Marking another milestone in an industry-led accelerated quest to commercialize accident tolerant fuels (ATF), nuclear fuel test rods containing Westinghouse’s EnCore technologies that completed their inaugural cycle at Exelon’s Byron nuclear plant have shipped to Oak Ridge National Laboratory (ORNL). While initial visual inspections show “no signs of degradation,” the irradiated fuel will now be examined over the next year to support licensing efforts with the Nuclear Regulatory Commission (NRC).
The test fuel rods represent the tenth batch of ATF lead test assemblies (LTAs) that have so far been loaded and removed from various nuclear plants across the country under the U.S. Department of Energy’s (DOE’s) ATF program.
Since the initiative was launched under a Congressional mandate in the wake of the Fukushima accident in 2011, it has spearheaded critical research developments and allowed three U.S. fuel suppliers—Framatome, GE’s Global Nuclear Fuel (GNF), and Westinghouse—to make remarkable gains in developing and piloting several ATF fuel concepts at existing nuclear plants.
The recent batch removal is also notable because it marks Westinghouse’s first milestone for two near-term ATF designs, which it is working to commercialize under its EnCore brand. However, with the closure of Exelon’s economically struggling Byron plant looming, how and where Westinghouse will continue its ATF testing remains in question.
Why Industry Is Banking on This New Generation of Nuclear Fuel Technology
ATF, an industry concept used to describe new technologies in the form of new cladding and/or fuel pellet designs, has morphed into a key priority for the U.S. nuclear power industry because it promises to improve the existing nuclear fleet’s cost competitiveness in power markets that are increasingly inundated with cheap renewables and natural gas power.
According to industry group Nuclear Energy Institute (NEI), ATF fuels could increase efficiency, improve performance, and reduce operating costs. They essentially leverage new materials that improve fission product retention and are structurally more resistant to radiations, corrosion, and high temperatures. And because they could potentially extend the time between refueling from 1.5 to 2 years, ATFs promise to reduce the amount of fuel needed by about 30%, resulting in less waste and used nuclear fuel.
While ATF technologies have been under development since the early 2000s, they received a marked boost in the wake of the March 2011 Fukushima accident as the DOE aggressively implemented plans under its Congressionally mandated Enhanced Accident Tolerant Fuel (EATF) program to develop ATFs for existing light water reactors (LWRs). The DOE has said these technologies are being developed under an “accelerated timeframe,” owing mainly to the status of the nation’s current fleet of reactors.
“Many of them have 60-year operating licenses and without renewal are set to expire in the 2030s,” the DOE noted. “Getting these ATFs to market before then would increase their performance and improve their prospects for extended operation.”
After five years of feasibility studies, the program identified promising concepts from Framatome, GNF, and Westinghouse. Pivotally, it also paved the way for LTAs—fuel assemblies that contain design features or materials—to be installed in reactor cores at select nuclear plants across the country.
Combined Progress for Cladding and Fuel
ATF concepts under development today under the DOE’s initiative generally fall into two categories—cladding and fuel.
Cladding for LWRs has historically been fabricated from zirconium alloys—Zircaloy-2, mainly, for boiling water reactors (BWRs) and Zircaloy-4 for pressurized water reactors (PWRs). But with a growing demand for higher burnup at LWRs, vendors are researching and testing new approaches to address in-reactor cladding corrosion. Among near-term concepts are new forms of zirconium alloy cladding (coated with a thin layer of either chromium or proprietary material), and iron-chromium-aluminum (FeCrAl) cladding.
At the same time, fuel vendors are also researching and testing fuel pellets that mix other materials—called “dopants”—into the pellet during manufacturing to essentially change the physical properties of the resulting fuel pellet. “Doped pellets” may reduce the rigidity of the fuel pellet and reduce cladding damage risks, as well as increase the ceramic grain size to promote fission gas retention, ultimately decreasing the release of radioactive gases during a postulated accident. The NRC notes that it has already approved doped pellets for BWRs made by GNF and Framatome, and doped pellets have already recently been batch loaded by at least one licensee at Brunswick Units 1 and 2.
