neptunium
noun
nep·tu·ni·um nep-ˈtü-nē-əm 
-ˈtyü-
: a radioactive metallic element that is chemically similar to uranium and is obtained in nuclear reactors especially as a by-product in the production of plutonium see Chemical Elements Table
Example Sentences
Recent Examples on the Web The amount of neptunium found in the air monitor at the school was a thousand times below the regulatory action level, said Mr. Edwards of the DOE. 
Kris Maher, WSJ, 28 Apr. 2021 Sometime in 2017, the radioactive element neptunium 237 was also detected near the school. The Cincinnati Enquirer, Cincinnati.com, 26 July 2019
Kris Maher, WSJ, 28 Apr. 2021 Sometime in 2017, the radioactive element neptunium 237 was also detected near the school. The Cincinnati Enquirer, Cincinnati.com, 26 July 2019 Word History
Etymology
New Latin, from International Scientific Vocabulary Neptune
First Known Use
1941, in the meaning defined above
Kids Definition
neptunium
noun
nep·tu·ni·um nep-ˈtü-nē-əm
-ˈtyü-
: a radioactive chemical element similar to uranium
Medical Definition
neptunium
noun
nep·tu·ni·um nep-ˈt(y)ü-nē-əm
: a radioactive metallic element that is chemically similar to uranium and is obtained in nuclear reactors as a by-product in the production of plutonium —symbol Np see Chemical Elements Table