No, it's in the missile object code (specifically, the NeutronMissileSlowDeathBehavior module). While there is a weapon associated with the Nuclear Missile superweapon, it is a dummy weapon that merely allows the missile to be targeted anywhere.
While you're there, make it do 2 damage waves, with the second one doing 500 damage out to the edge of the damage area. This will clear away those annoying GLA holes that make the Chinese nuke useless against GLA buildings.