What’s so hot about TRISO?
Let’s unwrap the forbidden poppyseed muffin and find out why so many advanced reactors call for it
Amazon recently announced it is putting US$500 million into X-energy, a Small Modular Reactor (SMR) start-up based in Maryland. This is hot on the heels of Google signing a Power Purchase Agreement for 500MW in new nuclear projects by Kairos Power. What does X-energy and Kairos have in common? Their SMRs both use TRi-structural ISOtropic (TRISO), an engineered fuel that is probably the most expensive fuel choice in nuclear, but also takes care of a lot of problems.
Enter the Dragon
A reactor using fuel like no other went into operation in Dorset, England in 1966. The Dragon Reactor used helium gas coolant, graphite as the moderator and fuel formed into tiny pellets coated in ceramics. This high-temperature reactor concept promised better thermal efficiency, better uranium utilization and better inherent safety. Unfortunately, the Brits fumbled it. The TRISO concept was sidelined despite the technical success of the Dragon because the Advance Gas-cooled Reactor (AGR) was deemed to have a smoother path to commercialization. While the AGR did provide decades of service, significant life-extension was impossible due to graphite core aging, making it uncompetitive with mainstream Light Water Reactors.
The Forbidden Poppyseed Muffin
Others soon saw the beauty of TRISO, which the Office of Nuclear Energy calls “the most robust nuclear fuel on earth.” The tiny grain of fissionable fuel is wrapped in three layers — a porous carbon buffer to absorb fission products, a denser, structural carbon layer, and finally a high-strength high-density layer of Silicon Carbide that serves as a pressure vessel, keeping radioactivity in and providing structural integrity.
“The outstanding safety of the TRISO fuel derives from the ability of the silicon carbon and pyrolysis carbon layers to withstand very high temperatures without releasing the fission products,” said Professor Jacopo Buongiorno of MIT, “So you could say that each TRISO particle has its own containment shell.”
Once you have a bunch of these particles, you embed them in a graphite matrix like poppyseeds in a poppyseed muffin. Then an overcoat of graphite goes over the whole fuel compact, which is typically in a spherical shape, just like you’d put cling wrap over a muffin before it goes in the picnic basket.
Even in the event of a reactor malfunction when all the active cooling is lost in the core, the TRISO particle is “meltdown proof.” The reactor shuts itself down with no operator intervention without the TRISO fuel reaching excessive temperatures.
In fact the safety profile of the TRISO fuel is so good that the US Nuclear Regulatory Commission (NRC) accepts that TRISO-fueled reactors don’t have to have the kind of bulky steel and concrete containment structures that regular Light Water Reactors have to have, containment so robust they must withstand the potential impact of an aircraft.
Simpler. Smaller. Safer. What’s there not to love?
The Downsides to TRISO
Here’s the big downside to TRISO: it’s very expensive to make. While I don’t have a concrete price, I’ve heard US$10,000/kg just for the fabrication cost.
“TRISO is the most expensive fuel there is out there,” said Buongiorno, “It has safety benefits, but also forces certain design choices like higher enrichment and low power density which are counter to economic competitiveness.”
Which brings me to another potential downside: the tiny kernel of fuel used in TRISO is High-Assay Low Enriched Uranium (HALEU), the whiskey to normal Low Enriched Uranium’s beer. The supply chain for HALEU, which can be up to 20% enriched, is less certain. Making it isn’t rocket science, but it’s plagued by the chicken-and-the-egg supply chain issues where the producers are reluctant to invest in facilities until they see the demand with their own eyeballs. The supply chain for the TRISO fabrication itself is also quite limited.
The high costs, uncertain supply chains and the untested nature of advanced reactors designs are all legitimate causes for concern, especially with the aggressive first-power dates attached to the recent announcements. (2030 for the Kairos project, 2029 for X-energy.)
THE ELEMENTAL TAKE
The key trade-off with TRISO-fueled reactors is this: your build should be much cheaper and faster because you don’t have to build the big hulking containment building, but in return you’re stuck with fuel costs that are much, much higher.
There’s a lot of uncertainty over how this trade-off will shake out in the long term, but in the short-term we can expect power from these new reactors to be incredibly expensive, to a degree that people might not fully realize yet. To me, the cheapness of the fuel is one of nuclear power’s best characteristics. By going to TRISO, much of that benefit is gone. In time we might have Gen-VI microreactors that doesn’t use TRISO, but for now they’re not yet on the market.
I can understand that Amazon et al are in a hurry. They probably like 2029/2030 a lot better than what a new-build AP-1000 can be delivered. They appear willing to pay a steep premium for the promise of that earlier delivery date. But I hope they are not putting all their eggs in one basket.
According to the Department of Energy’s latest updated Liftoff report on Advanced Nuclear, the lowest-cost path to nuclear is clear — a standardized fleet build of traditional gigawatt-scale Light Water Reactors. I hope that we will hear some news of deals related to that effort soon.
Thanks for the clear writing on an important topic... One comment. Expensive? How do we know that? Until 2 companies have factories making something at scale, and have 5 years to streamline the supply chain, no human on Earth can say what the cost will be. There has never been a serious factory making Triso.
So we don't know whether it's expensive or not. It would be expensive as part of a research program, assembled by graduate students or by hand, one at a time by human workers.
But how much would it cost in a factory?
What I can say is all the materials are cheap. It doesn't look much harder to make than an IC chip.
Seems like a TRISO fueled reactor would be more like a mid-merit plant in the disptch stack.
If (big if) the Capex and fixed Opex can be reduced enough, that could be interesting.
I think they have a more obvious fit for remote sites applications and in shipping, where the competing fuel costs are high, and minimizing staffing / install complexity is the top priority.