Irradiation Effects on Flow Localisation in Zirconium Alloys

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 safe operation of nuclear fuel assemblies requires a complete understanding of the mechanical properties of irradiated material. For instance, when strained plastically, irradiated materials display severe flow localisation and in the case of zirconium also a change from prismatic to basal slip. 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.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.The research will be undertaken in a collaborative effort between the UK and India with the aim to train Indian researchers to undertake advanced electron back scatter diffraction (EBSD) and synchrotron x-ray diffraction experiments. Neutron irradiated material will be provided by the TIFR/BARC facility, Mumbai, India while ion irradiation will be carried out at the new Dalton Cumbria Facility in the UK.

Mechanical properties effected by irradiation defects in cladding material as well as cladding corrosion and the associated hydrogen pickup are generally recognized as the major limitations on the lifetime (burnup) achievable from light water reactor fuel. Today, nuclear fuel, which goes into a nuclear reactor, will not be fully burned because it is not clear if the material that contains the fuel is robust enough to sustain such high burnup . Once, fuel cladding material has gone into a reactor, the mechanical properties of the material changes dramatically, which is clearly of relevance for the performance of the fuel assemblies. The utilities, i.e. operators of nuclear power plants, have an interest to keep the fuel assemblies in a reactor for as long as possible since it will enable them to operate nuclear power stations in a more efficient way (fewer shut downs) and produce less nuclear waste. Regulators demand from the nuclear industry that they operate their reactors in a safe manner. Today, regulators increasingly expect utilities and fuel manufacturers to develop more physically based predictions, which are only possible if the deformation mechanisms of irradiated clad is understood. In summary, operating nuclear power stations in the most efficient and safe way while producing as little as possible of nuclear waste must be clearly in all our interest and consequently the society as a whole will benefit from this project. A second crucial aspect of this project is that it will train young people in an area where there is a severe shortage of expertise in the UK and globally. It cannot be emphasised enough that the understanding of material performance under irradiation is crucial and it is the nuclear industry, the regulators, national laboratories and academia that all require engineers and scientists that are educated to work in this environment. Finally, the UK needs to link up with other nations in a worldwide effort to use nuclear power in the safest and most efficient way. The UK will train Indian researchers in using some of the most advanced experimental methodologies to characterise irradiated material.