There are bombs where a fission stage ignites a fusion stage which sends neutrons to start another fission reaction. From: https://en.wikipedia.org/wiki/Thermo

There are bombs where a fission stage ignites a fusion stage which sends neutrons to start another fission reaction. From: https://en.wikipedia.org/wiki/Thermonuclear_weapon#Summary https://en.wikipedia.org/wiki/Thermonuclear_weapon#Summary "(3) The fusion fuel of the secondary stage may be surrounded by depleted uranium or natural uranium, whose U-238 is not fissile and cannot sustain a chain reaction, but which is fissionable when bombarded by the high-energy neutrons released by fusion in the secondary stage. This process provides considerable energy yield (as much as half of the total yield in large devices), but is not considered a tertiary "stage". Tertiary stages are further fusion stages (see below), which have been only rarely used, and then only in the most powerful bombs ever made."