The effect of oxygen and nitrogen content of tantalum on its resistance to corrosion by molten plutonium-cerium-cobalt alloys was investigated. The impurity content of the tantalum ranged from 100- to 4400-ppm oxygen and from 20- to 1000-ppm nitrogen. The test was completed in five cycles over a period of 4700 h at 650°C. Plutonium penetration occurred at the closure welds through cracks attributed to mechanical stress from expansion of the fuel alloys on freezing and did not occur in capsules prepared from high-purity tantalum.