For the past three years, Idaho National Laboratory researchers David Hurley and Robert Schley have been tinkering away on a new kind of microscope.
Yet looking at the complex contraption now — a series of lenses, mirrors, cameras and fiber-optic cables rigged to metal bars — you might never know what it was. To top it off, the microscope relies on a pair of high-powered lasers to do its job.
Hurley and Schley built the first-of-its-kind Thermal Conductivity Microscope for one purpose: to measure how heat travels through specimens of irradiated nuclear fuel. Thermal conductivity is crucial to the success of a nuclear reactor, but nuclear scientists have long had a tough time measuring it with any level of precision.
“In a reactor, the ability to conduct heat is very important for the fuel,” Schley said. “If you have a fuel rod, when it’s in the reactor and fissioning, it generates a lot of heat. And that heat has got to get from inside the material out to the cooling fluid — the water in the reactor.”
The microscope isn’t measuring highly radioactive nuclear fuel at INL just yet. But the invention has already impressed INL leaders so much that Hurley and Schley took home the Outstanding Innovation Award last month at the lab’s annual awards ceremony.
As a nuclear fuel rod is gradually irradiated inside a reactor, imperfections begin to form, such as cracks and bubbles. Those imperfections change how heat is transferred through various parts of the fuel rod, and in turn how efficient the fuel is at heating the water and ultimately creating energy.
“In order to develop new fuels and make them as efficient as you can, you need to know how thermal conductivity changes over time with (fuel) in the reactor,” Schley said.
That’s where the Thermal Conductivity Microscope comes in. It is able to measure how heat travels through small areas of a fuel sample, and how that heat transfer changes when the fuel is partially or fully irradiated.
The microscope utilizes a pair of lasers. One is a “pump beam” laser that heats up part of the nuclear fuel sample. The other is a “probe beam” laser that is pointed directly on top of the heating laser, and measures how the heat propagates through the fuel. The second laser effectively acts as a thermometer, Hurley said.
Cameras mounted to the microscope show progress on a screen, and allow the researchers to accurately point the lasers to the place on the sample they want to measure.
Right now, the researchers are using a nonradioactive fuel surrogate to test the microscope at a laboratory in Idaho Falls. But in the next year or two, it’s expected the microscope will be ready to study real radioactive fuel samples at a facility located at the Materials and Fuels Complex.
The microscope will be placed in a walled-off “hot cell” used for examining radioactive fuel. It will be operated remotely using robotic arms and a computer.
As safer, more efficient nuclear fuels are developed, Hurley and Schley said their microscope will help confirm whether those fuels really do have the right thermal conductivity features. The microscope will also help develop more accurate computer models that show how heat is transferred through nuclear fuels, and how that heat transfer changes as the fuel is gradually irradiated.
“For us to be able to go in and say, ‘This is the thermal conductivity of this certain area of the fuel’ — that’s really important for validating and understanding these (computer) models,” Hurley said.