 Thin plates or foils of beryllium are sometimes used in nuclear weapon designs as the very outer layer of the plutonium pits in the primary stages of thermonuclear bombs , placed to surround the fissile material . These layers of beryllium are good " pushers " for the implosion of the plutonium @-@ 239 , and they are also good neutron reflectors , just as they are in beryllium @-@ moderated nuclear reactors . 
