1 - Impact of Temperature on Materials
Bret Kugelmass: Tell me about your backstory.
Rosaura Ham-Su: Rosaura Ham-Su was born and raised in Mexico City, leaving for Canada in her early 20’s. Her family lived in small towns without electricity and Ham-Su was able to see the amount of effort required to get water into the house without electricity and a pump. The type of pollution that was generated in order to get energy stuck with Ham-Su and she was always fascinated by how energy is produced and what is left behind. She studied engineering physics in Mexico at La Universidad Iberoamericana and received her PhD in material science from McMaster University. The focus of Ham-Su’s work was the processing of ceramic material. The way any material is processed as an impact on its behavior and properties afterward. Ceramics were looked at for structural applications to be used at high temperatures. Creep properties change a lot depending on how it was processed. When uranium is extracted from the ground, it has uranium, oxygen, and other things, so it must go through a separation process to go to different purifications to produce the wanted uranium. Metallic uranium is often produced and brought back to uranium oxide. When uranium oxide is put at a very high temperature in air, it will oxide more. To make a pellet UO2, the powder must be compressed and put in the sintering process in a hydrogen atmosphere.
2 - Magnetic Shape Memory Alloys
Bret Kugelmass: How are nuclear ceramics made?
Rosaura Ham-Su: Ceramics start with a very fine powder that is pressed into granules. Granules need to be big enough so gravity will take over their behavior. The granules get pressed again and becomes a green pellet, which is not ready yet. The green pellet goes through the sintering process in a hydrogen atmosphere and comes out 95% dense with 5% pores. This solid ceramic finished product is used for most nuclear reactors. Rosaura Ham-Su has always been focused on how the processing affects materials and how processing can be used to get different products. One of her jobs was to figure out how to make ceramic armour. Ham-Su went to work for Canada’s Department of National Defence to work on magnetic shape memory alloys, which change shape based on magnetic field. Their shape can also be changed to create a different magnetic field. This single-phase alloy was nickel manganese gallium. Their magnetic domains align with a crystal structure and are able to twin, in which a crystal changes shape very quickly. A changing magnetic field can create a current in the right conditions. These energy harvesting devices were to be used to power buoys that detect activity along the expansive Canadian coast.
3 - Recycling Spent Nuclear Fuel
Bret Kugelmass: Tell me about some of the challenges in fuel processing and development.
Rosaura Ham-Su: Fuel development is dedicated to making fuel more proliferation efficient, more efficient, more recyclable, and in general, more sustainable. The economics landscape changes depending on what’s happening around the world. Some concepts are worked in the lab that may not have the economics in the moment, but are still worked on in anticipation of changes that it will bring to the nuclear industry and could change the landscape of economics. Recycling of fuel has been worked on by the physicists in the lab for a long time and always starts with physics saying something is possible. Spent nuclear fuel has many interesting elements in it that could be used for radioisotopes for cancer treatments or more energy, but it is very radioactive. The conditions of the new fuel to produce more electricity are given by the physicists and the next challenge is to make it into a shape to demonstrate that it can be done. Fuel is not an infinite resource; even though there is enough for the reactors now, it is a source coming out of the ground and eventually it will be depleted. The concerns about nuclear waste are always the plutonium or the daughters of plutonium. When plutonium is removed from the fuel, the problems of the waste change in timescale by orders of magnitude. Part of the driver of using plutonium is to use it and not create any more. Some isotopes in spent fuel are useful, but some might be better destroyed and the creation of nuclear weapons is also a concern.
4 - New Nuclear Fuel Development
Bret Kugelmass: How detailed is the characterization of spent fuel?
Rosaura Ham-Su: Historically, every radioactive isotope is characterized in spent fuel. There are other things created that could be nonradioactive, but are not as well characterized. In long term storage, the fuel could create something that is corrosive, but not radioactive. One of the drivers for new fuels is to be safer, such as accident tolerant fuels (ATF). New technologies like additive manufacturing may also contribute to fuel development. The middle of a fuel pellet gets to approximately 2,000 degrees Celsius. Ages ago, the Russians designed a fuel that was hollow. The Canadian Nuclear Laboratories designed a fuel that was internally cooled where the water went through the pellet to remove the heat. One of the big criticisms for that fuel design was that it would be difficult to manufacture, but 3D printing may change that story. There are lots of interesting concepts in fuel development. Some concepts are used for isotope targets instead of producing energy. The elements important for the target need to be separated so they can be used for medical applications.
5 - Fuel Development at CNL
Bret Kugelmass: How does the ratio between Pu-239 and Pu-240 change over the life of the fuel within the reactor?
Rosaura Ham-Su: Every reactor is different, but the ratio is dependent on the residence time of that fuel in a given reactor. A steady state can be achieved based on the burnup and the physics of the reactor. In water reactors, there is a residence time that is ideal for getting energy out and changing the ratio. Civilian plutonium has more plutonium-240 and military-grade plutonium has more plutonium-239. Rosaura Ham-Su currently has a project in 3D printing at Canadian Nuclear Laboratories (CNL) and just finished up a big project on thorium. Thorium can be used to deplete plutonium from the world and is a plutonium consumer. Thorium does produce u-233, which has a strong gamma signal. Part of the concern with plutonium is that, when it is pure and young, it doesn’t have a gamma signal so it is hard to detect. There are different proliferation concerns with u-233. Thorium is a lot more abundant that uranium and can also be used to recycle fuel. It can be used in pellets or in molten salt. CNL goes very quickly into producing electricity, but also has research reactors that use different types of fuel, such as liquid metal. The advantage of uranium-moly fuel instead of uranium silicide is that it has a higher density. This allows low enriched uranium, which has less uranium-235, to power a research reactor instead of high enriched uranium. The marriage between fuel design and spent fuel management is very important. Current reactors were designed thinking about the production of electricity and the waste or spent fuel management was something to be thought about afterwards. As new reactors with new fuels are built, the whole fuel cycle will have to be thought about and well-planned to avoid leaving another generation with this problem.