Nuclear medicine is an imaging specialty focusing on those imaging modalities performed after administration of radioactive particles. Radiologists receive 6 months of Nuc med training during residency and can do all of the same procedures. They can also do a nuc med fellowship.
Nuclear medicine specialists do not read MR (nothing nuclear there, just magnets), CT or radiographs. This puts them at a disadvantge in for private practice jobs since the volume of nucs is not big enough for many hospitals so they hire radiologists who can do nucs and more. Academics has more pure nucs folks. Not the best market.
Radiation oncology uses radiation, either directly applied (brachytherapy) or external beam to treat mostly cancers but also some non malignant conditions. Great specialty with lots of carrer opportunities, research (even in private practice), money, and great lifestyle, all while still seeing patients in clinic.
In summary: Very separate specialties.