Rethinking nuclear fuel design
Safer fuel could spur sweeping changes in reactor design
Nuclear engineer Kurt Terrani and lab technician Stephanie Curlin at the controls of an ion-beam milling machine used to prepare samples of both new and irradiated TRISO fuel for study. Photo: Jason Richards
Scientists differ on what actions might have prevented the near destruction of three of the Fukushima Daiichi nuclear plant's six reactors in the wake of the tsunami that struck Japan in March 2011. They agree, however, that the design of the reactors' fuel elements contributed to both the hydrogen explosions that heavily damaged the facilities as well as to the subsequent contamination of land and water near the facility.
The design of the fuel used in the Fukushima reactors, like that used in most commercial reactors, hasn't changed much in 60 years. Its lineage can be traced directly to the reactor designs favored by Admiral Hyman Rickover to power the first nuclear submarines in the 1950s. These systems were, in turn, scaled up for use in commercial power plants.
"Today's nuclear reactors are designed around the strengths and weaknesses of their fuel," says Kurt Terrani, a nuclear engineer in ORNL's Nuclear Fuels Materials group. "The current fuel is extremely reliable under normal circumstances, and it has enjoyed 60 years of development and improvement. However, under extreme accident scenarios, it becomes a reactor's Achilles' heel."
A safer alternative
But what if someone developed a fuel that didn't have this Achilles' heel? How would that affect the US nuclear industry?
"An inherently safe fuel would fundamentally change reactor design and the cost of reactors," says scientist and Materials Science and Technology Division Associate Director Lance Snead. "In current reactors, a lot of expensive systems are dedicated to ensuring that the fission products from burned fuel do not escape from the plant under any circumstances. As a result, nuclear power plants can't compete with natural gas plants in terms of cost. Until we develop a less expensive fuel technology, the nuclear industry will lag behind other energy alternatives, and we will continue to burn fossil fuels."
So what would an inherently safe nuclear fuel look like? Snead suggests that it would have a lot in common with the tristructural-isotropic (TRISO) fuels ORNL has spent the last decade pushing to record levels of performance.
Originally developed for use in rocket engines, TRISO fuel designs have been around for 50 years. They are made up of microspheres of fuel coated with layers of carbon and a radiation-resistant shell of silicon carbide. Each millimeter-wide sphere is basically its own small pressure vessel that traps the radioactive byproducts of nuclear fission, such as xenon and cesium.
Interest in TRISO technology has waxed and waned over the years. However, in 2002, Gary Bell's research group at ORNL was tasked with applying TRISO technology to the development of small cylindrical containers of fuel, called "compacts"—first for testing in Idaho National Laboratory's Advanced Test Reactor and eventually for use in a proposed high-temperature gas cooled reactor.
"When this fuel was placed in the Advanced Test Reactor for three years," Snead says, "the compacts made by John Hunn and his colleagues in our fuels group performed flawlessly. I think that established our TRISO as the new gold standard. It doubled the burn-up, or fuel efficiency, of previous fuels."
Rethinking fuel design
In 2010, an ORNL research team investigating advanced fuels for light water reactors began looking for other ways to apply TRISO technology. One proposal was to encapsulate the long-lived components of commercial nuclear waste in TRISO spheres and then burn them as fuel in commercial reactors. The group calculated that this recycling process would decrease the waste volume by 80 percent, enabling waste repositories to last five times longer.
"While we were developing this variation on TRISO fuel, it was always clear that this kind of fuel design could also be used to replace traditional reactor fuel," Snead explains. "However, because the fuel would have been considerably more expensive, there was little motivation to explore that possibility."
The events at Fukushima in the following year and the obvious shift in research emphasis toward safer, accident-tolerant nuclear fuels caused Snead's group to switch its focus.
One key advantage of TRISO fuels in Fukushima-like accident scenarios is that they completely enclose both the fuel kernel and its waste products in two layers of protection. The fuel itself is wrapped in a SiC sphere, which is then contained within a dense, impermeable SiC matrix. The fuel cladding, which historically has been the primary barrier to release, provides a third barrier.
Another shortcoming of conventional fuel is that its zirconium-based cladding can actually burn under extreme conditions. In fact, the explosions that happened at Fukushima resulted from hydrogen released by steam interacting with burning cladding.
Snead explains that steam attacks SiC exceedingly slowly when compared to its effects on zirconium-based cladding. "If TRISO fuel were subjected to conditions like those at Fukushima, after burning through the cladding, the steam would have to penetrate both the SiC matrix and the TRISO spheres to reach the harmful fission products. It would take dozens or hundreds of hours for that to happen."
Safer for this generation and the next
Bell, leader of the lab's Nuclear Fuel Materials Group, notes that a key consideration in developing a new fuel for use in existing commercial power plants is to design something that looks and performs like the fuel they're already using. That's what motivated Snead and ORNL materials scientist Yutai Katoh to develop a new type of TRISO fuel called FCM (fully ceramic microencapsulated fuel), which is designed to replace the fuel in any fission reactor.
Of course, added safety comes at a price. "FCM production is a multi-step process and requires uranium enrichment of about 19 percent—about four times the level of standard fuel. This would increase the fuel costs for existing reactors, although it could also enable savings in reactors using next-generation designs," Snead says.
"Our goal with FCM is to show that TRISO fuel is safer and more efficient for both current-generation reactors and next-generation designs."
Part of the difficulty of introducing a new fuel to the nuclear industry is the extent to which every aspect of reactor operation is tied to the behavior of the current fuel. "In the 1970s, prior to Three Mile Island, the nuclear energy community hypothesized accident scenarios," Terrani says. "Then they defined design criteria to manage those scenarios. Their philosophy is defense in-depth. Reactors have multiple barriers, multiple systems and multiple levels of safety—all designed to respond when anything goes wrong within the reactor.
"When we talk about using TRISO fuel, or something similar, for the next generation of reactors, we are talking about an opportunity to redefine reactor design based on what we expect to happen under scenarios involving the new fuel."
TRISO is also a potential game-changer for waste disposal. When today's fuel is removed from a reactor, it has turned into a sandy substance called "rubble" that has to be to be processed before it can be disposed of permanently. On the other hand, silicon carbide-based FCM fuel comes out of the reactor essentially "repository-ready" and looking exactly as it did when it went in—at least that has been the case with the surrogate fuel pellets that have been tested so far.
The development of a safer, more efficient alternative to traditional nuclear fuel could bring sweeping changes to the safety, design and operation of commercial nuclear reactors.
"I think in the very near term we will be able to clearly demonstrate that we can manufacture stable FCM fuel," Snead says. "People in the nuclear industry are interested in testing this fuel in a commercial reactor, but the industry is also very cautious. Inserting the FCM fuel into a commercial reactor for the purpose of eventually licensing it would be historic, but it also would be just the start of a very long road to implementing the technology. However, once this fuel or one like it is proven in the current generation of reactors, it will give designers of next-generation reactors a fuel option that doesn't exist today."
That new fuel option, his team believes, will give the nuclear industry the opportunity to rethink reactor design from the ground up for the first time since the very early days of nuclear power. óJim Pearce