Radiation is a danger you cannot see, smell or touch, so the trefoil sign is read as a worry that works quietly in the background. It can also stand for an influence that spreads further than expected.
Where in your waking life have you met something like this lately?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.