"Trace quantities occur naturally in uranium ores." Neptunium - Element information Neptunium Periodic table "Trace quantities occur naturally in uranium ores." Np http://www.rsc.org/periodic-table/element/93/neptunium Neptunium "Trace quantities occur naturally in uranium ores." http://www.rsc.org/periodic-table/element/93/neptunium Neptunium Royal Society of Chemistry