Learn about half life, in this clip from Uranium: Twisting the Dragon's Tail. Half Life is the rate at which radioactive atoms decay and change themselves. The rate of decay varies which is why some radioactive substances can remain dangerous for a very long time. In 1986 after the Chernobyl nuclear accident the damaged reactor was encased in a concrete sarcophagus but now it's cracking and needs to be replaced. Watch Derek show a new confiement structure being built to last one hundred years. In 5 billion years, only half of that left over uranium will be decayed. Hear how scientists developed a way to use the half life of the radioactive atoms for medinical purposes in cancer patients with medicine Technetium 99m.