Ph.D. student Michael Bull receives UC Laboratory Research Fellowship

uc davis chemical engineering los alamos national laboratory graduate nuclear fellowship
Bull in the Kuhl Lab. The fellowship will allow him to conduct his research both at UC Davis and Los Alamos National Laboratory. Photo: Noah Pflueger-Peters/UC Davis.

Chemical engineering Ph.D. student Michael Bull recently received an In-Residence Graduate Fellowship from the UC Laboratory Research Fees Program. Bull will receive a stipend and mentoring from researchers at Los Alamos National Laboratory in New Mexico while conducting research both at the national laboratory and at UC Davis in professor Tonya Kuhl’s laboratory.

His project focuses on improving evaluation and maintenance of the nation’s nuclear stockpile through understanding the interactions between the polymers and sensitive explosive crystallites that make up a warhead’s outer shell. Bull will conduct tests at UC Davis and then use Los Alamos’ neutron scattering facilities to verify his results.

“Mike has come up with some really clever ideas for making high-resolution measurements possible and I have no doubt he will continue to innovate and create throughout the fellowship,” said Kuhl. “His work should lead to better polymer composites spanning everyday materials to polymer bonded explosives.”

Nuclear warheads are made of a uranium core surrounded by a polymer-bonded explosive, which is made of explosive nanocrystals embedded in a matrix of polymers. The matrix helps the explosive detonate in just the right way so it compresses the uranium and makes it fissionable.

However, it’s also the weak point of the system. Polymers are sensitive to temperature and the environment and can degrade over time, slowly moving away from the explosive crystallite and forming voids in the material. These voids change the interface between the polymers and crystallites, creating hotspots and increasing explosive sensitivity. Keeping track of these voids is crucial for making sure warheads are both safe and usable.

“The goal is to demystify this interface,” said Bull. “So far, scientists haven’t been able to make accurate interfacial energy measurements, which has limits our ability to predict the explosive material’s behavior. If we know what’s going on, we can optimize it and make polymer-bonded explosives that are safer and longer-lasting.”

To do this, Bull will pioneer a new technique using the Kuhl lab’s special surface force apparatus. The device, which combines microscopy and interferometry, allows them to measure the energy of the interface between the polymers and crystallites to see when and how it changes and track how fast polymers move when the interface starts to break apart.

The technique offers the potential to examine polymers from over 250 degrees Celsius to cryogenic temperatures and measure their interfacial energy at each step. With this, they can collect valuable data for aging models and computer simulations. These models can help researchers predict the behavior of polymers in a warhead and make maintaining, refurbishing or decommissioning them a much easier and more accurate process.

“If we can learn at what temperature the polymers start moving around, we can better target safe temperatures for storing the polymer-bonded explosives and warheads,” he said.

This fellowship begins a partnership between the research group and Los Alamos and streamlines collaboration on this problem. It also offers valuable experience for Bull, who hopes to work as a research scientist in a national laboratory after he completes his Ph.D.

 “We have a really strong synergy going,” he said. “We can use that to really paint a much more accurate picture of what’s going on in this material and together solve the problem of polymer aging.”