Neutron capture by materials of the nuclear reactor (shielding, cladding, etc.) or the environment (seawater, soil, etc.) produces activation products (not listed here). These are found in used nuclear reactors and nuclear fallout. A small but non-negligible proportion of fission events produces not two, but three fission products (not counting neutrons or subatomic particles). This ternary fission usually produces a very light nucleus such as helium (about 80% of ternary fissions produce an alpha particle) or hydrogen (most of the rest produce tritium or to a lesser extent deuterium and protium) as the third product. This is the main source of tritium from light water reactors. Another source of tritium is Helium-6 which immediately decays to (stable) Lithium-6. Lithium-6 produces tritium when hit by neutrons and is one of the main sources of commercially or militarily produced tritium. If the first or only step of nuclear reprocessing is an aqueous solution (as is the case in PUREX) this poses a problem as tritium contamination cannot be removed from water other than by costly isotope separation. Furthermore, a tiny fraction of the free neutrons involved in the operation of a nuclear reactor decay to a proton and a beta particle before they can interact with anything else. Given that protons from this source are indistinguishable from protons from ternary fission or radiolysis of coolant water, their overall proportion is hard to quantify.