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

Lead Research Organisation: University of Manchester
Department Name: Materials

Abstract

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

Planned Impact

Dimensional stability of cladding (i.e. irradiation growth and creep) 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 . Consequently the fuel manufacturers are interested to develop better fuel cladding. However, unless the role of alloy chemistry and microstructure on the mechanisms leading to dimensional instabilities are understood, the fuel manufacturers can only guess what kind of alloy development might lead to the desired improvement. 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 mechanisms leading for example to irradiation growth and creep are understood. The exploitation plans set up by our project partners are clearly described in their support letters and are explained in detail in the Impact Plan. 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. The project is designed that it studies the fundamentals of irradiation growth and creep. Consequently, the postgraduate students and research fellows will enjoy working on an intellectually challenging task while the nuclear industry will be eager to discuss their findings with them. To date, I have a 100% track record with students who have obtained a PhD on zirconium research now working in the nuclear industry. The strong support by my industrial project partners demonstrates that there is a clear avenue for disseminating findings to the nuclear industry. Quarterly progress meetings with all project partners and additional meetings with individual project partners will ensure that there is an excellent knowledge transfer in both ways. Our project partners also typically hold annual meetings with all their university partners. The project will operate within the framework of the Materials Performance Centre and the Dalton Nuclear Institute and it will be well linked with a range of new Manchester initiatives such as Nuclear Advanced Manufacturing Research Centre, the Centre for Nuclear Energy Technology, the Dalton Cumbria Facility and the National Nuclear Laboratory. Today, materials for Formula 1 and aeroengines are the most fascinating materials for prospective students. We need to gain the same fascination for nuclear materials and explain in what type of hostile environment they operate. I also plan to engage the public by working together with an artist to create artistic images of irradiation damage in material in order to communicate with a non-scientific community.

Publications

10 25 50

publication icon
Francis E (2013) Ultrahigh Resolution EDX Spectrum Imaging: Nuclear Materials Applications in Microscopy and Microanalysis

 
Description - We have found evidence of irradiation-induced nano-precipitation minimising c-loop formation, and therefore irradiation growth. At the close of the project, we will be better able to summarise key findings.
- We have been able to identify critical temperatures for irradiation damage to anneal out (become unstable).
- We have developed experimental protocols for the use of x-ray diffraction line-profile analysis to determine dislocation loop density.
- We have determined micro-segregation of alloying elements towards dislocation loops and other crystal defects.
Exploitation Route - Our findings could influence future alloy development.
- Our research provides experimental evidence than can be used in model development.
- Our experimental findings provide information that can be used to predict life of cladding.
Sectors Aerospace, Defence and Marine,Energy

 
Description In the initial project proposal, a key objective of this Fellowship was to, "build a world-class research group in the field of irradiation damage of Zr cladding, which is strongly supported by the nuclear industry ranging from fuel manufacturers to utilities". That this objective has been achieved is in evidence through the size of the group (now the largest Zr research group in the world), our high number of UK and international partners, the career destinations of our graduates to-date, and not least by Manchester's selection to host the 2019 ASTM Zr Symposium on behalf of the UK (not previously hosted here since 1978).
First Year Of Impact 2010
Sector Aerospace, Defence and Marine,Energy
Impact Types Societal,Economic

 
Description AMEC 
Organisation AMEC
Country United Kingdom 
Sector Private 
PI Contribution Contribution to R&D, trained future staff
Collaborator Contribution funding of PhD students, autoclave testing
Impact Phd students now working for AMEC, enhanced general understanding of their product
 
Description EDF 
Organisation EDF Energy
Department EDF Innovation and Research
Country France 
Sector Private 
PI Contribution Contribution to R&D knowledge
Collaborator Contribution funding of PhD students and providing access to EDF facilities
Impact PhD student now working for EDF, provided improved understanding of their product
Start Year 2007
 
Description NNL 
Organisation National Nuclear Laboratory
Country United Kingdom 
Sector Public 
PI Contribution Contribution to R&D
Collaborator Contribution funding of PhD student
Impact training of staff and PhD students now working for NNL
Start Year 2007
 
Description Rolls-Royce plc 
Organisation Rolls Royce Group Plc
Country United Kingdom 
Sector Private 
Start Year 2007
 
Description Westinghouse 
Organisation Siemens AG
Department Siemens Westinghouse Power Corp
Country United States 
Sector Private 
PI Contribution contribution to R&D knowledge
Collaborator Contribution top up of PhD students, fully funded PhD students and materials including irradiated materials
Impact Some of our PhD students now work for Westinghouse in Sweden and the USA. We also improved WH's understanding of their product.