Left: Nickel alloy forgings are frequently found in nuclear applications Source: CanForge
industry is difficult. Says Miao: “We’re trying to figure out a way to combine different components, including simulation, experiments, and cost analysis to have this framework to accelerate this process.” He adds: “The focus of this project was to not only develop this kind of coating solution, but also develop a framework to help evaluate and optimize this coating solution. We also needed to provide a demonstration case”. The team selected a molten salt reactor as the proposed demonstration case given the high anti-corrosion performance needed to operate in such an environment. “The idea is we need some material with high performance for an advanced nuclear reactor, like molten salt,” he says. Using a multi physics approach to drive experimentation and determine which potential combinations or alloys of coatings could be used, the Argonne approach aims to address multiple challenges by finding alternative materials that have the high corrosion resistance suitable for these environments with high temperature, pressure, and potentially neutron flux, all while in the presence of corrosive molten salts. Miao explains: “First, we need to identify a coating solution that can meet all the comprehensive set of performance criteria we need for molten salt fast reactors. We need to identify what kind of material would work and then also other coating parameters, like how thick the coating needs to be and if we have multi-layer coating, the thickness of each layer and then the exact composition we need to use.” He contrasts the multi-physics model with the
conventional approach to managing this kind of materials challenge, saying: “People typically just use a matrix of conditions which could be a very big set of data on a group of examples. Samples are prepared and then experimentation is used to find the optimal one. What we are trying to do is to develop a framework that would benefit from a multi-physical model, establishing a reactor model which can mimic the actual operating conditions for the coating – in this case that would be a molten salt reactor – and trying to put all the relevant information into the model. That model is also informed in real time by experimental data.” The framework assesses the qualities of different alloys and immediately rejects those that do not meet key criteria, such as resistance to the corrosive environment. This eliminates large parts of the experimentation that would otherwise be required. The idea is this multi- physical model, together with the experiment data, will
help to make the experimentation phase more efficient so that the optimal solution can be found faster with fewer experiments. “After we identify the best solution, we can also use the
same framework to further collect data and demonstrate the effectiveness of the solution so it will also help us with the qualification and the licensing process,” adds Miao
Taking coating to the next level Having run the programme for close to two years, the researchers have succeeded in identifying a novel coating material and have also now demonstrated that the new material could indeed withstand reactor conditions and resist the molten salt corrosive environment. However, the research framework goes beyond discovery of the basic coating material. Miao explains: “We are actually developing the coating rather than use a commercially-available coating and are aiming to optimise the composition. It’s not adding new elements, we’re more adjusting the percentage of different elements. We know that some combination of elements will work but we don’t know what the best composition is. We also want to know the optimal thickness to sustain the stress and also the corrosion.” He continues: “Currently we are focusing on how thick each layer of coating needs to be, for example, one micron or two microns. We put everything into the model which is informed by previous experimental data. We know, for example, that a one micron-thick layer would be under high stress and that it cannot endure the reactor conditions so we may need a two or more micron-thick coating instead. These are existing coating materials that have already been tested in some respects as a potential choice but that haven’t been applied in this environment.” The goal then is to develop a coating that can withstand the corrosive high temperature molten salt environment under reactor conditions, including the challenges of thermal expansion. Following a programme of analysis and experimentation developing promising compositions the next phase of the research will be testing the materials in the Argonne Tandem Linac Accelerator System (ATLAS) device. “We have already identified an optimal composition for
the coating and then we have testing, not only after we identify it, before it too. For example, we also use testing to help screen the optimal solution by testing thermal stress and resistance by thermocycling. We also tested with molten salt at high temperature to test corrosion resistance. We also put it into an ionic irradiation materials testing station at the ATLAS called the [ATLAS Material
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