What is the chemical formula for concentrated plutonium exploding and igniting hydrogen and how could it be stabilized? i'm not doing this irl, so don't worry!

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  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.