Tritium

• 1. Introduction

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  Tritium is a rare radioactive form of hydrogen, and in March 2016 the entire stock of it at Fukushima Daiichi weighed only 2.1 grams. Those grams were dispersed through 860,000 cubic meters of stored water. A speck by weight, yet it became the subject of a government disposal plan and an international protest. That is the odd position tritium occupies. It is scarce, feeble and slow to harm, and still it draws the attention of weapons designers, fusion physicists, oceanographers and watchmakers. This documentary asks where such an atom comes from, why anyone wants it, and what it leaves behind.

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• 2. Discovery and Decay

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  In 1934, Ernest Rutherford, Mark Oliphant and Paul Harteck bombarded deuterium with deuterons, the nuclei of deuterium, and detected a new atom. Deuterium is hydrogen of mass 2, and it occurs naturally at an abundance of only 0.015%. The trio could not isolate what they had found. That feat came in 1939 from Luis Alvarez and Robert Cornog, who also realized the new atom was radioactive. In 1954, Willard Libby recognized that it could serve for radiometric dating of water and wine.

  One proton and two neutrons make up the nucleus of tritium, which is sometimes called a triton. Ordinary hydrogen-1, known as protium, has one proton and no neutrons. Deuterium has one proton and one neutron and is not radioactive. Tritium is the heaviest hydrogen isotope that holds together as a bound particle. It is one of the few nuclides with a distinct name, since the more systematic label, hydrogen-3, is much less common.

  A half-life of 12.32 years governs how fast tritium fades. Each decay turns it into helium-3 and releases 18.6 kiloelectronvolts, a very small amount of energy. The emitted electron averages 5.7 kiloelectronvolts. A nearly undetectable particle called an electron antineutrino carries off the remainder.

  Beta particles from tritium cross only about 6 mm of air. They cannot pass through the dead outermost layer of human skin. Their low energy also keeps bremsstrahlung, a braking radiation, lower than it is for other beta particles.

  Rhenium-187 and tritium share an unusual trait. Both decays release little energy, which suits attempts to measure the absolute mass of the neutrino. None of those attempts has yet succeeded. The same weakness makes tritium-labeled compounds hard to detect, and they generally require liquid scintillation counting.

  Diatomic tritium is a gas at standard temperature and pressure, and combined with oxygen it forms tritiated water. Its atomic mass is 3.01604928. It melts at 20.62 K, against 13.99 K for natural hydrogen, and boils at 25.04 K, against 20.27 K.

  The decay product, helium-3, has a neutron-capture cross section of 5330 barns, a fact that matters inside every reactor.

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• 3. Breeding From Lithium

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  Lithium-6 is the usual raw material, and nuclear reactors turn it into tritium by neutron activation. The tritium and helium that form can release and diffuse within ceramics known as breeder ceramics. Neutrons of any energy will do the job. Slower ones work best, with a cross section above 900 barns for thermal neutrons. The reaction is exothermic and yields 4.8 MeV.

  ITER and other proposed fusion energy reactors are developing pebbles of lithium-bearing ceramics for this purpose. The pebbles sit in a helium-cooled pebble bed, which is also called a breeder blanket.

  The 1954 Castle Bravo nuclear test exposed a second route. It produced an unexpectedly high yield because high-energy neutrons can also make tritium from lithium-7. That reaction is endothermic and consumes 2.466 MeV. Before the test, planners wrongly assumed that lithium-7 would absorb a neutron and then decay through a chain of steps lasting far longer than the explosion.

  After the lithium-7 reaction, the slowed neutrons it leaves behind can still trigger the first, exothermic reaction with lithium-6. Lithium can therefore generate more tritium atoms than the neutrons it absorbs.

  Helium-3 captures thermal neutrons so readily that inside a reactor it converts straight back into tritium, expelling a proton. In principle, that lets operators recycle decayed tritium and hold the stock at the level they want.

  The rest of a reactor's tritium arrives uninvited, from boron, heavy water and fission itself.

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• 4. Cosmic Rays and Byproducts

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  Worldwide, natural sources make about 4 megacuries of tritium per year, which is roughly 415 grams. Cosmic rays create it when a fast neutron with energy above 4.0 MeV strikes atmospheric nitrogen. The global equilibrium inventory stays near 70 megacuries, about 7,250 grams, because decay balances the fixed rate of production.

  In pressurized water reactors, boric acid serves as a chemical shim. Relatively fast neutrons striking boron-10 in that acid occasionally yield tritium. These reactors only partly slow their neutrons, so the quantities are small but non-negligible. A more common outcome of boron-10 neutron capture is lithium plus a single alpha particle.

