Team Fin. Left to Right: Collin Grischott, Yuji Williams, and Jason Wood pose with one of Oregon State's Slocum G2 Gliders and their 3D -printed RTG model at the 2017 Undergraduate Engineering Expo.
Across the vast and varied expanse of the Pacific Ocean, from Vietnam to the Pacific Northwest, Slocum G2 Gliders sawtooth up and down. Under the direction of Kipp Shearman, associate professor of oceanography at Oregon State, and his fellow researchers, these gliders take physical measurements of salinity, temperature, and oxygen saturation and optical measures of plankton and other biota. Powered by either alkaline or lithium battery systems to run their buoyancy pumps, they can operate nonstop for between 10 days to 2 months before needing a fresh set.
In the Radiation Center on the Oregon State Corvallis campus, nuclear engineering undergraduate students Collin Grischott, Yuji Williams, and Jason Wood put the finishing touches on their senior design capstone research project in June 2017. They designed and 3D modeled a radioisotope thermoelectric generator (RTG) battery alternative for the gliders.
“We wanted to do something that has some practical applications, and collaborate with different departments in the [university],” says Williams. “We wanted to focus on actually being able to design something, to have something tangible to present. I mean, our 3D model isn't the actual RTG, that's going to be produced, but having a 3D model is pretty powerful for our design projects.
The team’s RTG model consists of a fuel pin surrounded by one centimeter layers of graphite and lead for shielding, with a seven-sided array of thermoelectric generators wrapping around the outside.
Diagram of the team's RTG model.
The team chose the radioisotope strontium-90 as the energy source for the fuel pin because of its low cost ($1 per gram), high power density (1.31 watts per cubed centimeter) and ready availability as a decay byproduct from commercial fission reactors. Strontium-90 decays into yttrium-90, which in turn decays into stable zirconium-90, generating heat along the way. Approximately 75% of the heat is generated in the yttrium-90 to zirconium-90 step of the process.
"That's actually [commercial reactors] number one problem, is strontium decaying into yttrium, creating a lot of heat in their storage tanks,” says Grischott. “It's a byproduct right now, but if it can be applied in a commercial application, and looked at as a useful material, then they can change what they're doing with it.”
Utilizing this decay heat, the team’s design would generate power through the thermoelectric generators. The RTG generates 45 watts--a three-fold increase over current batteries. In addition, it would generate power for 10 years, compared to the two months maximum with traditional batteries. It also weighs about two-and-a-half pounds less.
“[We] tripled the power, quadrupled the speed that it can travel, and extended the lifetime by forty,” says Grischott. Additionally, the team designed the RTG to fit in the same footprint as current batteries and estimates the total cost for materials would be less than $2,800. “I was totally excited to have people from the engineering college coming to me to talk to me about the research problems that I face, and the sort of potential for figuring out a solution,” says Shearman.
Problems like the upwards of $100,000 Shearman spends on batteries in a typical year’s worth of operations. “A set of alkaline batteries is a couple thousand, I think, for just a set of batteries. And then the lithiums are on the order of $20,000.”
A different power source, like the team’s RTG, would also open up new research possibilities for Shearman’s glider fleet. “So, one thing that's happening in ocean glider autonomous underwater vehicle research is full-depth ocean gliders are being created, so gliders that can withstand going down to 6,000 meters,” explains Shearman. “However, that dive may take a full day to do, and if you're going to run your sensors at a high rate, doing that for a full day is going to consume a lot of energy. Typically, the solution is to sample densely in the upper part of the ocean, and then sample a lot less frequently as you get deeper. With an [RTG] like this, you could run your sensors flat-out, all the time, and have energy always available." Those types of missions also have the potential to stay in continuous operation for years, gathering longer-term data than what’s currently available.
Purely a design concept at this point, the RTG would need to go through extensive design verification and licensing with the U.S. Nuclear Regulatory Commission (NRC) to be put into operation. “So, for us, the [question] is, how do we get this license, what kind of licensing do we need, and what are the requirements,” says Wood. “The biggest hurdle has just been looking at what kind of precedents has there been in the past for using strontium as a fuel for an RTG, and also using an RTG underwater? Because that is the biggest challenge, is justifying can we put a radioactive isotope in an unmanned, underwater vehicle and send it out in the ocean?”
The team did design the RTG to be well under regulatory shielding limits as these limits would be the first obstacle to navigate.
But as Shearman notes, whether something like this is ever realized in the real world, the point of capstone projects is to allow students to explore possibilities and questions that might otherwise never even be broached. “We all get stove-piped in our own areas, in our own work, and you need students like this bridging the gap,” he says. “They're so good at doing this, because they have classes and projects where they have to do stuff, but they're also not encumbered by, ‘Oh, I'm a physical oceanographer, I have to write my proposals and go to sea and do my research,’ right? They're wide open--why not build a nuclear battery for a glider, you know? I encourage more of that.”
— Jens Odegaard.