New research from the UK National Nuclear Laboratory and the University of Liverpool in the UK has used a novel technique to explore how thermal oxidation affects the structure of a key part of nuclear reactors so they can -engineered to work longer than ever before. This research is reported in Nuclear Engineering and Design.
The main component is graphite. It is made up of small crystals that come together to form “grains” or “filler particles”. Grain size, along with porosity, is used to distinguish nuclear graphite into different grades. It plays an important role in many types of nuclear reactors. It is used in the current operating fleet of reactors that have supplied almost 20% of the UK’s carbon-neutral electricity since the 1950s and will soon be used in advanced reactors planned to play a key role in future energy systems. environmentally friendly energy around the world.
Researchers at the UK National Nuclear Laboratory and the University of Liverpool examined the porosity of superfine grained nuclear graphite and linked it to a chemical change known as thermal oxidation. They focus on the very high temperatures that will be produced in some advanced reactors, known as high temperature reactors (HTR). The discoveries can be used to design efficient nuclear reactors that will last longer than the original fleet.
Nassia Tzelepi, Senior Fellow at the National Nuclear Laboratory, explains the importance of this work, “Thermal oxidation is one of the two mechanisms of HTR graphite components that affect the performance of reactors. A good understanding of its evolution and the effect of The properties of graphite are necessary to qualify a graphite for use in an HTR.”
Inside a nuclear reactor
A nuclear reactor relies on neutrons to split atoms, releasing large amounts of energy. Neutrons move very quickly so a moderator is used to slow them down, making them more likely to be absorbed by a fuel atom. Solid graphite blocks are excellent moderators.
A coolant takes energy and sends it to a place where it can be used. Current reactors send coolant to a turbine to generate electricity; future reactors may be used to generate low-carbon heat needed by some industries. In a high temperature gas reactor (HTGR) helium gas is used as a coolant, which reaches temperatures above 700 °C. Helium, even once it is purified, contains substances such as air or moisture. These impurities, combined with high temperatures, can slowly cause the graphite to oxidize over the predicted 40 to 60 year life of a reactor.
Oxidation can change the porosity of graphite, a key characteristic that affects its performance. Somewhat paradoxically, the extent of oxidation depends on porosity: how the pores are connected, their size, and the overall permeability of the material. Oxidation can be caused by high temperatures, known as thermal oxidation, or by radiation.
British scientists have extensive experience with radiation-induced oxidation in a historic fleet of graphite moderated reactors operating at around 350 to 450 °C. Current research focuses on what will happen at the higher temperatures produced by the new generation of reactors.
Examination of the microstructure using a specialized technique
Understanding what happens to graphite is not straightforward. As with many other composite materials, the manufacturing process and the type of raw material play a role in the properties of the final product.
Graphite is made up of small crystals that usually have a random orientation. Different grades of graphite can be produced characterized by the average particle size of the filler and the porosity. According to Nassia, “Each graphite is different depending on the raw material and how it is produced. We want to know what makes graphite grades with similar characteristics behave differently under thermal oxidation. ” There are many possible reasons for the different behavior, and these can be investigated by looking at the microstructure under the microscope.
Nassia’s team focused their study on superfine grained graphite with an average grain size of 10–20 micrometers, as it is being considered for next-generation reactors. Using a high temperature furnace and a carefully controlled atmosphere, four grades of superfine grained graphite were oxidized at 700-800 °C.
A technique developed specifically by British scientists to analyze nuclear graphite from historic reactors has been adapted to penetrate the smaller pores present in superfine grained graphite. This technique involves saturating “open” pores with a fluorescent dye. These open pores can penetrate the reactor coolant so it is important to distinguish them from “closed” porosity which is not accessible. Using a microscope, images of open and closed pores are collected and analyzed.
By combining this technique with other standard techniques clues are found to explain different behaviors. The total oxidation depth, shown graphically for three different samples, is related to the presence of relatively large, interconnected pores but on the surface of the graphite block a fine network of narrow, open pores develops oxidation near the surface.
The team also found that the oxidation rate seems to be lower in graphite where some parts of the microstructure come together to form small agglomerates. When grains are stacked in this way there are fewer exposed edges that can oxidize.
All this information about the microstructure can be used to determine what exactly the graphite is inside future reactors. For Nassia, this new research shows why continuous learning and discovery—and the development of novel techniques—is important: “The UK operates graphite moderated reactors subjected to high levels of ‘radiolytic’ oxidation for more than 50 years. The mechanism of radiolytic oxidation is different, a lot of experience is gained to understand not only the effect of oxidation on the properties of graphite but also to develop the methods of its study.”
The insight gained from these new techniques supports the development of future reactors, ensuring that they always operate in optimal conditions and allowing them to play a leading role in a future that is completely dependent of carbon neutral energy.
Athanasia Tzelepi et al, Evolution of the microstructure of superfine grain graphites under thermal oxidation, Nuclear Engineering and Design (2023). DOI: 10.1016/j.nucengdes.2023.112421
Provided by the National Nuclear Laboratory
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