Basically, an atomic bomb relying on pure fission like a plutonium bomb works by chain reactions. However, something must initiate the chain of reactions : that's where Hydrogen comes in:
the fission reaction of Uranium is [tex]U+n\rightarrow Sr+Xe+2n[/tex] where n is a neutron. Hydrogen is basically a neutron and an electron; it's thus quite easy to start the reaction with hydrogen.
There is no known way to significantly reduce radioactive decay as far as I know (I might be wrong), or to turn radioactive plutonium into non-radioactive plutonium without tearing the molecules apart.