  Heavy water reactors make tritium whenever a deuterium nucleus captures a neutron. Deuterium has a small absorption cross section, which makes heavy water a good moderator, so relatively little tritium forms. Even so, cleaning the moderator after several years can reduce the risk of escape to the environment. Ontario Power Generation's Tritium Removal Facility can process up to 2,500 t of heavy water a year and separates out about 2.5 kg of tritium.

  CANDU reactors typically produce 130 g of tritium per year. It is recovered at the Darlington Tritium Recovery Facility, attached to the 3,512 MW Darlington Nuclear Generating Station in Ontario. Between 1989 and 2011, that facility produced 42.5 kg, an average of about 2 kg per year.

  India also runs a large fleet of pressurized heavy water reactors, which began as CANDU technology and were later indigenized as IPHWR designs. India removes at least some tritium from the moderator and coolant. Less is publicly known than in Canada, because tritium has dual uses and India has a nuclear bomb program.

  Uranium-235, plutonium-239 and uranium-233 all yield tritium as an uncommon product of ternary fission, so every reactor makes some. The production is a side effect, never a goal. Some power plants discharge small quantities to the atmosphere. Reprocessing and spent fuel storage must account for it too.

  Voloxidation is an optional step in reprocessing that removes volatile fission products, including all isotopes of hydrogen, before the aqueous process begins. In principle, it permits economic recovery of tritium. Even if the tritium is merely disposed of, removing it reduces contamination of the process water. Tritiated water cannot be separated from ordinary water except by isotope separation.

  Its specific activity is 9650 curies per gram, so one terabecquerel of tritium weighs roughly 2.8 mg.

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• 5. Inside the Bomb

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  Nuclear weapons use tritium in three ways, and the neutron initiator is the simplest. In that device, an ultrafast switch such as a krytron actuates a small particle accelerator. The accelerator drives ions of tritium and deuterium to energies above the roughly 15 kiloelectronvolts needed for fusion. It fires them into a metal target, where the two are adsorbed as hydrides. The resulting fusion neutrons radiate in all directions, and some strike plutonium or uranium nuclei in the pit of the bomb. That starts the chain reaction, after explosives have compressed the pit to critical mass. The burst is large in absolute terms, so the pit reaches neutron levels that would otherwise take many more generations of chain reaction.

  A few grams of tritium-deuterium gas are injected into the hollow pit of fissile material before detonation in a process called boosting. The early stages of the fission chain reaction supply enough heat and compression to start deuterium-tritium fusion. Fission and fusion then proceed in parallel. Fusion supplies highly energetic neutrons of 14.1 MeV, which make the fission last longer and run faster. The added yield comes overwhelmingly from extra fission. The fusion energy itself is much smaller, because the fusion fuel is so small.

  Boosting brings several effects. The yield rises for the same amount of fission fuel. Varying the amount of fusion fuel makes variable yield possible. The bomb needs less of the very expensive fissile material. Nearby nuclear explosions can no longer set it off early. The implosion setup can be less demanding, with a smaller and lighter amount of high explosive.

  Four grams per warhead is the estimated quantity of tritium a boosted bomb needs. The tritium decays continually, and its product helium-3 absorbs neutrons. Too much helium-3 can offset or reverse the intended effect. So boosted bombs need fresh tritium periodically, about 0.20 g per warhead per year to hold the level constant. One mole of deuterium-tritium gas holds about 3.0 g of tritium and 2.0 g of deuterium. By comparison, the 20 moles of plutonium in a nuclear bomb come to about 4.5 kg of plutonium-239.

  A true hydrogen bomb needs a far larger charge of heavy hydrogen isotopes in its secondary stage. Because tritium decays, the secondary uses solid lithium deuteride as the source of both deuterium and tritium. During the Teller-Ulam staging, the primary fission bomb detonates first. The sparkplug, a cylinder of uranium and plutonium at the center of the fusion stage, then begins to fission with excess neutrons channeled from the primary. Its neutrons split lithium-6 into tritium and helium-4. Lithium-7 splits into helium-4, tritium and one neutron. Photons from the primary and fission of the surrounding jacket compress the fusion stage meanwhile. The stage breeds its own tritium as the device detonates, and some of it fuses with deuterium, releasing still more neutrons.

  Lithium deuteride is not used in boosted bombs. Its fusion needs extremely high ignition temperature, and it produces fewer and less energetic neutrons.

  The special heavy water reactors at the Savannah River Site made tritium for American nuclear weapons until their closure in 1988. After the Strategic Arms Reduction Treaty and the end of the Cold War, existing supplies were enough for the smaller number of weapons for some time. The United States produced 225 kg of tritium from 1955 to 1996. Decay left about 75 kg at the time of the report and about 16 kg as of 2023.

