Enhancing nuclear fuel efficiency through improved understanding of irradiation damage in zirconium cladding

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This project focuses on energy and more specifically on nuclear fission. Core material such as fuel assemblies are exposed to irradiation from the moment a nuclear reactor is switched on. The bombardment of material with neutrons creates collision cascades that immediately produce point defects and dislocations in the material. This results in very significant changes of the material properties compared to non-irradiated material.Nuclear fuel for light water reactors is contained by so-called cladding tubes, which are made from zirconium alloys because of their excellent corrosion resistance, sufficient mechanical properties and their low neutron absorption coefficient. Nuclear fuel is enriched initially with 5% 235U. However, the fuel cannot be fully burned due to the uncertainty of clad material degradation and dimensional instability of fuel assemblies. The dimensional instabilities are related to irradiation growth and creep of zirconium alloys. Irradiation growth occurs in zirconium alloys without applying any external load and is due to the hexagonal close packed crystal structure of zirconium. Irradiation creep is significantly faster than thermal creep due to the increased density of vacancies in irradiated material. The safe operation of nuclear fuel assemblies requires dimensional stability to ensure sufficient coolant flow and the safe operation of control rods when needed. Irradiation growth and creep can lead to bowing and buckling of fuel assemblies, which is of concern with current plants and even more a concern for increased burnup of the nuclear fuel. Consequently, we need to develop a detailed understanding of the mechanisms leading to these phenomena and how they are affected by material chemistry and the microstructure evolution during irradiation.Traditionally, microstructure and damage characterisation of irradiated material is mainly carried out by electron microscopy. However, in the last decade, very powerful 3rd generation synchrotron radiation sources have been built, which represent a tremendous opportunity to develop complementary tools or quantitative characterisation of irradiation damage and microstructure evolution.During the 1960s and 70s many countries including the UK had test reactors that allowed scientists to undertake research on irradiated material. However, most of these test reactors are gone now and it is unlikely that the UK or other countries will build many new test reactors. For this reason, governments have invested in proton/ion accelerators to simulate neutron irradiation. The advantage of such facilities is that they are by many order of magnitudes cheaper to run than a test reactor. However, our understanding of how well neutron induced damage is related to proton/ion induced damage is limited. Since Zr alloys are relatively mildly active when irradiated by neutrons, they represent also an ideal material to calibrate proton/ion against neutron irradiation.During the fellowship my research group will:- identify the role of alloy chemistry and microstructure on irradiation growth and creep of fuel clad,- for the first time extensively use synchrotron radiation to characterise irradiation damage and- calibrate proton/ion irradiated against neutron irradiated cladding material in order to use the convenience of the former (non-active material, easily irradiated to different levels in a short time) to identify the route cause for loop formation resulting in breakaway growth