Backed by the DOE and monitored by the NRC, since spring 2018, GNF, Westinghouse, and Framatome have loaded LTAs containing “near-term” ATF technologies at 10 nuclear plants. Tests of loaded fuel are ongoing or planned at another four nuclear plants.
A Brief Recap of ATF Achievements
So far, GNF has tested several different iron-chromium-aluminum (FeCrAl) alloys for cladding. In December 2020, ORNL said it received the first GNF-developed nuclear fuel test rods, which spent 24 months at Southern Co.’s Edwin Hatch nuclear plant. The test samples feature IronClad, an FeCrAl fuel cladding, and ARMOR, a hard, oxidation-resistant coating layered on top of zirconium cladding with uranium oxide (UO2) fuel (which GNF originally created outside of the DOE’s ATF program).
Framatome, meanwhile, is working to commercialize a near-term ATF design that involves chromium-coated zirconium alloy cladding with chromia (Cr2O3)–doped UO2 fuel. Four GAIA LTAs containing Framatome’s advanced chromium coating (which is added to its proprietary M5 zirconium alloy cladding) and the chromia-doped pellets were removed after an 18-month cycle at Southern Co.’s Vogtle 2 plant in August 2020. Two other 18-month cycles of operation for the LTAs are planned. Framatome’s concepts are also being tested at Entergy’s Arkansas Unit 1. In the fall of 2019, Arkansas 1 inserted 32 Framatome Cr-coated lead test rods. Two full assemblies with Cr-coated M5 cladding and Cr2O3-doped UO2 fuel were also planned for insertion at Calvert Cliffs in spring 2021.
Westinghouse is also testing several “near-term” technologies developed under its EnCore Fuel brand. These include “improved chromium-coated cladding that inhibits the zirconium-steam reaction and increases maximum temperature by an additional 300C,” as well as Westinghouse’s proprietary ADOPT fuel pellets, which are chromia and alumina (Cr2O3+Al2O3)—doped UO2. The additives in the doped fuel “facilitate greater densification and diffusion during sintering, resulting in a higher density and an enlarged grain size as compared to undoped UO2,” the company said in topical report that the NRC accepted for review in September 2020.
A Milestone for Westinghouse
The fuel rods now being examined at ORNL were loaded at Byron 2 in April 2019, removed during a fall 2020 refueling outage, and were shipped by NAC International to ORNL this June. “The shipment was made possible through coordinated efforts between ORNL, the lab’s DOE Site Office, INL, Westinghouse, NAC International, and the commercial operator,” the DOE said.
The LTAs reportedly contained 12 rods with the chromium-coated ZIRLO cladding and standard UO2 fuel; four rods with standard cladding and segmented uranium silicide (U3Si2) fuel; and four rods with chromium-coated Optimized ZIRLO cladding and ADOPT fuel. Westinghouse began using Optimized ZIRLO cladding, a reduced-tin zirconium alloy material, in 2000, first in Spain, and then in the U.S. in 2010.
According to Pacific Northwest National Laboratory (PNNL), which is providing technical assistance to the NRC related to the newly proposed nuclear fuel and cladding designs, Westinghouse was expected to install further LTAs containing chromium-coated cladding with ADOPT, UO2, and U3Si2 fuels for irradiation in 2022.
However, it is unclear how Westinghouse’s test schedule will fare, given that Exelon on July 28 announced told the NRC it planned to permanently close the 1985-built Byron plant in September. POWER has reached out to Westinghouse for detail, and it will update this article when more insight is received.
Westinghouse Reportedly Prioritizing Uranium Nitride Development Over Silicide Pellets
As NRC documents indicate, Westinghouse’s ATF development priorities may also be in flux. Among some much-watched initiatives is that Westinghouse has been working with Idaho National Laboratory (INL) to develop uranium nitride (UN) to replace UO2 in fuel pellets.
But according to the NRC, while the company was originally exploring U3Si2 for use with LTAs, after further research, “Westinghouse determined U3Si2 was not viable for further development and changed their efforts towards the advancement of UN.” UN pellets, the NRC says, may offer a 40% higher uranium density over UO2 (equivalent to 7% U-235 enrichment). That attribute could promote higher power or longer fuel cycles and increase fuel thermal conductivity. However, UN needs a rare isotope of nitrogen and it has a “high chemical reaction rate with LWR coolants at nominal operating temperatures,” the NRC said.
