Radioactivity
Radiation
noun
放射 is "radiation." Add the ability of radiation and you have radioactivity, for some reason. I guess things don't have radioactivity until you have given them the ability of radiation, right?
ほうしゃのう
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
放射能に関する会議を招集する前に、出席者を決める方がいいだろう。
Before calling the meeting about the radioactivity, we should decide who should attend.