Structural integrity characterisation of nuclear materials via nano additive manufacturing

We need to know the behaviour of novel materials in the presence of high irradiation and high temperature before we could embark on building advanced Generation IV and fusion nuclear systems. However, health and safety issues prevent us from testing macromechanical irradiated coupons in the laboratory. The solution is to test very small volumes of irradiated material, i.e. micromechanical coupons, and use multi-scale modelling to extrapolate the measured behaviour to macromechanical components. Because of their very low volume such specimens can be lab tested, even when irradiated to low or medium level of activity. This offers a possibility of testing multiple specimens to investigate stochastic effects, e.g. effects of irradiation on the shift of the ductile to brittle transition. The manufacturing technology is fast moving from subtractive methods to additive methods. Additive manufacturing can produce geometries which so far have not been possible using the traditional subtraction (e.g. milling) methods. The advances of additive manufacturing, so far, have not been replicated in micromechanical testing. Currently the common method for fabricating micromechanical coupons is to use Gallium or Helium Focused Ion Beam (FIB) micro-milling. In FIB milling, charged ions of helium or gallium are focused on the sample, sputtering the parent material in a pre-defined geometry until the desired shape is milled. The subtractive FIB milling method not only is not representative of additive manufacturing foreseen to be used in future nuclear complete fabrication, it leaves damages such as helium bubbles or gallium implantation in the milled micromechanical samples. It is therefore highly desirable to develop a new method to fabricate micro-scale micromechanical testing coupons that do not suffer from FIB damage (i.e. helium or gallium implantation or in severe cases, parent material amorphisation).
In this feasibility study, we will investigate the applicability of a novel nano-additive manufacturing methodology, originally developed for tuneable optical systems, to fabricate micromechanical specimens. We will be using three-dimensional direct laser method to produce a 3D polymer scaffolding of the negative desired structure, we will then deposit the parent material (tungsten, iron or carbon) on the polymer scaffolding using electron beam induced deposition. We then remove the polymer by inductively coupled oxygen plasma, and finally fill out the scaffolding with parent material using thermal evaporation, electron beam induced deposition or chemical vapour deposition depending on the material. This method allows us to produce a micromechanical test coupon with desired geometry with an accuracy of at least one order of magnitude better than FIB milling. This is especially important for fabricating specimens that contain cracks as the natural cracks occurring in service components, for example due to corrosion, are very sharp which are hard to replicate using FIB milling. We will investigate the fracture behaviour of nanometre cracks in our micro-scale specimens by X-ray nano-tomography. Using X-ray nano-tomography will allow us to observe, in real time, the interaction of the crack with the surrounding microstructure. The information obtained from micro-fracture tests will validate our cellular automata finite element model which we then use to extrapolate the results to a macro-scale component.
If successful, in future we will neutron irradiate the nano-additively manufactured specimens to investigate the effects of irradiation damage on the structural integrity of components with complex geometries. Complex geometry specimens irradiated with a high dose are important for fusion plants as the geometry of many structural components is complex and dictated by physics. Thus in the follow-on research we will be working with Culham Centre for Fusion Energy, National Nuclear Laboratory and Nuclear Advanced Manufacturing Research Centre.

The ultimate impact of the proposed research will on the society. UK has made a commitment to reduce the greenhouse gases by 80% by 2050. It is therefore predicted that burning fossil fuels for energy will be stopped or significantly reduced by then. Immense progress in renewable energy has been made in the past decades and with their cost reducing considerably, it can be envisaged that they will provide most of UK energy by then. However, wind is not always blowing and the sun is not always shining and therefore, there must be more reliable method of providing the base energy demand to have a prosperous nation with access to cheap and reliable energy source. The UK government has made a strategic decision on an energy mix that includes nuclear to cover the base demand supporting a resilient economy not perturbed by future energy shortage or price fluctuation. Our research underpins the scientific understanding required for designing and building the next generation of nuclear power plants, fission and fusion, from novel materials by innovative fabrication technologies, i.e. additive manufacturing.
The UK economy will benefit from our research as it will, in short term, help with the UK lead in nuclear materials worldwide. This is an area which both recent EPSRC independent review and the Nuclear Innovation and Research Advisory Board report identified as areas in which UK has a global advantage which should be maintained and expanded. In long term, this project will pave the way for selecting the correct materials and manufacturing technologies for the next generation of advanced nuclear systems.
The project will contribute significantly to the UK knowledge in nuclear materials and therefore is beneficiary to the UK nuclear research community. This is the first time that crack interaction with the microstructure is observed at this length scale. This will allow for the materials scientists to design the material in a way it is more resilient to inevitable fabrication flaws, ensuring the structural integrity of components which will be manufactured by using UK technology. The push-button cellular automata finite element modelling technique that is incorporated into this project will be made available to the wider community as it is the RCUK policy allowing for the researchers in the field to use it freely to design their micro-specimens and extrapolate the result of their micromechanical experiments to macromechanical industrial component behaviour.
Finally, people will benefit from this research. This is not limited to individuals who are directly involved in this project including the investigators, researcher and the advisory board members. The University of Bristol Mechanical Engineering Department has more than 180 undergraduate students every year as well as a number of Master degree students in Nuclear Science and Technology. We will communicate the wider aspects of the project, via individual undergraduate and MSc research projects, to both undergraduate and postgraduate students to increase their knowledge of the nuclear field and provide the nuclear community with young enthusiastic prospective engineers.