what is explosive compaction process and why it is used?

Answer 1

I don't know that I am answering the question you are asking. In a nuclear weapon a sphere of conventional explosive charges is focused toward the center. When the charges explode they compact the radioactive material in the center and (by "crowding") create a critical mass. A chain reaction (fission- atomic bomb) begins. If that's not enough that reaction can power a similar compaction to trigger a fusion (hydrogen bomb) reaction.