Tritium consists of about 4.5 kilograms of plutonium-239. Since tritium undergoes radioactive decay, and is also difficult to confine physically, the much larger secondary charge of heavy hydrogen isotopes needed in a true hydrogen bomb uses solid lithium deuteride as its source of deuterium and tritium, producing the tritium "in situ" during secondary ignition. During the detonation of the primary fission bomb stage in a thermonuclear weapon (Teller-Ullam staging), the sparkplug, a cylinder of U-235/Pu-239 at the center of the fusion stage(s), begins to fission in a chain reaction, from excess neutrons channeled from the primary. The neutrons released from the
Tritium MeV. In comparison, the fusion of deuterium with tritium releases about 17.6 MeV of energy. For applications in proposed fusion energy reactors, such as ITER, pebbles consisting of lithium bearing ceramics including LiTiO and LiSiO, are being developed for tritium breeding within a helium cooled pebble bed (HCPB), also known as a breeder blanket. High-energy neutrons can also produce tritium from lithium-7 in an endothermic (net heat consuming) reaction, consuming 2.466 MeV. This was discovered when the 1954 Castle Bravo nuclear test produced an unexpectedly high yield. High-energy neutrons irradiating boron-10 will also occasionally produce tritium: A more common result