Ep 228: Janelle Wharry - Professor, Nuclear Engineering, Purdue University
Becoming interested in materials (0:16)
0:16-6:33 (Janelle explains how she ended up in the nuclear industry, discussing how her interest in materials arose.)
Q. Can you tell us about growing up in Hawaii?
A. Janelle Wharry grew up in Hawaii and had always wanted to enter science and engineering. As a high school student, she attended a camp for prospective engineers at the University of Illinois. This is where she was first introduced to nuclear, motivating Janelle to apply to universities with nuclear engineering programs for university. After completing her undergraduate degree at Michigan State, Janelle entered the power industry, working at Duke Energy in core design. There, she realized that unnecessary plant shutdowns were due to materials issues. Fuel pin leakages, for example, are material issues that can cause a shutdown. This is when the coolant water comes into contact with the fuel pellets in the fuel pin, releasing fission gases into the rest of the coolant water. Janelle also looked into fuel cladding to understand the effects of different types of materials on overall reactivity. Cladding is typically zirconium based alloy and Janelle explored alternative alloy options. These material challenges sparked Janelle’s interest and she returned to school to study the effects of radiation on materials during her PhD. She is now a professor of nuclear engineering at Purdue University.
How radiation changes materials (6:34)
6:34-11:37 (Janelle explains the impacts of radiation on materials.)
Q. What are the effects of radiation on materials?
A. Most materials in a reactor are metallic or ceramic. The materials are made of atoms arranged in a lattice. Neutron radiation knocks atoms out of their lattice position. Gamma radiation does not knock atoms out of place but some electronics can. When atoms are moved, defects are formed. Some small defects will clump together to form larger defects, causing voids. These voids are pockets of vacuum, which cause materials to swell. Other atoms reposition around these voids, creating a more stable structure. Materials research is focused on intelligently designing materials to take advantage of this to create new materials that are more resistant to radiation. The fuel, clad and steel are most subject to radiation damage.
Material lifetimes (11:38)
11:38-17:51 (Janelle discusses the lifetime of materials and how welding pieces of a pressure vessel rather than forging can extend the overall lifetime.)
Q. How long are the lifetimes of the different materials?
A. Cladding and fuels are replaced every few cycles. Fuel assemblies are usually replaced every three fuel cycles, which are typically 18 to 24 months long each. These materials only need to last for about 6 years. Pressure vessels are not replaced and are therefore more critical when it comes to resisting radiation damage. These materials are not replaced because they are forged as a single piece, which has a high cost associated with it. Janelle believes figuring out a way to build the vessel in multiple pieces, allowing for the more damaged parts to be replaced, would greatly extend the lifetime of a reactor. Radiation exposure causes the vessel to become brittle over time, meaning it can absorb less mechanical stress. On an atomic level, the brittleness in pressure vessel steel is caused when different chemical species cluster together. The alloy begins with some other elements within it, such as copper, silicone, nickel and manganese. During radiation, these elements cluster together. Dislocation loops also form, which are faults within the material, causing the material to become brittle. Annealing will get rid of these dislocation loops, greatly decreasing embrittlement and increasing the lifespan of the reactor vessel. The problem arises when attempting to anneal the entire 20 foot structure, but building the core in multiple pieces would enable easier annealing.
Welding versus forging (17:52)
17:52-28:18 (Janelle explains how welding is cheaper than forging and how the new electron-beam welding method is on the path to NRC acceptance.)
Q. What is the push back on welding rather than forging?
A. Janelle notes that forging is what has always been done but believes welding pieces on site would be easier, faster and cheaper. Utilities must present new ideas in the US to the Electric Power Research Institute (EPRI), which represents all US utilities. EPRI has been exploring welding rather than forging pressure vessels using electron-beam welding, which does not use a filler metal and is low heat. This new welding method creates welds that look no different than if the vessel had been forged. Janelle is currently working on radiating these materials to produce fracture tests to understand the embrittlement of the material. She will then report her findings to show that this new welding method can produce vessels that can withstand radiation. After these tests, Janelle hopes to move the material through the qualification process to make sure it follows pressure vessel codes and meets all specifications. The American Society of Mechanical Engineers (ASME) will then write the electron-beam welding method into code, which the Nuclear Regulatory Committee (NRC) can then reference. This would set the industry on a path to the NRC accepting this technology for reactor use.
Improving materials with radiation (28:19)
28:19-35:08 (Janelle discusses the various projects she is working on, primarily looking into how irradiation can be used to induce positive defects to improve materials.)
Q. What else do you work on?
A. Janelle works on many “cook and look” projects where an object is irradiated and they see what happens. Many tests are carried out at the Advanced Test Reactor at Idaho National Labs. Janelle also works in the tailoring of mechanical and material properties using irradiation. Janelle believes this is a future path for nuclear science. Instead of thinking about radiation mitigation, Janelle is interested in exploring if radiation causes positive effects that scientists can take advantage of to improve materials. Janelle is looking at lithium ion batteries for energy storage. Defects in the metal oxide within a battery can cause more lithium to enter the metal oxide, increasing the energy storage capacity of the battery. Janelle is exploring if radiation can be used to induce these positive defects. She is also researching how to use irradiation to build specific features into metals to influence how it will respond to mechanical stress.
Strengthening materials with deformation (35:09)
35:09-44:16 (Janelle explains the different crystal structures found in materials and how irradiation can be used to change these structures, strengthening the material.)
Q. What are the different deformation modes?
A. Localized deformation, or twinning, occurs in metals at low temperatures. This is when only localized regions are affected. Face transformation occurs under high pressure zones and is when the crystal structure transforms into another crystal structure. This could potentially increase ductility, strength or toughness of a material. The primary crystal structures of face-centered cubic and body-centered cubic. Face-centered cubic is a typical crystal structure for steels and is when all atoms are arranged in a cube. Body-centered cubic structure is similar to the face-centered cubic structure, but has one atom in the center of the cube. A material with different crystal structures tends to be stronger. It is possible that we will be able to irradiate materials to precisely change structures someday. We can change the energy level and particle type used in irradiation to induce different microstructure changes at different depths within the material.
Deploying technological advancements to move the industry forward (44:17)
44:17-45:23 (Janelle discusses her view of the industry ten years from now. She calls for significant innovation and for the industry to deploy these advancements.)
Q. Where do you think the industry will be 10 years from now?
A. Janelle sees the need for significant innovation. She also calls for the industry to harness and deploy the technological advancements made in reactor designs. Janelle believes this will transform the way the industry works in the US, influencing changes in other countries.