The third hydrogen
Hydrogen has three isotopes and they are the only ones with names of their own, because doubling and tripling the mass of the lightest atom changes its behaviour in a way that adding a neutron to carbon does not.
Protium is a single proton, 99.98 % of all hydrogen. Deuterium adds a neutron. Tritium adds two, and is unstable — it beta-decays to helium 3 with a half-life of 12.32 years.
⚠️ That beta particle is exceptionally weak, averaging 5.7 kiloelectronvolts. It cannot penetrate skin, or paper, or the wall of the glass tube it is usually kept in. Tritium is one of the few radioactive materials that is harmless to stand next to.
Natural production is small: cosmic rays make a few kilograms a year in the whole atmosphere, in equilibrium with what decays. Everything used is manufactured.
Light without power
A tritium exit sign is a sealed glass tube with tritium gas inside and phosphor on the inner wall. Beta particles strike the phosphor and it glows.
Nothing else is required. No battery, no wiring, no bulb to replace. The sign works during a power cut, which is precisely when an exit sign is needed, and it keeps working for about fifteen years before the tritium has decayed enough to matter.
The same principle lights gun sights, watch faces and compass markings, replacing the radium that once did the job — with a fraction of the hazard, since tritium’s beta cannot escape the glass and radium’s alpha daughters could escape the paint.
⚠️ The signs are regulated because of what happens at the end of their life. A broken tube releases tritium as gas or water vapour, and disposal in ordinary waste is prohibited in most countries.
The fusion problem
The easiest fusion reaction to achieve is deuterium with tritium:
D + T → He-4 + n + 17.6 MeVIt ignites at a lower temperature than any alternative, which is why every major fusion project uses it. Deuterium is abundant — a fraction of all the hydrogen in seawater. Tritium is not.
World inventory is measured in tens of kilograms, produced mainly as a by-product of Canadian heavy water reactors. A single power-station-scale fusion reactor would consume more than that in a year.
So a fusion reactor must make its own. The plan is a blanket of lithium surrounding the plasma:
Li-6 + n → He-4 + TEvery fusion reaction produces one neutron, and every neutron must produce more than one tritium atom for the cycle to close — some are lost, absorbed elsewhere or escape.
⚠️ Nobody has demonstrated a breeding ratio above one in a working reactor. ITER will test blanket modules rather than breed its own supply, and the question of whether the tritium cycle can close is among the least settled parts of the fusion programme. It receives far less attention than plasma confinement, and may prove harder.
Dating water
Tritium in the atmosphere combines with oxygen to make tritiated water, which falls as rain and enters groundwater.
Once underground, it is cut off from resupply and decays with its 12.32-year half-life. Measuring how much is left tells you roughly when the water last saw the sky.
The atmospheric weapons tests of the 1950s and 1960s raised the level by two orders of magnitude and then let it fall — a sharp spike that hydrologists use as a marker. Water carrying bomb tritium fell after 1952 and before about 1970.
⚠️ The spike is fading as the tritium decays, and the method with it. Groundwater dating is moving to other tracers, though tritium remains the standard for water younger than about sixty years.
The weapons use
Tritium is not itself a fission or fusion fuel in a weapon in the way the name “hydrogen bomb” suggests. Its role is boosting.
A small quantity of deuterium and tritium gas injected into the core of a fission device fuses under the heat of the initial reaction and floods the core with fast neutrons, causing much more of the fissile material to react before the device disassembles.
⚠️ Because tritium decays at 5.5 % a year, every boosted weapon needs its tritium reservoir replaced every few years. This is a maintenance burden no other component imposes, and it is one of the reasons arsenals require continuous industrial support rather than simply sitting in storage.