Tritium Management

Tritium is one half of the D-T fusion fuel and is a radioactive isotope of hydrogen. This page walks through what kind of nuclide tritium is, why it must be handled with care, and what engineering tricks are used to safely confine and account for it, starting from intuition and going all the way to the research frontier. It is one of the central engineering themes for making fusion “safe to realize.”

Start with Intuition (High School)

Tritium is a member of the hydrogen family (an isotope). While ordinary hydrogen (protium) has only 1 proton, tritium is a heavy hydrogen with 1 proton plus 2 neutrons, written as T or $^3\mathrm{H}$. It is also called triple hydrogen (tritium).

Tritium is not stable on its own and gradually breaks down over time. This “breaking down” is a decay that emits radiation, and tritium undergoes beta decay with a half-life of 12.3 years. The half-life is the time it takes for exactly half of a given amount of tritium to break down. After 12.3 years it drops to half, and after another 12.3 years to half of that (a quarter of the original). Once it has decayed, it turns into a stable substance called helium-3.

The important point here is that the energy of the beta rays (fast electrons) that tritium emits is very low. How low? Low enough to be stopped by a single sheet of paper or the surface of your skin. In other words, even if you are exposed to tritium from outside your body, the radiation does not reach inside, and external exposure is hardly a problem.

So why is care needed? It is when tritium is taken into the body. If it is swallowed or inhaled and enters the body, even weak beta rays keep hitting cells from the inside. Moreover, because tritium is hydrogen, when it takes the form of water (tritiated water) it easily blends into the water in our bodies and becomes part of us. That is why the watchword of tritium management is “keep it out of the body rather than off the skin.”

Another troublesome issue is that hydrogen is a very small and light element, so it gradually slips through (permeates) even metal walls. Just as air slowly leaks out of a balloon, tritium tries to pass through the walls of containers and piping. So “just putting it in a box” is not enough for peace of mind, and special measures are needed to confine it.

Understand the Physics (Undergraduate)

Tritium decay is the following beta-minus decay:

$$^3\mathrm{H} \rightarrow \, ^3\mathrm{He} + e^- + \bar{\nu}_e$$

This means that one neutron turns into a proton, an electron $e^-$ and an electron antineutrino $\bar{\nu}_e$ are emitted, and it becomes helium-3 ($^3\mathrm{He}$). The energy of the emitted beta rays (electrons) has a continuous distribution, with a maximum of about 18.6 keV and an average of about 5.7 keV. This is a strikingly low value even among radioactive nuclides, and the range of the electrons (how far they can travel) is a few mm in air and only a few micrometers in water or biological tissue. This fact physically underpins the conclusion that “external exposure is negligible and internal exposure is the main concern.”

The speed of decay can be expressed with an exponential function. The number of atoms $N$ at time $t$ can be written as

$$N(t) = N_0 \, e^{-\lambda t}, \qquad \lambda = \frac{\ln 2}{T_{1/2}}$$

where $N_0$ is the initial number of atoms, $\lambda$ is the decay constant, and $T_{1/2}$ is the half-life. Plugging in the half-life $T_{1/2} = 12.3$ years shows that tritium naturally decreases by about 5.5% per year. For fuel stored over the long term, accounting must factor in this loss.

The specific activity, which expresses how much radioactivity there is per unit mass, is very large for tritium at about 360 TBq/g, and even just 1 g produces an enormous number of decays. This is a consequence of the short half-life and the light atoms.

