The blast from a primary fission component triggers a secondary fusion explosion in a thermonuclear bomb or warhead. Encyclopædia Britannica, Inc.
…of the fissile isotopes uranium-235, plutonium-239, or some combination of these; however, some explosive devices using high concentrations of uranium-233 also have been constructed and tested.
…latter isotope eventually decays into plutonium-239—a fissile material of great importance in nuclear power and nuclear weapons. Another fissile isotope, uranium-233, can be formed by neutron irradiation of thorium-232.
…used to create quantities of plutonium-239 from uranium-238, the principal constituent of naturally occurring uranium. Absorption of a neutron in the uranium-238 nucleus yields uranium-239, which decays after 23.47 minutes through electron emission into neptunium-239 and ultimately, after 2.356 days, into plutonium-239.
…group, he discovered the isotope plutonium-239, which he found to be fissionable, much like uranium-235. Plutonium-239 was used in the first atomic bomb and in the bomb dropped on Nagasaki.