radiation, the more decomposed from β-Zr to β-Nb phase they become, and the slower that these regions are oxidised. With all these factors combined together, the reasons for the improved resistance of the Zr-Nb alloys to in-reactor corrosion is discussed.

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