The effect when it enters the body varies greatly with chemical form. Molecular tritium gas ($\mathrm{HT}$) is mostly exhaled without being absorbed, but tritiated water ($\mathrm{HTO}$) mixes completely with the water in the body and is excreted with a biological half-life of about 10 days. The biological half-life is the time it takes for the amount that entered the body to be halved by metabolism. Furthermore, organically bound tritium (OBT), bound to organic matter, stays in the body much longer. To estimate the effective dose from intake, one uses the dose coefficient (committed effective dose coefficient) $e$, computing the dose $E$ (unit Sv) as

$$E = e \times A$$

by multiplying the intake activity $A$ (unit Bq) by the coefficient $e$ (unit Sv/Bq). For ingestion or inhalation of $\mathrm{HTO}$, a value of about $1.8 \times 10^{-11}$ Sv/Bq is used for adults. In actual management, the tritium concentration in urine is measured to back-calculate the body burden and assess the exposure.

Permeability can also be treated quantitatively. Diffusion of hydrogen isotopes in metals can be approximated by Fick’s law, in which the flux is proportional to the concentration gradient, and the diffusion coefficient increases sharply with temperature in the Arrhenius form $D = D_0 \, e^{-E_a/(k_B T)}$. This means that the hotter the wall, the faster tritium soaks in and the faster it escapes.

Deepen the Theory (Graduate)

Tritium safety is built on the philosophy of multiple confinement barriers rather than a single wall. The first barrier is the piping and containers that directly handle tritium, the second barrier is the glovebox or double piping that encloses them, and the third barrier is the building itself. Each barrier functions independently, and they are designed to the single-failure criterion so that even if one of them fails, it does not lead to a release into the environment. The spaces between barriers are stepped down in pressure (cascading negative pressure) so that contaminated air does not flow back outward.

The glovebox of the second barrier is operated with its interior in an inert gas (nitrogen or argon) atmosphere. Keeping the oxygen and moisture concentrations low serves two purposes. One is to suppress the oxidation of $\mathrm{HT}$ into the highly bioavailable $\mathrm{HTO}$. The other is to avoid explosions, because the combustion range of hydrogen in air is wide at about 4-75% by volume and the minimum ignition energy is extremely small at about 0.02 mJ.

Tritium that leaks into the atmosphere is recovered by an atmosphere detritiation system. The standard method is a combination of catalytic oxidation and moisture adsorption. First, a catalyst such as a platinum-group metal oxidizes $\mathrm{HT}$ into $\mathrm{HTO}$, and then a molecular sieve captures it as moisture. A single-stage removal factor of 99.9% or better can be aimed for. The recovered tritiated water is returned to the process that separates hydrogen isotopes via electrolysis or isotope exchange. For isotope separation, cryogenic distillation at ultra-low temperatures of 20-25 K is used, separating $\mathrm{H_2}$, $\mathrm{D_2}$, and $\mathrm{T_2}$ by their slight differences in boiling point. Palladium membrane permeation, which selectively passes only hydrogen, is also effective for purification. For storage, metal hydrides such as ZrCo are used, allowing reversible operation that releases on heating and absorbs on cooling.

Tritium accountancy also involves theoretical challenges. From the standpoint of nuclear material protection, it is necessary to always know where and how much tritium exists throughout the system, but tritium changes form among gas, water, and solid materials, and moreover dissolves into the walls. The amount that accumulates in materials is called retention. When tritium is taken up on the surface or inside plasma-facing materials (tungsten, or the carbon-based materials once considered), the fuel not only goes “missing” but also becomes waste requiring detritiation. Detritiation uses baking (heating to drive out tritium), isotope exchange, surface treatment, and so on, and its effectiveness varies greatly depending on material temperature, irradiation defects, and the presence of co-deposited layers. Modeling of retention and detritiation uses reaction-diffusion equations that couple diffusion, trapping, and desorption (a framework solved, for example, by codes such as TMAP or TESSIM).

Where tritium comes from also matters. In a fusion reactor, tritium is bred in the blanket by hitting lithium with neutrons. Because DEMO-class reactors cannot rely on external supply, they need to produce more tritium than they consume, and the indicator for this is the tritium breeding ratio (TBR). Only when the TBR is about 1.1 or higher does the entire fuel cycle become self-sustaining.

