We often hear about the “fuel cycle” challenge of fusion – how big of a challenge is that, really? In short: a big one! But more often than not, we hear the challenge described in ways that don’t fully reflect Proxima’s perspective on the topic. So let’s dive in and briefly talk about tritium.

Tritium, the heaviest form of hydrogen, is a naturally unstable element that decays into helium-3 on a timescale of approximately 12 years. There are few uses of tritium outside of fusion aside from self-luminous watches and exit signs, which makes the demand for tritium very limited. The world’s total tritium supply is estimated at approximately 50 kg, and most of it comes from the Canadian heavy-water fission reactors (“CANDU”). Yes, you read that right: 50 kg in total, world-wide!

Let’s put that into context:

• a 100 MW-electric magnetic confinement fusion power plant would consume approximately 17 kg of tritium per year, running continuously.
• A 1 GWe power plant like Stellaris would consume approximately 400 grams of tritium per day.

So we clearly can’t build thousands of fusion power plants using the existing tritium supply chain – one way or another, we need to increase the availability of tritium if we want to make our vision of powering humanity through fusion a reality.

But wait! Do we really need tritium? Can’t we use other combinations of fuels, like deuterium-deuterium, deuterium-helium-3, or proton-boron? Unfortunately, none of these options are realistic as fuel sources for the first generation of fusion power plants, because the “cross section” (probability) of deuterium-tritium fusion reactions is much, much higher (two orders of magnitude!) compared to any other fusion reaction with alternative fuels. So, we’d better get enough tritium.

Modifying existing fission power plants to produce more tritium is one option, but relying on tritium production from fission power plants would create a dependency of fusion on fission, which defeats the purpose if we want to establish fusion as the ultimate source of clean energy that we believe it is.

Researchers have long recognized that it’s highly desirable for fusion power plants to be self-sufficient in their tritium supply.

What that means is that fusion power plants burning tritium should make their own tritium. That’s the role of the so-called “tritium-breeding blankets” that surround the plasma.

The purpose of a blanket is to capture neutrons from deuterium-tritium fusion reactions in order to:

• absorb their energy;
• shield the magnets;
• amplify the fusion power via secondary reactions;
• breed tritium from lithium.

If done successfully, the tritium breeding function of a blanket would produce more than 1 tritium atom for every neutron that is produced by “burning” a tritium atom in the plasma, resulting in net production of tritium. Achieving a Tritium Breeding Ratio (TBR) that is greater than 1 (ideally, with some margin!) is a key challenge of blanket design. That means...

The tritium challenge is a technology challenge, not a challenge of access to natural resources.

In other words, we don’t expect tritium to be mined from a deep seabed or extracted from asteroids. We need to develop blanket technology that we can install in our fusion power plants, with the goal that each plant will only need a little tritium to get started before becoming entirely self-sufficient, and creating a bit of surplus for the next plant in line.

So clearly, the key question for blankets is one of technical feasibility. And let’s be clear: blankets are hard. Very hard. But who ever said that fusion was going to be easy?

The Stellaris engineering concept that Proxima Fusion and its partners published in February 2025 includes a tritium-breeding blanket simulated at TBR>1 (i.e. with tritium self-sufficiency). In other words, each Stellaris-like fusion power plant would not need externally sourced tritium in order to produce power, during the many years of its operation.

Now, it’s important to realize that the Stellaris blanket is a “concept,” not a complete engineering design. Moreover, the Stellaris design aimed at demonstrating that a water-cooled lead-lithium blanket concept can be coherently included in a stellarator power plant, without suggesting that this is the optimal choice. There’s lots of work ahead of us over the next few years to increase the Technology Readiness Level (TRL) of blankets, but we see no roadblock.

Significant work is underway across the world to advance the design of fusion blankets. Both private companies and public institutions are doing research on multiple options – an ecosystem is taking shape. Today, it’s not yet clear which type of blanket design will eventually be deployed in commercial systems.

It’s worth noting that net energy devices like our Alpha demo stellarator won’t need to breed their own tritium, since they’re demonstrators rather than full-fledged power plants. They’ll only need a few tens of grams of tritium overall. The ITER tokamak is expected to test important tritium breeding blanket technology and increase its TRL. It’s now critical that as private industry accelerates the demonstration of fusion systems that produce net energy at an unprecedented speed, our research ecosystem also accelerates on the tritium technology challenge, with more experiments and more funding.

Fusion blankets, as an engineering challenge, are just as complex as they are fascinating. In this short post, we haven’t even touched on the extraction of tritium from blankets, tritium storage, lithium-6 separation, and many more related topics including how to secure a starting inventory of tritium for a fusion power plant.

To get fusion to the grid in the 2030s, there’s no time to waste.