In 2013, Phelps became the manager of Oak Ridge's programs that make the isotopes nickel-63 and selenium-75. She has also studied isotopes of plutonium and neptunium, which are used by the National Aeronautics and Space Administration (NASA) to power spaceships. Phelps also studies isotopes that are used for medical research and treatments, like actinium, lanthanum, europium, and samarium. Phelps has studied electrodeposition with californium-252 for the Californium Rare Isotope Breeder Upgrade project (CARIBU). Oak Ridge makes most of the world's californium, which is used in cancer treatments.