Research Frontier (PhD)

Here are some themes still under active research, together with keywords often encountered in papers. Please understand them as challenges toward a real reactor rather than as completed technologies that can be stated with certainty.

For tritium behavior in materials, tritium accumulation in and removal from plasma-facing materials (fuel retention and removal) is the central issue. The questions are how to predict and how to detritiate trap sites in tungsten, capture at defects produced by neutron irradiation, and uptake into co-deposited layers. The experimental validation of reaction-diffusion models and the uncertainty of extrapolation to real-machine scale are points of contention.

For confinement and reduction of permeation, the durability of permeation barrier coatings (such as oxide films) and the maintenance of performance under high temperature and neutron irradiation are being studied. The recovery efficiency of tritium from piping and breeding materials is also a challenge for scaling up with continuous operation in mind.

For the entire fuel cycle, the TBR margin needed to achieve tritium self-sufficiency, the minimization of the initial inventory required at start-up (start-up inventory), and the trade-off between processing-system response time and inventory are being discussed. The operating experience of ITER’s tritium plant (comprising storage and supply systems, exhaust processing systems, isotope separation systems, water processing systems, and atmosphere detritiation systems) is expected to be valuable input for future designs.

On the accounting and regulatory side, improving the accuracy of tritium accountancy for tritium that changes form, and developing safety assessments and a regulatory framework that reflect the passive safety characteristic of fusion, are being pursued in various countries. Fusion facilities have no chain reaction like fission and are hard to run away, but because they handle large amounts of the highly mobile radioactive material tritium, an evaluation approach different from that of fission reactors is required. In assessing environmental releases, the environmental behavior of $\mathrm{HTO}$ and the transfer of organically bound tritium (OBT) through ecosystems are subjects of modeling.

Check Your Understanding

Q1. The beta rays of tritium are weak enough to be shielded by a single sheet of paper, so why does tritium need to be handled with care?

A Because the beta rays penetrate the skin and reach the organs, making external exposure serious B Because once taken into the body, even weak beta rays keep hitting cells from the inside, and tritiated water in particular easily blends into the water in the body C Because the beta rays permeate metal and cause the surrounding equipment to fail D Because the helium-3 produced in the decay emits strong radiation

Q2. The half-life of tritium is 12.3 years. About how many years does it take for an initial amount to drop to one eighth?

A About 24.6 years B About 98.4 years C About 36.9 years D About 12.3 years

Q3. Which is the correct combination of reasons for keeping the oxygen and moisture concentrations low in a glovebox atmosphere?

A To suppress conversion of HT into HTO by oxidation, and to prevent explosions from the wide combustion range and small ignition energy of hydrogen B To prevent degradation of the catalyst, and to slow down the decay of tritium C To prevent corrosion of the metal walls, and to make the walls absorb the beta rays D To promote isotope separation, and to lower the specific activity

Q4. What does retention refer to, and why is it a problem?

A A phenomenon where tritium dissolved in the coolant corrodes the piping, shortening the life of the device B A phenomenon where tritium leaks from storage tanks, leading to environmental release C A phenomenon where tritium accumulates inside plasma-facing materials and the like, making the fuel unrecoverable, complicating accounting, and producing waste that requires detritiation D A phenomenon where tritium diminishes through natural decay, requiring the fuel to be replenished

Q5. What indicator is needed for tritium self-sufficiency to hold in a DEMO-class fusion reactor, and what value is the target?

A The fusion gain Q, with a target of about 10 or higher B The tritium breeding ratio (TBR), with a target of about 1.1 or higher C The energy confinement time, with a target of about 1 second or longer D The specific activity, with a target of about 360 TBq/g or higher

Related Topics

• Fuel Cycle - The overall flow of the system that recovers, purifies, and recycles tritium
• Blanket - The mechanism that breeds tritium from lithium, and the TBR
• Radioactive Waste - Detritiation and waste management of tritium-containing materials