“Our mission is reliable energy anywhere,” Dr Kurt Terrani begins.
The former National Technical Director at Oak Ridge National Laboratory (ORNL) speaks to TCT in his latest role as Director at Seattle-based Ultra Safe Nuclear Corporation (USNC). He believes USNC is on the ‘bleeding edge’ of nuclear fuel and reactor design, with the commercial availability of Fully-Ceramic-Microencapsulated (FCM) fuel set to represent a ‘watershed moment in zero-carbon energy production’ in the US and beyond.
Supporting the company’s ambition is ExOne’s binder jetting technology, which facilitates a key step in the manufacture of UNSC’s FCM fuel. USNC is working with two value propositions. The first is nuclear reactor designs that, as long as gravity is present and hot fluid rises to the top while cold fluid goes to the bottom, are inherently safe. And the second is the FCM fuel form which brings multiple barriers to radionuclide release, thus ensuring nuclear safety.
FCM sees industry standard TRISO fuel, which contains the radioactive by-products of fission within layered ceramic coatings, encased within a fully dense carbide matrix. Terrani explains: “The whole game is to keep the radiation inside the nuclear reactor core. In some of the [conventional] designs, they build big vessels and big concrete containment domes [but] these folks decided way back when to pass that burden to the fuel itself, so they made small particles of fuel –about a millimetre in diameter – and they put various ceramic coating layers around them.”
Historically, fuel particles might be placed in a carbon glue that keeps small pressure vessels together, but this isn’t deemed to provide ‘a lot of benefit on radionuclide release.’ Mitigating that release is USNC’s goal, and to do that it has sought to place fuel particles in silicon carbide shells, which are mechanically, thermally, and environmentally stable. It has previously been a challenge to manufacture high purity silicon carbide, but additive manufacturing is considered the enabling technology that is facilitating the transition at USNC.
“The gold standard is to sinter the material at very, high temperatures –over 2,000°C – or do this process called chemical vapour deposition, [which is] extremely expensive,” Terrani says. “Then you get big chunks of material that you have to machine. That’s another challenge [because] the only thing that can cut silicon carbide is diamond, so you get down into this game of extremely expensive material, extremely expensive machining. Anybody who looks at that eventually decides to walk away. Well, we couldn’t walk away. Silicon carbide is a material that we need in our value proposition.”
Binder jet 3D printing is the solution here because of its ability to process materials at room temperature – silicon carbide would start dissociating at higher temperatures, so processes that use an electron beam or laser beam can be counted out.
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“We use binder jet to form silicon carbide in extremely complex structures and what’s unique about this technique is it gives you the full 3D freedom,” Terrani continues. “A lot of times with additive, there’s a certain space where you can make parts, but with binder jet, any kind of geometry is achievable. So, then we take this silicon carbide, highly porous, and we take it to another process called chemical vapor infiltration. We have a body that’s made up [of] silicon carbide particles glued to each other. We pick very pure, highly crystalline silicon carbide – because that’s what we want for our application – and then the small amount of binder goes away as you heat it up. Then, you deposit more silicon carbide in the pores via chemical vapour infiltration.”
During the chemical vapour infiltration process, the silicon carbide structures are poured into the furnace, with more silicon carbide being used to fill all the open space. The outcome is a ‘relatively dense, fully sealed on the surface, strong and highly pure silicon carbide in very complex geometry’ that forms the shells that become part of USNC’s FCM fuel form. For the thousands of fuel elements coming off ExOne’s binder jet 3D printers, unique barcodes are integrated during the print process to allow USNC to manage quality. From the outside, these elements look like a chunk of silicon carbide, but inside, the fuel particles have been arranged in a very particular way to integrate things like cooling channels where applicable.
These fuel particles are then used in USNC’s Micro Modular Reactor (MMR) energy systems which, uses helium as an inert gas because it does not react chemically with the fuel or reactor core components. USNC believes its MMR design has the lowest power density and highest surface area to power ratio of any reactor ever commercialised. When multiple MMR systems are linked together, they will be able to power chemical plants, large industrial sites, remote communities, and entire cities. Testing is currently underway in a Dutch reactor, with advanced licensing in progress in Canada for first application and demonstration units scheduled for first nuclear power in 2026.
USNC
Inside the USNC facility.
As USNC plugs away at the development of its MMR systems and FCM fuel form ready for future implementation, the company anticipates investing in more ExOne binder jet systems. It currently runs both the X160Pro machine, which is used for the printing of large non-fuel structures and the X25Pro for making the structures that the fuel is integrated into. USNC also has an Advanced Technology Division focused on space and propulsion applications which also leans on the in-house 3D printing capacity. Some machines are running silicon carbide, and others are running zirconium carbide, with the same chemical vapour infiltration process occurring post-print.
“With these machines, we are doing nuclear quality work,” Terrani says. “Once it goes with a certain kind of powder, forever thou shalt not be changed. We’re not going to switch even though the systems are designed that way. [With] the nuclear quality rigor that’s imposed on us, we need to keep the systems very clean and dedicated to a very specific process. That’s why we have multiple systems; for different components and also different ceramic carbides that lend themselves to our different designs.”
Terrani believes that previous reactor designs, for example, have been starved by the constraints of conventional manufacturing processes, citing the array of rods that make up traditional light water reactors which have only been produced uniformly because the manufacturers didn’t want to change their tooling for each rod. Additive manufacturing provides a solution here for UNSC, just as it does in the silicon carbide shells for its FCM fuel form.
Today, USNC is confident it has overcome all the key research and development hurdles in its path and is now embarking on engineering, licensing, and deployment. As the climate emergency intensifies, the need for cleaner, more reliable energy increases. USNC set out on its mission in 2011, the same year the Fukushima Daiichi nuclear power facility saw all but one of its power generators destroyed by a tsunami. It was the most severe nuclear accident since the Chernobyl disaster in 1986, with 154,000 people having to be evacuated.
Those incidents, while rare and unforeseen, significantly damaged the reputation of nuclear energy. Many sites were decommissioned in the aftermath, while the likes of Greenpeace have long campaigned against the use of nuclear energy. But by bringing about advances in the technology that powers its MMR and FCM – and emphasising the low carbon output of nuclear energy – USNC senses a potential change in the mood music and is one of those pushing forward with a modern and more efficient reactor, powered by newer and cleaner fuel.
“Climate change is upon us,” Terrani finishes. “Anybody who says there’s one solution, I think they’re being insincere, but nuclear is clearly a very significant part of this. Look at nuclear energy as it’s available today, they are 1950s/1960s designs that have been modified a little bit in the 70s and 80s. There are two things that are out of tune with where we are in 2022. Number one, economically, they’re very expensive – you're talking about spending 10-20 billion dollars to build one of these things – and number two, the philosophy of safety was different. In the 50s and 60s, they came up with some bounding scenarios and said, ‘we’ll make sure the reactor is safe under these boundary scenarios […] and anything beyond that, we don’t care about it.’ And I’m not talking about the Eastern designs that dismissed even those basic principles, like Chernobyl.
“The principles of design and philosophy are outdated, and economically, nuclear as it was done previously is doomed. What’s needed? I can tell you. A lot of the time, people think that nuclear is not there as a matter of public opinion. That’s an excuse. Nuclear is not there because the vendors aren’t able to offer it.”
