Complex sentence: The explosion of the gun powder will drive the two hemispheres of U- 235 together: to form a common ball of supercritical mass. The force of impact will weld them together.
Split into simpler sentences: The explosion of the gun powder will drive the two hemispheres of U- 235 together to form a common ball. This common ball will have a super critical mass. The force of impact will weld them together.