Without this company’s technology, future fusion power plants might never light up

Proponents of nuclear fusion have long promised to create nearly limitless power here on earth by harnessing the same reaction that powers the sun. Today, fusion’s biggest hurdle is ensuring that any fusion power plant produces more power than it needs to operate. The second is ensuring that it has enough fuel to run.

Many fusion reactors are designed to run on a mix of two isotopes of hydrogen, deuterium and tritium. (Common hydrogen atoms have no neutrons; deuterium atoms have one, and tritium have two.) There’s plenty of deuterium, which can be found in seawater, but not nearly enough tritium, which is so rare that it essentially has to be manufactured.

“There’s only 20 kilograms of tritium anywhere in the world right now,” Kyle Schiller, CEO of Marathon Fusion, told TechCrunch. A single commercial-scale power plant will require a few kilograms just to start up, meaning the world has enough tritium for a dozen at most. His startup, which has been operating stealthily, thinks it has a solution to that problem.

Today, the world’s tritium supply is a waste byproduct of a small number of nuclear plants running on fission, the type of nuclear power that has been harnessed for energy since the middle of the 20th century. Assuming that scientists can harness nuclear fusion to create viable power on earth, the first fusion plants will use this supply. Future reactors will depend on the first crop of fusion power plants, which will be designed to generate additional fuel.

“Deployment of fusion devices is this doubling process,” said Adam Rutkowski, Marathon’s CTO. “You’re breeding enough tritium to maintain the steady state consumption by the device, but you also need to breed excess tritium to start up the next reactor.”

That breeding will take place when neutrons unleashed during fusion strike a blanket of lithium. The impact will release helium and tritium, and those products will then be routed out of the reactor core where they can be filtered. Some of the tritium will be injected back into the reactor, while another portion will be reserved as fuel for other reactors.

There’s existing equipment for the task, but it’s only useful for experimental work. It’s efficient and effective, but because experimental reactors run for short periods, it doesn’t have the throughput needed for a commercial power plant. To get to that point, the filtration systems will need “a few orders of magnitude improvement,” Schiller said.

That’s where Marathon hopes to come in. It’s working to refine a 40-year-old technology known as superpermeation that uses solid metal to filter impurities from hydrogen.

It works something like this: The hydrogen and other stuff that needs to be filtered out is first turned into a plasma, though not one as hot as inside the reactor. Using pressure from the reactor exhaust, it’s pressed up against the metal membrane, which allows hydrogen (including tritium) to pass through while blocking everything else. The membrane also happens to compress the hydrogen on the other side, a nifty side benefit.

“The whole idea here is just getting maximal throughput as fast as possible,” Rutkowski said.

Rutkowski and Schiller have been working on the problem for a couple of years now, receiving early support from the Department of Energy’s ARPA-E program and the Breakthrough Energy Fellows program. Recently, Marathon raised a $5.9 million seed round, the company exclusively told TechCrunch. The round was led by the 1517 Fund and Anglo American with participation from Übermorgen Ventures, Shared Future Fund and Malcolm Handley.

Marathon said it has letters of intent from both Commonwealth Fusion Systems and Helion Energy, two fusion startups which have raised $2 billion and $607 million, respectively.

Given that commercial fusion power is still years away — if it’s even possible — Marathon’s bet might seem a bit early. After all, only one fusion experiment has hit breakeven in the scientific sense, which discounts the facility’s overhead, something a commercial power plant can’t do. 

Schiller disagrees that his company is too far ahead of the curve. “We’ve been pretty continuously surprised over the last decade or so just how fast progress [with fusion] has gone,” he said. “I really think that if we wake up one morning and get to breakeven, we’re going to wish we had started even sooner.”