natural radioactivity

21 LETTERS · STARTS WITH N

natural radioactivity is a valid 21-letter word worth 29 points in Scrabble and 33 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS

N1A1T1U1R1A1L1R1A1D2I1O1A1C3T1I1V4I1T1Y4

1 + 1 + 1 + 1 + 1 + 1 + 1 + 0 + 1 + 1 + 2 + 1 + 1 + 1 + 3 + 1 + 1 + 4 + 1 + 1 + 4 =29 PTS

DOUBLE WORD → 58TRIPLE WORD → 87

MEANING

What does natural radioactivity mean?