Safety Analysis of Modified CANDU Reactors (0:46)
0:46-14:17 (Jacob Eapen recalls his time working on safety analysis of the advanced heavy water reactor at the Bhabha Atomic Research Center in India)
Q: How did it all begin for you?
A: Jacob Eapen received his mechanical engineering degree from the University of Kerala in India. While looking for a place to do research, he took the opportunity to work at the Bhabha Atomic Research Center in Mumbai where nuclear reactor research was going on. In India, nuclear is looked at in a very positive manner and is a very prestigious industry, attracting some of the top minds from across the country. Since Jacob had no background in nuclear, he spent the first year in a nuclear training program. He then joined a division called the Division of Reactor Safety, mostly dealing with CANDU-type reactors that went through multiple modifications in India. Jacob worked on an offshoot of the CANDU reactor called the advanced heavy water reactor. In the U.S., the capacity factor has climbed from 91 to 92% and higher over the years. In India, the capacity factor is approaching 90%, but was close to 70% earlier on since there was an emphasis on shutting down reactors.
Jacob starting working on safety codes to model the refueling of a CANDU reactor, which can be refueled without a shutdown. CANDU reactors have long, pressurized tubes. The fuel elements are the shape of short fuel bundles that sit horizontally in the tubes, unlike the vertical fuel assemblies in the U.S. The reactor is heavy water cooled and heavy water moderated. The tubes have refueling machines on both ends which push and receive the fuel. The reactor does not have one pressure boundary like a light water reactor. Since the fuel is smaller, things like flow induced vibrations are minimized and tend to have fewer problems. There is a big neutron field around the reactor, both light water and CANDU. The problems related to fission gas are prevalent in both reactors, but geometry plays a big role. Jacob’s code development focused on accident analysis, including core meltdown.
Computational Material Science in Nuclear (14:17)
14:17-28:05 (How Jacob expanded his career in the U.S. nuclear academic and industrial spaces to focus on computational material science)
Q: When did you come to the U.S.?
A: Jacob Eapen was in the middle of a job search to pursue his Master’s or PhD to do research at the highest level, looking at opportunities in India and the U.S. He chose NC State where he started studying computational flow of plasma dynamics. Mohammed Bourham had a project from the Navy focused on electrothermal plasma launching which Jacob supported from the computational side. Projectiles were thrust to very high speeds, up to 7 km/s. The Navy wanted to get as high as 10 km/s to shoot projectiles to space. Jacob was planning on doing his PhD at NC State, but it was a low point for nuclear at that time in 1998. He took a job to experience the U.S. nuclear industry, leaving NC State to work for Framatome in Lynchburg, Virginia. Jacob worked on many little projects that usually lasted 4-6 weeks, solving client problems related to safety analysis or fluid flow heat transfer. In his two years at Framatome, Jacob learned a lot about the inner workings of the industry, but decided he wanted to get back into academia. Jacob went to MIT for his PhD and his first instinct was to go into thermohydraulics. While there, he learned about material science and had the natural inclination to work on the computational side of materials. The pillars of science used to be just experimental and theoretical. Once the first computers were built, a new pillar evolved which was a mix between theory and experimentation. This computational theory branch takes the concepts from theory and performs experiments on the computer.
One thing Jacob worked on at NC State was graphite. Graphite has a planar structure meaning the carbon atoms are arranged in a honeycomb lattice and are very strongly bonded on a plane. There are many layers of these planes that are loosely tied together. Graphite was the first moderator used by FERMI and Chicago Pipe and was also used by the British. The graphite moderator program in the U.S. in the 1950’s in the aircraft reactor program, which didn’t pan out. In the 1960’s, Oak Ridge took initiative and build a molten salt reactor with a graphite moderator, which ran for a few years. The DOE is currently looking at the possibility of the next generation reactors that can go to higher temperatures to achieve thermodynamic efficiency. Graphite is a great moderator that can withstand high temperatures. The microstructure changes with neutron irradiation and fission gases create big voids in graphite. One of Jacob’s projects was focused on creep of graphite, looking at how different defects were formed in the graphite. When exposed to radiation, graphite expands along one axis and shrinks along another axis.
