Complex sentence: Fission products are as deadly as neutron-activated cobalt in the first week or two following detonation, after a month to six weeks, the fission products from a thermonuclear weapon decay to levels tolerable by humans.
Split into simpler sentences: Fission products are as deadly as neutron-activated cobalt in the first week or two following detonation. After a month to six weeks, the fission products from a thermonuclear weapon decay to levels tolerable by humans.