Most of the energy from large H-bombs still comes from fission. You can surround the fusion stage with natural uranium. While natural uranium has to much U-238

Most of the energy from large H-bombs still comes from fission. You can surround the fusion stage with natural uranium. While natural uranium has to much U-238 to undergo a runaway chain reaction, the fission reaction emits enough high energy neutrons to induce a significant fission reaction.