High Temperature Properties of Uranium Oxide Fuel (28:05)
28:05-44:53 (An overview of Jacob’s current and past research projects in the material science space, including analysis of high temperature properties of UO2 fuel)
Q: What other type of work does your lab focus on?
A: When Jacob Eapen joined NC State in 2008, he was looking for material science projects because he did computational material science in his PhD. Following that, he went to Los Alamos for a short time where he worked with Art Voter, a well-known scientist in the area of accelerated molecular dynamics. After this, Jacob went to NC State. One of his materials projects was on graphite and another project was on high temperature properties of uranium oxide (UO2) fuel, greater than 2,000 degrees Kelvin UO2 has a peculiar property called superionic property. Beyond a certain temperature, one of the ions starts diffusing like a liquid. It is solid state and the uranium is keeping the lattice in place, but the oxygen ions are diffusing in and around uranium pillars. When Jacob and his team started looking at it, they were able to simulate this behavior in the UO2. Simulations are a big help when there are a lot of conflicting theories. He also used a spectroscopic technique to get data about dynamic structure function. Models are used to make sense of this very abstract data.
In the early 80’s, the UK, who were the leaders in neutron scanning techniques, predicted that the oxygens were leaving the lattice. However, they couldn’t fit the placement of the ions into any known models. One person said that the oxygen ions were just jumping from one place to another, not flowing. Jacob Eapen picked this work up and saw that the ions were jumping. This led him to the world of supercooled liquids. When atoms are cooled very quickly, they are in a frustrated state and tend to follow things along a line, in a string-like motion. If the temperatures are raised even higher, it goes to a pseudo phase transition called lambda transition. Around 2,700 degrees Kelvin the specific heat property value shoots up and comes back down, representing a second-order phase transition. Jacob was able to show that, as early as 2,000 degrees Kelvin, the ions slowly start moving from the lattice. At low temperatures, there are a small number of very long strings. As temperature increases, there are more strings but of a shorter nature. Many of the fission products take the place of the uranium, not the oxygen. The thermal conductivity of graphite was also looked at and another student looked at transfers in non-metallic insulators. Jacob does a little bit related to the quantum effects, such as bonding, but Jacob is more interested in the collective nature of atoms.
Cladding Materials in Accident Tolerant Fuel Design (44:53)
45:53-54:44 (An explanation of why accident tolerant fuels are being developed and some of the challenges faced in choosing a material for the fuel cladding)
Q: Are there no quantum effects that are not being accounted for?
A: Jacob Eapen does a little bit related to the quantum effects, such as bonding, but Jacob is more interested in the collective nature of atoms. Material scientists are more interested in the prediction of structures and new structures. There are very good approximations, such as density functional theory. One of his ongoing projects does this, but it is very expensive to complete the calculations. Jacob tries to find the interatomic potential that would mimic the electronic behavior to calculate the forces between two atoms. Quantum effects are smaller at higher temperatures.
Jacob has a project ending soon on accident tolerant fuels. Accident tolerant fuels (ATF) started from Fukushima in 2011 and the Department of Energy (DOE) reorganized the fuel program to include ATF programs. One main focus was how to control hydrogen, which was the big problem with zirconium. An exothermic chemical reaction is happening which produces a lot of heat, which is the downside of the zirconium that makes up the cladding material. Zircaloy does not absorb neutrons like steel, which is why steel is not used in cladding material in light water reactors. Another option for the cladding include silicon carbide composites, which has small cross-sections and does not react with water. Long filaments of silicon carbide are put in a matrix of silicon and carbide, which creates a silicon carbide composite. There is a good chance that this was developed in the 60’s at Los Alamos or Oak Ridge. In the 60’s, there was a tremendous interest by the Navy to develop nuclear reactors for their submarines and aircraft carriers, which pumped a lot of money into nuclear development. At the time, there was a need to replace the noisy diesel generators that could easily be detected by the Russians. That kind of need is not there any more.
Climate change is real and fossil fuels are adding a lot of carbon into the atmosphere and the oceans. A carbon neutral source like nuclear is needed to counter climate change. Other renewable sources cannot be scaled to the level that nuclear can go with current technology. Jacob’s wish is that the next generation of people can bring the next generation of reactors that can scale up.