Question

Why lead metal is used to store radioactive substanes

Answer 1

Radioactive wastes continue to emit alpha, beta and gamma radiation. While alpha and beta particles can be easily stopped, gamma is highly penetrative. It is most efficient stopped by elements with high atomic weight. As Pb is among the heaviest stable element (most of the radioactive elements decay into Pb to become stable), it is used for shielding gamma radiation.

Answer 2

because it has a  high density and it is effective to stop gamma rays,x-rays.