At the same time, the company continues work with General Atomics to develop silicon carbide (SiC) fiber–reinforced SiC matrix (SiC/SiC) composite cladding. SiC composites (which Framatome is also exploring) could maintain structural integrity at very high temperatures, even beyond the melting point of UO2, and it could improve high-temperature steam oxidation. On the flip side, however, SiC cladding still needs to overcome its increased permeability to fission gases, corrosion during normal reactor operation, a lack of ductility, and manufacturing challenges. Owing to their challenges, industry (as well as the NRC) generally view silicon carbide cladding and UN pellet technologies as “longer-term” ATF technologies.
Yet another notable longer-term ATF concept—but one that is not backed by the DOE’s ATF program—is Lightbridge’s fuel design, which incorporates an extruded metallic bar that comprises a zirconium-uranium matrix within a zirconium alloy cladding. Lightbridge suggests that because the fuel design allows new reactors to operate at “significantly lower temperatures while extracting more heat from the fuel core and delivering greater electricity output,” existing PWRs could achieve 10% power uprates and extend their fuel cycles from 18 to 24 months. The NRC says it is following developments in Lightbridge technology. However, while it recognizes the design’s fuel thermal conductivity and safety attributes, it also highlights a key challenge—that the fuel may need up to 19.8% of uranium-235 enrichment.
As POWER has reported, high-assay, low-enriched uranium (HALEU) is currently neither commercially available in the U.S. or allowed under current NRC regulations, because current regulations limit power reactor fuel at 5% by weight (with exceptions). However, the NRC says it is preparing to review exemptions and amendments regarding higher enrichment across the entire fuel. The DOE is also funding efforts to demonstrate HALEU production by May 2022 at Centrus Energy Corp.’s enrichment facility in Piketon, Ohio. The Centrus demonstration effort this June, notably, also received the NRC’s approval.
DOE Still Shooting for Commercial ATF Introduction by 2025
Still, optimism about the near-term commercialization of ATF technologies continues. Keeping with ambitions rolled out in a January 2021–issued technology blueprint, the DOE’s Office of Nuclear Energy confirmed to POWER on Aug. 4 that it is “still on track for initial commercial introduction of accident tolerant fuel by 2025.”
The Biden administration, too, appears committed to the effort. In the DOE’s June 2021–issued budget request for fiscal year 2022, the administration urged Congress to increase funding for its ATF efforts—which are now contained in a “subprogram” under the agency’s Fuel Cycle Research and Development (FCR&D) program—from $105,800 to $115,000. The funding increase will help expand test capabilities at INL and replace lost capabilities stemming from the 2018 closure of the Halden test reactor in Norway. “These capabilities are boiling water reactor conditions, highly-instrumented test trains, ramp testing, and dry out testing,” the request says.
But funding also reflects increased R&D planned by the various fuel vendors to support their near-term ATF concepts. “This subprogram supports the industry’s objective to install the first reload quantities of accident tolerant fuel in pilot plants by the mid-2020s and qualify the fuel for use at higher burnup levels,” the budget request says.
“In FY22 this will involve cost-shared testing and examination of fuel and cladding material performance to generate data that can be used by industry partners to support their NRC licensing efforts, research and development of pilot fuel pellet and cladding manufacturing equipment, analysis and redesign of fuel fabrication processes, and revising fuel performance codes and methods.”
Meanwhile, as directed by Congress under the 2019-enacted Nuclear Energy Innovation and Modernization Act (NEIMA), the NRC has developed an ATF Project Plan—essentially a strategy that outlines how the regulatory agency will perform efficient reviews of submittals “within the expedited timelines requested by the vendors and licensees through a new paradigm.”
The NRC says it is already capable of reviewing ATF submittals: “The near-term technologies can be licensed if submitted today without any changes to the current regulations and guidance,” it says.
—Sonal Patel is a POWER senior associate editor (@sonalcpatel, @POWERmagazine)