  Production resumed from 2003 to 2005, when reactors at the commercial Watts Bar Nuclear Plant irradiated rods containing lithium. The rods replaced the usual control rods of boron, cadmium or hafnium. Extraction at the Tritium Extraction Facility at the Savannah River Site began in November 2006. Tritium leaks from the rods during operation, which caps how many any one reactor can hold without exceeding the maximum allowed tritium levels in its coolant.

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• 6. Fusion and Glowing Lights

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  The deuterium-tritium reaction releases 17.6 MeV, and that is why tritium figures so prominently in fusion research. Nuclei carry positive charge and repel one another under Coulomb's law. At high enough temperature and pressure, as in the core of the Sun, random motion overcomes the repulsion. The strong nuclear force then takes effect and fuses the nuclei into heavier atoms.

  A triton has the same charge as any hydrogen nucleus, and it feels the same electrostatic repulsion. Its two neutrons, however, add to the strong nuclear force once it is close to another nucleus. So tritium fuses with light atoms more easily than ordinary hydrogen does. Deuterium shows the effect to a lesser degree. That is why brown dwarfs, the failed stars, cannot fuse normal hydrogen yet do fuse a small minority of their deuterium nuclei.

  The National Ignition Facility burns deuterium-tritium fuel, and the experimental reactor ITER will do so as well. Both magnetic and inertial confinement designs rely on it. The reaction has the largest fusion cross section of any candidate fuel, about 5.0 barns. It reaches that maximum at the lowest energy, about 65 kiloelectronvolts at center of mass. The Tritium Systems Test Assembly at Los Alamos National Laboratory was built to develop and demonstrate the technologies for fusion-relevant deuterium-tritium processing.

  Beta particles from small amounts of tritium make chemicals called phosphors glow, a process called radioluminescence. Self-powered lights known as betalights use it for night illumination. They appear in firearm sights, watches, exit signs, map lights and knives. Navigational compasses, such as the current-use M-1950 U.S. military compasses, also carry them. Commercial demand for tritium in such products is 400 g per year, at a cost of 30,000 dollars per gram or more. Novelty items such as self-illuminating key chains use it too.

  NanoTritium batteries show that tritium can also generate electricity. Their betavoltaic design is an atomic battery that converts the beta particles into current. The Zellweger ZE22/3 glow tube is one of several electron tubes that use tritium differently. A small amount ionizes the fill gas, typically neon or argon. That gives a steady current when high voltage is applied and keeps the tube performing reliably. The tritium is sealed in a glass envelope with two electrodes, one of them coated with the radioactive material to create an ion path.

  Tritiation of drug candidates allows detailed analysis of how they are absorbed and metabolized. In one study, retinyl acetate labeled with tritium was traced through the bodies of rats.

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• 7. Risk and Release

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  About 12 days is how long the human body retains tritiated water, with a small portion remaining behind as organically bound tritium. Plants convert tritiated water into the bound form, and animals eat the plants, so tritium can travel along the food chain. The metabolism of the bound form is less well understood. Tritium can also incorporate into RNA and DNA molecules, with somatic and genetic effects that may emerge in later generations. The isotope is a radiation hazard when inhaled, swallowed in food or water, or absorbed through the skin.

  The biological half-life of tritiated water in the body is 7-14 days. That reduces the effect of a single ingestion and precludes long-term bioaccumulation. Studies of radiation workers in coastal Karnataka, India, found that the half-life in winter was twice that in summer. After a suspected exposure, drinking uncontaminated water helps replace the tritium. Sweating, urination and breathing also help the body expel it. Care is needed to avoid dehydration or loss of electrolytes, because those can have more severe short-term consequences than tritium exposure.

  Tritium has leaked from 48 of 65 nuclear sites in the United States. In one case, leaking water held 7.5 microcuries per liter. That is 375 times the current EPA drinking water limit and 28 times the World Health Organization recommendation. It equals roughly 0.8 parts per trillion by mass. The US Nuclear Regulatory Commission reports that in normal operation in 2003-56 pressurized water reactors released 40,600 curies in liquid effluents, an average of 725 curies each. Twenty-four boiling water reactors released 665 curies, an average of 27.7 curies each. The larger figure weighs about 4.207 grams.

  Legal drinking water limits vary widely. Australia allows 76,103 becquerels per liter, a level matched to 1,000 microsieverts of dose per year. Japan allows 60,000 and Finland 30,000. The World Health Organization and Switzerland both set 10,000, Russia 7,700 and Ontario 7,000. The United States allows 740. Norway and Germany allow only 100.

