Both the codons uuu and uuc specify the amino acid phenylalanine. what is the term for this phenomenon?

This is called codon degeneracy. It is also said that the genetic code is redundant, meaning that multiple codons can code for the same amino acid, like for example CCC and CCA both coding Proline.

radioactive decay occurs in unstable atomic nuclei – that is, ones that don't have enough binding energy to hold

the nucleus together due to an excess of either protons or neutrons.