L in any kind of nuclear reaction there is a transmutation from one elemental nucleus into one or more others. Natural decay of uranium goes through a rather lengthy complicated breakdown ending with an isotope of lead. Fission, like that found in pwoer plants breaks the heavy uranium into isotopes of smaller nuclei. Nuclear reactions are also used to produce the heavier elements like plutonium, americium, and einstienium.
The chernoble disaster burned through the floor of the reactor and was feared to cause a China Syndrom, in fact workers tunnelled under the reacor in an attempt to create a concrete barrier. However it is now known that the nuclear pile became so spread out that its heat dissapated and the material hardened on its own.
