In addition, small plutonium bombs are often used to trigger larger hydrogen weapons.
From the nature.com
That compresses the plutonium while it is simultaneously bombarded with neutrons from a trigger made of polonium, and thus begins the second stage.
From the economist.com
Uranium can also be used to trigger nuclear fusion of heavy isotopes of hydrogen, unleashing vastly more explosive power than is possible with uranium or plutonium alone, Alvarez says.