  The United States limit gives 4.0 millirems, or 40 microsieverts, per year under EPA regulation 40CFR141. It rests on dose calculations from National Bureau of Standards Handbook 69, dating from about 1963. Four millirem is about 1.3% of the average natural background of roughly 3 millisieverts. Updated standards from the International Commission on Radiological Protection, used in NRC regulation 10CFR20, give 0.9 millirem, or 9 microsieverts, at the same 740 becquerels per liter.

  In June 2016, the Tritiated Water Task Force reported on tritium in the cooling water stored at Fukushima Daiichi. Tritium in water extracted from the buildings fell tenfold over the five years from 2011 to 2016. That is a drop from 3.3 to 0.3 megabecquerels per liter, after correcting for the 5% annual decay of tritium.

  An expert panel weighed the options for disposal. It reported, "Tritium could be separated theoretically, but there is no practical separation technology on an industrial scale. Accordingly, a controlled environmental release is said to be the best way to treat low-tritium-concentration water." After a public information campaign sponsored by the Japanese government, the gradual release into the sea began on the 24th of August 2023. It was the first of four releases through March 2024. China protested. The IAEA endorsed the plan, and the water is diluted to under 1500 becquerels per liter, far below the limit recommended by the WHO for drinking water. The full process is expected to take decades.

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• 8. Reading the Oceans

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  In 1963, Valentia Island in Ireland recorded 2,000 tritium units in precipitation. One tritium unit equals about 0.118 becquerels per liter. Before the atmospheric weapons tests of the late 1950s and early 1960s, the surface of the Earth held only about 3-4 kg of tritium. After the tests, which took place mainly in the Northern Hemisphere at high latitudes, that amount rose by two to three orders of magnitude. Natural background levels were exceeded by about 1,000 units in 1963 and 1964. Rainwater before the fallout is estimated at between 5 and 10 units.

  Oceanographers found the surplus unexpectedly useful. The tritium oxide that entered the upper layers of the oceans before the Partial Nuclear Test Ban Treaty has been used to measure how fast those layers mix with the water below. Together with chlorofluorocarbons, it acts as a transient tracer that outlines the biological, chemical and physical paths through the world's oceans. In the Northern Hemisphere, it also helps estimate the age of groundwater and build hydrogeologic models.

  In the North Atlantic, tritium in the stratosphere oxidized to water molecules and fell in rainfall. The Transient Tracers in the Ocean program used that bomb-tritium to quantify the replenishment and overturning of deep water. Bomb-tritium enters the ocean through precipitation, vapor exchange and river runoff, and it also enters the deep ocean around the Antarctic. It works as a tracer for time scales of up to a few decades.

  Data from 1981 place the 1-unit isosurface between 500 and 1,000 meters deep in the subtropics. South of the Gulf Stream it reaches 1,500-2,000 meters, because of recirculation and ventilation of the upper Atlantic. Farther north it deepens until it reaches the floor of the abyssal plain, linked to ventilation over 10-20 year time scales.

  Near Bermuda, the tritium maximum moved downward from the surface in the 1960s to 400 meters in the 1980s, about 18 meters per year. Increases at 1,500 meters in the late 1970s and 2,500 meters in the mid-1980s matched cooling events in the deep water. A 1991 study used tritium to follow newly formed North Atlantic Deep Water, which spills over the sills dividing the Norwegian Sea from the North Atlantic. Values there rose to 4 units, and the sub-polar gyre showed more than 1.5. Tritium in the deep western boundary current fell by a factor of 10 from the Labrador Sea to the Tropics, which indicates loss to the ocean interior through turbulent mixing and recirculation.

  A 1998 study sampled surface seawater and the water vapor 10 meters above it at the Sulu Sea, Fremantle Bay, the Bay of Bengal, Penang Bay and the Strait of Malacca. Surface seawater tritium was highest at Fremantle Bay, about 0.40 becquerels per liter, probably from freshwater runoff. Water vapor held roughly ten times as much, from 0.46 to 1.15. The study concluded that this vapor tritium came from natural tritium moving down from the stratosphere.

  In the North Pacific, bomb tritium spread in three dimensions. Mapped on three surfaces of constant potential density, it appeared well mixed at 6-7 units on the 26.81 surface in the subarctic gyre. Exchange with the gyre to the south looked slow. The depth penetration separates into three layers. The shallowest holds about 28% of the total, the middle layer about 58% and the deepest the remaining 14%.

  A 2004 study examined tritium in the Ohio, Missouri and Arkansas rivers, all part of the Mississippi River Basin. Concentrations peaked in 1963. The Missouri River exceeded 1,200 units that year, while the Arkansas never passed 850 and fell below 10 in the mid-1980s. About 780 grams of tritium flowed from the Mississippi into the Gulf of Mexico between 1961 and 1997, an average of 21.7 grams per year. Current flows are 1-2 grams per year, against roughly 0.4 grams per year in the years before the bombs.

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