Sizing up nuclear fuel

Posted: May 7, 2014 - 5:50pm | Y-12 Report | Volume 10, Issue 2 | 2014

When is small too small? Y‑12 researchers analyzing samples of uranium materials would answer, “Almost never.”

In response to a request from the International Atomic Energy Agency, a team from Y‑12’s Analytical Chemistry and Development organizations evaluated natural and low-enriched-uranium samples — most no larger than 15 grams, or roughly comparable to a teaspoon of powder.

The IAEA called on Y‑12 because it is one of only a few Department of Energy/National Nuclear Security Administration facilities with expertise in fusing particles of uranium oxide, by means of a process called sintering, and in studying the densification of solids. Equally important, Y‑12 has the analytical facilities to handle these types of samples. In addition, the relatively tiny samples, coupled with the relatively large number of analyses of each one, eliminated many laboratories.

For this project, the team examined both uranium oxide powder and compressed fuel pellets to evaluate isotopic content, elemental impurities and physical characteristics, such as response to the high temperatures encountered in commercial nuclear reactors.

Analytical Chemistry received about half the material that came to Y‑12, which had to accommodate as many as 12 analyses each. That portion represents a half or, for some analyses, even a tenth of the amount chemical technicians usually work with.

Because of the number of examination techniques required — for instance, the inorganic and radiochemical analyses of every element on the periodic table — managing their sequence was essential. The IAEA relies on Y‑12 expertise to analyze the powder and pellets that fuel nuclear reactors. Analytical Chemistry’s Darrin Mann said, “Moving the sample between groups was the biggest hurdle. Nondestructive techniques came first, but then we had to decide what came next because the sample size dwindled with each analysis.”

Adding to the work stress was the need for extraordinary measures to avoid cross contamination. “Although we maintain sample integrity with all our analyses, we had to be particularly careful with these samples,” said Vicki Belt, also of Analytical Chemistry. “We knew we had only one chance because there wasn’t enough material for any reanalysis.”

Development needed to determine how well the uranium oxide powder, when pressed into a fuel pellet and sintered, could withstand the heat of a nuclear reactor. “We used accepted Nuclear Regulatory Commission procedures, which simulate reactor conditions, to ensure the pellets would remain stable during operation,” said Greg Schaaff, a materials scientist in Development. Too much expansion would break a fuel rod containing the pellet, causing a change in reactor core geometry and creating criticality safety issues.

The fact that the pellets Development formed were smaller than those used in a reactor did not affect quality. “We made sure they were scaled appropriately, according to the sample amounts we received,” Schaaff explained.

The yearlong project was an opportunity to showcase Y‑12’s strengths and broaden experience. As Schaaff said, “It was a nice change of pace for us. People learned new things — how to apply unfamiliar procedures to the certification of fuel, for instance. And it was great for our newer staff to be involved in something out of the ordinary.”