In general, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts (lighter nuclei). There is also a fission process known as ternary fission. Ternary fission is a comparatively rare (0.2 to 0.4% of events) type of nuclear fission in which the fissioning nucleus splits into two relatively large fragments plus a light fragment such as a hydrogen, tritium, or helium nucleus, plus several neutrons.
Ternary fission may happen during neutron-induced fission or in spontaneous fission (the type of radioactive decay). About 25% more ternary fission happens in spontaneous fission compared to the same fissioning system formed after thermal neutron capture, illustrating that these processes remain physically slightly different. Note that, similarly as for alpha decay, also spontaneous fission occurs due to quantum tunneling and it is one of decay modes rather than induced nuclear reactions.
The smallest of the charged products may range from so small a charge and mass as a single proton (Z=1), up to as large a fragment as the nucleus of argon (Z=18).
Alpha Particles and Ternary Fission
Especially energetic alpha particles (except artificially accelerated helium nuclei) are produced in a nuclear process, which is known as a ternary fission. In this process, the nucleus of uranium is splitted into three charged particles (fission fragments) instead of the normal two. The smallest of the fission fragments most probably (90% probability) being an extra energetic alpha particle. This high incidence is related to the stability (high binding energy) of the alpha particle, which makes more energy available to the reaction.
Tritium and Ternary Fission
Tritium is a byproduct in nuclear reactors. Most important source (due to releases of tritiated water) of tritium in nuclear power plants stems from the boric acid, which is commonly used as a chemical shim to compensate an excess of initial reactivity. Tritium is also a fission product (ternary fission) of the splitting of fissionable materials. In fact, fission probably produces more tritium than all other sources in Light Water Reactors. Its production (yield) is of about one atom per each 10,000 fissions. The experimentally measured tritium production rate is 1.08 x 10-4 tritium atoms per 235U fission and 1.41×10-4 tritium atoms per 239Pu fission. It has been calculated that a 3000 MWt reactor produces approximately 42 Curies per day of tritium. On the other hand only a very small fraction of the fission-product tritium diffuses out of the fuel matrix and fuel cladding into the primary coolant. It is a major fuel reprocessing concern